SYNTHESIS OF THIOALDEHYDES HAVING OPTICALLY-ACTIVE ALKOXY MOIETY AND THEIR ASYMMETRIC HETERO-DIELS-ALDER REACTION

Optically active alpha-alkoxycarbonylthioaldehydes (2a-g) were prepare d using 8-arylmenthols as chiral auxiliaries. Their asymmetric hetero Diels-Alder reactions with cyclopentadiene gave the endo cycloadducts (3 and 4) and exo cycloadducts (5 and 6) with moderate diastereomeric excesses. However, the major endo cycloadducts (3b-g) were isolated in optically pure form. This is the first chiral synthesis of 2-thiabicy clo[2.2.1]hept-5-ene ring system. The absolute configuration of the cy clic carbonate (12), which was prepared from the major endo cycloadduc t (3c) via the epimerization, or the minor exo compound (6c), was dete rmined as 1R by X-ray analysis, The cycloadduct (3c) was transformed t o a potential intermediate (14) for the synthesis of carbocyclic homon ucleosides.