The reduction of conjugated aldehydes and ketones by sodium borohydride leads, in general, to substantial amounts of fully saturated alcohol products. In alcohol solvents the formation of saturated /3-alkoxy alcohols (involving solvent addition to the double bond) is observed. This product is enhanced by added solvent conjugate base and depressed by addition of trialkyl borate. The structural features which control the extent of simple carbonyl reduction, 1,4 reduction, and solvent addition have been examined, as well as the effects of different solvents on the course of the reaction.Sodium borohydride reduction of carbon-carbon double bonds has been observed in conjugated esters,3 nitroalkenes,4 and enol acetates.6 These examples are apparently widely regarded as exceptions to the general rule that double bonds are inert to sodium borohydride. Based on the early literature report6 that crotonaldehyde, cinnamaldehyde, and mesityl oxide yield only allylic alcohols with this reagent, most recent textbooks7 either state or imply that carbonyl-conjugated double bonds are unaffected by sodium borohydride. Conversely, lithium aluminum hydride is often viewed as a less selective reagent based on the well-documented complete reduction of cinnamyl derivatives.8

This review (Part II) is topically organized around the dienophile that is generated in a retro [4 + 2] reaction, and to the extent possible follows the principles adopted for Part I, which covers reactions in which both dienophile bonding centers are carbon atoms. The present chapter encompasses retro–Diels–Alder (rDA) reactions in which one or both of the dienophile reaction centers are heteroatoms. Any of the other atoms in the starting material (cycloadduct) may be carbon or heteroatom. Substituents on all positions are encompassed, and any bond order available to any oxidation state of an element is included, as is bonding to non‐nearest neighbor atoms (e.g., bicyclics). The reader is directed to Part I (Vol. 52) for general discussion of the rDA reaction, the features that affect rates and outcome, and the use of acids, bases, other catalysts, and scavengers. Certain subclassifications (e.g., processes proposed under MS conditions, polymer applications) of rDA reactions have been omitted from both sections; these topics are listed in the Introduction to Part I. Citations to over 100 reviews on various aspects of the rDA reaction are collected at the beginning of the References to Part I. Those that are especially pertinent to Part II will be repeated here, in the context of the particular dienophile under discussion. The decision to split the review of the rDA reaction into two parts was based on the volume of literature encountered, as outlined in the Introduction to Part I. Active literature searching was concluded for both Parts in April 1995, although occasional more recent references have been included.

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5% HC1. The benzene layer was then washed successively with three 700-ml solutions of 5 % NaOH followed by water, and dried over sodium sulfate. The benzene was distilled, and the residue distilled at a range of 75-94°( 18 mm). Vpc analysis revealed the presence of alcohol contaminant, and a pentane solution of the product was passed once through a column of acid-washed alumina for purification. After evaporation of the pentane, the resultant ketone (8.6 g, 75.4%) was pure by vpc and nmr standards: nmr (CC14) 0.86-1.04 (doublet, 6 ), 1.32-3.23 (multiplet, 6 H). Anal.