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The reaction of 2.3-dichloro-5.6-dicyanobenzoquinone with steroidal 4-en-3-ones is catalysed by acids. 1.2-Dehydrogenation predominates in the presence of weak acids, as it does in the uncatalysed reaction, whereas strong acids promote exclusive formation of 6,7-dehydro-derivatives. These results are rationalised by a mechanism involving oxidation of the alternative enolic forms of the ketone. Thus, in the absence of acid or with weak acids, the kinetically determined 2.4-dien-3-01 undergoes C-1 hydride ion abstraction with dichlorodicyanobenzoquinone; strong acid accelerates the formation of the thermodynamically more stable 3.5-dien-3-01. which readily suffers hydride abstraction from C-7 by the quinone. In the uncatalysed reaction the rate of 1.2-dehydrogenation depends upon both steroid and quinone concentration, whereas, in the presence of weak acids, the rate depends only on the concentration of ketone. Dehydrogenation of methylated and deuteriated 3-ketones reveals preferential elimination of the trans-diaxial 2pand 1 a-hydrogen atoms. Specific loss of the 1 a-hydrogen is ebserved with 5a-3-ones, but the oxidation of 4-en-3-ones is less specific and involves some attack a t the 1 P-position.

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