Chemical Investigation on the Volatile Part of the CO2 Supercritical Fluid Extract of Infected Aquilaria sinensis (Chinese Agarwood)

This work is focused on the characterization of the composition of a CO2 supercritical fluid extract of Aquilaria sinensis (Chinese agarwood) collected in the Dongguan area (China) and infected by mechanical methods. The constituents of this extract were analyzed by gas chromatography-mass spectrometry (GC-MS) and quantified accurately by gas chromatography with a flame ionization detector (GC-FID), using an internal reference and predicted response factors. Since a significant number of components of this extract remained non-identified after the initial GC-MS analysis of the whole extract, its fractionation by chromatography on silica gel helped to characterize several additional constituents by isolation and structural analysis by NMR spectroscopy. The main components are the classical agarwood chromones (Flindersia chromone and its mono-, di-, and trimethoxylated analogues (respectively, 11.01% and 0.11-4.02%) along with sesquiterpenic constituents typically found in agarwood essential oils, like baimuxinal (1.90%) and kusunol (1.24%), as well as less common selinane dialdehydes (1.58-2.27%) recently described in the literature. Moreover, the structure and stereochemistry of a new sesquiterpenic alcohol, 14β,15β-dimethyl-7αH-eremophila-9,11-dien-8β-ol (0.67%), was determined unambiguously by the combination of structural analysis (NMR, MS), hemisynthesis, and total synthesis, leading to dihydrokaranone and a neopetasane epimer.

Enantioselective Total Synthesis of (+)-Heilonine

Chemical transformations that rapidly and efficiently construct a high level of molecular complexity in a single step are perhaps the most valuable in total synthesis. Among such transformations is the transition metal catalyzed [2 + 2 + 2] cycloisomerization reaction, which forges three new C-C bonds and one or more rings in a single synthetic operation. We report here a strategy that leverages this transformation to open de novo access to the Veratrum family of alkaloids. The highly convergent approach described herein includes (i) the enantioselective synthesis of a diyne fragment containing the steroidal A/B rings, (ii) the asymmetric synthesis of a propargyl-substituted piperidinone (F ring) unit, (iii) the high-yielding union of the above fragments, and (iv) the intramolecular [2 + 2 + 2] cycloisomerization reaction of the resulting carbon framework to construct in a single step the remaining three rings (C/D/E) of the hexacyclic cevanine skeleton. Efficient late-stage maneuvers culminated in the first total synthesis of heilonine (1), achieved in 21 steps starting from ethyl vinyl ketone.

Concise Total Synthesis of Peyssonnoside A

Peyssonnoside A is a marine-derived sulfated diterpenoid glucoside with a unique 5/6/3/6 tetracyclic skeleton with a highly substituted cyclopropane ring deeply embedded into the structure. Herein, we report the first total synthesis of this natural product in a concise, efficient, scalable, and highly diastereoselective fashion. The aglucone peyssonnosol was synthesized in 21% overall yield after 15 steps, featuring a Simmons-Smith cyclopropanation and Mukaiyama hydration, fully controlled by the spatial structure of the substrates.

Total synthesis of five natural eremophilane-type sesquiterpenoids

The first total syntheses of five natural eremophilane-type sesquiterpenoids were achieved in 4-12 steps via a common synthetic intermediate. The syntheses feature a double Michael addition, Robinson annulation, α-enolization of an unsaturated ketone, and Pd-catalyzed Suzuki coupling reaction to install the side chain. This synthetic strategy could be easily extended to other eremophilane-type sesquiterpenoids with similar bicyclic skeletons.

Total Synthesis of the Sesquiterpenoid Periconianone A Based on a Postulated Biogenesis

The first enantioselective total synthesis of the complex tricarbocyclic sesquiterpenoid periconianone A based on a postulated biogenesis is reported. Key elements of the synthetic route include the use of an isopropenyl group as a removable directing group for stereoselective synthesis, a sequence featuring a Rh-mediated O-H insertion/[3,3]-sigmatropic rearrangement and subsequent α-ketol rearrangement, and a late stage aldol reaction to furnish the complex cage-like framework.

Enantioselective Access to Robinson Annulation Products and Michael Adducts as Precursors

The Robinson annulation is a reaction that has been useful for numerous syntheses since its discovery in 1935, especially in the field of steroid synthesis. The products are usually obtained after three consecutive steps: the formation of an enolate (or derivative), a conjugate addition, and an aldol reaction. Over the years, several methodological improvements have been made for each individual step or alternative routes have been devised to access the Robinson annulation products. The first part of this Review outlines the most relevant developments towards the formation of monocarbonyl-derived Robinson annulation products (MRA products, MRAPs) and activated monocarbonyl-derived Robinson annulation products (AMRA products, AMRAPs). The following sections are then devoted to the diastereoselective and enantioselective synthesis of these products, while the last section describes the enantiomeric resolution of racemic mixtures.

Total synthesis of the G2/M DNA damage checkpoint inhibitor psilostachyin C

A concise total synthesis of the G2/M DNA damage checkpoint inhibitor psilostachyin C is reported using a 1,4-addition-aldol condensation-ring-closing metathesis (RCM) strategy. Initial biological studies indicate that psilostachyin C could enhance the sensitivity of the HeLa cell toward camptothecin (CPT) treatment via the activation of the caspase-3 mediated apoptosis pathway.

High diversity of Ligularia dictyoneura in chemical composition and DNA sequence

The chemical composition of root extracts of the title species collected at 20 different places in the Hengduan Mountains of China was examined. From these samples, a total of 17 eremophilane derivatives were isolated, three of which were new franoeremophilane derivatives: 3beta-acetoxy-6beta-(angeloyloxy)furanoeremophilan-10beta-ol, 1alpha-acetoxyfuranoeremophilan-15,6alpha-olide, and 6beta-[2-(hydroxymethyl)prop-2-enoyloxy]furanoeremophil-1(10)-ene. Based on the chemical composition, the samples could be classified into as many as seven types: one type containing non-furanoeremophilane derivatives and the other six containing furanoeremophilane derivatives with different oxidation levels. Results of DNA sequencing of the atpB-rbcL region and the internal transcribed spacers of the ribosomal RNA gene also indicated a high diversity in the species.

Total synthesis of guanacastepene a: a route to enantiomeric control

[reaction: see text] The goal of the total synthesis of guanacastepene A served as a focus to bring together several chemical inquiries. One involved the synthesis of fused 5,7-hydrazulenones (see structure 20). Another issue had to do with the mechanistic intermediates in reductive cyclizations (see 17 to 18 and 19). The total synthesis required a mastery of an intramolecular Knoevenagel condensation of a beta,gamma-unsaturated ketone (see compound 41). Actually, cyclization was best accomplished when the terminal double bond of 41 was first converted to an epoxide. Further issues related to the stereochemistry at C5 and, rather surprisingly, the propensity for beta-face acetoxylation at C13. Crystallographic verification of the assigned beta-stereochemistry at C13 is provided. Finally, a route to optically active material is provided (see compound 20). A key element in this construction was an enantioselective addition of isopropenyl cuprate to 2-methylcyclopentenone (see compound 99).

Total synthesis of ent-cholesterol via a steroid C,D-ring side-chain synthon

For the first time, one of the two enantiomers of cholesterol (ent-cholesterol) has been synthesized by a synthetic route that starts from a precursor containing the D-ring and entire side chain of cholesterol. As part of the reported synthetic route, a method of general utility for the large scale (>10 g) preparation of each enantiomer of [1 alpha(R*),7a alpha]-1-(1,5-dimethylhexyl)-1,2,3,6,7,7a-hexahydro-7a-methyl-5H-inden-5-one, C,D ring-side chain synthons that can be used for the synthesis of enantiomers of vitamin D(3), cholesterol, and their analogues was also developed. Using the enantiomer of the C,D-ring side-chain synthon that leads to ent-cholesterol, the A- and B-rings were elaborated from a linear fragment that is sequentially cyclized to form the steroid B- and A-rings. Using this route, ent-cholesterol was prepared in 23 steps from the methyl ester of (1 alpha,5 alpha,6 alpha)-(+/-)-6-methyl-2-oxo-bicyclo[3.1.0]hexane-1-carboxylic acid in a total yield of 2.6%.

A four-step synthesis of the hydroazulene core of guanacastepene

[reaction: see text] A concise, four-step conversion of 2-methylcyclopentenone to hydroazulene 2 is described. An alternative approach that led to an unexpected [5,5]-bicyclic carbon framework is also discussed.

On roads not taken in the evolution of protein catalysts: antibody steroid isomerases that use an enamine mechanism

Reactive immunization has emerged as a new tool for the study of biological catalysis. A powerful application resulted in catalytic antibodies that use an enamine mechanism akin to that used by the class I aldolases. With regard to the evolution of enzyme mechanisms, we investigated the utility of an enamine pathway for the allylic rearrangement exemplified by Delta5-3-ketosteroid isomerase (KSI; EC 5.3.3.1). Our aldolase antibodies were found to catalyze the isomerization of both steroid model compounds and steroids. The kinetic and chemical studies showed that the antibodies afforded rate accelerations up to a factor of 10(4) by means of an enamine mechanism in which imine formation was the rate-determining step. In light of our observations and the enzyme studies by other workers, we suggest that an enamine pathway could have been an early, viable KSI mechanism. Although this pathway is amenable to optimization for increased catalytic power, it appears that certain factors precluded its evolution in known KSI enzymes.