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Formyl group-containing compounds are important building blocks in organic synthesis since the formyl group can be transformed to other various functional groups. Aldehydes are the most typical formyl group-containing compound and are widely used as a substrate for carbon-carbon bond forming reactions such as the aldol reaction and the Wittig reaction.1-2) Also, their oxidation/reduction converts them to the related alcohols and carboxylic acids respectively, while the reductive amination is an efficient functional transformation of the formyl group into amines.3-5) As above reactions,the formyl group is a versatile precursor of organosynthetic reactions and a number of various formyl group introducing reagents have been developed. Reagents for the formylation are classified into three types according to their reactivity and substrates: (1) electrophilic aromatic substitution, (2) reaction with carbon nucleophiles, and (3) direct formylation at oxygen or nitrogen atoms.
(1) Electrophilic aromatic substitution
Vilsmeier reagent is a strong electrophile and formylates electron-rich alkenes and aromatic rings by electrophilic aromatic substitution.6) This reagent is widely used and can be prepared by the reaction of DMF or N-methylformanilide with phosphoryl chloride.7) The Duff reaction using hexamethylenetetramine is effective to formylate electron-rich aromatic compounds such as phenols and indoles.8) Furthermore, the Riecke reaction using dichloromethyl methyl ether is suitable for formylation at the sterically hindered site of substances.9)
(2) Formylation by the reaction with carbon nucleophiles
Formyl group-containing compounds can be also synthesized by the reaction of Grignard reagents or organolithium compounds with N,N-disubstituted formamides, orthoformates, N-methoxymethyleneaniline and so on (Bouveault/ Bodroux-Chichibabin aldehyde synthesis).10,11) In addition, it is known that the direct C-formylation of ketones can be performed by the reaction of related ketone enolates with TFEF.12)
N-Formylsaccharin, cyanomethyl formate,13-15) and 1-formyl-1H-benzotriazole work as O/N-formylating reagents, and successfully react with alcohols and amines under mild conditions to afford the desired O/N-formylated compounds. Especially, N-formylsaccharin can be used not only as a formylating reagent but also as a carbon monoxide introducing reagent.