The reactions of 2-methoxy-5, 6-dihydropyrans (2) and 2-methoxy-4H-pyrans (3) with a variety of electrophilic olefins R4R5C= C (CN) X have been studied. Cyclobutanes are formed in an equilibrium reaction from 2 as well as 3. In general, the compounds 3 give ultimately the thermodynamically stable heptadienoic esters 10. 2-Methoxy-4H-pyrans, substituted at carbon atom 6, yield, however, a pyrone derivative (11) when they react with ...