Enantiospecific Total Synthesis of l-2‘,3‘-Dideoxyisonucleosides via Regioselective Opening of Optically Active C2-Symmetric 1,4-Pentadiene Bis-epoxide1

Intramolecular dehydration of biomass-derived sugar alcohols in high-temperature water

The intramolecular dehydration of biomass-derived sugar alcohols d-sorbitol, d-mannitol, galactitol, xylitol, ribitol, l-arabitol, erythritol, l-threitol, and dl-threitol was investigated in high-temperature water at 523–573 K without the addition of any acid catalysts.

Nucleosides with Transposed Base or 4'-Hydroxymethyl Moieties and Their Corresponding Oligonucleotides

This review focuses on 4'-hydroxymethyl- or nucleobase-transposed nucleosides, nucleotides, and nucleoside phosphonates, their stereoisomers, and their close analogues. The biological activities of all known 4'-hydroxymethyl- or nucleobase-transposed nucleosides, nucleotides, and nucleoside phosphonates as potential antiviral or anticancer agents are compiled. The routes that have been taken for the chemical synthesis of such nucleoside derivatives are described, with special attention to the innovative strategies. The enzymatic synthesis, base-pairing properties, structure, and stability of oligonucleotides containing nucleobase- or 4'-hydroxymethyl-transposed nucleotides are discussed. The use of oligonucleotides containing nucleobase- or 4'-hydroxymethyl-transposed nucleotides as small oligonucleotide (e.g., human immunodeficiency virus integrase) inhibitors, in applications such as antisense therapy, silencing RNA (siRNA), or aptamer selections, is detailed.

A Fortuitously Straightforward Synthesis of 4-Acetoxy-2-Propyltetrahydrothiophene

4-Acetoxy-2-propyltetrahydrothiophene was synthesised from 1-hepten-4-ol by a three-step route involving epoxidation and mesylation to 1,2-epoxy-4-heptyl mesylate and then reaction with thioacetate. An acetoxylated cyclic product was formed instead of the expected thioacetate, and a mechanism for its formation using an intramolecular transesterification is proposed.

C4‘-Spiroalkylated Nucleosides Having Sulfur Incorporated at the Apex Position

Methodology based on the concept of thionium ion-initiated pinacolic ring expansion has been developed for accessing C4'-spirocyclic thionucleosides. The readily available racemic ketones 6 and 37 are conveniently resolved via their acetals with (R)-mandelic acid. Subsequent reactions beginning with utilization of the Pummerer rearrangement lend themselves to functionalization of the spirocyclic core and ultimately incorporation of the nucleosidic bases. Limitations to this strategy are pointed out. Acquisition of the alpha- and beta-isomers at C4' is equally facile. Absolute configurational assignments have been made possible by X-ray crystallography.

Biocatalytic reduction of beta,delta-diketo esters: a highly stereoselective approach to all four stereoisomers of a chlorinated beta,delta-dihydroxy hexanoate

A stereoselective chemoenzymatic synthesis of all four stereoisomers of tert-butyl 6-chloro-3,5-dihydroxy-hexanoate (6a) is presented. The key step of the sequence is a highly regio- and enantioselective single-site reduction of tert-butyl 6-chloro-3,5-dioxohexanoate (1a) by two enantiocomplementary biocatalysts. Alcohol dehydrogenase from Lactobacillus brevis (recLBADH) afforded a 72% yield of enantiopure tert-butyl (S)-6-chloro-5-hydroxy-3-oxohexanoate [(S)-2a]. The enantiomer (R)-2a was prepared with 90-94% ee by Baker's yeast reduction in a biphasic system (50% yield). Both biotransformations were performed on a gram scale. The beta-keto group of the enantiomeric delta-hydroxy-beta-keto esters 2a thus obtained was reduced by syn- and anti-selective borohydride reductions. Permutation of the reduction methods yielded all four stereoisomers of the crystalline target compound 6a (> or = 99.3% ee, dr > or = 205:1), which is a versatile 1,3-diol building block. recLBADH accepts a variety of beta,delta-diketo esters as was determined in a photometric assay. tert-Butyl 3,5-dioxo-hexanoate (1b) and tert-butyl 3,5-dioxo-heptanoate (1c) were reduced on a preparative scale as well to afford the corresponding delta-hydroxy-beta-keto esters (R)-2b and (R)-2c with 99.4% ee and 98.1% ee, respectively.

A Versatile Enantioselective Strategy Toward L-C-Nucleosides: A Total Synthesis of L-Showdomycin

Strategies for the synthesis of nucleosides that can provide either L or D isomers become more important as a result of the increasing number of such compounds that are therapeutically useful. The lower toxicity and reduced susceptibility toward metabolism of the L isomers make them particularly interesting. A strategy toward the C-nucleoside analogues has been explored in the context of the synthesis of L-showdomycin. The route involves an asymmetric desymmetrization using palladium catalysis of cis-2,5-diacyloxy-2,5-dihydrofurans available in one step from furan, with carbon nucleophiles. Nucleophilic synthons for a maleimide unit and a methoxycarbonyl unit have been designed. Two sequential palladium-catalyzed reactions introduce both substituents with excellent chemo-, regio-, diastereo-, and enantioselectivity. The presence of a double bond in this doubly alkylated compound at C-3 and C-4 allows easy structural variation. The use of an ester as a hydroxymethyl precursor also introduces a diversity element as well as having importance in its own right. The successful completion of a synthesis of L-showdomycin validates this approach as a viable strategy to C-nucleosides.

Synthesis and testing of new modified nucleosides

New efficient routes for the high-yielding synthesis of several classes of modified nucleosides have been developed. We have prepared both the D- and L-enantiomers of the methylene-expanded oxetanocin isonucleosides 1a-c and the L-2',3'-dideoxy isonucleosides 2abc (both the oxa and thia analogues) as well as new routes for the preparation of L-ribose and 2-deoxy L-ribose 3ab and their modified nucleosides 4.