Dehydration and rearrangement studies in decahydronaphthalene systems

Abstract

The H₂SO₄-Ac₂O-AcOH and thionyl chloride-pyridine induced dehydrations of C5 -acetoxy and -chloro substituted 8a-methyldecahydronaphthalen-4a-ols have been investigated. An attempt is made to rationalise the results of this study by consideration of the conformations of substrates, intermediates, and transition states, and the relative rates of the various molecular processes. The ¹³C n.m.r. spectra of a series of substituted octa- and decahydronaphthalenes together with those of analogous steroids have been assigned. Comparison of substituent effects calculated in each system demonstrates the applicability of steroid ¹³C n.m.r. data as an aid for the prediction and assignment of chemical shifts in the ¹³C n.m.r. spectra of octa- and decahydronaphthalenes. The H₂SO₄-Ac₂O-AcOH induced dehydration of (4aα,5β,8aβ)-5-acetoxy-8a-methyldecahydronaphthalen-4a-ol was shown to occur regiospecifically with rearrangement and is interpreted in terms of the intermediacy of a tight ion pair in which there is sufficient mobility for the departing oxyanion to act as a base in the removal of the α-proton at Cl.

The effect of steroid rings C,D and side chain on the octa- or decahydronaphthalene skeleton has been determined for several series of substrates and this data can now be used to facilitate the assignment of other octa- and decahydronaphthalenes, cholestenes and cholestanes. The conformationally labile cis-decahydronaphthalenes cis-4a-chloro-8a-methyldecahydronaphthalene, cis-8a-methyldecahydronapthalen 4a-yl acetate, and (1α,4aα,8aα)-4a-methyldecahydronaphthalen- 1,8a-diyl diacetate, have been examined by ¹³C n.m.r. at low temperature and thermodynamic parameters for the inversion of equilibrating conformers determined from the rate data which was extracted by lineshape analysis of the recorded spectra.