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47.1.1.1.3.3.2 Variation 2: Alkenation with In Situ Silylated Alkoxide Ylides

DOI: 10.1055/sos-SD-047-00004

Schobert, R.; Hölzel, C.; Barnickel, B.Science of Synthesis, (20104745.

Although nonstabilized alkoxide ylides generally need not be protected for Wittig alkenations, a few exceptions are worthy of note. If (3-hydroxypropyl)triphenylphosphonium salts are treated with 2 equivalents of base, the corresponding alkoxide ylides tend to yield predominantly the E-alkenes 93 upon reaction with aldehydes due to an intramolecular proton transfer according to a Schlosser-type mechanism (Scheme 32).[‌344‌,‌345‌] This phenomenon and the effects of other nucleophilic groups in ylides have been extensively studied.[‌346‌,‌347‌] The isomeric Z-homoallylic alcohols 95 can be obtained from the corresponding 3-siloxy ylides 94 (Scheme 33). To avoid tedious protection and deprotection steps, the hydroxy group is usually protected in situ by addition of chlorotrimethylsilane after generation of the alkoxide ylide.

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