Solutions intramolecular hydrogen bonds

The amino form 66a of 5-pyridylamino-l,2,4-triazole stabilized by two intramolecular hydrogen bonds is the only tautomer observed by the X-ray study of the crystal (90KGS1632). However in DMSO-dg, DMF-d7, and HMPA-di8 solutions, the equilibrium involves three tautomeric forms 66a-66c (Scheme 31) (90KGS1632). 

Apparently, cyclization involves the formation of open-chain intermediates 342, 343, further closing up to imidazolidines 344 and oxazolidines 345 which eliminate the secondary amine, thus leading to imidazolines 346 and oxazolines 347. The latter exist in the solution exclusively in the enolic forms 348, 349 which are stabilized by conjugation and intramolecular hydrogen bonds. 

In summary, although subtle conformational differences between the various cycloamyloses and the effect of intramolecular hydrogen bonds on their solution conformations remain to be accurately resolved, overall structural features have been clearly defined. This is particularly advantageous and, in fact, a prerequisite if the cycloamyloses are to be profitably used as models for enzymatic or other catalytic processes. In subsequent sections of this article, various aspects of binding and catalysis will be explained on the basis of the chemical nature and geometrical dimensions of the cycloamylose cavity which is, in fact, their active site. 

Treatment of the phosphoric ester 224 with triethylamine breaks the intramolecular hydrogen bonding in the solution structure and promotes cyclization to give spirophosphorane 43 (Equation 44) <2003PS2117>. 

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