Second generation α-enones from a pyranosidic α-enone

The Diels-Alder product from the reaction of methyl 2,3,6-trideoxy-α- d - glycero -hex-2-enopyranosid-4-ulose ( 1b ) with trans -1-methoxy-3- tert -butyl-dimethylsilyloxy-1,3-butadiene is 3b (93%). Reaction of 3b with sodium borohydride causes reduction of the C-4 carbonyl group only, but, with lithium aluminum hydride, further reactions occur which can be rationalized by fragmentation brought about by hydride cleavage on the silicon-oxygen bond, with simultaneous ejection of the β-methoxyl group complexed to a trivalent aluminum species. The enone resulting from this fragmentation also reacts further with lithium aluminum hydride, and several products result. The behavior of postulated intermediates, which have been prepared separately and subjected to the reaction conditions, supports the proposed reaction mechanisms. The “second generation” enone (methyl 2,3,6-trideoxy-α- d -talopyranosido)-[3,2- d ]-2-cyclohexenone ( 10a ), arising from the first generation precursor 1b , has been prepared by two routes.