Abstract: The invention provides a process for the preparation of 5,6-dihydro-2H-pyran derivatives, wherein an aldehyde of formula I: ##STR1## wherein R.sub.1 represents an alkyl, cycloalkyl, aralkyl, alkaryl or aryl group having at most 15 carbon atoms and a diene of formula II: ##STR2## wherein R.sub.2 and R.sub.3 represents a hydrogen atom or an alkyl group having 1-6 carbon atoms are reacted in a Diels-Alder-type reaction under the influence of an inorganic Lewis acid catalyst, producing a compound of formula III wherein R.sub.1 -R.sub.3 have the meaning given above. ##STR3## Preferably R.sub.1 represents a 1-6 carbon atom alkyl group, a phenyl group or an o-, m- or p-tolyl group. Isoprene or 2-methyl-1,3-pentadiene is preferably used as the diene. The preferred catalyst is AlCl.sub.3 or SnCl.sub.4 and an organic nitro compound may be used as the co-catalyst.