A comparison of the Wittig and Knoevenagel–Doebner reactions for the chain extension of aldoses

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Abstract
Reaction conditions are described for the chain extension of 2,3 : 5,6-di-O-isopropylidence-D-mannose (1) with a Wittig ylide (2) to give the acyclic manno-octenoate (3) or the C-glycofuranosides (4 and 5) and with the Knoevenagel–Doebner reaction to afford the isomeric gluco-octenoate (6); the isomerisation proved to occur in the latter reaction provides an explanation for an anomaly in the literature and indicates that the C-glycosides (4 and 5) are formed by a Michael-type ring closure rather than via a betaine intermediate.

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