Zinc acetate Knoevenagel reaction

Olefinic esters may be obtained directly by the Knoevenagel reaction. Alkyl hydrogen malonates are used in place of malonic acid. Decarboxylation then gives the ester directly as in the preparation of ethyl 2-heptenoate (78%) and methyl m-nitrocinnamate (87%). Alkyl hydrogen malonates are readily available by partial hydrolysis of dialkyl malonates. The use of malonic ester in the condensation leads to olefinic diesters, namely, alkylidenemalonates such as ethyl heptylidenemalonate (68%). A small amount of organic acid is added to the amine catalyst since the salts rather than the free amines have been shown to be the catalysts in condensations of this type. Various catalysts have been studied in the preparation of diethyl methylenemalonate. Increased yields are obtained in the presence of copper salts. Trimethylacetalde-hyde and malonic ester are condensed by acetic anhydride and zinc chloride. Acetic anhydride is also used for the condensation of furfural and malonic ester to furfurylidenemalonic ester (82%). ... 

Based on a domino Knoevenagel/cne reaction, Tietze and Steinmetz developed a stereoselective solid-phase synthesis of cyclopentane and cyclohexane derivatives of type 326 and 327 using a Merrifield resin modiiled with a propandiol linker 320 as shown in Scheme 4.6.3. Subsequent reaction with monomethyl malonoyl chloride 321 afforded the polymer-bound malonate 322, which, in a two-component domino reaction was treated with unsaturated aldehydes 323 in the presence of a catalytic amount of piperidinium acetate and zinc chloride. Except for a-substituted aldehydes, the initial Knoevenagel condensation occurred without addition of dehydrating agents and the subsequent intramolecular ene reaction gave cyclopentane and cyclohexane derivatives 325 after cleavage firom the resin either by reduction or transesterification. 

Cavaleiro et al. reported the reaction of vinyl-substituted zinc porphyrins 76 with Knoevenagel products of formaldehyde 72 with several quinones 75 in refluxing 1,4-dioxane and ort/jo-dichlorobenzene (Scheme 13.22) [37]. The desired products 77 could be obtained in moderate to high yields where the exclusive formation of the para-benzoquinone is noteworthy (see also [36]). A catalytic amount of acetic acid reduces the reaction time and increases the product yield. 

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