Vallesamidine and schizozygane alkaloids: rearranged monoterpene indole alkaloids and synthetic endeavours

Covering 1963 to 2023Monoterpene indole alkaloids are the main sub-family of indole alkaloids with fascinating structures, stereochemistry, and diverse bioactivities (e.g., anticancer, anti-malarial and anti-arrhythmic etc.). Vallesamidine alkaloids and structurally more complex schizozygane alkaloids are small groups of rearranged monoterpene indole alkaloids with a unique 2,2,3-trialkylated indoline scaffold, while schizozygane alkaloids can generate a further rearranged skeleton, isoschizozygane, possessing a tetra-substituted, bridged tetrahydroquinoline core. In this review, the origin and structural features of vallesamidine and schizozygane alkaloids are introduced, and a discussion on the relationship of these alkaloids with aspidosperma alkaloids and a structural rearrangement hypothesis based on published studies is followed. Moreover, uncommon skeletons and potential bioactivities, such as anti-malarial and anti-tumour activities, make such alkaloids important synthetic targets, attracting research groups globally to accomplish total synthesis, resulting in impressive works on novel total synthesis, formal synthesis, and construction of key intermediates. These synthetic endeavours are systematically reviewed and highlighted with key strategies and efficiencies, providing different viewpoints on molecular structures and promoting the extension of chemical space and mining of new active scaffolds.

New Sustainable Synthetic Routes to Cyclic Oxyterpenes Using the Ecocatalyst Toolbox

Cyclic oxyterpenes are natural products that are mostly used as fragrances, flavours and drugs by the cosmetic, food and pharmaceutical industries. However, only a few cyclic oxyterpenes are accessible via chemical syntheses, which are far from being ecofriendly. We report here the synthesis of six cyclic oxyterpenes derived from ß-pinene while respecting the principles of green and sustainable chemistry. Only natural or biosourced catalysts were used in mild conditions that were optimised for each synthesis. A new generation of ecocatalysts, derived from Mn-rich water lettuce, was prepared via green processes, characterised by MP-AES, XRPD and TEM analyses, and tested in catalysis. The epoxidation of ß-pinene led to the platform molecule, ß-pinene oxide, with a good yield, illustrating the efficacy of the new generation of ecocatalysts. The opening ß-pinene oxide was investigated in green conditions and led to new and regioselective syntheses of myrtenol, 7-hydroxy-α-terpineol and perillyl alcohol. Successive oxidations of perillyl alcohol could be performed using no hazardous oxidant and were controlled using the new generation of ecocatalysts generating perillaldehyde and cuminaldehyde.

Transformations of Monoterpenes with the p-Menthane Skeleton in the Enzymatic System of Bacteria, Fungi and Insects

The main objective of this article was to present the possibilities of using the enzymatic system of microorganisms and insects to transform small molecules, such as monoterpenes. The most important advantage of this type of reaction is the possibility of obtaining derivatives that are not possible to obtain with standard methods of organic synthesis or are very expensive to obtain. The interest of industrial centers focuses mainly on obtaining particles of high optical purity, which have the desired biological properties. The cost of obtaining such a compound and the elimination of toxic or undesirable chemical waste is important. Enzymatic reactions based on enzymes alone or whole microorganisms enable obtaining products with a specific structure and purity in accordance with the rules of Green Chemistry.

Structure-Odor Activity Studies on Derivatives of Aromatic and Oxygenated Monoterpenoids Synthesized by Modifying p-Cymene

Thymoquinone was recently reported as having a unique pencil-like odor and being the impact compound for the cedar-like and cedar wood-based product smell such as pencils. The compounds thymol and carvacrol are structurally related odorants commonly found in plants and foods such as thyme and oregano, also having a significant contribution to their overall aroma. However, a systematic elucidation of the sensory properties in this class of oxygenated, aromatic monoterpenoids has not been carried out. To close this gap and gain new insights into structure-odor relationships leading to pencil-like and woody odors, 19 structurally related derivatives of p-cymene starting from thymol and carvacrol were synthesized and characterized. The compounds had odor thresholds ranging from 2.0 ng/L air to 388.8 ng/L air, being lowest for thymol and carvacrol and highest for thymohydroquinone. The compounds smelled mostly thyme-like, oregano-like, and pencil-like with phenolic, earthy, and medicinal variations in their odor character, which could be successfully linked to structural motifs.

Promising New Inhibitors of Tyrosyl-DNA Phosphodiesterase I (Tdp 1) Combining 4-Arylcoumarin and Monoterpenoid Moieties as Components of Complex Antitumor Therapy

Tyrosyl-DNA phosphodiesterase 1 (Tdp1) is an important DNA repair enzyme in humans, and a current and promising inhibition target for the development of new chemosensitizing agents due to its ability to remove DNA damage caused by topoisomerase 1 (Top1) poisons such as topotecan and irinotecan. Herein, we report our work on the synthesis and characterization of new Tdp1 inhibitors that combine the arylcoumarin (neoflavonoid) and monoterpenoid moieties. Our results showed that they are potent Tdp1 inhibitors with IC₅₀ values in the submicromolar range. In vivo experiments with mice revealed that compound 3ba (IC₅₀ 0.62 µM) induced a significant increase in the antitumor effect of topotecan on the Krebs-2 ascites tumor model. Our results further strengthen the argument that Tdp1 is a druggable target with the potential to be developed into a clinically-potent adjunct therapy in conjunction with Top1 poisons.

Chemoenzymatic Synthesis of the Intermediates in the Peppermint Monoterpenoid Biosynthetic Pathway

A chemoenzymatic approach providing access to all four intermediates in the peppermint biosynthetic pathway between limonene and menthone/isomenthone, including noncommercially available intermediates (-)- trans-isopiperitenol (2), (-)-isopiperitenone (3), and (+)- cis-isopulegone (4), is described. Oxidation of (+)-isopulegol (13) followed by enolate selenation and oxidative elimination steps provides (-)-isopiperitenone (3). A chemical reduction and separation route from (3) provides both native (-)- trans-isopiperitenol (2) and isomer (-)- cis-isopiperitenol (18), while enzymatic conjugate reduction of (-)-isopiperitenone (3) with IPR [(-)-isopiperitenone reductase)] provides (+)- cis-isopulegone (4). This undergoes facile base-mediated chemical epimerization to (+)-pulegone (5), which is subsequently shown to be a substrate for NtDBR ( Nicotiana tabacum double-bond reductase) to afford (-)-menthone (7) and (+)-isomenthone (8).

High Add Valued Application of Turpentine in Crop Production through Structural Modification and QSAR Analysis

Turpentine is a volatile component of resin, which is an abundant forest resource in Southern China. As one of the most important components, the integrated application of β-pinene has been studied. The broad-spectrum evaluation of β-pinene and its analogues has, therefore, been necessary. In an attempt to expand the scope of agro-activity trials, the preparation and the evaluation of the herbicidal activity of a series of β-pinene analogues against three agricultural herbs were carried out. In accordance with the overall herbicidal activity, it is noteworthy that compounds 6k, 6l, and 6m demonstrated extreme activity with IC₅₀ values of 0.065, 0.065, and 0.052 mol active ingredients/hectare against E. crus-galli. The preliminary structure-activity relationship (SAR) was analyzed and the compounds with the appropriate volatility and substituent type that had beneficial herbicidal activity were analyzed. Simultaneously, the quantitative structure-activity relationship (QSAR) model was built and the most important structural features were indicated, which was, to a certain extent, in line with the SAR study. The study aimed to study the application of the forest resource turpentine in agriculture as a potential and alternative approach for comprehensive utilization.

A new approach to 19-nor-A-ring phosphine oxide for the convergent synthesis of 19-nor-calcitriol

A new approach to 19-nor-A-ring phosphine oxide 5 together with a convergent synthesis of the vitamin D₃ analogue 1α,25-dihydroxy-19-norvitamin D₃ (3) have been developed. The 19-nor-A-ring is constructed from (S)-carvone. The triene system is assembled by a Wittig-Horner coupling.

Total Synthesis of (±)-Isoperbergins and Correction of the Chemical Structure of Perbergin

On the basis of its reported chemical structure, perbergin, a Rhodococcus fascians virulence quencher from the bark of Dalbergia pervillei, and its isomer were synthesized in nine steps with a 13.5% yield. However, the NMR spectra of the synthetic products were inconsistent with those reported in the literature. Re-evaluation of the 1D and 2D NMR spectra of the natural product perbergin revealed that the geranyl moiety of this compound is located at C-6 and has an E-configuration, instead of the reported C-8 geranylation with a Z-configuration. Interestingly, the synthetic isoperbergins demonstrated good antibacterial activity against R. fascians, Mycobacterium smegmatis, and Staphylococcus aureus, but not against the Gram-negative bacteria Pseudomonas aeruginosa and Escherichia coli.

[Inhibitory Properties of Nitrogen-Containing Adamantane Derivatives with Monoterpenoid Fragments Against Tyrosyl-DNA Phosphodiesterase I]

It was found that compounds combining diazaadamantane and monoterpenoid fragments are potent inhibitors of new structural type of human recombinant DNA repair enzyme Tyrosyl-DNA phosphodiesterase I (Tdp1). It was demonstrated that the inhibition efficiency depended on the length and flexibility of the aliphatic chain of the substituent.

Enantioselective divergent syntheses of several polyhalogenated Plocamium monoterpenes and evaluation of their selectivity for solid tumors

The family of polyhalogenated monoterpenes from Plocamium counts over a hundred known members. Using glyceraldehyde acetonide as a chiral-pool precursor, an enantioselective and divergent strategy was developed that provides a blueprint for the synthesis of many of the small yet complex acyclic members of this family. The broad applicability of this approach is demonstrated with the short, eight-step synthesis of four natural products and three analogues. These syntheses are the first of any members of the acyclic polyhalogenated Plocamium monoterpenes and permitted the evaluation of their selectivity against a range of tumor cell lines.

First total syntheses and antimicrobial evaluation of penicimonoterpene, a marine-derived monoterpenoid, and its various derivatives

The first total synthesis of marine-derived penicimonoterpene (±)-1 has been achieved in four steps from 6-methylhept-5-en-2-one using a Reformatsky reaction as the key step to construct the basic carbon skeleton. A total of 24 new derivatives of 1 have also been designed and synthesized. Their structures were characterized by analysis of their 1H NMR, 13C NMR and HRESIMS data. Some of them showed significant antibacterial activity against Aeromonas hydrophila, Escherichia coli, Micrococcus luteus, Staphylococcus aureus, Vibrio anguillarum, V. harveyi and/or V. parahaemolyticus, and some showed activity against plant-pathogenic fungi (Alternaria brassicae, Colletotrichum gloeosporioides and/or Fusarium graminearum). Some of the derivatives exhibited antimicrobial MIC values ranging from 0.25 to 4 μg/mL, which were stronger than those of the positive control. Notably, Compounds 3b and 10 showed extremely high selectively against plant-pathogenic fungus F. graminearum (MIC 0.25 μg/mL) and pathogenic bacteria E. coli (MIC 1 μg/mL), implying their potential as antimicrobial agents. SAR analysis of 1 and its derivatives indicated that modification of the carbon-carbon double bond at C-6/7, of groups on the allylic methylene unit and of the carbonyl group at C-1, effectively enhanced the antimicrobial activity.

First Synthesis of (±)-Robinlin

The first synthesis of (+/-)-robinlin (1), a novel homo-monoterpene with strong bioactivity in the brine shrimp lethality test, was achieved by starting from 3-isobutyloxy-2,6,6-trimethyl-2-cyclohexen-1-one (2).

Synthesis, antimicrobial evaluation, and structure-activity relationship of α-pinene derivatives

Several (+)- and (-)-α-pinene derivatives were synthesized and evaluated for their antimicrobial activity toward Gram-positive bacteria Micrococcus luteus and Staphylococcus aureus, Gram-negative bacterium Escherichia coli, and the unicellular fungus Candida albicans using bioautographic assays. (+)-α-Pinene 1a showed modest activity against the test organisms, whereas (-)-α-pinene 1b showed no activity at the tested concentration. Of all the α-pinene derivatives evaluated, the β-lactam derivatives (10a and 10b) were the most antimicrobial. The increase in the antimicrobial activity of 10a compared to 1a ranged from nearly 3.5-fold (C. albicans) to 43-fold (S. aureus). The mean ± standard deviation for the zone of inhibition (mm) for 10a (C. albicans) was 31.9 ± 4.3 and that for S. aureus was 51.1 ± 2.9. Although (-)-α-pinene 1b was not active toward the test microorganisms, the corresponding β-lactam 10b, amino ester 13b, and amino alcohol 14b showed antimicrobial activity toward the test microorganisms. The increase in the antimicrobial activity of 10b compared to 1b ranged from 32-fold (S. aureus) to 73-fold (M. luteus). The mean ± standard deviation for the zone of inhibition (mm) for 10b (S. aureus) was 32.0 ± 0.60 and that for M. luteus was 73.2 ± 0.30.

An expedient synthesis of linden ether

We here describe a comprehensive study on the preparation of the intensive flavor 3,9-epoxy-p-mentha-1,4(8)-diene (1). Key steps of the presented synthesis are the selective addition of MeLi to the keto-ester 7, the regioselective cyclization of the obtained triol to give the ethers 4 and 8 and the selective dehydration of ether 4 through the use of POCI3 and pyridine. It is worth noting that the presented synthesis represents the first expedient and reliable entry to ether 1. Being present in linden honey, 1 is also known as linden ether and it has been regarded as a potential marker for the authentication of the linden honey origin. Therefore, ether 1 can be used as a useful reference standard for the analysis of the natural flavors, as we demonstrated by means of its identification in a sample ofunifloral linden honey.

Monoterpene citral derivatives as potential antimalarials

Conjugated acid derivatives containing nitrogen have been synthesized from the simple acyclic monoterpene citral, using various reactions, including Wittig, Baylis Hillman, amide and ester condensations. Similarly, amine peroxides were synthesized by subjecting citral to Mannich type reaction with amines and t-butyl hydroperoxide. Molecules 3-10 were evaluated for antimalarial activity against erythrocytic stages of chloroquine sensitive P. falciparum strain 3D7. Four derivatives displayed interesting activity with an IC50< 2.5 microM, and warrant further investigations.

Design, synthesis and antiviral activity studies of schizonepetin derivatives

A series of schizonepetin derivatives have been designed and synthesized in order to obtain potent antivirus agents. The antiviral activity against HSV-1 and influenza virus H3N2 as well as the cytotoxicity of these derivatives was evaluated by using cytopathic effect (CPE) inhibition assay in vitro. Compounds M2, M4, M5 and M34 showed higher inhibitory activity against HSV-1 virus with the TC₅₀ values being in micromole. Compounds M28, M33, and M35 showed higher inhibitory activity against influenza virus H3N2 with their TC₅₀ values being 96.4, 71.0 and 75.4 μM, respectively. Preliminary biological activity evaluation indicated that the anti-H3N2 and anti-HSV-1 activities improved obviously through the introduction of halogen into the structure of schizonepetin.

Total synthesis of (+)-pleuromutilin

The first enantiospecific total synthesis of the antibacterial natural product (+)-pleuromutilin has been achieved. The approach includes the synthesis of a non-racemic cyclisation substrate from (+)-trans-dihydrocarvone, a highly selective SmI2-mediated cyclisation cascade, an electron transfer reduction of a hindered ester, and the first efficient conversion of (+)-mutilin to the target.

Novel limonene and citral based 2,5-disubstituted-1,3,4-oxadiazoles: a natural product coupled approach to semicarbazones for antiepileptic activity

Two novel series of N(4)-(5-(2/3/4-substituted-phenyl)-1,3,4-oxadiazol-2-yl)-N(1)-(2-methyl-5-(prop-1-en-2-yl)cyclohex-2-enylidene)semicarbazide and N(4)-(5-(2/3/4-substituted-phenyl)-1,3,4-oxadiazol-2-yl)-N(1)-(3,7-dimethylocta-3,6-dienylidene)-semicarbazide were synthesized to meet structural prerequisite indispensable for anticonvulsant activity. The anticonvulsant activities of the compounds were investigated using maximal electroshock seizure (MES), subcutaneous pentylenetrtrazole (scPTZ) and subcutaneous strychnine (scSTY) models. The rotorod test was conducted to evaluate neurotoxicity. Some of the selected active compounds were subjected to GABA assay to confirm their mode of action. The outcome of the present investigations proved that the four binding sites pharmacophore model is vital for anticonvulsant activity. The efforts were also made to establish structure-activity relationships among test compounds.

Surprising formation of p-cymene in the oxidation of α-pinene in air by the atmospheric oxidants OH, O3, and NO3

Anthropogenic sources release into the troposphere a wide range of volatile organic compounds (VOCs) including aromatic hydrocarbons, whose major sources are believed to be combustion and the evaporation of fossil fuels. An important question is whether there are other sources of aromatics in air. We report here the formation of p-cymene [1-methyl-4-(1-methylethyl) benzene, C6H4(CH3)(C3H7)] from the oxidation of α-pinene by OH, O3, and NO3 at 1 atm in air and 298 K at low (<5%) and high (70%) relative humidities (RH). Loss of α-pinene and the generation of p-cymene were measured using GC-MS. The fractional yields of p-cymene relative to the loss of α-pinene, Δ [p-cymeme]/Δ [α-pinene], were measured to range from (1.6±0.2)×10(-5) for the O3 reaction to (3.0±0.3)×10(-4) for the NO3 reaction in the absence of added water vapor. The yields for the OH and O3 reactions increased by a factor of 4-8 at 70% RH (uncertainties are ±2s). The highest yields at 70% RH for the OH and O3 reactions, ∼15 times higher than for dry conditions, were observed if the walls of the Teflon reaction chamber had been previously exposed to H2SO4 formed from the OH oxidation of SO2. Possible mechanisms of the conversion of α-pinene to p-cymene and the potential importance in the atmosphere are discussed.

2-azapinanes: aza analogues of the enantiomeric pinyl carbocation intermediates in pinene biosynthesis

The enantiomeric 2-azapinanes, aza analogues of the pinyl carbocation intermediates in pinene biosynthesis, were synthesized from (-)- and (+)-cis-pinonic acids. The individual reactions in the 5-step sequence were Beckmann rearrangement of the pinonic acid oximes, cyclization to the N-acetyl lactams, hydrolysis to the NH-lactams, N-methylations, and LiAlH(4) reductions. The anti stereochemistry of the N-methyl groups in the salts with respect to the gem-dimethyl bridge was established by NOE measurements and by X-ray diffraction analysis.

Synthesis and in vitro antibacterial activity of thymol and carvacrol derivatives

Fourteen esters of thymol and carvacrol were synthesized and characterized on the basis of spectral data. The NMR data for some of these are being given for the first time. The antibacterial activity screening of thymol, carvacrol and their esters were carried out against four Gram-positive (Streptococcus mutans MTCC 890, Staphylococcus aureus MTCC 96, Bacillus subtilis MTCC 121, Staphylococcus epidermidis MTCC 435) and one Gram-negative (Escherichia coli MTCC 723) bacteria. The enhancement in activity was noticed in the thymyl ester derivatives 4a-c (against S. mutans, B. subtilis and S. epidermidis) in comparison to thymol, whereas the carvacrol derivatives were found to be much less active than carvacrol.

Synthesis and antibacterial activity of carvacryl ethers

Structural modifications of phenolic monoterpenoid obtained by reacting carvacrol with various substituted alpha-chloro acetanilides, to improve biological activities which give the product with better yield and higher purity under mild reaction conditions with the help of microwave irradiation techniques.

Modifications of the chemical structure of terpenes in antiplasmodial and antifungal drug research

Pure natural monoterpenes were evaluated in vitro for their antiplasmodial activities against Plasmodium falciparum. Chemically modified terpenes were also tested to see whether the introduction of an alkyne, a cyclopropane, a diene, or a cyclopentenone moiety had an influence on the biological activity. The IC(50) obtained on a chloroquine-resistant strain of Plasmodium (FcM29-Cameroon) showed moderate activity, but with the alkyne and the cyclopentenone derivatives showing a promising enhancement of activity compared with the parent molecules. On the contrary, no antifungal activity was found in vitro using Candida albicans. Given the observed antiplasmodial activity of some of these modified monoterpenes, new monoterpene derivatives could be the basis for new antimalarial drugs to be researched.

Heterogeneous reactions of ozone and D-limonene on activated carbon

If released in significant amounts, products formed by reactions between ozone (O3) and volatile organic compounds (VOCs) sorbed on activated carbon (AC) filters could degrade indoor air quality (IAQ). Heterogeneous reactions were investigated in laboratory experiments aimed at characterizing reaction products. Effluent air of AC loaded with limonene and exposed to O3 (5.8 ppm) yielded unreacted limonene (501+/-197 microg/m3), low levels of 4-acetyl-1-methylcyclohexene (AMCH) (20+/-2 microg/m3), and limonene oxides (25+/-7 microg/m3). Most of the O3-limonene products remained on the AC, and most (58%) of the limonene remained unreacted on the AC after exposure to a stoichiometric excess of O3 for 48 h. Thus, in addition to known homogenous reactions, O3-limonene reactions occur heterogeneously on AC but to a much lesser extent. However, the fate of 95% of the depleted limonene was not determined; much of the missing portion was attributed to desorption from the AC, but the formation of other secondary indoor air pollutants is possible. VOC-loaded AC air filters exposed to O3 seem unlikely, however, to constitute a significant emission source of reaction products. More studies are necessary to investigate other pollutants, effects of environmental conditions, and VOC releases from AC that may be enhanced by O3 exposure.

PRACTICAL IMPLICATIONS

Reactions between ozone and certain volatile organic compounds such as limonene (a common ingredient of many consumer products) occurring on the surface of ventilation filters could impact indoor air quality if products are released in significant amounts. This study suggests that although very small amounts of limonene adsorbed on a filter will react with O3, ventilation filters are not likely to be significant sources of ozone oxidation products. More studies are needed to investigate whether ozone exposure enhances desorption of pollutants from ventilation filters and to measure the formation of formaldehyde and other products that are not easily retained by charcoal filters.

Diastereoselective Formal Synthesis of a Monoterpene Alkaloid, (−)-Incarvilline

Diastereoselective formal synthesis of a monoterpene alkaloid, (-)-incarvilline, the key intermediate for the synthesis of (-)-incarvillateine, was achieved by using an intramolecular Pauson-Khand reaction of (S)-N-[(E)-2-butenyl]-N-(3-butynyl-2-methoxymethoxy)-p-toluenesulfonamide as a key step.

Studies of reaction parameters on synthesis of Citronellyl laurate ester via immobilized Candida rugosa lipase in organic media

Immobilized Candida rugosa lipase was used for the synthesis of citronellyl laurate from citronellol and lauric acid. Screening of different types of support (Amberlite MB-1 and Celite) for immobilization of lipase and solvent (n-hexane, n-heptane, and iso-octane) and optimization of reaction conditions, such as catalyst loading, effect of substrates molar ratio and temperature, have been studied. The maximum enzyme activity was obtained at 310 K. The immobilized C. rugosa lipase onto Amberlite MB-1 support was found to be the best support with a conversion of 89% of citronellyl laurate ester in iso-octane compared to Celite 545. Deactivation of C. rugosa lipase at 313, 318 and 323 K were observed. Ordered bi bi mechanism with dead end complex of lauric acid was found to fit the initial rate data and the kinetic parameters were obtained by non-linear regression analysis.

The synthesis of L-carvone and limonene derivatives with increased antiproliferative effect and activation of ERK pathway in prostate cancer cells

Thirty-one novel derivatives of carvone, carveol, and limonene were designed and synthesized using L-carvone as a starting material via chlorination, nucleophilic substitution, and reduction. The structures of these derivatives were characterized by MS and 1H NMR. The antiproliferative effect was evaluated in human prostate cancer LNCaP cells. L-carvone, L-carveol, and L-limonene were weak cell growth inhibitors and introduction of 4-(2-methoxyphenyl)piperazine to carvone, carveol or limonene significantly increased their antiproliferative effect. The antiproliferative effect was correlated with ERK activation and p21(waf1) induction.

Catalytic mechanism of limonene epoxide hydrolase, a theoretical study

The catalytic mechanism of limonene epoxide hydrolase (LEH) was investigated theoretically using the density functional theory method B3LYP. LEH is part of a novel limonene degradation pathway found in Rhodococcus erythropolis DCL14, where it catalyzes the hydrolysis of limonene-1,2-epoxide to give limonene-1,2-diol. The recent crystal structure of LEH was used to build a model of the LEH active site composed of five amino acids and a crystallographically observed water molecule. With this model, hydrolysis of different substrates was investigated. It is concluded that LEH employs a concerted general acid/general base-catalyzed reaction mechanism involving protonation of the substrate by Asp101, nucleophilic attack by water on the epoxide, and abstraction of a proton from water by Asp132. Furthermore, we provide an explanation for the experimentally observed regioselective hydrolysis of the four stereoisomers of limonene-1,2-epoxide.

Use of the Oxazole-Olefin Diels-Alder Reaction in the Total Synthesis of the Monoterpene Alkaloids (-)-Plectrodorine and (+)-Oxerine

A full account of the total synthesis of two monoterpene alkaloids, (-)-plectrodorine [(-)-1] and (+)-oxerine [(+)-3], is presented. The key steps involved are the formation of the oxazole alcohol 10 from the gamma-butyrolactone 9 and the intramolecular Diels-Alder reaction of the oxazole-olefins 13a, b. Since the sign of specific rotation for the synthetic (+)-3 was different from that reported for natural oxerine, the absolute configuration of this alkaloid is not yet fully understood.

Cycloisomerization of dienes with carbophilic lewis acids: mimicking terpene biosynthesis with Pt(II) catalysts

[reaction: see text]. Dicationic Pt(II) complexes containing triphosphine pincer ligands are excellent catalysts for the cycloisomerization of 1,6- and 1,7-dienes into bicyclopropane carbocycles. In analogy to the biosynthetic route to these monoterpene-like compounds, carbocation intermediates are proposed and supported by trapping experiments. Reactivation of the trapped intermediates indicates that cation generation by C-C bond formation is both rapid and reversible.

Photochemical Release of Aldehydes from α-Acetoxy Nitroveratryl Ethers

Photolabile aldehyde-releasing precursors (alpha-acetoxy ethers) were prepared by reduction of the corresponding esters with DIBAL and quenching the intermediate aluminum hemiacetal with acetic anhydride. These species smoothly released aldehydes upon irradiation with UV light at 350 nm. Using this method, not only simple model aliphatic aldehydes were liberated but also specimens relevant for the flavor and fragrance industry (methional, phenylacetaldehyde, and (R)-citronellal). [reaction: see text]

Total Syntheses of Four Stereoisomers of 4α-Hydroxy-1β,7β-peroxy- 10βH-guaia-5-ene

[reaction: see text]. The first total syntheses of four stereoisomers of 4alpha-hydroxy-1beta,7beta-peroxy-10betaH-guaia-5-ene are reported starting from the readily available (+)-dihydrocarvone. These compounds have been synthesized from dienes (-)-isoguaiene and (-)-10-epi-isoguaiene by tandem ene hydroperoxylation-[4 + 2] cycloaddition with O(2) followed by selective reduction. The structure of the natural 4alpha-hydroxy-1beta,7beta-peroxy-10betaH-guaia-5-ene isolated from Liabum floribundum has been confirmed.

A Convenient 3-Step Synthesis of (R)-7-Hydroxycarvone from (S)-α-Pinene

A convenient 3-step synthesis of (R)-7-hydroxycarvone (2) has been developed starting from (S)-alpha-pinene (7), using photooxygenation, oxidation, and fragmentation reactions. An improved synthesis of epoxy alcohol 6 and an unusual Ti(OiPr)(4) catalyzed hydroxy epoxide to keto alcohol rearrangement are also described.

Stereoselective Synthesis ofcis-p-Menth-8-ene-1,7-diol,cis-p-Menthane-1,7-diol, andcis-p-Menthane-1,7,8-triol

The natural products cis-p-menthane-1,7-diol (cis-IV), cis-p-menth-8-ene-1,7-diol (cis-I) and cis-p-menthane-1,7,8-triol (cis-II) are obtained starting from the corresponding cis-cyanohydrins, cis-2 and cis-7, respectively, by chemical transformation of the cyano into the hydroxymethyl group. The key step of the synthesis is the very high cis-selectivity (> or = 96 %) of the MeHNL-catalyzed HCN addition to 4-alkylcyclohexanones. From 4-isopropylcyclohexanone (1) the cyanohydrin cis-2 and from 4-(1-methylvinyl)cyclohexanone (6) the cyanohydrin cis-7 result almost quantitatively. Regioselective hydroxylation of cis-I affords the triol cis-II. X-ray crystal structure determinations of the final products confirm their cis-configuration.

New Insight into Solvent Effects on theFormal HOO. + HOO. Reaction

The 2,2'-azobis(isobutyronitrile)(AIBN)-induced autoxidation of gamma-terpinene (TH) at 50 degrees C produces p-cymene and hydrogen peroxide in a radical-chain reaction having HOO* as one of the chain-carrying radicals. The kinetics of this reaction in cyclohexane and tert-butyl alcohol show that chain termination involves the formal HOO. + HOO. self-reaction over a wide range of gamma-terpinene, AIBN, and O2 concentrations. However, in acetonitrile this termination process is accompanied by termination via the cross-reaction of the terpinenyl radical, T., with the HOO. radical under conditions of relatively high [TH] (140-1000 mM) and low [O2] (2.0-5.5 mM). This is because the formal HOO. + HOO. reaction is comparatively slow in acetonitrile (2k approximately 8 x 10(7) M(-1) s(-1)), whereas, this reaction is almost diffusion-controlled in tert-butyl alcohol and cyclohexane, 2k approximately 6.5 x 10(8) and 1.3 x 10(9) M(-1) s(-1), respectively. Three mechanisms for the bimolecular self-reaction of HOO. radicals are considered: 1) a head-to-tail hydrogen-atom transfer from one radical to the other, 2) a head-to-head reaction to form an intermediate tetroxide, and 3) an electron-transfer between HOO. and its conjugate base, the superoxide radical anion, O2-.. The rate constant for reaction by mechanism (1) is shown to be dependent on the hydrogen bond (HB) accepting ability of the solvent; that by mechanism (2) is shown to be too slow for this process to be of any importance; and that by mechanism (3) is dependent on the pH of the solvent and its ability to support ionization. Mechanism (3) was found to be the main termination process in tert-butyl alcohol and acetonitrile. In the gas phase, the rate constant for the HOO. + HOO. reaction (mechanism (1)) is about 1.8 x 10(9) M(-1) s(-1) but in water at pH< or =2 where the ionization of HOO. is completely suppressed, this rate constant is only 8.6 x 10(5) M(-1) s(-1). The very large retarding effect of water on this reaction has not previously been explained. We find that it can be quantitatively accounted for by using Abraham's HB acceptor parameter, beta(2)(H), for water of 0.38 and an estimated HB donor parameter, alpha(2)(H), for HOO. of about 0.87. These Abraham parameters allow us to predict a rate constant for the HOO. + HOO. reaction in water at 25 degrees C of 1.2 x 10(6) M(-1) s(-1) in excellent agreement with experiment.

Total synthesis of (-)-incarvilline, (+)-incarvine C, and (-)-incarvillateine

The first total syntheses of new monoterpene alkaloids (-)-incarvilline, (+)-incarvine C, and (-)-incarvillateine, corresponding to the natural enantiomers, have been accomplished. The strategy for the synthesis of these natural products utilized 6-epi-incarvilline as a common precursor, which was assembled by a three-component coupling reaction using (4S)-4-siloxy-2-cyclopenten-1-one to construct an appropriately trisubstituted cyclopentanone, followed by ring closure to the cis-perhydro-2-pyrindine skeleton by means of a reductive Heck-type reaction. Furthermore, topochemically controlled [2 + 2] photodimerization of cinnamic acid derivatives in the solid state for the stereospecific construction of a 1,2,3,4-tetrasubstituted cyclobutane ring was also investigated as a means to access (-)-incarvillateine.

Relative Effectiveness of Pretreatments on Performance of <I>Rhizomucor miehei</I> Lipase in Nonpolar Reaction Media

Enzymes can be used in nonpolar reaction media to modify water-insoluble substrates. A variety of pretreatments, applied to the enzyme prior to introduction to the nonpolar media, can improve enzyme activity. However, the various pretreatments have not been studied using directly comparable conditions, nor have they been applied simultaneously to test for interactive effects. This work evaluates pretreatment of lipase with various classes of additives. The pretreated lipase is used to catalyze esterification between citronellol and acetic acid in a medium of n-hexane. The effectiveness of a particular pretreatment is presented in terms of relative performance (RP), which is equal to the number of times faster the pretreated lipase catalyzes the reaction relative to untreated lipase. The individual and interactive effects of the pretreatment factors were studied and compared. Buffer salts had a much stronger performance-enhancing effect than nonbuffer salts; pretreatment with 90% (w/w) sodium phosphate yielded lipase with an RP of approx 64. A strong interaction was found between the treatments with sodium phosphate and pH adjustment. These treatments may mitigate the inhibitory effect of acetic acid. Activating effects of phase interfaces and active-site protectants are shown to be complementary to other treatments, demonstrating that they likely act by distinct mechanisms.

Synthesis and cytostatic properties of monoterpene derivatives of cyanuric and isocyanuric acids

Symmetric, trisubstituted derivatives of cyanuric and isocyanuric acid 3a-e and 4a-e, respectively have been prepared by treatment of cyanuric chloride 1 with appropriate terpenic alcohols 2a-e. Under less basic conditions involving treatment of appropriate alcohols 2a-e with metallic sodium, less polar cyanuric acid derivatives 3a-e were obtained in 74-85 yield. Under more basic conditions, in the presence of sodium hydride, isocyanuric acid derivatives 4a-e were prepared in 71-88 yield. Cytostatic activity of (IR,2S,5R)-menthol derivative (3a) has been evaluated on 9 cancer subtypes including 62 tumor cell lines. The studies have shown that contrary to expectations, 2,4,6-trimentoloxy-1,3,5-triazine revealed a weak or moderate activity against most of the cell lines.

Synthesis of the sex pheromone of the citrus mealybug, Pseudococcus cryptus

The sex pheromone of the citrus mealybug (Pseudococcus cryptus), [(1R,3R)-3-isopropenyl-2,2-dimethylcyclobutyl]methyl 3-methyl-3-butenoate, was synthesized from (+)-alpha-pinene in five operational steps in a 43% overall yield. The synthetic pheromone was identical with the natural pheromone in (1)H-NMR and mass spectroscopic properties, and showed almost the same pheromonal activity as the natural pheromone.

13C-CPMAS and1H-NMR Study of the Inclusion Complexes ofβ-Cyclodextrin with Carvacrol, Thymol, and Eugenol Prepared in Supercritical Carbon Dioxide

Beta-cyclodextrin (beta-CD) inclusion complexes with carvacrol (1), thymol (2), and eugenol (3) (components of essential oils of vegetable origin) were prepared by the supercritical CO2 technique, and their structural characterization was achieved by means of 1H-NMR in aqueous solution and 13C-CPMAS NMR in the solid state. Evidence of the formation of the inclusion complexes for all the examined systems was obtained by 1H-NMR in solution, while 2D-ROESY-NMR experiments were used to investigate the geometry of inclusion. In addition, the dynamics of these inclusion complexes in the kHz timescale was investigated by analysis of the 1H and 13C spin-lattice relaxation times in the rotating frame.

Synthesis of 4-aryl substituted semicarbazones of some terpenes as novel anticonvulsants

PURPOSE

A series of 4-aryl substituted semicarbazones of citral and R- (-) carvone were designed and synthesized to meet the structural requirements essential for anticonvulsant activity.

METHODS

TLC evaluated purity of synthesized compounds and their structure confirmed by infrared spectroscopy, proton magnetic resonance spectroscopy and by nitrogen estimation. All the compounds were evaluated for anticonvulsant activity by maximal electroshock (MES) and subcutaneous metrazol (ScMet) induced seizure methods and minimal motor impairment was determined by rotorod test.

RESULTS

All the synthesized compounds exhibited significant protection after intraperitoneal (i.p.) administration in MES. Seventy two percent of the compounds exhibited protection in ScMet test. Some of them also showed good activity after oral administration. The results showed that anticonvulsants with cyclic and acyclic terpenoid moiety retain activity in MES as well as ScMet test. The p-fluoro aryl substituted semicarbazones emerged as the most active analogue in both cyclic and acyclic terpenes.

CONCLUSION

Semicarbazones with terpenoid as the lipophilic moiety resulted in compounds with broad spectrum of anticonvulsant activity and therefore, they may be utilized for the future development of novel anticonvulsants with broad spectrum of anticonvulsant activity. The results also validated pharmacophore model with four binding sites essential for anticonvulsant activity.

General Approach to Polycyclic Meroterpenoids Based on Stille Couplings and Titanocene Catalysis

We describe a novel convergent procedure that has proved useful in the synthesis of a wide range of meroterpenoid-related structures containing a mono-, sesqui-, or diterpenoid moiety linked to a nonfused aromatic subunit with various substitution patterns. The key steps were the Stille-type coupling of aryl stannanes and allylic carbonates, followed by the titanocene-catalyzed domino cyclization of aryl epoxypolyprenes. The coupling reaction was perfectly compatible with preformed epoxides, while the sequential cyclization, which presumably proceeded via alkyl radicals inert to benzene derivatives, selectively provided exocyclic alkenes.

α- and β-Thujone Radical Rearrangements and Isomerizations. A New Radical Clock

Radical clocks have been extensively used in chemical and biochemical mechanistic studies. The C4 radicals of alpha- and beta-thujone can undergo two distinct rearrangement reactions that could, in principle, serve as simultaneous but independent radical clocks. We have therefore generated these C4 radicals by photolysis of the corresponding N-hydroxypyridine-2-thione ester precursors and have investigated their fates and lifetimes. Photolysis of either alpha- or beta-thujone generates the same 6:100 mixture of alpha- and beta-thujone when the radicals are quenched by thiophenol. Hydrogen atom transfer from thiophenol to the radical thus occurs preferentially from the less sterically hindered alpha-face to give beta-thujone. The third product formed in the photolysis via opening of the cyclopropyl ring is 2-methyl-5-isopropylcyclopent-2-enone. The ratio of ring opened to unopened products gives very similar values of kralpha = 4.4 x 10(7) s(-1) and krbeta = 1.0 x 10(8) s(-1) for ring opening of the radicals generated from alpha- and beta-thujone, respectively. If the C4 cation rather than radical is generated, it is converted to carvacrol, a phenol that is not obtained in the radical reactions. Thujone therefore differentiates between radical and cation pathways and provides a measure of the radical lifetime.

Sex pheromone of the pink hibiscus mealybug, Maconellicoccus hirsutus, contains an unusual cyclobutanoid monoterpene

Two compounds that together constitute the female sex pheromone of the pink hibiscus mealybug (PHM), Maconellicoccus hirsutus, were isolated, identified, and synthesized. They are (R)-2-isopropenyl-5-methyl-4-hexenyl (S)-2-methylbutanoate [common name is (R)-lavandulyl (S)-2-methylbutanoate] and [(R)-2,2-dimethyl-3-(1-methylethylidene)cyclobutyl]methyl (S)-2-methylbutanoate [which we refer to as (R)-maconelliyl (S)-2-methylbutanoate]. Maconelliol is an unusual cyclobutanoid monoterpene, and its structure has been established by enantioselective synthesis from precursors of known structure and configuration. A 1:5 synthetic mixture of the two RS esters (1 microg per rubber septum) proved to be a potent attractant of males in field bioassays. The pheromone component, maconelliyl 2-methylbutanoate, represents a heretofore undescribed natural product.

Biogenetic-type synthesis of (+)-cymbodiacetal, a constituent of Cymbopogon martinii

A biogenetic-type synthesis of (+)-cymbodiacetal (1), a novel bismonoterpenoid dihemiacetal, is described.

Enzymatic esterification of lavandulol – a partial kinetic resolution of (S)-lavandulol and preparation of optically enriched (R)-lavandulyl acetate

The asymmetric esterification of the racemic primary alcohol lavandulol was achieved using lipase B from Candida antarctica and acetic acid as acyl donor in 80% yield. The enantioselectivity of the process was characterised, and a preparative resolution of 25 mM racemic lavandulol, stopped at approx. 55% conversion, yielded (S)-lavandulol in 42% yield and 52% e.e. and (R)-lavandulyl acetate in 51% yield and 48% e.e.

Effect of reaction parameters on synthesis of citronellyl methacrylate by lipase-catalyzed transesterification

The methacrylate ester of citronellol was synthesized using various lipases as catalyst. The effect of different reaction parameters such as amount of lipase, solvent, temperature, and acylating agent on the conversion of citronellol to citronellyl methacrylate was studied. Methyl methacrylate, vinyl methacrylate, and 2,3-butanedione mono-oxime methacrylate were used as acylating agents. Porcine pancreatic lipase (PPL), Candida rugosa lipase (CRL), and Pseudomonas cepacia lipase (Amano-PS) were used as biocatalysts. Diisopropyl ether (DIPE) was found to be the most suitable solvent. The stereoselectivity of CRL in transesterification of (+/-)-citronellol was tested for the optimized reaction parameters.

An efficient procedure for the preparation of (E)-alpha-alkylidenecycloalkanones mediated by a CeCl(3) x 7H(2)O-NaI system. Novel methodology for the synthesis of (S)-(-)-pulegone

2-Alkylidenecycloalkanones are powerful synthons used as the key intermediates in many important syntheses. Because of their potential, a general method of preparation from readily available starting materials, under very mild conditions, was considered to be worthwhile. Cerium(III) chloride heptahydrate in combination with sodium iodide in refluxing acetonitrile promotes a regio- and stereoselective beta-elimination reaction to (E)-2-alkylidenecycloalkanones in 2-(1-hydroxyalkyl)cycloalkanones. The synthetic value of the present procedure is demonstrated by the synthesis of monoterpene (S)-(-)-pulegone (8) in its optically active form.

Synthesis of both enantiomers of isorobinal, a novel cyclic monoterpene isolated from the astigmatid mite, Rhizoglyphus sp

Both enantiomers of isorobinal, a cyclic monoterpene isolated from the astigmatid mite (Rhizoglyphus sp.), were synthesized from the enantiomers of perillaldehyde in four steps by using PCC-oxidation of a tertiary allylic alcohol intermediate as the key step.