engleski

Mechanism of 1, 2-hydride shift in some carbocations involved in steroid biosynthesis

The mechanism of 1, 2-hydride shift in protosteryl C(20) cation (1A) and in dammarenyl C(20) cation (2A) was investigated by the semi-empirical AMI method and ab initio quantum chemical calculations (HF/3-21G level). Stationary points 1A/1B and 2A/2B, and the corresponding transition hydrido-bridged structures ITS and 2TS were located on the energy surface. Process 1A --> 1B turned out to be energetically more favorable than process 2A --> 2B by ca. 9 kcal mol(-1), mostly due to the unfavorable steric repulsive interaction between the methyl group at C(14) and the beta -oriented side chain at C(17) in 1A and the lack of CC-hyperconjugative stabilization in 1A. The exothermicity of processes 1A --> 1B and 2A --> 2B was increased by subsequent introduction of substituents (H, Me, i-Pr, and t-Bu) at C(14). The more pronounced trend in 1A --> 1B proves that the origin of the relative stability of IB comes from the steric interactions in IA. Introduction of the halogen atom (F, Cl, and Br), due to its -I effect and relatively small size, changed the direction of the equilibrium 1A(Y)/1B(Y), and 1A(Y) was found to be by ca. 3 kcal mol(-1) more stable than 1B(Y). [References: 23]

hydride shift

nije evidentirano