Twist-boat conformation molecular model

Consider the cis and trans isomers of 1,3-di- err-butylcyclohexane (build molecular models). What unusual feature accounts for the fact that one of these isomers apparently exists in a twist boat conformation rather than a chair conformation ... 

Mechanistically, the requisite betaine intermediates [from dioxaphospholanes N and O] adopt either the chair ( C -> 4) or twist-boat ( 85, 82) conformations so that the C-O" and -O-+PPh3 groups can assume the requisite pre-transition state andperiplanar arrangement for suitable displacement of Ph3PO. From the results of molecular modeling studies (/.e., MacroModel II) on both the chair e.g., IC4) and twist-boat betaine e.g., 85, 82) conformers, the small energy differences between them suggest that the twist-boat ( 85 and 82) betaine conformers may also be... 

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