presence of acid catalyst, reactions yielded regular protection products. However, butyl vinylether and 4-methoxy-3-butenone could not give intermolecular cycloaddition reactions under the acidic conditions, because both decomposed to the new products with acids. Hetero-Diels–Alder products obtained only under thermal conditions but in low yields. J. Heterocyclic Chem., 46, 226 (2009).