π-Face-selective hetero Diels–Alder reactions of 3,4-di-tert-butylthiophene 1-oxide. An excellent trapping reagent for thioaldehydes and thioketones

Hetero Diels–Alder reactions of 3,4-di-tert-butylthiophene 1-oxide (1) with thioaldehydes and thioketones take place exclusively, except the reaction with thiobenzophenone, at the syn-π-face of 1 with respect to the SO bond. The π-face selectivity was explained in terms of the extent of conformational changes of 1 that are brought about in the process to the transition states.