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25.8.1.1.12.2 Variation 2: Wittig Reaction

DOI: 10.1055/sos-SD-025-00531

Escher, I.; Glorius, F.Science of Synthesis, (200725750.

Aldehydes readily undergo two-carbon-atom homologation with the appropriate Wittig reagent in the presence of a strong base such as potassium tert-butoxide.[‌122‌,‌123‌] Phosphonium salts including (1,3-dioxolan-2-ylmethyl)(trialkyl)phosphonium halides 41 (Scheme 22) have been employed successfully in the total synthesis of natural products, including a frog-skin alkaloid.[‌124‌] Other phosphonium species employed in related homologations include the diethyl acetals 42. In such cases hydrolysis of the acetal intermediates usually gives the 2-enals without difficulty (for the hydrolysis of α,β-unsaturated acetals see Section 25.8.1.1.7). Formylphosphoranes such as 43 or 44 also react with aldehydes to form the corresponding 2-enals in high yields and they have been employed in the total syntheses of macrolides such as swinholide and ()-borrelidin.[‌125‌,‌126‌] A typical procedure leading to 3-(5-amino-2-thienyl)propenals 46, from the thiophene-2-carbaldehydes 45, uses an acetal phosphorane, generated in situ from the corresponding phosphonium bromide; here hydrolysis of the intermediate α,β-unsaturated acetals is achieved by the action of aqueous oxalic acid (Scheme 22).[‌123‌]

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