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The 14α-demethylation of biosynthetic reactions is carried out exclusively by lanosta-8,24-dien-3β-ol (lanosterol). Theoretical calculations were done to determine the importance of this structure with the Hartree–Fock scheme. It was modeled on structures similar to lanosterol with variations on the endo between C8&bond;C9 (lanosta-7,24-dien-3β-ol and lanosta-6,24-dien-3β-ol) as compared with the exo between C24&bond;C25 (Lanosta-8-en-3β-ol) and the size of the side chain (27-norlanosta-8-en-3β-ol, 26,27-dinorlanosta-8-en-3β-ol, 25,26,27-trinorlanosta-8-en-3β-ol, 24,25,26,27-tetranorlanosta-8-en-3β-ol, 23,24,25,26,27-pentanorlanosta-8-en-3β-ol, 22,23,24,25,26,27-hexanorlanosta-8-en-3β-ol). Energetic, electronic, and orbital evidence was found. The theoretical data were analyzed and were found to be important for the 14α-demethylation, the presence of HOMO on the region C8&bond;C9 and C14&bond;C141, a negative charge on the outgoing methyl C141. This atomic charge varies with the size of the side chain, and with the modification of the endo and exo double bonds. The conformation of the structure of the steroid is also related to its reactivity. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [ABSTRACT FROM AUTHOR]