The present invention concerns a process for preparing each of the 4 individual diastereomers of formula (I) in stereochemically pure form from a single enantiomerically pure precursor. The tetracyclic ringsystem having cis-fused five and seven membered rings is formed in a base-catalysed cyclization reaction. The invention further relates to the thus obtained cis-fused tetracyclic alcohol intermediates and methanamine end-products, and the methanamine end-products for use as a medicine, in particular as CNS active medicines.