The water soluble steroid 1 acts as a catalyst for the enolization of the ketones 2a-c. The rate enhancement for 1-catalyzed reaction relative to imidazole-catalyzed reaction (R), increases as the size of the aromatic ring system of the substrate increases; a plot of log Rvs π has a slope of 0.43. This is interpreted in terms of hydrophobic binding between steroid α-surface and aromatic ring system which lowers the free energy level of the transition state for steroid-catalyzed enolization.

. Can. J . Chem. 60,747 (1982). The steroid dimer a,a'-bis(17~-(4'-imidazolyl)-ll-keto-5a-androstan-3~-amino)-p-xylene, 3. has been synthesized by reductive amination o f 17P-(4'-imidazolyl)-5a-androstane-3,11-dione by p-xylenediamine in the presence o f sodium cyanoborohydride, and by reductive amination o f terephthalaldehyde by 3P-amino-I7P-(4'-imidazolyl)-5a-androstan-lI-one. Introduction We have previously (1) reported studies of the imidazolyl steroid, 1, which can act as a catalyst, in aqueous solution, for active esters %a-d, and which shows a marked specificity for esters with a hydrophobic acyf group, such as 2b-d, with 2d being the best substrate found so far. Careful analysis of the kinetic behavior of these reactions showed that there was a potential rate enhancement, due to favorable hydrophobic interactions between steroid and ester, of as much as 156-fold for a phenanthrene ring interacting with the steroid, but that much of this was masked by unavoidable rate-retarding steric effects in the vicinity of the reaction center, so that only a 3.6-fold rate enhancement was observable. In related studies of enolization catalyzed by 1 (2) it was found that the same hydrophobic effect was present and the steric retardation was less, so that more of the potential effect became apparent; in fact a 118fold rate enhancement relative to imidazole was directly observable.A clear conclusion from these experiments was that in order to get a large rate enhancement for ester hydrolysis, a greater area of hydrophobic contact between substrate and catalyst was essential. A conceptually simple way to achieve such an increased area of contact would be to use dimeric

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