mntsdinGcat8riuin ano-s7.oiha.oo The discovery, by Danishefsky and hia group, t that Lewis acids such as Z&l2 and BF5.OEt2 can induce the cyclocoadcnsation of l~xy-3-siloxybuta-1,3~enm and aldchydes to #ve 2,3dihydropymna. e.g. m-1. hae bad a notable impact on the synthesis of pyranoid ay~tems.~ Although cycloaddition products are considered to intervene in certain cases,3 the oppornkty to study such labile species became possible when it was found that they could be iaolatcd from reactiona catalyscd by adublc lanthanidc comptcxca.4 In a quest for stereopure 2Jdihydropyran4-onea such aa 1. the Yale workers prepared a series of dienes incorporating menthyl auxiliark and studied their reaction8 with henzaldehyde in the presmce of eurcpium( III) compkxea.~ The beat nzault was achieved using the diene ti and (+)-Et@+8 in hexanes at -20 l C; a 96~4 mixture of the ckcycloadducta 3a and 4a (arising from endo-cycloadditions) was produced, from which the major product was isolated in 60% yield after ctystallisation. Treatment of compound 3a with trifluoroact tic acid afforded the enantiopure dihyw 1. which served BB a precursor of ~gluccee.~ Having developed the dienw 5a7 and sbs and shown that they undergo endocycloadditiona in a stereoselective manner with cyclic dienophilea under thermal conditions, we were intensed in amassing their reactivity towards aromatic aldehydes. Baaed upon the known diaatereofacial selectivity of the dienes Sa and S towards electron-deficient alkencs7-9 and the established preference for aromatic aldehydes to undergo c&o-selective cycloadditione with l-oxy3-siloxybuta-1 &Genes in the presence of europium(m) cunplexea.4~ we expected that cycloadducts of type 6 would predominate over their counterparts of typea 7-9.
As well as laying the foundations for a new protocol for the elaboration of (l-l)-linked dimccharides, we envisaged that the technology would lead to the synthesis of antipodes of the dihydropyranone 1 and its relatives. We now report on 8ome of our findings.