Synthetic equivalents of functional groups

FUNCTIONAL GROUPS. TRANSFORMATIONS WITHIN AND BETWEEN THE OXIDATION LEVELS. SYNTHETIC EQUIVALENCY OF FUNCTIONAL GROUPS... 

Isohypsic Transformations. Synthetic Equivalency of Functional Groups of the Same Oxidation Level... 

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. 

Vinyl sulfones are reactive as dienophiles. The sulfonyl group can be removed reductively with sodium amalgam (see Section 5.6.2). In this two-step reaction sequence, the vinyl sulfone functions as an ethylene equivalent. The sulfonyl group also permits alkylation of the adduct, via the carbanion. This three-step sequence permits the vinyl sulfone to serve as the synthetic equivalent of a terminal alkene.68... 

Examples of this approach to the synthesis of ketones and carboxylic acids are presented in Scheme 1.6. In these procedures, an ester group is removed by hydrolysis and decarboxylation after the alkylation step. The malonate and acetoacetate carbanions are the synthetic equivalents of the simpler carbanions lacking the ester substituents. In the preparation of 2-heptanone (entries 1, Schemes 1.5 and 1.6), for example, ethyl acetoacetate functions as the synthetic equivalent of acetone. It is also possible to use the dilithium derivative of acetoacetic acid as the synthetic equivalent of acetone enolate.29 In this case, the hydrolysis step is unnecessary, and decarboxylation can be done directly on the alkylation product. 

In view of myriad of options available for the interconversions of functional groups at oxidation level 1, one can safely consider all of these functions to be synthetically equivalent. In essence, this means that if it is necessary to introduce a certain functional group into a given structure, the task can be considered achievable if a constructive reaction chosen to create the C-C bond leads to the formation of a double bond or hydroxyl group at the desired location. 

In another approach to enantioselective 2 + 2 photocycloadditions, the easily available chiral l,3-dioxin-4-ones have been used as synthetic equivalents of acetylacetates in de Mayo reactions. Chiral information is introduced through a ketalic functional group and menthone derivatives efficiently control the facial selectivity, as indicated in Scheme 30 [155-157]. 

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