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

Scientific paper - Original scientific paper

• Author: Valerije Vrček
• Author: Ivana Vinković Vrček
• Author: Olga Kronja
• Author's institution: University of Zagreb; Faculty of Pharmacy and Biochemistry; (Department of organic chemistry)
• Scientific / art field, discipline and subdiscipline: NATURAL SCIENCES; Chemistry; Organic Chemistry

Abstract (english)

The mechanism of 1,2-hydride shift in protosteryl C(20) cation (<b>1A</b>) and in dammarenyl C(20) cation (<b>2A</b>) was investigated by the semi-empirical AM1 method and <i>ab initio</i> quantum Chemical calculations (HF/3-21G level). Stationary points <b>1A/1B</b> and <b>2A/2B</b>, and the corresponding transition hydrido-bridged structures <b>1TS</b> and <b>2TS</b> were located on the energy surface. Process <b>1A→1B</b> turned out to be energetically more favorable than process <b>2A→2B</b> by <i>ca</i>. 9 kcal mol^-1, mostly due to the unfavorable steric repulsive interaction between the methyl group at C(14) and the <i>β</i>-oriented side chain at C(17) in <b>1A</b> and the lack of CC-hyperconjugative stabilization in <b>1A</b>. The exothermicity of processes <b>1A→1B</b> and <b>2A→2B</b> was increased by subsequent introduction of substituents (H, Me, i-Pr, and <i>t</i>-Bu) at C(14). The more pronounced trend in <b>1A→1B</b> proves that the origin of the relative stability of <b>1B</b> comes from the steric interactions in <b>1A</b>. Introduction of the halogen atom (F, Cl, and Br), due to its -I effect and relatively small size, changed the direction of the equilibrium <b>1A_Y/1B_Y</b>, and <b>1A_Y</b> was found to be by <i>ca</i>. 3 kcal mol^-1 more stable than <b>1B_Y</b>.

• Keywords (english): steroid biosynthesis; quantum-chemical calculations; carbocations; hydride shift
• Language: english
• Publication type: Scientific paper - Original scientific paper
• Publication status: Published
• Peer review: Peer review - international
• Publication version: Published version
• Journal title: Croatica Chemica Acta
• Numbering: vol. 74, no. 4, pp. 801-813
• p-ISSN: 0011-1643
• e-ISSN: 1334-417X
• URN:NBN: urn:nbn:hr:163:127588
• Publication: 2001
• Document URL: https://hrcak.srce.hr/131956
• Type of resource: Text
• Access conditions: Open access
• Created on: 2020-04-29 11:06:46