Hexofuranose Hexopyranosides

An analogous methyl 2,3-dideoxy-3-C-methylene-a-L-en/thro-hexopyranoside (20) was obtained10 from methyl 4,6-O-benzylidene-2-deoxy-a-L-eri/t/iro-hexopyranosid-3-ulose (19) it was used in the synthesis of olivomycose (see p. 242). There was also reported32,34 the preparation of 3-deoxy-l,2 5,6-di-0-isopropylidene-3-C-methyl-ene-a-D-rifco-hexofuranose (22) from 1,2 5,6-di-0-isopropylidene-a-D-nfeo-hexofuranos-3-ulose (21) in about 60% yield, followed by transformations into branched-chain sugar derivatives, including... 

Reduction of methyl 4,6-0-benzylidene-2,3-di-0-p-tolylsulfbnyl-a-D-glucopyranoside with lithium aluminum hydride in tetiahydrofuran affords methyl 4,6-0-benzylidene-3-deoxy-a-D-fifco-hexopyranoside (in 66% yield) by a mechanism analogous to that proposed by Overend and coworkers for the formation of 5-deoxy-l,2-0-isopropyli-dene-a-D-x /lo-hexofuranose. With the corresponding /3-d anomer, hydride reduction in tetrahydrofuran proceeds less readily, giving methyl 4,6-0-benzylidene-3-deoxy-/8-D-rtho-hexopyranoside in 38% yield. This compound was also obtained (in 22% yield) when hydride reduction was performed in p-dioxane, together with a second crystalline compound, identified as methyl 4,6-0-benzylidene-2,3-dideoxy-/3-D-er /thro-hexoside formed by direct displacement at C-2 and C-3 with hydride. 

A convenient synthesis of hex-5-enopyranosides from 6-deoxy-6-halo-hexopyranosides involves treatment with sodium hydride-DMF with ben l halide present simultaneous benzylation of free hydroxy-groups occurs. Reference is made to 5,6-unsaturated hexofuranose derivatives having a dimethoxyphosphinyl group at C-3 in Chapter 17. 

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