Locking groups

Locking groups—t-butyl groups, decalins, and steroids... 

What do you think would happen if a cyclohexane derivative underwent an Sn2 reaction If the conformation of the molecule is fixed by a locking group, the inversion mechanism of the S j2 reaction, means that, if the leaving group is axial, then the incoming nucleophile will end up equatorial and vice versa. 

These locking groups work because the f-butyl group can never be axial (p. 375) and the frans-decaiin can never ring-flip (p. 378). 

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