Steric and Inductive Effects on the Hydrolysis of Quinone Bisketals

Article abstract of DOI:10.1021/jo00223a029

The effects of allylic substituents on the regiochemistry of monohydrolysis of tetralin-type quinone bisketals 12 have been studied.The requisite bisketals were prepared by anodic oxidation of the corresponding 1-substituted 5,8-dimethoxytetralin.Product studies establish that hydroxyl and ether functions at the allylic position preferentially afford quinone monoketals of type 13 wherein the ketal function nearest to the allylic substituent is hydrolyzed.The fluoro system also preferentially forms the monoketal 13 (R = F).A series of alkyl substituents were also studied, and increasing the size of the group led to increasing regioselectivity in favor of 13.Only the Δ^1,2-unsaturated systems 12j,k preferentially gave monoketals in which the more distant ketal function had hydrolyzed.Kinetic studies established at least two major factors in the regiochemistry of the bisketal hydrolysis.While both the oxygenated and alkylated substituents gave monoketals 13 in which hydrolysis had selectively occurred at the nearer ketal function, the origins of the observed regioselectivity are different.Oxygenated systems gave the observed regiochemistry due to a rate retardation of the hydrolysis of the more distant ketal by what is proposed to be an inductive effect.However, alkyl substituents exerted their effect by increasing the rate of hydrolysis of the nearer ketal function due to a relief of strain energy.