Stereochemical Course of the Allylic Hydroxylation : Reaction of 1-tert-Butyl-4-alkylidenecyclohexanes with Selenium Dioxide

Abstract

Selenium dioxide-mediated oxidation of substituted olefins is regarded as one of the most reliable and predictable methods for introducing a hydroxy group into the allylic position. The reaction reveals a very useful regioand stereo-selectivity when applied to trisubstituted olefins, producing (E)-allylic alcohol predominantly. Many aspects concerning selectivity of this reaction has been summarized with pertinent examples in a review. The mechanism of this highly selective process had been a subject of interest for many years until Sharpless proposed a mechanism, a generally accepted, consisting of two consecutive pericyclic reactions (an electrophilic ene reaction followed by the 2,3sigmatropic rearrangement) (Scheme 1). A recent mechanistic study on the selenium dioxidemediated allylic oxidation of the acyclic olefin, 2-methyl-2butene clarified the stereochemical aspect of the ene step in this reaction successfully. Later, the overall profile of the allylic oxidation of 2-methyl-2-butene has been explained based on ab initio studies by us. Synthetically very useful aspect in this conversion of the nonactivated C=C double bond into the allylic alcohol intermediate lies in its high stereoselectivity as demonstrated in the synthesis of transpinocarveol and a steroid compound. Although this reaction has been proved very useful for producing an allylic alcohol from an olefinic moiety in various cyclic systems, the systematic account on the stereochemical course for cyclic systems has been scarce. The above cyclic systems, due to their stereochemical features already crafted as highly biased, might be prone to produce a high stereoselectivity and still remain as specific cases. We were interested in understanding detailed features of the stereochemical control in the allylic oxidation for more general cyclic systems. Thus, we have performed stereochemical studies on selenium dioxide-mediated oxidation of 1-tert-butyl-4-alkylidene cyclohexanes 3 whose structural elements would be built in such a way that the selenium reagent may respond aptly. Accordingly, the selenium reagent would arrange the hydrogen of three positions (a-c) to the corresponding hydroxyl functional group (4-6) (Scheme 2). Parallel ab-initio calculation studies for transition states of two major steps (an ene step and a [2,3]-sigmatropic rearrangement) were made to analyze the stereochemical course of this reaction using HF/3-21G* method. Calculation results were compared with the experimental observations and the overall stereochemical course of the whole transformation could be described on the basis of the comparative analysis of two data. 4-tert-Butylalkylidenecyclohexanes 3 were prepared from 4-tert-butylcyclohexanone by known procedure. Allylic oxidation was carried out by reacting 3a-e with 1 equivalent of selenium dioxide in ethanol for 15 h at room temperature. Reaction products were purified by flash column chromatography on silica gel. Allylic alcohols were isolated as a mixture of diastereomeric isomers. The ratios of diastereomeric isomers were determined by analyzing areas of distinct