Conformations of Fused Rings

FIGURE 2.29 The chair (C) and twist-chair (TC) pseudorotational itinerary of the septanoid ring. 

FIGURE 2.31 Conformations of two cw-fused five-membered rings. 

FIGURE 2.32 Cis and trans anellated five- and six-membered rings. 

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Fig. 9. Possible conformations of fused-ring compounds (e.g. (58)). Nitrogen atoms and ring substituents are omitted. The first two letters for each structure refer to the ring conformations (b = boat, c = chair) and the second two letters refer to the phosphorus-exo-substituent (a = axial, b = bowsprit, e = equatorial, f = flagpole)...

Rules of the Fischer Projection Mutarotation The Haworth Projection The Mills Projection The Reeves Projection Conformations of the Six-Membered Rings Conformations of the Five-Membered Rings Conformations of the Seven-Membered Rings Conformations of Fused Rings Steric Factors... 

The pattern of formation of fused-ring diacetals of glycitols having a trans junction of the rings conforms to expectations, the known stable acetals of this type all having the residues in equatorial positions. 

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. 

Polycyclic compounds are common in nature, and many valuable substances have fused-ring structures. For example, steroids, such as the male hormone testosterone, have 3 six-membered rings and 1 five-membered ring fused together. Although steroids look complicated compared with cyclohexane or decalin, the same principles that apply to the conformational analysis of simple cyclohexane lings apply equally well (and often better) to steroids. 

The isotropic equivalent thermal parameters are on the whole larger than in the PbTX-1 dimethyl acetal structure or the structure of the natural product. The B values for atoms on the fused ring skeleton range from 4.7 to 12.6 A (mean square amplitudes of 0.059 and 0.16 A ). Curiously, the largest values are associated with C17-C20 of the 9-membered E ring—the ring that adopts two conformations in crystalline PbTX-1. The acyclic atoms do not have appreciably higher thermal parameters, with the exception of hydroxyl 013, which has a B of 22.4 A 2. 

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