e with S-allyl S-methyl dithiocarbonate gave the endo cycloadduct whereas phencyclone gave a mixture of the endo and exo cycloadducts. The X-ray analysis of the cycloadduct of phencyclone and S-allyl S-methyl dithiocarbonate indicated the presence of a short contact of Ar–H···OC< type. A possible role of the interaction in determining the endo/exo selectivity was discussed based on the X-ray crystallographic