Lanosterol Biosynthesis: The Critical Role of the Methyl‐29 Group of 2,3‐Oxidosqualene for the Correct Folding of this Substrate and for the Construction of the Five‐Membered D Ring

Lanosterol synthase catalyzes the polycyclization reaction of (3S)‐2,3‐oxidosqualene ( 1 ) into tetracyclic lanosterol 2 by folding 1 in a chair‐boat‐chair‐chair conformation. 27‐Nor‐ and 29‐noroxidosqaulenes ( 7 and 8 , respectively) were incubated with this enzyme to investigate the role of the methyl groups on 1 for the polycyclization cascade. Compound 7 afforded two enzymatic products, namely, 30‐norlanosterol ( 12 ) and 26‐normalabaricatriene ( 13 ; 12 / 13 9:1), which were produced through the normal chair‐boat‐chair‐chair conformation and an atypical chair‐chair‐boat conformation, respectively. Compound 8 gave two products 14 and 15 ( 14 / 15 4:5), which were generated by the normal and the unusual polycyclization pathways through a chair‐chair‐boat‐chair conformation, respectively. It is remarkable that the twist‐boat structure for the B‐ring formation was changed to an energetically favored chair structure for the generation of 15 . Surprisingly, 14 and 15 consisted of a novel 6,6,6,6‐fused tetracyclic ring system, thus differing from the 6,6,6,5‐fused lanosterol skeleton. Together with previous results, we conclude that the methyl‐29 group is critical to the correct folding of 1 , with lesser contributions from the other branched methyl groups, such as methyl‐26, ‐27, and ‐28. Furthermore, we demonstrate that the methyl‐29 group has a crucial role in the formation of the five‐membered D ring of the lanosterol scaffold.