Aldol condensation reactions of (dienone) tricarbonyliron complexes .3. Enantioselective total synthesis of 3,6-dideoxyhexoses from lactaldehyde.

The trimethylsilyl enol ether 2 of racemic (3,5-heptadien-2-one) tricarbonyliron 1 undergoes a completely stereoselective cross aldol reaction with TiCl4-coordinated (S)-(-)-benzyloxylactaldehyde (-)-3, yielding two separable diastereomeric ketols, differing only in the configuration of the metal-ligand attachment (resolution). Both were converted into protected 3,6-dideoxyhexoses [(+)-16 (R,S,S) and (-)-17 (S,S,S)] by successive stereoselective reduction into 1,3-diols, transformation into diacetates, decomplexation and ozonolysis. The same dideoxyhexoses, as well as the enantiomeric series, can be obtained from the racemic lactaldehyde and the pure enantiomers of 2. (C) 1997 Elsevier Science Ltd.