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8.2.4.4.1 Method 1: Generation and Reactions of Carbanions

DOI: 10.1055/sos-SD-008-00826

Jonczyk, A.; Kowalkowska, A.Science of Synthesis, (200681110.

Usually, sodium acetate is not a sufficiently strong base for effective deprotonation of CH acids. However, condensation of aromatic aldehydes with carboxylic anhydrides in the presence of an alkali metal salt of a carboxylic acid corresponding to the anhydride leads to α,β-unsaturated carboxylic acids (Perkin reaction).[‌1123‌] Reactions with acetic anhydride are carried out in the presence of sodium or potassium acetate (e.g., the preparation of 3-nitrocinnamic acid from 3-nitrobenzaldehyde, acetic anhydride, and sodium acetate in yields of 7477%[‌1124‌]), but the effect of either salt depends upon the aldehyde structure.[‌1125‌] A related process, the reaction of aromatic aldehydes with N-acylglycine derivatives, in a mixture of acetic anhydride and sodium acetate, gives 1,3-oxazol-5(4H)-ones (Erlenmeyer reaction),[‌1123‌] e.g. the 1,3-oxazol-5(4H)-one from α-(benzoylamino)-β-(3,4-dimethoxyphenyl)acrylic acid (6973% yield).[‌1126‌] Condensation of 4-oxo-4H-1-benzopyran-3-carbaldehyde with 1,3-dicarbonyl compounds carried out in acetic anhydride in the presence of sodium acetate affords the corresponding α,β-unsaturated 1,3-dicarbonyl compounds.[‌1127‌] N-Bromoacetamide undergoes addition to α-nitroalkenes and also to carbohydrate derivatives. When the nitro group is situated in a terminal position, the adducts undergo bromination with the formation of β-(acylamino)-α,α-dibromo-α-nitroalkanes. The process is performed with a catalytic amount of sodium acetate (without which it fails), and is likely of ionic character.[‌1128‌] 2,4,6-Triphenylpyrylium perchlorate undergoes condensation with ethyl pyruvate, in the presence of 2 equivalents of acetate, forming ethyl 2-benzoyl-3,5-diphenylbenzoate, or with 1 equivalent of this salt to give ethyl 3,5-diphenylbenzoate. In the latter case, other condensing reagents give better results.[‌1129‌] The reactions of but-2-ynoic acid derivatives,[‌1130‌,‌1131‌] or alkyl buta-2,3-dienoates,[‌1131‌] with 1,3-dicarbonyl compounds, disulfonylmethane, cyanomethyl phenyl sulfones, 2-nitrocyclohexanone, or ethyl 2-cyanopropanoate, e.g. 201 and 247, a catalytic amount of phosphine, and sodium acetate in acetic acid afford products, e.g. 246, with the formation of a new CC bond at C4 of the alkynyl or the allenic ester and simultaneous conversion of the intermediate alkynyl (or allenic) moiety into a vinylic group, in good yield (Schemes 158 and 159). When the process is performed with a chiral phosphine, e.g. 248, the products are obtained in up to 81% ee.[‌1131‌] The postulated mechanistic pathway[‌1130‌,‌1131‌] assumes deprotonation of the CH acid by the intermediate betaine, but deprotonation by sodium acetate cannot be excluded. In fact, without this salt, the reaction in acetic acid does not take place; however, it proceeds in good yield with sodium acetate itself.[‌1131‌]

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