Three-membered rings conformation

Disubstituted cyclopropanes exemplify one of the simplest cases involving stabil ity differences between stereoisomers A three membered ring has no conformational mobility so the ring cannot therefore reduce the van der Waals strain between cis sub stituents on adjacent carbons without introducing other strain The situation is different m disubstituted derivatives of cyclohexane... 

This chapter will deal exclusively with three-membered rings containing the hetero atoms O, S and N, and fused to the steroid skeleton. Because of the conformational requirements in steroids, not all of the usual methods of synthesis of three-membered rings are applicable to the fused ring system. For the synthesis of steroids to which an aziridine, oxirane or thiirane is attached either in the side chain or at a ring position but not directly fused to the nucleus, the methods discussed in this chapter, as well as others, are applicable. 

The molecular structure of the parent compound was investigated in the vapor and in the solid phase using X-ray, XN and GED methods. The reported data are shown in Table 16. In both phases a clear bond length separation could be detected with a localized three-membered ring and its three adjacent double bonds. The symmetry-equivalent cyclopropane bonds are rather long in C3v-symmetric BUL (1.533-1.542 A), which can be explained by the common electron-withdrawing effect of the 7r-systems in a. svM-ciinal conformation. For comparison, the unaffected bonds in unsubstituted cyclopropane are 1.499 A in the crystal and 1.510 A in the gas phase. Therefore, the bond lengths in BUL... 

A further feature of three-membered rings is that they must be planar, and a consequence of this is that, in cyclopropane, all C-H bonds are in the high-energy eclipsed state. There can be no conformational mobility to overcome this. 

In the synthesis of cyclohexene oxide from cyclohexene shown, this does implicate the less favourable diaxial conformer in the epoxide-forming step. Cyclohexene oxide contains a c/s-fused ring system, the only arrangement possible, since the three-membered ring is necessarily planar (see Section 3.5.2). 

In view of this conformational behavior and of the generally accepted properties of three-membered rings to conjugate with unsaturated groups, it seems more appropriate to consider acyl derivatives of these rings in the context of conjugated carbonyl heterocycles than in that of derivatives of saturated carbon. 

In C- and N-acyl derivatives of three-membered rings, the geometry of the conformers still seems rather undetermined and, as a result, the same may be said of conformer populations. This is true of simple acetyl derivatives and, of course, of the conformationally more complex aroyl derivatives. For the N-acylaziridines, assignment of nitrogen invertomers seems sufficiently reliable... 

C-Acyl and N-acyl derivatives of three-membered rings have received little attention in recent investigations, especially with regard to up-to-date experimental techniques for determining dynamic and structural parameters of the conformational equilibrium. 

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