Heterocyclic Acyl and Formyl Anion Equivalents

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Introduction

Although many carbonyl derivatives act as acyl cation equivalents, R(C = O)+ in synthetic chemistry, the inherent polarity of the carbonyl group makes it much more difficult to find compounds that will act as equivalents of acyl anions, R(C = O). Since the 1960s, major progress has been made in this area, and there are now a wide variety of compound types that can react in this way. As in so many areas of organic chemistry, heterocyclic compounds take pride of place and form the basis of many of the most useful methods. In recent years there has been particular interest in developing chiral acyl anion equivalents that will show high diastereoselectivity in their addition to prochiral electrophiles. In this chapter we have attempted to describe the most important acyl and formyl anion equivalents in which the anionic carbon atom is either part of a heterocyclic ring or is attached to one or more adjacent heterocyclic groups. The examples are organized in order of (1) increasing ring size, (2) increasing number of heteroatoms, and (3) decreasing priority of heteroatoms.

Section snippets

Oxiranes

The phenylsulfonyloxirane 1 is readily deprotonated at low temperature, and the resulting anion reacts with a wide vaiety of electrophiles to give 2. Nucleophilic ring opening of this by magnesium bromide is accompanied by loss of PhSO2 to give the functionalized ketone products 3, thus making 1 a synthetic equivalent for the acyl anion 4 [88JCS(CC)645; 91JCS(P1)897]. The oxirane 5 with additional functionality behaves similarly, giving first 6 and then 7 [89JCS(P1)835; 92JCS(P1)2863]; and

FURANS AND DIHYDROFURANS

The ease with which furans may be formed from and hydrolyzed back to 1,4-dicarbonyl compounds, together with ready 5-lithiation of 2-substituted furans, makes 10 a convenient equivalent of the γ-oxoacyl anion 11. An early example of this is provided by Büchi and Wüest’s synthesis of Z-jasmone 12 (66JOC977). Lithiation of 2,3-dihydrofuran at the 5 position is also readily achieved, making it a potential γ-funtionalized butyryl anion equivalent although this has only been reported for a limited

DIHYDROPYRANS

Dihydropyrans 8890 are deprotonated at the vinylic position adjacent to oxygen by t-BuLi and the resulting anions add readily to alkyl halides, aldehydes, and ketones. Subsequent acid hydrolysis provides the products expected from reaction of an ω-functionalized pentanoyl anion: 88 acts as HOCH2(CH2)3CO, 89 as OCH(CH2)3CO, and 90 as MeCO(CH2)3CO (77TL4187; 81T3997).

PYRIDINES AND PIPERIDINES

Pyridinium p-toluenesulfonylmethylide 91 has been used as a formyl anion equivalent for conjugate addition to N-substituted

Aromatic Fused Dithiepines and Their S-oxides

The benzodithiepines 126 and 127 have been used as formyl and acyl anion equivalents, respectively, although the range of electrophiles was restricted to alkyl halides (75S720) and epifluorohydrin (72TL1837). The carbonyl products were formed by hydrolysis with either mercury or copper salts.

There has been recent interest in naphtho-fused dithiepines as chiral acyl anion equivalents, particularly since the starting dithiol 128 can be obtained in enantiomerically pure form (89TL2575). This is

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