TANDEM CARBONYL COUPLING-REARRANGEMENTS PROMOTED BY THE NIOBIUM(III) REAGENT - DUAL REDUCTIVE AND LEWIS ACID PROPERTIES OF NBCL3(DME)

Benzaldehyde (1a) and acetophenone (6) are shown to undergo highly stereoselective reductive couplings by NbCl3(DME). When the reaction is performed at - 10-degrees-C, the threo diols can be isolated. The intermediate niobiopinacols reveal the ability of subsequent transformations, due to the Lewis acid properties of the reagent used. Starting from la stereospecific acetalization leads to threo-2,4,5-triphenyl-1,3-dioxolane (3a), whereas 3,3-diphenyl-2-butanone (7) is obtained from 6 in a pinacol-pinacolone type rearrangement. Alkenes may also be formed in the competing, temperature controlled deoxygenation. The stereoselectivity and the easy control of NbCl3 (DME) mediated reactions make ii a promising reagent for further synthetic applications.