Reactions of acid anhydrides-III. Dimerizations of 3-oxo-steroids

Steroidal ketone 1 reacts with trichloroacetyl p-toluenesulphonate to give in addition to the enol tosylate 3 a dimeric ketone 4 together with its enol esters 5 and 6. The dimeric ketone 4 was also obtained when ketone 1 was treated with trichloroacetic anhydride and p-toluenesulphonic acid. High concentrations of reactants and presence of acid favoured the formation of the dimeric products. It is suggested, that the dimerization occurs by an acid catalysed aldol-type condensation, while the anhydrides act as dehydrating agents converting the initially formed dimeric ketol to the β,γ-unsaturated ketone 4. Formation of dimeric ketone 4 was also observed when the α-bromoketone 8 was treated with zinc and ethyl chlorocarbonate. In the presence of zinc and zinc chloride however, ketone 8 yielded the dimeric β-diketone 12.