Conformation flattened 1,2-alternate

The skeletal geometry of cyclohexene and 1,2-epoxy cyclohexane systems corresponds to a partially flattened chair or half-chair (8). The alternative "half-boat conformation has... 

In chemistry somewhat related to the above, //-octa-bromo-/w.vo-octamethyIcalix 4 pyiToIe 10 was. synthesized in 90% yield by reacting / . vo-octamethylcalix[4 pyrrole with N-broniosuccinimide in hot, dry THF. In this instance, an X-ray structure (Figure 12) revealed the calixpyrrole product (10) exists in a chair-like flattened 1,2-alternate conformation in the solid state (that is, the dihedral angles between pyn ole rings and plane through the calixpyrrole... 

Figure 12. X-ray crystal structure of /i-oclabromo-rneso-octamethyl-calix[4]pyrrole compound 10. The molecule adopts a chair-like flattened 1,2-alternate conformation in the solid state. Crystal structure originally published in Gale, P. A. Sessler, J. L. Allen, W. E. Tvermoes, N. A. Lynch, V. Chem, Commun. 1997, 665, Diagram produced using data from the Cambridge Crystallographic Database.

The mechanistic work on the hydrolysis of acetals provides a stereoelectronic rationale for the earlier suggestions that this essential conformational change shonld be involved in the enzyme-catalyzed p-glycosyl transfer reactions either as a part of the initial binding process or in one of the subsequent isomerization steps. The lone pair-a overlap gradually increases as the ring flattens into the half-chair conformation and an alternative mode of stereoelectronic assistance becomes possible. 

For compoimds 16, this conformation is in slow exchange (AG = 70-88 kJ/mol at 328K in CDCI3) with the 1,2,3-alternate conformation, the latter being less stable (AG° = 2-7 kJ/mol). ° Therefore it is possible to conclude that three important factors stabilize the flattened cone structure, viz. (i) the self-inclusion of the methoxy groups in the apolar cavity driven by CH-ti interactions, (ii) the presence of three bulky groups in 2,4,6 positions at the lower rim, and (iii) the presence of the ter/-butyl groups at the upper... 

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