Alternative Methods for Diastereoselective Olefin Reductions

A number of alternatives to catalytic hydrogenation have been developed for stereoselective reductions of olefins. They include reductions with diimide, dissolving metal reductions, ionic reductions with organosilanes, and enzymatic reductions [10-13]. 

An illustrative example of a stereoselective reduction with diimide is the conversion of the steroid derivative 82 into 83 as the only observed isomer (Equation 19) [61]. This transformation was used as a convenient stereo-and regioselective method for isotopic labeling of 83. Use of diimide avoided the formation of diastereomeric mixtures and prevented partial unwanted isotopic labeling at the allylic position, both of which had otherwise been observed with use of standard heterogeneous catalytic hydrogenations. 

The reduction of carbon-carbon multiple bonds by dissolving metals was first introduced by Campbell in 1941 for the partial and stereoselective reduction of alkynes to olefins [63]. This was an important advance, as it represented a practical and highly selective synthetic method for the formation 

In careful optimization of the regio- and stereoselective reduction of the diene component in ergosterol (3), Barton found that sodium metal in a f-BuOH/THF mixture readily afforded regioisomer 4 in 96% yield (Equation 22) [66]. Only traces of the epimeric cis-isomer were observed. 

Another elegant example is the frans-selective reduction of lisuride (88) [67], reported by researchers at Sobering in the context of a synthesis of the partial dopamine receptor antagonist 89 (Equation 23) [68]. In this transformation, in situ deprotonation of the allylic urea side chain prevents its excision, which had been obsen/ed under catalytic hydrogenation conditions. 

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