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20.5.14.1.3.3 Variation 3: Carbonyl Homologation by Siloxyalkynes and Alkoxyalkynes

DOI: 10.1055/sos-SD-020-01227

Vanderwal, C. D.; Jacobsen, E. N.Science of Synthesis, (2007201319.

Siloxyalkynes and alkoxyalkynes are nucleophilic agents that, by virtue of having the ester oxidation state, are capable of functioning as surrogates for ester enolates. Thus, Lewis acid promoted addition of these reagents to aldehydes or ketones can deliver alk-2-enoic acid esters, often with high levels of geometrical selectivity. Siloxyalkynes can be generated from esters by sequential treatment with dibromomethyllithium, butyllithium, and a chlorosilane.[‌49‌] Reaction of aldehydes with these nucleophiles in the presence of titanium(IV) chloride, followed by the addition of methanol, generates trisubstituted alkenes with high selectivities and in reasonable yields; this reactivity is exemplified by the synthesis of alk-2-enoic acid ester 82 from siloxyalkyne 80 and 3-phenylpropanal (81) in which the E-isomer is formed exclusively (Scheme 16).[‌49‌] The use of alkoxyacetylenes obviates the need for the addition of alcohol, as the ester is formed directly. This alk-2-enoic acid ester synthesis performs well even with hindered ketones, as ketone 83 is effectively homologated with ethoxyacetylene in good yield to afford alk-2-enoic acid ester 84.[‌50‌]

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