Steric Course of Some Cyclopropanation Reactions of L-Threo-Hex-4-Enopyranosides

Abstract: Completely protected 4-deoxy-alpha-L-threo-hex-4-enopyranosides 1c,d undergo the dichlorocarbene addition affording exclusively diastereomeric adducts 5c,d with the cyclopropane ring anti to the C-3 alkyloxy substituent, while the reaction with 3-unprotected derivatives 1a,b affords a mixture of syn and anti derivatives. Under the Simmons-Smith cyclopropanation adducts 2a-d with a syn stereochemistry are obtained. Starting from 5b, the cyclopropanated sugar 3b is obtained by reduction with LiAlH4, thus the two diastereomers 2b and 3b can be stereoselectively obtained through the two different pathways. For a useful comparison, 4-deoxy-beta-L-threo-hex-4-enopyranoside le was also subjected to the above two cyclopropanation methods affording the expected cycloadduct 2e and a diastereomeric mixture of dichlorocycloadducts 4e and 5e (4e/5e=2.8:1).