Photochemically Induced Addition of 2-Propanol to Hex-2-enono-δ-lactones Followed by Radical Cyclization: A Novel Entry to Branched Cyclohexanes and Cyclopentanes from Carbohydrates

Photochemically induced conjugated addition of 2-propanol to carbohydrate-derived hex-2-enono-δ-lactones 7−10, substituted at C-6 or C-7 to give electron rich unsaturation, takes place at C-3 generating a radical at C-2 (carbohydrate numbering) which then undergoes efficient radical cyclization onto the pendant olefin to give the corresponding carbocycle. The reaction allows a rapid access to highly functionalized cyclohexanes and cyclopentanes from simple carbohydrate precursors and takes place in a highly stereocontrolled manner. Only one isomer is obtained on the cyclization of lactones from the gluco series, 7−9, where three new stereogenic centers are created. On the other hand, galactose-derived lactone 10, furnished upon application of the protocol a ≈4:1 mixture of two epimers at C-7.