Propynal diethylacetal

A different route to the intermediary Michael acceptor was used in the synthesis of 3-(hetero)aryl-lH-pyrazoles 28. In contrast to previously described approaches, the terminal alkyne, namely propynal diethylacetal (5d), was utilized to introduce the carbonyl group. Propynal diethylacetal (5d) acts as a synthetic equivalent of propargyl aldehyde and, thus, Sonoga-shira cross-coupling of (hetero)aryl iodides 6 and propynal diethylacetal (5d) and subsequent acetal cleavage and concomitant cyclocondensation in the presence of para-toluenesulfonic acid and hydrazine hydrochloride (20a) furnishes the title compounds (Scheme 15) (201 lMOM(l6)9340). 

Oxidative carbonylation was coupled with reduction to afford catalytic carbonylation (see also Sect. VI.4.4.2). Thus, the diethylacetal of 2-propynal was carbonylated in a 65% yield by combining oxidative dicarbonylation and reductive splitting of an ethoxy group (Scheme 34). 

An interesting amine migration followed by a Claisen rearrangement occurs on heating the condensation product of 2-methyl-3-propyn-2-ol with dimethylaceta-mide diethylacetal (Scheme 41). Acid hydrolysis of the products leads to a 4-oxohexanoate. 

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See also in sourсe #XX -- [ ]

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