Regio- and Chemoselective Reductions

Hydride reductions of a, 3-unsaturated aldehydes and ketones may proceed via 1,2- or 1,4-additions to furnish the corresponding ally lie alcohols or saturated aldehydes and ketones, respectively. 

and Li[AlH(0R)3] effect both 1,2 and 1,4-additions (Table 4.4). The regioselectivity of enone reductions can be strongly influenced by the nature of the reagent, the presence of substituents on the substrate, and the reaction conditions. 

For example, K-Selectride reduces -unsubstituted cyclohexenones to cyclohexanones (1,4-addition) and (3-substituted cyclohexenones to the corresponding ally lie alcohols (1,2-addition). 

A number of chemoselective reductions of the more reactive R-CHO group in the presence of an R2CO group have been reported using 9-BBN pyridine or K[BH(OAc)3], as exemplified below.  

Zinc borohydride is a mild reducing agent permitting the reduction of aldehydes in the presence of ketones. Moreover, it selectively reduces a nonconjugated keto group in the presence of a conjugated keto group. 

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A practical synthesis of 1,3-OX AZEPINES VIA PHOTOISOMERIZATION OF HETERO AROMATIC V-OXIDES is illustrated for 3,1-BENZOXAZEPINE. A hydroboration procedure for the synthesis of PERHYDRO-9b-BORAPHENALENE AND PERHYDRO-9b-PHEN-ALENOL illustrates beautifully the power of this methodology in the construction of polycyclic substances. The conversion of LIMONENE TO p-MENTH-8-EN-YL METHYL ETHER demonstrates a regio-and chemoselective method for the PHOTOPROTONATION OF CYCLOALKENES. An efficient method for the conversion of a ketone to an olefin involves REDUCTIVE CLEAVAGE OF VINYL PHOSPHATES. A mild method for the conversion of a ketone into the corresponding trimethylsiloxy enol ether using trimethylsilyl acetate is shownforthe synthesis of (Z)-3-TRIMETHYLSILOXY-2-PENTENE. 

Functional group transformations classical and chemoselective methods for oxidation and reduction of organic substrates, and the availability and utilization of regio-, chemo-, and stereoselective agents for reducing carbonyl compounds... 

Introduction Chemo-, Regio-, and Stereoselectivity Chemoselectivity Controlling Chemoselectivity Regioselectivity Markovnikov Hydration Mercuration-reduction Wacker oxidation... 

This is the case with NAD(P)H-dependent dehydrogenases, where enzymes find applications in several synthetic processes (comprising the reduction of aldehydes, ketones, carboxylic acids, double, and triple carbon-carbon bonds), aimed at the preparation of chiral enantiopure bioactive compounds and of building blocks for fine chemicals and pharmaceutical products. Moreover, dehydrogenase-catalyzed oxidation reactions are gaining increasing interest as an environmentally friendly alternative to chemical oxidation processes, especially in those cases where a defined selectivity (either stereo-, regio-, or chemoselectivity) is required as well [1]. 

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