Issue in Honor of Prof. Joan Bosch ARKIVOC 2007 (iv) 8-19
cyclopentenone, 7, in 43% yield. Although the overall yields of both approaches are very similar, the new one does not involve purification of any intermediate by column chromatography, so it is more convenient for the large scale preparation of 6. Reaction of enone 6 with tosylhydrazide led to the corresponding hydrazone, 5, in 75% yield.
It is well-known that reduction of tosylhydrazones to hydrocarbons with hydride reagents such as LiAlH4, NaB(CN)BH3, or NaBH4 in acidic medium (“Caglioti reaction”) is a mild alternative to the classical Wolff-Kishner and Clemmensen procedures.4 In a,ß-unsaturated derivatives, the carbon-carbon double bond migrates to the original carbonyl carbon atom.5 In our hands, reduction of 5 with NaBH4 in acetic acid at 70 ºC led to 4 in 60% isolated yield.
The epoxidation of 4 with m-CPBA in dichloromethane led to a mixture of two stereoisomeric epoxides in the ratio of 7:1.6 Column chromatography allowed us to separate both stereoisomers in 61% and 9% yield, respectively. Both stereoisomers were fully characterized by spectroscopic techniques and elemental analyses. The 1H and 13C NMR spectra of these epoxides could be fully assigned on the basis of 1H/1H COSY and 1H/13C HETCOR experiments. Moreover, most of the 1H/1H coupling constants were measured, from which values the relative configuration and preferential conformation for each epoxide was proposed. Table 1 collects the experimental 1H/1H coupling constants for each epoxide and the preferred conformation suggested for these epoxides. In both cases, an envelope conformation with the C4 carbon atom on the same side of the oxygen oxirane atom seems to be the preferred one, no matter if the diphenylphosphinoylmethyl group occupies a pseudoequatorial (8) or a pseudoaxial (3) arrangement. Contrary to our expectations, the main epoxide, 8, has the diphenylphosphinoylmethyl group in a cis arrangement with respect to the epoxide function.
Oxidation of 4 with excess of dimethyldioxirane in acetone7 gave a mixture of both epoxides 8 and 3 in the ratio of 5:1, in 83% yield. Thus, both epoxidation procedures show similar stereoselectivity in favor of epoxide 8.
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