Aldol addition of lithium and boron enolates of 1,3-dioxan-5-ones to aldehydes. A new entry into monosaccharide derivatives

Methods allowing control of stereoselectivity in aldol reactions of enolate s derived from 1,3-dioxan-5-ones (4) are described. Boron enolates, generat ed in situ, react with benzaldehyde to give the corresponding anti aldol se lectively (the anti:syn ratio of up to 96:4) and in high yield. Lithium eno lates give high anti selectivity only with aldehydes branched at the a-posi tion. Enantioselective deprotonation of Cs symmetrical dioxanones (e.g., 4b ) can be accomplished efficiently, with enantiomeric excess of up to 90%, w ith chiral lithium amide bases of general structure PhCH-(Me)N(Li)R (9, 10) if the R group is sufficiently bulky (e.g, R = adamantyl) or is fluorinate d (e.g., R = CH2CF3). Dioxanone boron and lithium enolates react readily wi th glyceraldehyde derivatives (19), yielding protected ketohexoses (20 and 21).