Masked carbonyl equivalent

Dithiane as a nucleophile, serving as a masked carbonyl equivalent. This is an example of umpolung. 

The sulfoxide (103) can be readily oxidised to the sulfate (104) in good yield. Subsequent reaction of compound (104) with a masked carbonyl equivalent nucleophile provided the mannopyranose sulfate (105) (Scheme 4) <92CAR(224)193>. 

An unusual rearrangement following cyclization using a masked a-carbonyl equivalent, 2-anilino-2-ethoxy-3-oxothiobutanoic acid anilide 118, to form 6,7-disubstituted pteridine 119 by simply refluxing in ethanol for 3h was reported (Scheme 24) <2002JOC4526>. The synthesis of pyridopyrazine and related compounds using 39 was also discussed. 

The chemistry of the thiazoles does, of course, bear some similarity to that of the oxazoles. Metallated thiazoles and thiazole derivatives have found use, for example, both as carbonyl equivalents and masked acetaldehyde enolates. Additionally, as will be exemplified below, thiazole derivatives have served as catalysts for benzoin-type condensations, and they have provided access to a unique class of activated acid derivatives. 

Tetr 36 2531 (1980) (carbonyl homologation and masked homoenolate equivalents)... 

Thus vinylsilanes may be regarded as masked carbonyl groups as exemplified by the two reaction sequences depicted in Equations Si5.22 and Si5.23. Note that in the second example the cuprate is acting as an acyl anion equivalent. 

The yields in these reactions are not wonderful and most syntheses planned with acyl anion or d1 synthons are realised with one of the reagents we are about to describe rather than with acyl-lithiums. Things may change as understanding of these rather reactive intermediates develops. There are three main types of acyl anion equivalent reagents which can be considered as modified acetals, that is protected aldehydes, masked carbonyl compounds such a nitroalkanes, and substituted vinyl-lithiums. The rest of this chapter will be devoted to these reagents. 

Due to the stabilizing effect of two sulfur atoms, 1,3-dithianes are easily lithiated at the a-position on treatment with n-BuLi (see 2-Lithio-l,3-dithiane) The 2-lithio-1,3-dithianes constitute an important class of acyl anion equivalents, permitting electrophilic substitution to occur at the masked carbonyl carbon. Hydrolysis of the 1,3-dithiane functionality into a carbonyl group is effected in the presence of mercury(II) ion (eq 4). ... 

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. 

The condensation of a,/3-unsaturated carbonyl compounds with phenols has received much attention as a route to chromenes. Problems associated with the instability of alkenals have been largely overcome by the use of masked forms of these reagents. The derived acetals have proved particularly useful in this respect. It is also advantageous to use the acetals of hydroxyalkanals and to introduce the double bond during the course of the reaction. Thus, 4,4-dimethoxy-2-methylbutan-2-ol serves as the equivalent of the unstable 3-methylbut-2-enal. 

The use of masked acyl anion equivalents in a synthetic protocol requires additional steps to unmask the carbonyl unit. Sometimes the deprotection procedures are incompatible with sensitive compounds thus, a direct nucleophilic acylation protocol is desirable. While C-nucleophilic carbonyl groups do not... 

A sequence in which a carbonyl group has been masked as a sulfur derivative, alkylated with an electrophile, and then revealed again is a nucleophilic acylation. These nucleophilic equivalents of carbonyl compounds are known as acyl anion equivalents. In the retrosynthetic terms of Chapter 50 they are d1 reagents corresponding to the acyl anion synthon. 

Pedro and coworkers developed a strategy for asymmetric Michael additions of a masked benzoyl anion equivalent (199) to a,/3-unsaturated carbonyl compounds (equation 54)9°. (5)-mandelic acid (198) as a source for the benzoyl anion and chiral information, the corresponding Michael adducts were obtained in good yields and with high... 

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