Squalene‐Hopene Cyclase: On the Polycyclization Reactions of Squalene Analogues Bearing Ethyl Groups at Positions C‐6, C‐10, C‐15, and C‐19

Squalene-hopene cyclase (SHC) has been found to convert acyclic squalene into 6,6,6,6,5-fused pentacyclic triterpenes hopene and hopanol. The enzymatic reactions of squalene analogues bearing ethyl groups in lieu of methyl groups at positions C-6, C-10, C-15, and C-19 have been examined to investigate whether the larger ethyl substituents (a C-1 unit increment) are accepted as substrates and to investigate how these substitutions affect polycyclization cascades. Analogue 6-ethylsqualene 19a did not cyclize, which indicates that substitution with the bulky group at C-6 completely inhibited the polycyclization reaction. In contrast, 19-ethylsqualene 19b afforded a wide spectrum of cyclization products, including mono-, bi-, tetra-, and pentacyclic products in a ratio of 6:6:1:2. The production of tetra- and pentacyclic scaffolds suggests that the reaction cavity for D-ring formation site is somewhat loosely packed and can accept the 19-ethyl group, and that a robust hydrophobic interaction exists between the 19-ethyl group and the binding site. In contrast to 19b, 10-ethylsqualene 20a and 15-ethylsqualene 20b afforded mainly mono- and bicyclic products, that is, the polycyclization cascade terminated prematurely at the bicyclic reaction stage. Therefore, the catalytic domains for the 10- and 15-methyl binding sites are tightly packed and cannot fully accommodate the Et substituents. The cyclization pathways followed by the ethyl-substituted substrates in the presence of SHC and lanosterol and -amyrin synthases are compared.