Chemistry of Ketene Acetals. 7. 2-Methoxy-4H-pyrans as Strong Hydride Donors in Reactions with Electrophilic Olefins

Article abstract of DOI:10.1021/jo00164a020

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 strongly electrophilic olefins substituted with three or four electron-withdrawing groups.Both conversions are supposed to occur via the abstraction of a hydride ion from C(4) in a rate-determining step leading to a pyrylium ion (14).The further course of the process is determined by the character of the nucleophile formed in the addition of the hydride ion to the olefin and by the substitution pattern of 3.