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29.6.1.2.8.9 Variation 9: From Enol Ethers with Acetals

DOI: 10.1055/sos-SD-029-00368

von Angerer, S.; Warriner, S. L.Science of Synthesis, (200729373.

Acetals are capable of adding across the C=C bond of alkoxyethenes to give new acetals with a chain length extended by two carbon atoms. This addition reaction is catalyzed by a variety of Lewis acids[‌248‌,‌546‌‌553‌] and the acidic montmorillonite K 10 clay (Scheme 106).[‌554‌] In the course of the reaction, one of the alkoxy groups of the reacting acetal 97 becomes part of the new acetal structure 98, whereas the other remains at the original carbon atom. Hydrolysis and subsequent elimination of the remaining alkoxy group as an alcohol give rise to the formation of trans-α,β-unsaturated aldehydes. This reaction also works with 1-alkoxybutadiene and isoprenyl building blocks to give the products of 1,4-addition, but the yields are only moderate.[‌551‌,‌555‌] Diacetals of dialdehydes, e.g. terephthalaldehyde or 1,1,4,4-tetramethoxybut-2-ene, undergo the same reaction and permit the simultaneous extension in two directions.[‌552‌,‌553‌,‌556‌] This method has been used for the preparation of vitamin A aldehyde and its congeners.[‌550‌]

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