A New Entry to the Stereoselective Introduction of an Ethynyl Group by a Radical Reaction: Synthesis of the Potential Antimetabolite 2′‐Deoxy‐2′‐C‐ethynyluridine

Abstract

Atom‐transfer 5‐exo cyclization and subsequent elimination occur when ethynyldimethylsilyl ethers of iodohydrins are treated with Et3B followed by tetrabutylammonium fluoride (TBAF) to give the corresponding cis‐configured α‐hydroxy ethynyl compounds in high yield (see scheme). This is the first example of the use of a radical reaction to introduce an ethynyl group.

Synthesis and applications of highly functionalized 1-halo-3-substituted bicyclo[1.1.1]pentanes

“…Atom transfer radical addition (ATRA) reactions using chemical initiators are an attractive alternative for TCP ring opening, but previously required a large excess of the radical precursor, or suffered from the formation of oligomeric ‘staffane’ byproducts due to multiple insertions into TCP. 10d , 13 Building on our studies of radical-mediated nucleoside alkynylation, 16 we questioned whether triethylborane could serve as an effective initiator 17 for the synthesis of 1-halo-3-substituted bicyclopentanes ( Fig. 1c ).…”

Acetylenosaccharides

“…Upon irradiation with UV light (300 nm), the 3-iodo-allofuranose 85 reacted with TIPSsCaCsSO 2 CF 3 to afford the gluco-configured acetylene 86 [78] (Scheme 5.20). In radical atom-transfer reactions, the iodides 87 and 89 were transformed into 5-[(trimethylsilyl)iodomethylene]-1-sil-2-oxolanes and thence, by treatment with TBAF, into the alcohols 88 and 90, respectively [144]. This intramolecular alkynyl transfer always results in 1,2-cis hydroxyacetylenes and complements the alkynylating opening of epoxides.…”

Radical Group Transfer of Vinyl and Alkynyl Silanes Driven by Photoredox Catalysis

“…Due to the controllable lability of organosilanes, the cyclic products of these reactions can undergo ring-opening, which leads to an overall group transfer reaction of the vinyl or alkynyl substituent from silicon to carbon. Radical group transfer reactions of vinyl and alkynyl silanes have previously been performed using traditional radical initiators, e.g., on cyclic alkyl iodide substrates (Scheme b). − Following the radical cyclization reaction, addition of a fluoride source results in elimination and desilylation. A similar strategy has also been applied to activated, tertiary bromides and in a radical cyclization cascade to set multiple contiguous stereocenters …”