Diastereoselective Olefin Reductions by Catalytic Hydrogenation

As a general working rule, olefin reductions catalyzed by heterogeneous transition metal catalysts are observed to take place preferentially from the sterically most accessible face [15, 16]. A demonstration of this can be found in work by Boyce, involving the regio- and stereoselective reduction of 1 (Equation 1) [28]. In the presence of Pd catalyst poisoned with CaCOs, this transformation proceeded to give enol ether 2 as the only diaste-reomer. 

Laubach s and Brunings monohydrogenation of the conjugated diene in ergosterol (3, Equation 2) constitutes a remarkable example of a reaction exhibiting both chemo- and diastereoselectivity [29]. This hydrogenation gave the major product 4 in 94% yield. 

During studies of catalytic hydrogenations of bicyclic a,/l-unsaturated ketones 8, Augustine observed that the stereochemical outcome of the reduction was sensitive to the pH of the reaction medium (Equation 3) [32], In this 

Subsequently, diester 19 was subjected to desymmetrization through the use of an enzymatic process before its transformation to 20. 

The reduction of condensed heterocycles represents a convenient approach to the preparation of their saturated counterparts. Catalytic hydrogenation of pyridines, for example, affords cis-substituted piperidines. Researchers from American Cyanamid highlighted this approach in the diastereoselective formation of the cis-substituted keto-piperidine 22 from 21 (Equation 4) [36]. 

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