BRANCHED-CHAIN CARBOHYDRATE LACTONES FROM A SAMARIUM(II) IODIDE-PROMOTED SERIAL DEOXYGENATION CARBONYL ADDITION-REACTION

A new deoxygenation-carbonyl addition reaction mediated by samarium(II ) iodide (SmI2) in THF/HMPA was examined with carbohydrate lactones an d several substrates containing an alpha-alkoxy carboxylic ester. In a single reaction, these compounds were deoxygenated and subsequently c oupled to several ketones by a carbonyl addition reaction. The first r eactions studied simple ester and ketone adducts which were later elab orated to more complex optically active carbohydrate lactones appended to terpene ketones. Simple esters smoothly afforded beta-hydroxy carb onyl products. Fully benzoate-protected 3-deoxycarbohydrate lactones w ere reacted with simple ketones to produce C2-branched sugars. The att endant carbonyl addition to the least sterically hindered face of the aldonolactone provided the major products. Moderate diastereoselectivi ties (up to 5:1) were observed in the simple ketone products as determ ined by difference NOE studies. Finally, the terpene ketones, (-)-ment hone or (+)-dihydrocarvone, were coupled to 3-deoxycarbohydrate lacton es which gave C2-branched sugars with very high diastereoselectivities (up to 99:1).