Enzymatic Resolution of Aminocyclopentenols as Precursors to d- and l-Carbocyclic Nucleosides

Synthesis of the Cyclopentane Core Skeleton of Cranomycin and Jogyamycin

Cranomycin and jogyamycin, two aminocyclopentitol natural products, possess complex structures and potential medicinal properties. This review describes synthetic studies about the process of making an advanced intermediate of cranomycin and jogyamycin. This highly functionalized intermediate, featuring three contiguous amine-substituted stereocenters, was constructed from cyclopentadiene through a series of reactions including the nitroso Diels-Alder reaction, nitrogen radical cyclization reaction, 1,2-nitrogen migration, and stereoselective nitrogen addition.

Biocatalytic routes to anti-viral agents and their synthetic intermediates

An assessment of biocatalytic strategies for the synthesis of anti-viral agents, offering guidelines for the development of sustainable production methods for a future COVID-19 remedy.

Total synthesis of (−)-agelastatin A: an SH2′ radical azidation strategy

A new synthetic approach to (−)-agelastatin A has been established through the strategic implementation of brominative olefin transposition of a silyl enol ether and subsequent S_H2′ radical azidation of the resultant allylic bromide.

Generation and Trapping of Nitrosocarbonyl Intermediates

The nitrosocarbonyls (R-CONO) are highly reactive species and remarkable intermediates toward different synthetic targets. This review will cover a research area whose impact in current organic synthesis is constantly increasing in the chemical community. This review represents the first and comprehensive picture on the generation and trapping of nitrosocarbonyls and is solidly built on more than 380 papers. Six different classes of key starting materials such as hydroxamic acids, N-hydroxy carbamates, N-hydroxyureas, nitrile oxides, and 1,2,4-oxadiazole-4-oxides were highlighted. The content of the review surveys all the methods to generate the nitrosocarbonyls through different approaches (oxidative, thermal, photochemical, catalytic, aerobic, and the less common ones) in the light of efficiency, yields, and mildness. The most successful trapping agents employed to catch these fleeting intermediates are reviewed, exploiting their superior dienophilic, enophilic, and electrophilic power. The work is completed by paragraphs dedicated to the detection of the intermediates, theoretical studies, and insights about the challenges and future directions for the field.

The nitrosocarbonyl hetero-Diels-Alder reaction as a useful tool for organic syntheses

Organic transformations that result in the formation of multiple covalent bonds within the same reaction are some of the most powerful tools in synthetic organic chemistry. Nitrosocarbonyl hetero-Diels-Alder (HDA) reactions allow for the simultaneous stereospecific introduction of carbon-nitrogen and carbon-oxygen bonds in one synthetic step, and provide direct access to 3,6-dihydro-1,2-oxazines. This Review describes the development of the nitrosocarbonyl HDA reaction and the utility of the resulting oxazine ring in the synthesis of a variety of important, biologically active molecules.

Enantioselective syntheses of carbocyclic nuleosides 5'-homocarbovir, epi-4'-homocarbovir and their cyclopropylamine analogs using facially selective Pd-mediated allylations

Carbocyclic nucleosides (-)-5'-homocarbovir and (+)-epi-4'-homocarbovir were prepared from an acylnitroso-derived hetero Diels-Alder cycloadduct. A kinetic enzymatic resolution generated an enantiopure aminocyclopentenol and Pd(0)-mediated decarboxylative allylations of allyl 2,2,2-trifluoroethyl malonates were used to install the 4'-hydroxyethyl groups. Late stage derivatization gave access to the cyclopropylamine congenors, (-)-5'-homoabacavir and (+)-epi-4'-homoabacavir. All carbonucleoside target molecules were evaluated for antiviral activity.

[4 + 2] Cycloadditions of 1-phosphono-1,3-butadienes with nitroso heterodienophiles: a versatile synthetic route for polyfunctionalized aminophosphonic derivatives

The hetero-Diels-Alder (HDA) reaction of 1-(diethoxyphosphonyl)-1,3-butadiene, 1-(dibenzyloxyphosphonyl)-1,3-butadiene, and 1-(diethoxyphosphonyl)-3-tert-butyldimethylsilyloxy-1,3-butadiene with various nitroso heterodienophiles has been investigated as a new synthetic route for aminophosphonic derivatives. The HDA cycloadditions regioselectively led to the proximal isomers, i.e., presenting the NR(3) group in the meta position regarding the phosphonate substituent. From the resulting 6-phosphono-3,6-dihydro-1,2-oxazine cycloadducts, a limited number of chemical steps were allowed to obtain a significant variety of aminophosphonic compounds of potential interest in medicinal chemistry. This has been illustrated through the synthesis of (Z)-4-(o-tolylamino)-1-hydroxybut-2-enylphosphonic acid, diethyl 3,4-dihydroxy-1-o-tolylpyrrolidin-2-yl-2-phosphonate, 4-(o-tolylamino)-1,2,3-trihydroxybutylphosphonic acid, diethyl 3-(2-(o-tolylamino)-1-hydroxyethyl)oxiran-2-yl-2-phosphonate, and diethyl 4,5-dihydroxymorpholin-6-yl-6-phosphonate.

Palladium(0)/indium iodide-mediated allylations of electrophiles generated from the hydrolysis of Eschenmoser's salt: One-pot preparation of diverse carbocyclic scaffolds

A formyl equivalent was generated in situ from Eschenmoser's salt in aqueous THF and was reacted with an allylindium species. Acylnitroso-derived hetero-Diels Alder adducts and related allyl acetates were shown to be substrates for Pd(0)/InI-mediated allylations of formaldehyde-related species to provide homoallylic alcohols. Hydroxymethyl groups were installed with regio- and diastereocontrol to provide relevant disubstituted carbocyclic scaffolds. Enantiopure anti-disubstituted cyclopentene products were prepared from a chiral allyl acetate.

Enzymatic deprotection of the cephalosporin 3'-acetoxy group using Candida antarctica lipase B

Cephalosporins remain one of the most important classes of antibiotics. A useful site for derivatization involves generation of and chemistry at the 3'-hydroxymethyl position. While 3'-acetoxymethyl-substituted cephalosporins are readily available, deacetylation to access the free 3'-hydroxymethyl group is problematic when the carboxylic acid is protected as an ester. Herein we report that this important transformation has been efficiently accomplished using Candida antarctica lipase B. Although this transformation is difficult to carry out using chemical methods, the enzymatic deacetylation has been successful on gram scale, when the cephalosporin is protected as either the benzhydryl or tert-butyl esters and on the corresponding sulfoxide and sulfone of the tert-butyl ester.

Regio- and stereochemically controlled formation of hydroxamic acids from indium triflate-mediated nucleophilic ring-opening reactions with acylnitroso-Diels-Alder adducts

Treatment of acylnitroso-Diels-Alder [2.2.1] bicyclic adducts 2a-b with indium triflate in an alcohol solvent induces ring opening reactions to afford monocyclic anti-1,2-, anti-1,4- and syn-1,4-hydroxamic acids with good to excellent regio- and stereoselectivity (up to 7:86:7). Treatment of [2.2.2] bicyclic nitroso adducts 2c-d under similar reaction conditions generates only anti-1,2- and anti-1,4-hydroxamic acids with anti-1,4-product predominant (up to 17:83).

Palladium-catalyzed decarboxylative rearrangements of allyl 2,2,2-trifluoroethyl malonates: direct access to homoallylic esters

Homoallylic esters are obtained in a single transformation from allyl 2,2,2-trifluoroethyl malonates by using a Pd(0) catalyst. Facile decarboxylation of allyl 2,2,2-trifluoroethyl malonates is attributed to a decrease in pK(a) compared to allyl methyl malonates. Subsequent reduction of the homoallylic 2,2,2-trifluoroethyl ester provides a (hydroxyethyl)cyclopentenyl derivative that represents a key intermediate in the synthesis of carbocyclic nucleosides. A select allyl 2,2,2-trifluoroethyl malonate undergoes a decarboxylative Claisen rearrangement to provide a regioisomeric homoallylic ester.

Regio- and stereoselective indium triflate-mediated nucleophilic ring-opening reactions of 3-aza-2-oxabicyclo[2.2.1]hept-5-ene and -[2.2.2]oct-5-ene systems

Indium triflate-mediated nucleophilic ring-opening reactions of 3-aza-2-oxabicyclo[2.2.1]hept-5-ene and -[2.2.2]oct-5-ene systems with alcohols and water were investigated. Reactions of [2.2.1] bicyclic systems gave anti-1,2-, anti-1,4- and syn-1,4-ring-opened products with moderate to excellent regio- and stereoselectivity. When [2.2.2] bicyclic systems were subjected to similar reaction conditions, only anti-1,2 and anti-1,4-ring-opened products were obtained.

Syntheses of new spirocarbocyclic nucleoside analogs using iminonitroso Diels-Alder reactions

N-Cbz- and Boc-protected spirocyclic dienes were prepared by dialkylation of cyclopentadiene. These dienes coupled efficiently in a series of iminonitroso Diels-Alder reactions to produce a series of new spirocyclic adducts. Hydrogenolysis of these adducts afforded new spirocycles that contain multiple handles for further functionalization. Furthermore, stereocontrolled dihydroxylation and reductive cleavage of the spirocyclic adducts generated versatile scaffolds for the syntheses and derivatization of novel spirocyclic carbocyclic nucleoside analogs.

Titanocene(III) chloride-mediated reductions of oxazines, hydroxamic acids, and N-hydroxy carbamates

Titanocene(III) chloride (Cp(2)TiCl), generated in situ, reduces N-O bonds of various substrates in good to excellent yields (72-95%). Reactions may be performed with stoichiometric Cp(2)TiCl or with catalytic Cp(2)TiCl.

Concise syntheses of enantiomerically pure protected 4-hydroxypyroglutamic acid and 4-hydroxyproline from a nitroso-cyclopentadiene cycloadduct

O-TBS-protected methyl trans-4-hydroxypyroglutamate and methyl trans-4-hydroxyproline ester were synthesized from nitroso-cyclopentadiene Diels-Alder cycloadducts. Enzymatic resolution of the key intermediate, 4-amino-cyclopent-2-enol, provides access to both L- and D- amino acids.

Substrate-dependent dihydroxylation of substituted cyclopentenes: toward the syntheses of carbocyclic sinefungin and noraristeromycin

Carbocyclic nucleosides are of considerable interest for the development of new therapeutic agents. A key reaction in the preparation of many such nucleoside analogues is dihydroxylation of appropriately substituted cyclopentenes. Although often considered a routine reaction, in this paper, we report the dramatic influence of substituents on the facial selectivity of dihydroxylations. The substituted cyclopentene substrates are derived from acylnitroso cycloaddition reactions of cyclopentadiene, followed by N-O reduction and efficient enzymatic resolution. The results are directly utilized in a very efficient asymmetric synthesis of an antiviral carbocyclic nucleoside, noraristeromycin 5. Extensions toward the synthesis of carbocyclic sinefungin 7 document the importance of realizing the substituent dependence of the dihydroxylation reaction.

Stereoselective Total Synthesis of (+)-Streptazolin by Using a Temporary Silicon-Tethered RCM Strategy

A stereoselective total synthesis of (+)-streptazolin 1 was accomplished starting from readily available aminocyclopentenol (-)-7. The synthetic sequence highlights an intramolecular aldol condensation strategy to construct the piperidine core and a silicon-tethered ring-closing metathesis strategy to install the Z exocyclic ethylidene side chain of streptazolin. Separate protodesilylation and Tamao oxidation of a common intermediate 32 afforded streptazolin and the precursor for 13-hydroxystreptazolin. The overall yield for (+)-streptazolin 1 from aminocyclopentenol (-)-7 was 4.8% for a total of 16 steps.

Synthesis of 2,3-Di- and 2,3,4-Trisubstituted Furans from 1,2-Dioxines Generated by an Enyne-RCM/Diels−Alder Reaction Sequence

An efficient method for the synthesis of 2,3- and 2,3,4-substituted furans starting from acyclic enynes was developed using an enyne-RCM/Diels-Alder reaction sequence. The reaction conditions for the transformation of 1,2-dioxines having an adjacent 1,2-oxazine ring into furans and the cleavage of N-O bonds are discussed.

Chemoenzymatic Asymmetric Total Synthesis of Phosphodiesterase Inhibitors: Preparation of a Polycyclic Pyrazolo[3,4-d]pyrimidine from an Acylnitroso Diels−Alder Cycloadduct-Derived Aminocyclopentenol

[reaction: see text] Enzymatic resolution of Boc-protected 4-aminocyclopenten-1-ol 4c gave both enantiomers 5c and 6c in high ee. Boc removal and separate condensation with chloropyrazolopyrimidine 18 provided elaborated 1,4-aminocyclopentenol derivatives 20 and 26, respectively. Separate treatment of 20 and 26 with Pd(0) under basic conditions induced cyclization to unsaturated polycycles 22 and 27, which, upon catalytic hydrogenation, were transformed to new cyclopentane-containing pyrazolopyrimidines 24 and 28, analogues of recently described novel phosphodiesterase inhibitors.

Synthetic Application of Acylnitroso Diels−Alder Derived Aminocyclopentenols: Total Synthesis of (+)-Streptazolin

Concise total syntheses of (+)-streptazolin 1 and its more stable dihydro derivative 2 were accomplished via an intramolecular aldol condensation strategy starting from readily available aminocyclopentenol (-)-7. The synthetic sequence included reductive amination, stereoselective epoxidation, intramolecular aldol (and condensation) reaction, and Wittig reaction. The overall yield for dihydro derivative 2 from aminocyclopentenol (-)-7 was about 7% for a total of 14 steps.

Chemoenzymatic Synthesis and Synthetic Application of Enantiopure Aminocyclopentenols: Total Synthesis of Carbocyclic (+)-Uracil Polyoxin C and Its α-Epimer

Carbocyclic uracil polyoxin C (+)-2 and its alpha-epimer (-)-3 were synthesized in an efficient fashion from cis-4-(N-tert-butylcarbamoyl)cyclopent-2-en-1-ol (+/-)-7. The synthesis incorporates a concise, inexpensive chemoenzymatic synthesis of enantiopure aminocyclopentenols, a Pd(0)-catalyzed substitution reaction, and a mild reduction of an alpha-nitro ester by TiCl(3)/sodium borohydride. Significantly, this process demonstrates the synthetic utility of the versatile enantiopure aminocyclopentenol building block (-)-4.

Concise Synthesis of 4-Acylamino Analogues of 2-Aminobicyclo[3.1.0]hexane-2,6-dicarboxylic Acids (LY354740) from an Acylnitroso Diels−Alder Cycloadduct

Concise total syntheses of 4-acylamino analogues of LY354740 were accomplished employing an N-Boc acylnitroso Diels-Alder cycloadduct as the starting material. The syntheses involved N-O bond cleavage, oxidation, intermolecular cyclopropanation, Bucherer-Bergs reaction, hydrolysis, and regioselective acylation with a temporary copper chelate. The synthesis of an optically active compound was also achieved.

Asymmetric Synthesis of the Carbocyclic Nucleoside Building Block (R)-(+)-4-Aminocyclopentenone Using δ-Amino β-Ketophosphonates and Ring-Closing Metathesis (RCM)

Amino keto-2,7-dienes undergo ring-closing metathesis (RCM) to give 4-aminocyclopentenones, valuable intermediates in the asymmetric construction of carbocyclic nucleosides. The key amino ketodienes were prepared using delta-amino beta-ketophophonates, a new sulfinimine-derived chiral building block, and HWE chemistry. [reaction: see text]

Regio- and stereoselective ring openings of 3-Aza-2-oxabicyclo[2.2.1]hept-5-ene systems with copper catalyst-modified grignard reagents: application to the synthesis of an inhibitor of 5-lipoxygenase

Treatment of acylnitroso hetero Diels-Alder cycloadducts 2 with organomagnesium reagents in the presence of a catalytic amount of copper induces ring opening to afford predominantly monocyclic anti-1,2-hydroxamic acids 12. Alkylmagnesium reagents were found to give superior regio- and stereoselectivities compared with vinyl and arylmagnesium reagents. This cycloadduct ring opening methodology was applied to the synthesis of a unique cyclopentenyl hydroxamic acid-based inhibitor of 5-lipoxygenase.

Synthesis of a conformationally restricted substrate analogue of siderophore biosynthetases

[reaction: see text] A conformationally restricted analogue (5) of N(omega)-acetyl-N(omega)-hydroxyornithine and -lysine was synthesized. The synthesis features an efficient acylnitroso hetero-Diels-Alder cycloadduct (1) ring opening with palladium(0) and methylnitroacetate.