Why doesn't all-trans-1,2,3,4,5,6-hexaspiro(THF)cyclohexane complex metal ions?

Despite the inherent preference for placing alkyl substituents, rather than alkoxy substituents, in equatorial positions, all-trans-hexaspiro(THF)cycl ohexane strongly favors the all-O-equatorial conformer. Ab initio and densi ty functional calculations on a series of cyclohexane derivatives containin g one, two, or three spiro(THF) units demonstrate that this preference resu lts from at least two important factors. First, when oxygen atoms are attac hed to adjacent carbons, the gauche effect favors the di-O-equatorial arran gement. In trans-1,2-dispiro(THF) cyclohexane, the single gauche interactio n overcomes the inherent steric preference for projecting the two oxygen at oms axially. Similarly, in the all-trans-hexaspiro(THF)cyclohexane the six gauche interactions in the all-O-equatorial conformer overpower the inheren t conformational biases of the six isolated spiro(THF) moieties. Neverthele ss, the gauche effect only partially accounts for the more than 20 kcal/mol conformational bias calculated for this molecule. There is also another fa ctor, the high energetic cost associated with projecting multiple alkoxy su bstituents axially on the same face of a cyclohexane scaffold. The calculat ions find the energetic cost of each 1,3-diaxial interaction is about 2 kca l/mol larger between alkoxy substituents than between alkyl substituents.