Alkoxycarbonyl function, reduction

The Homer-Emmons addition of dialkyl carboalkoxymethylenephosphonates to aldehydes [22] has been widely used to generate a,p-unsaturated esters which, in turn, can be reduced to allylic alcohols. Under the original conditions of the Homer-Emmons reaction, the stereochemistry of the oc,(3-unsaturated ester is predominantly trans and therefore the trans allylic alcohol is obtained upon reduction. Still and Gennari have introduced an important modification of the Homer-Emmons reaction, which shifts the stereochemistry of the a,[i-unsaturated ester to predominantly cis [23], Diisobutylaluminum hydride (DIBAL) has frequently been used for reduction of the alkoxycarbonyl to the primary alcohol functionality. The aldehyde needed for reaction with the Homer-Emmons reagent may be derived via Swern oxidation [24] of a primary alcohol. The net result is that one frequently sees the reaction sequence shown in Eq. 6A. 1 used for the net preparation of 3E and 3Z allylic alcohols. 

In a bifunctional compound, if a reagent reacts with one functional group preferentially, even though the other is apparently susceptible to the reaction conditions, the reaction is said to be chemoselective. Two illustrative examples are the reduction of a carbonyl group in the presence of a cyano, nitro or alkoxycarbonyl group (Section 5.4.1, p. 519 see also Metal hydrides, Section 4.2.49, p. 445) and the acylation of an aromatic amino group in the presence of a phenolic group (Section 6.9.3, p. 984). 

Samarium iodide is a reagent capable of highly selective reduction of epoxy ketones and esters to the corresponding alcohols (Table 12). Diene monoepoxides are converted with high regio- and stereo-selectivity to the ( )-allylic alcohols, and the reaction can be carried out at -90 C under neutral conditions. As a result functional groups such as alkoxycarbonyl, carbonyl and cyano survive. 

Thus, with an alkoxycarbonyl, carboxyl or cyano group on all three corners, the rearrangement to the corresponding 3,6,9-trisubstituted all-cw-cyclonona-l,4,7-triene requires elevated temperatures, while upon reduction of all three ester or nitrile functionalities to hydroxymethyl or aminomethyl groups, respectively, the compounds rearrange to the respective cyclonon-atrienes below... 

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