Pyryliumverbindungen. XXI. Struktur und Tautomerie von Pseudobasen unsymmetrisch substituierter 2,4,6‐Triarylpyryliumsalze

Abstract

Pyrylium Compounds. XXI. Structure and Tautomerism of Pseudobases of Unsymmetrically Substituted 2,4,6‐Triarylpyrylium SaltsHydrolytic ring opening of unsymmetrically substituted 2,4,6‐triarylpyrylium salts 11 results in a mixture of two tautomeric pseudobases, the penten‐1,5‐diones 13 and 14. In crystalline state as a rule one of these tautomers markedly predominates, whereas in solution a rapid equilibrium establishment occurs which can be followed by ¹H‐n.m.r. and u.v. spectroscopy (equilibrium ratio nearly 1:1). The ¹H‐n.m.r. spectroscopic structural assignment of the two isomeric forms was supported by u.v. spectroscopic studies using 1:1 mixtures of the chromophoric partial systems ArCOCHCHPh and ArCOMe or the pseudobases 10 of symmetrically substituted 2,4,6‐triarylpyrylium salts 9 as reference systems. From the mass spectra of the pseudobases 10 and 13/14 follows that – if hydrolysis of 11 is performed in media containing H₂¹⁸O – mass spectrometric isotopic analysis of the resulting ¹⁸O‐labelled pseudobases allows no information to be made about the regioselectivity of the ring opening. – Novel starting pyrylium salts of type 9 and 11 are characterized by u.v./vis data and by transformation into the corresponding pyridine derivatives.