Quantum-Chemical Study of the Mechanism of Assembly of Spiroketal Derivatives from 2-Methylcyclohexanone and Phenylacetylene in a KOH/DMSO Medium

The article presents a quantum-chemical analysis of the mechanism of formation of dispiroketals from 2-methylcyclohexanone and phenylacetylene in a superbasic KOH/DMSO medium. The conformations of the starting ketone, the formation pathways of intermediate products and isomers, and the energy barriers of the key stages were studied by the B2PLYP-D2/6-311+G**//B3LYP/6-31+G* methods. It was established that the reaction occurs in three stages: nucleophilic ethynylation of ketone, formation of semiketal, and intramolecular O-vinylation. The rate-determining process is the two-stage process of addition of the second ketone molecule and subsequent O-vinylation. Of the eight possible diastereomers, only three are localized (SSSS, RSSR, SSSR), the final ratio of which is 7 : 2 : 1. The predominance of the SSSS isomer is associated with the thermodynamic stability and kinetically advantageous formation of the preceding alkoxide ion.