Asymmetric synthesis. Part 6. Copper salt promoted grignard reagent additions to ethyl 2,3-dideoxy-4,5:6,7-di-O-isopropylidene-D-arabino-trans-hept-2-enonate and subsequent formation of optically active 2-alkyl (or aryl) butane-1,4-dioic acids and butyro-1,4-lactones

Abstract

The copper-salt promoted 1,4-additions of aryl and t-butyl Grignard reagents to ethyl 2,3-dideoxy-4,5:6,7-di-O-isopropylidene-D-arabino-trans-hept-2-enonate are highly stereoselective yielding products with the D-manno-configuration. In contrast isopropyl and ethyl Grinard reagents give products preponderantly with the D-gluco-configuration. Cyclohexylmagnesium bromide does not react stereoselectively but gives a 55 : 45 mixture of the D-gluco- and D-manno-isomers. Controlled degradation of the carbohydrate molecule affords 2-aryl (alkyl) butane-1,4-dioic acids and 3-aryl (alkyl)-butyro-1,4-lactones. The enantiomeric purity of these products is established by reference to known products or by use of optically active n.m.r. shift reagents.