From Furan to Nucleosides

Towards a better understanding of the electrosynthesis of 2,5-dicarboxy-2,5-dihydrofurans: structure, mechanism and influence over stereochemistry

2,5-Dicarboxy-2,5-dihydrofurans are key constituents of a number of natural products and have roles as intermediates in the formation of other such compounds of interest. Typically, these species are synthesized using toxic Pb(IV) salts. Electrochemical syntheses of 2,5-diacetoxy-2,5-dihydrofuran that do not require the use of lead have been reported, but a general lack of experimental detail has prevented these procedures from being more widely adopted. Moreover, no electrochemical study has yet reported the ratio of cis and trans isomers produced. Herein, we compare the chemical, lead-based route to 2,5-diacetoxy-2,5-dihydrofuran with a fully described electrosynthesis method. In doing so, we have discovered that the cis and trans isomers of this compound were previously incorrectly assigned in the literature, an error that we correct by obtaining the crystal structure of cis-2,5-diacetoxy-2,5-dihydrofuran. This allows the ratios of the isomers as prepared by the chemical (2 : 1 cis : trans) and electrochemical (7 : 5 cis : trans) methods to be obtained. Through experimental and computational insights, we propose a mechanism for the electrochemical synthesis of 2,5-dicarboxy-2,5-dihydrofurans and go some way towards validating this mechanism by synthesizing 2,5-dibutoxy-2,5-dihydrofuran electrochemically for the first time. We hope that these findings will provide some greater clarity to the literature surrounding the electrosynthesis and potential applications of 2,5-dicarboxy-2,5-dihydrofurans.

Regioselective vicinal functionalization of unactivated alkenes with sulfonium iodate(i) reagents under metal-free conditions

Metal-free, molecular iodine-free direct 1,2-difunctionalization of unactivated alkenes has been reported. The sulfonium iodate(i) reagent efficiently promoted the intermolecular vicinal iodo-functionalization of a diverse range of olefins in a stereo and regioselective manner. This method enables the divergent and straightforward preparation of synthetically useful functionalities; β-iodocarboxylates, β-iodohydrins, and β-iodoethers in a one-step process. Further interconversion of iodo-functionalized derivatives allows easy access to valuable synthetic intermediates en route to biologically active molecules.

Asymmetric synthesis of allylic amines via hydroamination of allenes with benzophenone imine

Rhodium-catalyzed highly regio- and enantioselective hydroamination of allenes is reported. Exclusive branched selectivities and excellent enantioselectivities were achieved applying a rhodium(i)/Josiphos catalyst. This method permits the practical synthesis of valuable α-chiral allylic amines using benzophenone imine as ammonia carrier.

Acetoxy Meldrum's acid: a versatile acyl anion equivalent in the Pd-catalyzed asymmetric allylic alkylation

Acetoxy Meldrum's acid can serve as a versatile acyl anion equivalent in the Pd-catalyzed asymmetric allylic alkylation. The reaction of this nucleophile with various meso and racemic electrophiles afforded alkylated products in high yields and enantiopurities. These enantioenriched products are versatile intermediates that can be further functionalized using nitrogen- and oxygen-centered nucleophiles, affording versatile scaffolds for the synthesis of nucleoside analogues. These scaffolds were used to complete formal syntheses of the anti-HIV drugs carbovir, abacavir, and the antibiotic aristeromycin.

Highly diastereoselective synthesis of modified nucleosides via an asymmetric multicomponent reaction

We have developed a practical synthesis of unique nucleoside derivatives via TiCl(4) promoted multicomponent reaction of optically active dihydrofuran, ethyl pyruvate/glyoxylate, and a TMS protected nucleobase in a single-pot operation.

Regio- and enantioselective N-allylations of imidazole, benzimidazole, and purine heterocycles catalyzed by single-component metallacyclic iridium complexes

Highly regio- and enantioselective iridium-catalyzed N-allylations of benzimidazoles, imidazoles, and purines have been developed. N-Allylated benzimidazoles and imidazoles were isolated in high yields (up to 97%) with high branched-to-linear selectivity (up to 99:1) and enantioselectivity (up to 98% ee) from the reactions of benzimidazole and imidazole nucleophiles with unsymmetrical allylic carbonates in the presence of single component, ethylene-bound, metallacyclic iridium catalysts. N-Allylated purines were also obtained in high yields (up to 91%) with high N9/N7 selectivity (up to 96:4), high branched-to-linear selectivity (98:2), and high enantioselectivity (up to 98% ee) under similar conditions. The reactions encompass a range of benzimidazole, imidazole, and purine nucleophiles, as well as a variety of unsymmetrical aryl, heteroaryl, and aliphatic allylic carbonates. Competition experiments between common amine nucleophiles and the heterocyclic nitrogen nucleophiles studied in this work illustrate the effect of nucleophile pK(a) on the rate of iridium-catalyzed N-allylation reactions. Kinetic studies on the allylation of benzimidazole catalyzed by metallacyclic iridium-phosphoramidite complexes, in combination with studies on the deactivation of these catalysts in the presence of heterocyclic nucleophiles, provide insight into the effects of the structures of the phosphoramidite ligands on the stability of the metallacyclic catalysts. The data obtained from these studies have led to the development of N-allylations of benzimidazoles and imidazoles in the absence of an exogenous base.

Efficient synthesis of 4'-cyclopropylated carbovir analogues with use of ring-closing metathesis from glycolate

The first synthetic route of novel 4'-cyclopropylated carbovir analgues is described. The construction of cyclopropylated quaternary carbon at 4'-position of carbocyclic nucleosides was successfully made via sequential Johnson's orthoester rearrangement and ring-closing metathesis (RCM) starting from ethyl glycolate. Synthesized compounds 15 and 16 showed moderate antiviral activity without any cytotoxicity up to 100 micromol.

Metal-catalyzed enantioselective allylation in asymmetric synthesis

Metal-catalyzed enantioselective allylation, which involves the substitution of allylic metal intermediates with a diverse range of different nucleophiles or S(N)2'-type allylic substitution, leads to the formation of C-H, -C, -O, -N, -S, and other bonds with very high levels of asymmetric induction. The reaction may tolerate a broad range of functional groups and has been applied successfully to the synthesis of many natural products and new chiral compounds.

Synthesis of novel 2’-methyl carbovir analogues as potent antiviral agents

In this study, the synthesis procedures of 2'-branched carbovir analogues were accomplished. The introduction of a methyl group in the requisite 2'-position was carried out by the addition of a carbonyl using isopropenyl magnesium bromide. The desired compound, cyclopentenol 10(beta), was synthesized via ring-closing metathesis using a second-generation Grubbs' catalyst. The nucleosidic bases (adenine, cytosine, thymine, uracil, 5-fluorouracil and 5-iodouracil) were efficiently coupled using a Pd (0) catalyst. When the synthesized compounds were examined for their activity against several viruses, including HIV-1, HSV-1, HSV-2 and HCMV, the 5-iodouracil analogue, 23, exhibited significant anti-HCMV activity.

Synthesis and Antiviral Evaluation of Novel 6′(α)-Methyl-Branched Carbovir Analogues

A new stereoselective synthesis of 6'-branched Carbovir analogues was accomplished in this study. The introduction of a methyl group in the requisite 6'(alpha)-position was carried out using an ester enolate alkylation (LHMDS, CH3I). The desired cyclopentenol 7 was synthesized via a Felkin-Anh-controlled Grignard addition and ring-closing metathesis using second-generation Grubbs' catalyst. The natural bases (adenine, cytosine) were efficiently coupled using a Pd(0) catalyst. When the synthesized compounds were examined for their activity against several viruses such as the HIV-1, HSV-1, HSV-2, and HCMV, the adenine analogue 12 exhibited moderate antiviral activity against the HCMV.

Toward amide-linked RNA mimics: total synthesis of 3'-C branched uridine azido acid via an ene-iodolactonization approach

[reaction: see text] A novel synthesis of 3'-C branched uridine azido acid has been accomplished by a sequence of stereoselective ene and iodolactonization reactions. Compared to traditional routes that start from carbohydrates, the present methodology is more flexible and can be further optimized by incorporation of novel future developments of synthetic chemistry. Because the key chemistry does not involve the 3'-C substituent, our route is a general approach to 3'-C alkyl nucleoside analogues.

A new concept for the preparation of beta-L- and beta-D-2',3'-dideoxynucleoside analogues

[reaction: see text] A new method for the synthesis of 2',3'-dideoxynucleoside analogues has been developed. An electrochemical activation of 2-substituted furans is followed by the coupling with a pyrimidine or purine base. This gives planar furyl nucleosides as key intermediates, which are hydrogenated cis-selectively to give the corresponding beta-2',3'-dideoxynucleosides as racemic mixtures. An enzymatic kinetic resolution gives rise to beta-D- and beta-L-configured derivatives in high optical purity. This is exemplified by the synthesis of beta-D- and beta-L-3'-deoxythymidine.

Application of the AAA reaction to the synthesis of the furanoside of C-2-epi-hygromycin A: a total synthesis of C-2-epi-hygromycin A

A strategy for stereocontrolled syntheses of furanoside type of natural products is developed for a glycosyl aryl ether. This strategy resolves the issue of low diastereoselectivity typical of normal glycosidation methods for furanosides. All the stereochemistry ultimately derives from a desymmetrization of a 2,5-diacyloxy-2,5-dihydrofuran using Pd catalyzed asymmetric allylic alkylation which sets both the absolute stereochemistry and 1,4-relative stereochemistry. Diastereo-controlled elaboration of the 3,4-double bond then completes the synthesis. A new conjunctive reagent, 1-nitro-1-phenylsulfonyl-ethane, is developed to serve as an acyl anion equivalent. The utility of a phenol as a nucleophile in the Pd catalyzed glycosylation is demonstrated. From this strategy emerged a short, practical synthesis of C-2-epi-hygromycin A.

Spirocyclic restriction of nucleosides. Synthesis of the first exemplary syn-1-oxaspiro[4.4]nonanyl member

[reaction: see text] The potential benefits associated with the spirocyclic restriction of nucleosides are summarized. Following exploration of a pi-allylpalladium route to 5'-alpha- or syn-dideoxy examples, we evaluated MOM protection of the 5'-hydroxyl as being suited to the synthesis of the first member of this new class of nucleoside mimic.