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26.9.1.1.8.2 Variation 2: Intermolecular Aldol Reaction of Enamines and Enol Ethers

DOI: 10.1055/sos-SD-026-00987

Marsden, S. P.Science of Synthesis, (2005261075.

The problems alluded to in Section 26.9.1.1.8.1 in the direct aldol condensations of unsymmetrical ketones and enolizable aldehydes mean that these substrates are frequently better dealt with through prior derivatization of the ketone to an isolable enolate equivalent which can be purified to give a single regioisomer. Enamines participate in aldol reactions, exemplified by the reaction of cyclopentanone-derived enamine 152 with butyl glyoxalate to give 153 (Scheme 33).[‌142‌] The ability to form selectively the less substituted enamines such as 154 from nonsymmetrical ketones adds utility to this process, although yields of the resulting enones are not always high.[‌143‌] Silyl enol ethers can also be formed regioselectively and undergo aldol reactions with carbonyl compounds under Lewis acid catalysis.[‌144‌] The reactions usually stop at the aldol stage, but elimination products can accompany these and occasionally they are the major product. The aldol adducts can usually be converted into enones upon workup, as in the formation of enone 155,[‌145‌] or in a separate elimination step (see Section 26.9.1.1.5.3). Alternatively, reaction of silyl enol ethers such as 156 under fluoride catalysis yields enones (e.g., 157) directly, although further condensation to yield dienones can complicate this protocol.[‌146‌] Enol acetates such as 158 have also been shown to be competent nucleophiles for the attack of acetals (e.g., 159) and carbonyls under Lewis acid activation, and the elimination product 160 is occasionally observed as the major product.[‌147‌]

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