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Please login or sign up for a free trial to access the full content. Method 3: N-tert-Butylhydrazones as Acyl Anion Equivalents

DOI: 10.1055/sos-SD-027-00576

Kim, S.; Yoon, J.-Y.Science of Synthesis, (200427689.

Aza anions 75, formed by the deprotonation of N-tert-butylhydrazones, react with electrophiles such as carbonyl compounds and alkyl halides at the carbon atom.[‌79‌,‌80‌] However, aza anions derived from N-phenylhydrazones react with alkyl halides principally at nitrogen.[‌81‌] Even the alkylation of aza anions obtained from N-tert-butylhydrazones 74 with alkyl halides is very sensitive to the nature of the electrophile, and both C- and N-alkylated products can form;[‌82‌] typically, iodomethane favors C-alkylation, whereas iodoethane or iodopropane give mainly N-alkylated products. When α,β-unsaturated esters and nitriles are used as electrophiles 1,4-addition can occur. The tert-butylhydrazones 74, which are unstable to chromatography and are stored in the absence of light and oxygen, have particular value in the formation of acyl anion equivalents; thus, they can be quenched at 0°C with aldehydes and ketones 76 and then protonated to afford unstable 2-(tert-butyldiazenyl)ethanols.[‌82‌] Although these products 77 revert to the parent hydrazones 74 and carbonyl compounds 76 on standing, they isomerize to hydroxyacetaldehyde tert-butylhydrazones 79 when treated with butyllithium, followed by the addition of water, via dilithium compound 78. Hydrolysis of these products to α-hydroxy ketones 80 is achieved by treatment with oxalic acid (Scheme 17). Yields are low when enolizable ketones, such as acetophenone and cyclohexanone, are used.

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