BIFUNCTIONAL CHIRAL AUXILIARIES .7. ALDOL REACTIONS OF ENOLATES DERIVED FROM 1,3-DIACYLIMIDAZOLIDINE-2-THIONES AND 1,3-DIACYLIMIDAZOLIDIN-2-ONES

Dibutylboron enolates of 1,3-diacylimidazolidine-2-thiones and 1,3-diacylimidazolidin-2-ones undergo highly syn-stereoselective aldol reactions with aldehydes to allow elaboration of both acyl sidechains. The reaction is proposed to occur via sequential enolisation rather than via a bisenolate and the stereochemistry of the product was elucidated by both X-ray crystallography and reductive cleavage of a homochiral aldol product to give (1R,2S)-1-phenyl-2-methylpropane-1,3-diol. In contrast, tin (II) triflate mediated aldol reactions result in only one of the two acyl sidechains reacting and exhibit reversed enantiofacial stereoselectivity.