1,3-Dipolar Cycloadditions of alpha-Diazo Ketones Derived from N-Protected (S)-Proline with Aromatic and Cycloaliphatic Thioketones

Abstract

Enantiomerically pure alpha-oxo diazo compounds derived from (S)-proline were used for 1,3-dipolar cycloaddition with aryl and hetaryl thioketones, as well as with cycloalkanethiones. Whereas the reactions with hetaryl thioketones in boiling THF yield alpha,beta-unsaturated ketones via a cascade of cycloaddition, 1,3-dipolar electrocyclization, and desulfurization, the analogous reactions with thiobenzophenone and cycloalkanethiones result in the formation of 1,3-oxathiole derivatives. In the latter case, the 1,5-dipolar electrocyclization of the intermediate thiocarbonyl ylide is the key step of the reaction sequence. In all cases, the isolated products are optically active, i.e., the multistep processes occur with retention of the stereogenic center incorporated via the use of (S)-proline as the precursor of the diazo compounds.