Ethanoic acid enolization with

Scheme 6.68. A potential pathway for the mercuric ethanoate (mercuric acetate [Hg(02CCH3)2]) catalyzed electrophilic addition of ethanoic acid (acetic acid [CH3CO2H]) to an alkyne to form an enol ethanoate (enol acetate) and then, with excess acid, the corresponding diester (an acylal) of ethanal (acetaldehyde, CH3CHO) or ketone hydrate.

Exercise 17-11 The direct halogenation of aldehydes under either acidic or basic conditions is complicated by side reactions involving either oxidation of the aldehyde —CHO group or additions to the—CH—0 double bond. Therefore the synthesis of a-halo aldehydes by the procedure described for ketones is not of much practical value. a-Haio aldehydes can be prepared indirectly from the enol ethanoate of the aldehyde. The enol ethanoate is made by treating the aldehyde with ethanoic anhydride and potassium ethanoate. The overall sequence follows ... 

Eschenmoser s pyrone 38 on treatment with cyclopropenone ketal 39 in refluxing benzene afforded lactone 40 (73%). Lactone 40 on hydrolysis with acetic acid at 100°C afforded, after deprotection and decarboxylation, tropone 37 (70%). Introduction of the tropolonic hydroxyl group was achieved with hydrazine hydrate in ethanol, to give a mixture of deacetyl-colchiceinamides 41 (53%) and 42 (37%), followed by reaction with ethano-lic potassium hydroxide, which afforded tropolones 43 and 44, respectively. Tropolone 43 was converted to 44 which, therefore, became the major reaction product. Methylation of 44 gave a mixture of enol ether 18 and 45 which were separated by chromatography. 

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