Abstract: A process for the preparation of a compound of formula (I) and of a acid salt (T) wherein R1 is selected from the group consisting of alkyl, aryl, cycloalkyl, heteroalkyl, heteroaryl and heterocycloalkyl, R2 and R3, are, independently of each other, selected from the group consisting of alkyl, aryl, cycloalkyl, heteroalkyl, heteroaryl and heterocycloalkyl, R4, R5, R6 and R7, are independently of each other, selected from the group consisting of H, alkyl, aryl, cycloalkyl, heteroalkyl, heteroaryl and heterocycloalkyl, and wherein the acid salt is a 2,3-Ditoluoyl tartaric acid salt, 2,3-Dibenzoyl tartaric acid salt, 2,3-Dianisoyl tartaric acid salt, 2,3-Dibenzoyl tartaric acid mono(dimethylamide) salt or a mixture of two or more thereof, wherein the tartaric acid salt (T) of the compound of formula (I) contains at least 90% by weight of the tartaric salt of the compound of formula (Ia) based on the total weight of the acid salt of the compound of formula (I).