The present invention concerns an intermediate of the following formula I in which R 1 and R 2 represent independently of each other a C 1 -C 12 alkyl group, a (C 1 -C 12 alkoxy)carbonyl group, optionally substituted by one, two or three halogen atom, a (C 2 -C 12 alkenyloxy) carbonyl group, an acyl group, a aryl(C 1 -C 12 )alkyl group, an arylalkoxy carbonyl group, a (C 1 -C 12 alkyl)sulfonyle group or an arylsulfonyl group and R 3 represents a O-protecting group. The present invention concerns also a process of preparation of the intermediate and its use for the preparation of Ecteinascidin 743 and Ecteinascidin-770.