Steric effects 2,3 -Wittig rearrangement

Scheme 17.21 Solvent effect on the steric course of r2.31-Wittig rearrangement Scheme 17.22 Aza-r2.31-V ttig rearrangement Scheme 17.23 Furanone via Wittig rearrangement.

Scheme 17.21 Solvent effect on the steric course of [2,3]-Wittig rearrangement.

These amidines have been extensively applied to dehydrohalogenation in organic synthesis and in some cases DBU (1) is more effective than DBN (2) [5]. A double bond can be also introduced into organic molecules by elimination of sulfonate ester instead of the halogen atom (i.e. dehydrosulfonation in addition to dehydrohalogenation). Furthermore, these amidines can be applied to the Wittig reaction [6], aldol condensation [6], 1,3-allyl rearrangement [7] and epimerization of the (3-lactam skeleton (at Ce of the penicillic acid derivatives). Sterically hindered phenols (e.g. 2,6-di(ferf-butyl)-4-fluorophenol) are (9-acetylated with DBU (1), which is superior to sodium hydroxide in the synthesis [8]. 

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