N-acylsultams 2 furnish, via aldolization of their enolates 16 with aldehydes, diastereomerically pure, crystalline syn aldols. The absolute configuration of the product is controlled by the choice of the enolate counterion: 16, M = B → 3; 16, M = Li or Sn(IV) → 5. Hydroperoxide-assisted hydrolysis/esterification or reductive cleavage provided enantiomerically pure methoxycarbonyl aldols (12 and 13) or 1,3-diols (11) with recovery of auxiliary 1. The chiral serricornin precursor 14 was thus prepared.