Investigation on the reactions of o-hydroxybenzyl alcohols with vinyl ethers under acidic and/or thermal conditions

The reaction of vinyl ethers with o-hydroxybenzyl alcohols under different reaction conditions was investigated. The aim of this attempt was to find out whether the protection reactions or the hetero Diels-Alder reaction of quinone methide in situ generated from o-hydroxybenzyl alcohol is more likely to occur. o-hydroxybenzyl alcohols can give hetero Diels-Alder reactions with dihydro-2H-pyran at high temperatures but only when used with acid catalysts. At room temperature, even in the presence of acid catalyst, reactions yielded regular protection products. However, butyl vinyl ether 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.