6-Membered rings prototropic tautomerism

Protoporphyrin-IX, N-methyl-, 4, 396 Protoporphyrins, 4, 382 photooxygenation, 4, 402 Prototropic tautomerism polyheteroatom six-membered rings, 3, 1055 Prozapine properties, 7, 545 Pschorr reaction carbolines from, 4, 523 dibenzazepines from, 7, 533 dibenzothiophenes from, 4, 107 phenanthridines from, 2, 433 Pseudilin, pentabromo-synthesis, 1, 449 Pseudoazulene synthesis, 4, 526 Pseudobases in synthesis reviews, 1, 62 Pseudocyanines, 2, 331 Pseudothiohydantoin synthesis, 6, 296 Pseudouracil structure, 3, 68 Pseudoyangonin IR spectra, 3, 596 Pseudoyohimbine synthesis, 2, 271 Psicofuranine biological activity, 5, 603 as pharmaceutical, 1, 153, 160... 

Prototropic Tautomerism of Heteroaromatic Compounds II. Six-Membered Rings... 

Prototropic Tautomerism of Heteroaromatic Compounds III. Five-Membered Rings and One Hetero Atom A. R. Katritzky and J. M. Lagowski... 

Prototropic Tautomerism of Heterparomatic Compounds IV. Five-Membered Rings with Two or More Hetero Atoms... 

The nature and position of the cyclic heteroatoms within the five-membered rings of azoles can affect the positions of the prototropic tautomeric eqihbria, particularly of hydroxy and mercapto derivatives. 

Several other review articles cover various aspects of tautomerism in six-membered heterocycles. Various aspects of prototropic tautomerism have been covered in a review article by Katritzky [91H(32)329] types of tautomerism, methods of study of aromatic tautomerism, and tautomeric equilibria in pyridines and other six-membered rings. Some generalizations have also been made. Another review article by Kurasawa, which appeared in two parts in 1995, covers, for the most part, the work of that author [95H(41)1805 95H(41)2057]. 

Katritzky, A. R., Lagowski, J. M., Prototropic Tautomerism of Heteroaromatic Compounds. I. General Discussion and Methods of Study, 1, 311 II. Six-Membered Rings, 1, 339 III. Five-Membered Rings and One Hetero Atom, 2, 1 IV. Five-Membered Rings with Two or More Hetero Atoms, 2, 27. 

Prototropic tautomerism is observed in the pyrrolopyridines. As a consequence, the endocyclic bond angle centered on the five-membered ring nitrogen closes while the endocyclic bond angle... 

Annular tautomers are prototropic tautomers in which the migrating proton is restricted to ring atoms. For these five-membered heterocycles, annular tautomerism can only occur with pyrrole 2 and its polycyclic derivatives. There is no authenticated case of the monocyclic pyrrolenine tautomeric forms 211 and 212 predominating, presumably due to the required loss of resonance energy in these nonaromatic tautomers . 

Prototropic Tautomerism-Five-Membered Rings with One Hetero Atom Suppl. 1,1976, 214-265 ... 

---