Equivalency concept ketene equivalents

The synthetic utility of the D-A reaction can be expanded by the use of dienophiles that contain masked functionality and are the synthetic equivalents of unreactive or inaccessible compounds. (See Section 13.1.2 for a more complete discussion of the concept of synthetic equivalents.) For example, a-chloroacrylonitrile shows satisfactory reactivity as a dienophile. The a-chloronitrile functionality in the adduct can be hydrolyzed to a carbonyl group. Thus, a-chloroacrylonitrile can function as the equivalent of ketene, CH2=C=0,63 which is not a suitable dienophile because it has a tendency to react with dienes by [2 + 2] cycloaddition, rather than the desired [4 + 2] fashion. 

Although metal ynoiates are equivalents of metallated ketenes, direct metallation of ketenes is often troublesome. Metallation of the precursors of ketenes, followed by transformation into the metallated ketenes, would be a better route to metal ynoiates. Based on this concept, an efficient method for the synthesis of lithium ynoiates has been developed, taking advantage of the properties of ester enoiates, which are easily converted into ketenes by elimination of alkoxides (equation 11) . The synthetic methods for lithium ynoiates via the cleavage of ester dianions are both convenient and quite general, because alkyl-, aryl- and sQyl-substimted lithium ynoiates can be synthesized in good yields. 

Before proceeding, it is important to note that it is the conversion of the a-chloronitrile to a ketone (a functional group transformation) that establishes the a-chloroacrylonitrile as a ketene equivalent in Diels-Alder chemistry. a-Chloroacrylonitrile is not the only ketene equivalent that has been used in Diels-Alder chemistry, nor in this strategy for prostaglandin synthesis. Reviews have been written on this topic. What is perhaps most important is the development of the concept of equivalencies that was occuring at the time this synthesis was undertaken. We will see this concept again. 

---