The photochemistry of ketones derived from carbohydrates. Part 10. A study of stereochemical influences on photo-induced rearrangements and ring expansions of 3-oxacyclopentanones using 1,2-O-isopropylidenefuranos-3-ulose derivatives

Abstract

U.v.-irradiation of the 3-oxacyclopentanone 5-deoxy-1,2-O-isopropylidene-β-D-threo-pentofuranos-3-ulose (9) in methanol gave at –60 °C, and to a small extent at +20 °C, [2S,3R,5S,6R]-2,3-isopropylidenedioxy-5-methoxy-6-methyl-1,4-dioxane (12), whereas in a benzene solution at +20 °C only the unsaturated lactones 2,2-dimethyl-5-[(Z)-prop-1-enyloxy]-1,3-dioxolan-4-one (13) and its (E)-isomer (14) were obtained. The photoproducts formed from (9) in methanol–diethyl ether (1:4) at –60 °C were a mixture of the dioxane (12) and the lactones (13) and (14). The isomeric erythro-ulose (20) gave only the lactones (13) and (14) when irradiated in either methanol–diethyl ether (1:4) at –60 °C or in benzene at +20 °C. The lactone mixtures from (9) and (20) possessed similar specific optical rotations.

U.v.-irradiation of a benzene solution of 1,2-O-isopropylidene-α-L-glycero-tetrofuranos-3-ulose (22) at +20 °C gave racemic 5-ethenyloxy-2,2-dimethyl-1,3-dioxolan-4-one (16). The optical activity exhibited by the lactone mixtures produced from ketones (9), (20) and (22) indicated that they were not formed via the respective oxacarbenes (11), (29) and (30), which are chiral, but through the intermediacy of the symmetric enals (31) from (9) and (20), and (32) from (22).

The uloses (9) and (20) formed hemiacetals (18), and (23) and (24) respectively, in methanolic solutions but only to a limited extent at low temperatures.