Preparation of 3,6-dihydro-2H-pyran-2-carboxylic esters

A process is provided for preparing 3,6-dihydro-2H-pyran-2-carboxylic esters via a thermal hetero Diels-Alder reaction of 1,3-dienes with glyoxylic esters without the use of catalysts and stabilizers using a special metering technique.