Flattening rule

On the other hand, puckering the ring will destroy it equatorial attack cannot approach antiperiplanarity to the C2—C3 and C5—C6 bonds. We are thus led to the rule the more flattened the ring, the more axial attack. This flattening rule may help to rationalize experimental results quite difficult to understand otherwise. 

Similarly, ring C is more rigid than ring A. It is also more puckered, being linked by a trans junction to a five-membered ring instead of a six-membered ring The flattening rule then predicts that axial attack will be easier on 1-ketosteroids than on 12-ketosteroids. This is indeed what is observed by Ayres et al. ... 

Pentanone is fairly flexible, so an antiperiplanar attack can occur at either face. The o cc orbital lies lower in energy than o CH so frontier orbital control favors the anti transition state by 3.73 kcal mol-1. Therefore, the flattening rule may be generalized as follows antiperiplanar attack and frontier orbital control in general are only important for reasonably flexible ketones. 

Hint. Reread the, in Section 6.2.1 above, the sub-sections The flattening rule (p. 155) and The importance of being flexible (p. 160). 

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