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 and several substrates containing an alpha-alkoxy carboxylic ester. In a single reaction, these compounds were deoxygenated and subsequently coupled to several ketones by a carbonyl addition reaction. The first reactions studied simple ester and ketone adducts which were later elaborated to more complex optically active carbohydrate lactones appended to terpene ketones. Simple esters smoothly afforded beta-hydroxy carbonyl products. Fully benzoate-protected 3-deoxycarbohydrate lactones were reacted with simple ketones to produce C2-branched sugars. The attendant carbonyl addition to the least sterically hindered face of the aldonolactone provided the major products. Moderate diastereoselectivities (up to 5:1) were observed in the simple ketone products as determined by difference NOE studies. Finally, the terpene ketones, (-)-menthone or (+)-dihydrocarvone, were coupled to 3-deoxycarbohydrate lactones which gave C2-branched sugars with very high diastereoselectivities (up to 99:1).