Abstract
High-pressure reactions of allyltin derivatives of furanoses with sugar aldehydes affords threo-homoallylic alcohols regardless of the configuration ( Z or E) across the double bond of the starting allyltin. Reaction of methyl 2,3-di- O-benzyl-5,6,7-trideoxy-7-(tri- n-butyl)stannyl-hept-5-( Z)-eno-β- d-ribofuranoside 13 Z with 2,3:4,5-di- O-isopropylidene- d-arabinose 6 was highly stereoselective and afforded only one threo-homoallylic alcohol 14, while the same process with E-organometallic 13 E furnished both threo-alcohols: 14 and 15 in the ratio 36:64. This result does not agree with the widely accepted six-membered (chair) transition state, which predicts the threo-isomers for the reaction of the E-allyltin and erythro for the Z-isomers; however, this result may be explained by a twisted six-membered transition state.
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