Synthesis of (+)-13-stemarene and (+)-18-deoxystemarin: Expeditious preparation of the key 6-exo-Hydroxybicyclo[2.2.2]octan-2-one ethylene dithioacetal

An expeditious preparation of the 6-exo-hydroxybicyclo[2.2.2]octan-2-one ethylene dithioacetal 2b, a key intermediate in the synthesis of (+)-13-stemarene (4) and (+)-18-deoxystemarin (5) is described. Compound 2b was obtained as the major product by equilibrating the endo rich mixture of 6-hydroxybicyclo[2.2.2]octan-2-one ethylene dithioacetals 2 with TsOH in benzene at reflux, easily available from the corresponding hydroxy ketones 9. The model experiments which preceeded the above transformation, not previously described in the literature, are also presented.