Comparing and taming the reactivity of HWE and Wittig reagents with cyclic hemiacetals
2011
Carrillo, Jokin | Costa, Anna M. | Sidera, Mireia | Vilarrasa, Jaume
A practical solution to the formation of mixtures of E/Z and open/cyclic isomers in the reaction of (2R,4S)-4-hydroxy-2-methylpentanal (as its hemiacetal, a lactol) with conjugated phosphoranes (stabilised Wittig reagents) and Horner–Wadsworth–Emmons reagents is disclosed. The HWE reaction has a strong bias to give oxolanes. On the other hand, stabilised Wittig reagents give unsaturated carboxyl derivatives of configuration E (major) and oxolanes (minor); the latter can be avoided by addition of CF₃CH₂OH or using morpholine amide phosphorane.
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