Enantioselective synthesis of the chromane moiety of vitamin E

Several new approaches for the enantioselective synthesis of the chromane m oiety of vitamin E are described. Sonogashira coupling of 3a with the alkyn e 4 and subsequent elimination gave 6, which was bis(hydroxylated) in 93% y ield and with 85% ee. Recrystallization gave enantiopure 7a, which was hydr ogenated and transformed into the vitamin E precursor 11. The bis(hydroxyla tion) of 18 and 21 to give 9 and 22, respectively, was less than satisfacto ry, proceeding with ee values of 28% and 18%. In contrast, stereoselective allylation of ketone 15 followed by removal of the protecting group or ozon olysis of the allyl moiety furnished the allyl alcohol 26 and the aldehyde 27, respectively, in almost enantiopure form, which again could be used as precursors for vitamin E. Partial hydrogenation of 5a gave the alkene 32a a nd that of 28 the alkene 30b, both of which show interesting atropisomerism .