Drieding models

Dichomine (161) has five asymmetric centers, and thus 32 stereoisomers are possible. However, an examination of Drieding models indicated that for steric reasons Qnly the structure shown in 161, which is already highly strained and rigid, and its enantiomer, are possible. H Coupling constants observed for various protons (H-14 with H-3 and H-17a, H-14 with H-3/3 and H-17j3, and H-14 with H-15a and H-15/3) agreed well with those... 

These annulations are governed by conformational factors. In 1968 Johnson predicted that the dominant conformation for C(3),C(4)-dialkyl-substituted cyclohex-enones, such as 5, has the C(4) substituent oriented axially because of A-strain (Eq. 7)P In addition, the diene moiety of S can exist in either a transoid or a cisoid conformation, cf. conformations lii and iv, respectively. In simple acyclic 1,3-dienes, the planar transoid conformation is more stable than the cisoid form. However, examination of a Drieding model of iii suggests that nonbonded steric interactions tween the equatorial C(4) methyl substituent and the C(3) vinyl group would force the dienone into a cisoid conformation. This orientation clearly precludes 1,6-nucleophilic attack by the silicate species thus, 1,4-addition occurs to generate a cyclopentane ring. 

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