The present invention relates to a new process for the preparation of thieno-indole derivatives of formula (Ia) or (Ib), exploiting an asymmetric synthesis for the preparation of key (8S) or (8R) 8-(halomethyl)-1-alkyl-7,8-dihydro-6H-thieno[3,2-e]indol-4-ol intermediates, and to useful intermediate compounds of such process. Thieno-indole derivatives are described and claimed in GB2344818, WO2013/149948 and WO2013/149946, which also disclose processes for their preparation. Thieno-indole enantiopure derivatives can now be advantageously prepared through a new asymmetric synthesis of the key 8-(halomethyl)-7,8-dihydro-6H-thieno[3,2-e]indol intermediates, which, avoiding the chiral resolution step, provides benefits in terms of reducing time and costs of the whole process for their preparation. The synthesis starts from the N-alkylation of 5-amino-4-halo-3-alkyl-1-benzothiophene-7-ol derivatives with enantiopure glycidyl 3-nosylate, followed by intramolecular 6-endo-tet cyclization using alkyl Grignard reagents; Mitsunobu activation of the secondary alcohol promotes internal spirocyclization, affording the 4,4a,5,6-tetrahydro-8H-cyclopropa[c]thieno[3,2-e]indol-8-one derivatives; finally, stereo-electronically controlled regioselective cyclopropane opening yields the key enantiopure 8-(halomethyl)-1-alkyl-7,8-dihydro-6H-thieno[3,2-e]indol-4-ol intermediates, which can be further derivatized following teachings disclosed in WO2013/149948 or WO2013/149946, to prepare the final thieno-indole derivatives of formula (Ia) or (Ib). Such compounds are disclosed to be alkylating compounds with cytotoxic activity, therefore useful as such in the treatment of a variety of cancers and in cell proliferative disorders, or, conjugated with different types of nucleophiles, in the preparation of Antibody Drug Conjugated derivatives.