Diequatorial ring formation

In an effort to induce diequatorial ring formation, hydrogen bonding was introduced as a constraint on the system in the preparation of a series of tetraoxyphosphoranes containing an imino function (13, 37). Some representative examples are displayed here. 

In the triggering of protein kinases by cAMP, enzyme-cAMP adduct formation via a P -TBP having a diequatorial ring orientation has been proposed (24-26, 34) as an essential step in the dissociation of the haloenzyme to the free catalytic subunit. Scheme 4. 

Recently, we synthesized the related phosphorane W and showed the formation of the (e-e) ring orientation by X-ray analysis (Prakasha, T. K. Day, R. O. Holmes, R. R., unpublished work). This represents the first solid state structural evidence of a diequatorial ring orientation in any cyclic oxyphosphorane system. However, when the phenyl ring substituents are replaced by protons. X-ray analysis of X (Prakasha, T. K. ... 

General Comments. The formation of deoxy sugars by hydrogenation over Raney nickel often leads to the abnormal isomer (namely, that formed by diequatorial opening of the oxirane ring) as the major product, in contrast to the product afforded by lithium aluminum hydride this suggests that a different mechanism is involved in the nickel-catalyzed reaction. 

Ring opening occurs in the presence of iodine in an SnI process, the oxirane oxygen being initially complexed with the iodine. The two oxygen atoms bound to C-l serve to destabilize positive charge formation at C-2, rendering nucleophilic attack to C-3 more favorable. This accounts for the diequatorial product observed. 

With X Y and R H, formation of isomers has been observed equilibration occurs on heating. Pseudorotation involves a trigonal-bipyramidal intermediate with the four-membered ring in the diequatorial position (221). 

The reaction of 1-phosphaindene (31) with arylacetylenes leads to the formation of (32) and a single crystal X-ray diffraction study reveals an almost regular tbp with axial chlorines and the benzophosphole ring in a diequatorial configuration. ... 

Cyclohexane 1,2-epoxide can be directly converted, in high yield, into the episulfide by the action of thiocyanate ion the initial product is free to shift its conformation to the diequatorial arrangement and thus to permit formation of the intermediate (91, n = 2). Pyranose epoxides which lack the conformation-locking, fused-acetal ring might also be expected to give episulfides readily by the direct route. 

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