Steric effects in the tetracyanoethylene catalysed methanolysis of some cyclohexane epoxides
Graphical Abstract
Introduction
Tetracyanoethylene (TCNE) is a mild π-acid catalyst which has proved to be of value in the ring opening of epoxides.1, 2 The stereochemistry of the reaction has been examined in the steroid series where neighbouring group participation by an adjacent cis hydroxyl group has been observed.3 Thus, a 5β-hydroxyl group has been shown to affect both the regiochemistry and stereochemistry of the cleavage of an adjacent 3β,4β-epoxide. Whereas the TCNE catalysed methanolysis of 17β-acetoxy-3β,4β-epoxy-5β-androstane gave the diaxial 3β-hydroxy-4α-methyl ether, methanolysis of 5β-17β-dihydroxy-3β,4β-epoxy-5β-androstane gave the diequatorial 3α-methoxy-4β,5β,17β-trihydroxy-5β-androstane. In this paper, we describe the results of the TCNE catalysed methanolysis of some cyclohexane and monoterpenoid epoxides.
The first compounds to be examined were the isomeric cis-2,3-epoxy-1,5,5-trimethylcyclohexan-1-ol 34 and cis-1,2-epoxy-1,5,5-trimethylcyclohexan-3-ol 5 in which the methyl groups provide a conformational ‘lock’.
Section snippets
Results and discussion
The epoxides 3 and 5 were prepared from the readily available isophorone 1. Treatment of isophorone epoxide with hydrazine hydrate gave 1,5,5-trimethylcyclohex-2-en-1-ol 2.5 This was epoxidized with m-chloroperbenzoic acid to afford cis-2,3-epoxy-1,5,5-trimethylcyclohexan-1-ol 3. Reduction of 1 with sodium borohydride in methanol followed by epoxidation of the alcohol 4 with m-chloroperbenzoic acid, gave cis-1,2-epoxy-1,5,5-trimethylcyclohexan-3-ol 5. The cis relationship of the epoxide and
Conclusion
We have shown that the regiochemistry and stereochemistry of the TCNE catalyzed methanolysis of some cyclohexane epoxides have been directed by the presence of a hydroxyl group. We have established the stereochemistry of the products by X-ray crystallography. The structures of these products indicate that when TCNE is used as a mild π-acid catalyst for the cleavage of hydroxy-epoxides, the hydroxyl group can participate in the reaction.
General
Light petroleum refers to the fraction bp 60–80 °C. Silica for chromatography was Merck 9385. Extracts were dried over anhydrous sodium sulfate. IR spectra were determined as nujol mulls. 1H NMR spectra were determined for solutions in deuteriochloroform at 300 MHz. High-resolution mass spectra were obtained on a Bruker Daltonics Apex III mass spectrometer operating in the electrospray mode.
Crystallographic data
Crystallographic data (excluding structure factors) for the structures in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication numbers CCDC 247473, 247474, 247475. Copies of the data can be obtained free of charge, on application to CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44-1223-336033 or e-mail: [email protected]).
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