Vinpocetine impurity C – ethyl(13aS,13bS)-13a-ethyl-9-methoxy-2,3,5,6,13a,13b-hexahydro-1H-indolo[3,2,1-de]pyrido[3,2,1-ij][1,5]naphthyridine-12-carboxylate (methoxyvinpocetine)