2-[(Trimethylsilyl)methyl]benzyl Methanesulfonates: Useful Precursors for the Generation of o-Quinodimethanes

Abstract

2-[(Trimethylsilyl)methyl]benzyl methanesulfonates react with potassium fluoride in the presence of a catalytic amount of 18-crown-6 at room temperature to give o-quinodimethanes, which are trapped with electron-deficient olefins to afford [4+2] cycloadducts in good yields.The Diels-Alder reaction of o-quinodimethane intermediates with alkenes is a versatile method for the synthesis of 1,2,3,4-tetrahydronaphthalene structures. Various methods for the generation of o-quinodimethanes have been developed, and these have been applied to the syntheses of natural products such as steroids. 1 Among them, 1,4-elimination in α′-heterosubstituted α-silyl-o-xylenes is a useful method for the generation of o-quinodimethanes under mild conditions compared with other methods. 2 However, some problems remain with this type of reaction; the precursors are difficult or troublesome to prepare and somewhat harsh conditions are required to generate the oquinodimethane intermediate in some cases. For example, it has been reported that 2-[(trimethylsilyl)methyl]benzyl acetate and the corresponding phenyl carbonate can be prepared in short steps, and the o-quinodimethane can be generated under thermal conditions using potassium fluoride (DMF, 100 °C) or potassium fluoride with an equimolar amount of 18-crown-6 (MeCN, 35-50 °C). 2g-j We envisioned that the silyl compound, which has a better leaving group than the ester or carbonate group, would generate the o-quinodimethane under milder conditions. In this study, we aimed to develop a new precursor, which can be prepared in short steps, and generate o-quinodimethane under mild conditions; we expected that 2-[(trimethylsilyl)methyl]benzyl methanesulfonate (2) would meet these requirements. 3In our preliminary experiment, we found that 2, which was derived from the corresponding alcohol 1, 2f reacts with two equivalents of potassium fluoride in the presence of 30 mol% of 18-crown-6 to give spiro-di-o-xylylene 4 in 68% yield (NMR) with a small amount of [4+4] cycloadduct of o-quinodimethane 5 (6%), 4 indicating that 2 works as a precursor of o-quinodimethane in the presence of a fluoride source (Scheme 1). Intermolecular cycloaddition of o-quinodimethane generated from 2 with electron-deficient olefins could be performed to give tetrahydronaphthalenes 3. We examined the reaction of 2 with dimethyl fumarate as a typical dienophile in the presence of potassium fluoride in acetonitrile at room temperature to optimize the reaction conditions (Table 1). Scheme 1