Remarkable rate enhancement is achieved by use of a weak Lewis acid catalyst such as zinc chloride or bromide in the hetero Diels–Alder reactions of 2-oxo-3-alkenylphosphonates as readily enolizable 1-oxa-1,3-dienes with vinyl ethers. Highly endo-selective cyclo-adducts, 2,4-cis-3,4-dihydro-2H-pyrans, are produced in good yields.
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Wada et al. (1989) studied this question.
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