Wittig-Homer cyclization intramolecular

Synthesis from xylose The xylose derivative 47, obtained from 5,5-bis-benzyloxy-7-oxa-bicyclo[2.2.1]hept-2-ene (45), has been used in the synthesis of (-P)-castanospermine (1) (Scheme 6). Bromination of 45 occurred exclusively on the less hindered convex face of 45, followed by stereoselective migration of the endo OBn group of the acetal to give 46, which subsequently converted to 47. Mesylation of 47 followed by cyclization with ammoifia gave 48, whose protection, hydrolysis, acetylation and cyclization by an intramolecular Wittig-Homer condensation gave 49. Conversion of 49 into epoxide 50... 

A tandem enzymatic aldol-intramolecular Homer-Wadsworth-Emmons reaction has been used in the synthesis of a cyclitol.310 The key steps are illustrated in Scheme 8.33. The phosphonate aldehyde was condensed with dihydroxyacetone phosphate (DHAP) in water with FDP aldolase to give the aldol adduct, which cyclizes with an intramolecular Horner-Wadsworth-Emmons reaction to give the cyclo-pentene product. The one-pot reaction takes place in aqueous solution at slightly acidic (pH 6.1-6.8) conditions. The aqueous Wittig-type reaction has also been investigated in DNA-templated synthesis.311... 

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