Process for the preparation of an n-phenylacetic derivative of tetrahydrothieno(3,2-c)pyridine and its chemical intermediate

A process for the preparation of the dextrorotatory isomer of the compound of ##STR8## wherein a formylating agent chosen from formaldehyde, its hydrate or its polymer forms, the compound of formula XCH.sub.2 Y in which X is a halogen atom, a C.sub.1 -C.sub.4 alkoxy group, a C.sub.1 -C.sub.4 alkylthio group or an amino group, and Y is a C.sub.1 -C.sub.4 alkoxy group, a C.sub.1 -C.sub.4 alkylthio group, amino or a C.sub.2 -C.sub.5 alkoxycarbonyl or phenoxycarbonyl group and a heterocyclic compound of formula ##STR9## in which Z is O, NH or S is reacted with the dextrorotatory isomer of the compound of formula ##STR10## and the compound formed is cyclised in the presence of an acid.