Preferred Conformations of the Cycloheptane Rings of A-Homosteroids

Abstract

The low-temperature measurements were accomplished by passing precooled nitrogen through a dewar-jacketed probe with monitoring of the temperature by a copper-constantan thermocouple. For the spectra taken with the A-56/60A spectrometer, the Varian V-6057 variable temperature accessory was used. The temperatures were calibrated by replacing the sample tube with a similar one containing another thermocouple, and also by the temperature dependence of the chemical shift of the methanol hydroxyl group. The temperatures are believed to be accurate to +2".The theoretical spectra and the Arrhenius plots were computed by an IBM 7094 I1 computer coupled to a Moseley X-Y plotter.All of the gem-fluorocyclohexanes used in this study (except 1,ldifluorocyclohexane) and 1,1,2,2-tetrafluorocyclohexane were synthesized by the reaction of sulfur tetrafluoride with the corresponding ketones.2n The ketones were synthesized as below or obtained commercially.

Steroids and steroidases. VIII. Further data on the preferred conformations of the cycloheptane rings of A-homosteroidketones

“…As observed previously (1)(2)(3) it is difficult to make any quantitative prediction regarding the total effect of inserting a carbonyl group into the seven-membered ring although it is fairly clear that its introduction results in a lowering of the total strain energy. The major factors contributing to this reduction are: (a) a reduced torsional barrier for methylene groups adjacent to the carbonyl group (5) and (6) the eliminatioil of certain R ++ H and H ++ H non-bonded interactions (cf.…”

“…Previous conformational analyses of the cycloheptane rings of A-homosteroid-3-and 4-ketones showed that, for compounds such as l(a-c)-4(a-c), an equilibrium mixture of ring A conformers exists at room temperature and that the main contributors are the twist chairs2 TC,, TC,, and possibly TC,, (1). Conformations TC, and TC, represent those twist chairs in which the carbonyl group is at the axis carbon, or adjacent to it, and of these two carbonyl locations, the previous data indicated that the axis carbon position might be very marginally favored.…”

“…However, the evidence was not unequivocal, and furthermore, it did not enable the relative importailce of TC,, to be estimated with any degree of certainty. The investigations described in this paper are an extension of the earlier study (1) and were undertaken in an attempt to define more accurately the conformational preference of the carbonyl group and to clarify the contribution made by TC,, to the room temperature equilibrium mixture.…”

“…ZThe conformational analytical terms and notations used are as defined previously (1,2). 3At about the same time Bixon and Lifson (4) also reported the results of their carefully designed calculations on cycloheptane.…”

“…Nevertheless this effect will distort the geometry of ring A to an unknown extent and accordingly the desirability of applying appropriate physical methods, such as optical rotatory dispersion (0.r.d.) and dipole moment determinations, to check the validity of the conformational predictions made remains uilchanged (1).…”