organic chemistry - Mechanism for basic hydrolysis of α-chloronitrile to ketone?

In Corey's 1969 prostaglandin synthesis,¹ compound 2 is formed from compound 1 by:

[...] treatment with 2.5 equiv of potassium hydroxide (added as a hot saturated aqueous solution) in dimethyl sulfoxide for 14 hr at 25-30°.

Can anybody explain the mechanism of this reaction? I thought of an $\mathrm{S_N2}$ reaction, followed by nitrile hydrolysis and elimination of $\ce{CO2}$. But $\mathrm{S_N2}$ there seems impossible, and the elimination of HCl should be more reasonable than decarboxylation.

Reference

(1) Corey, E. J.; Weinshenker, N. M.; Schaaf, T. K.; Huber, W. Stereo-controlled synthesis of dl-prostaglandins F2α and E2. J. Am. Chem. Soc. 1969, 91 (20), 5675–5677. DOI: 10.1021/ja01048a062.

Answer

Your analysis is slightly incorrect, actually. If you do a $\mathrm{S_N2}$ on the chloride, hydrolyse the nitrile, and decarboxylate, you end up with the wrong oxidation state:

Instead, if you actually get to the cyanohydrin, you could (in theory) simply lose $\ce{CN-}$ to form the ketone:

However, a $\mathrm{S_N2}$ reaction at that centre is going to be difficult, to say the least.

The reaction has been studied by Shiner et al.,¹ who determined that the primary site of attack is likely to be the nitrile. They wrote:

No mechanistic studies of the hydrolysis step have previously been reported, although pathways involving nucleophilic displacement of chloride or cyanide, affording intermediate cyano- or chlorohydrins, have been proposed. A priori we regarded both $\mathrm{S_N2}$ and $\mathrm{S_N1}$ displacements as improbable in these systems.

The immediate product of nitrile hydrolysis is the amide, and the α-chloroamide intermediates were actually isolated:

The mechanism after this involves a cyclisation of the amide onto the $\ce{C-Cl}$. The exact steps are not entirely clear. However, the following was proposed:

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Reference

(1) Shiner, C. S.; Fisher, A. M.; Yacoby, F. Intermediacy of α-chloro amides in the basic hydrolysis of α-chloro nitriles to ketones. Tetrahedron Lett. 1983, 24 (51), 5687–5690. DOI: 10.1016/S0040-4039(00)94173-X.