A new form of secondary interaction in diels-alder reactions

1.	The diene condensation of 1-alkoxy-1,3-butadienes with aliphatic aldehydes at high pressures leads to predominate formation of 2-alkoxy-6-alkyl-3-dihydropyrans, corresponding to the endo-orientation of the diene and dienophile in the transition state.
2.	The observed effect is explained by the presence of interaction of the partially protonated hydrogen on the -carbon atom of the aldehyde with the -system of the diene.