RING-CHAIN TAUTOMERISM AND CRYSTAL-STRUCTURE OF SOME 1,3-OXAZACYCLANES, AND THE ELECTROPHILIC SUBSTITUENT CONSTANTS FOR SOME HETEROARYL GROUPS IN SOLUTION AND IN THE GAS-PHASE

In principle, the ring-chain tautomeric ratios of a number of 2-hetero aryl-substituted oxazolidines and tetrahydro-1,3-oxazines are well sui ted for determination of the Hammett-Brown sigma+ constants of heteroa ryl substituents such as 2-furyl, 3-furyl, 3-thienyl and 2-pyrrolyl in CDCl3 solution and in the gas phase. However, a number of factors, an d especially hydrogen-bonding, can change the monomeric character of e ither the ring or the chain form (or both), leading to deviations from the ideal sigma+ values. X-Ray analysis has demonstrated the chain st ructures of the products of the reactions of L-(-)-norpseudoephedrine and (+/-)-trans-2-aminomethylcyclohexanol with pyrrole-2-carboxaldehyd e.