1.2.3- Triazoles 4-hydroxy

H-1, 2,3-Triazol 4-Hydroxy-2-phenyl- E8d, 357 (1-Oxid-Red.) 111-1,2,4-Triazol 5-Oxo-3-phenyl-4,5-dihydro- X/2, 159... 

In peptide syntheses, where partial racemization of the chiral a-carbon centers is a serious problem, the application of 1-hydroxy-1 H-benzotriazole ( HBT") and DCC has been very successful in increasing yields and decreasing racemization (W. Kdnig, 1970 G.C. Windridge, 1971 H.R. Bosshard, 1973), l-(Acyloxy)-lif-benzotriazoles or l-acyl-17f-benzo-triazole 3-oxides are formed as reactive intermediates. If carboxylic or phosphoric esters are to be formed from the acids and alcohols using DCC, 4-(pyrrolidin-l -yl)pyridine ( PPY A. Hassner, 1978 K.M. Patel, 1979) and HBT are efficient catalysts even with tert-alkyl, choles-teryl, aryl, and other unreactive alcohols as well as with highly bulky or labile acids. 

HydrOxy-THISs react with diethoxycarbonylazine producing a 1,2,4-triazole via addition, elimination of carbonyl sulfide (29). and subsequent loss of the ester groups (Scheme 20) (30). 

BAS 111 l-Phenoxy-3-(lH-l,2,4-tria2ole-l-yl)-4-hydroxy-5,5-dimethylhexane [9003-11-6] (BAS 111) (46) is a triazole that has plant growth inhibiting properties. It exerts its influence by inhibiting the production of gibbereUic acid in plants this has been demonstrated in canola (31,37). 

Pyrazolino[2,3-c][l,2,3]triazoles, 5, 702 Pyrazolium hydroxide, l,2-dimethyl-3,5-diphenylanhydro-4-hydroxy-IR spectra, 5, 201 Pyrazolium salts dequatemization, 5, 269 H NMR, 5, 185 hydrogen exchange at ring carbon, 5, 245 mesoionic compounds, 5, 171 nitrodebromination, 5, 237 reactivity, 5, 217 reduction, 5, 68, 243 synthesis, 5, 156 UV spectra, 5, 199 Pyrazolium salts, amino-reactions, 5, 262 Pyrazolium salts, bromo-nucleophilic displacements, 5, 266 Pyrazolium salts, 1,2-dimethyl-deuteration, 5, 175, 245 hydrogen exchange, 5, 71 acid-catalyzed, 5, 239 reactions... 

AHC(S1), pp. 384,385]. For some derivatives of 119 and 120, the relative stability of oxo isomers may be enhanced. Thus, the predominance of the 5,5-diphenyldihydro-4//-l,2,3-triazol-4-one structure 121 in solution has been confirmed by UV and and NMR studies (93CB103). Also, 1-methyl-5-hydroxy-l,2,3-triazole exists mainly in the tautomeric form 120 (R = Me, R = H) [76AHC(S1), p. 385], although the existence of a minor amount of oxo form 122 was postulated to explain the exchange of the 4-position proton in D2O. 

Most 3-hydroxy-l,2,4-triazoles exist in the hydroxy form 123 [76AHC(S1), pp. 377,389 84CHEC-I(5)733]. An exception is the 5-nitro derivative, which in the solid state possesses the 0x0 structure 124 (97JPC3605). 

Type 125 0x0 forms are characteristic for 5-hydroxy-l,2,4-triazoles [76AHC(S1), pp. 379, 388], These forms are additionally stabilized by an electron-withdrawing substituent, R = NO2 (98MRC343). Both hydroxy and 0x0 tautomers are capable of forming stable dimers owing to the in-termolecular hydrogen bonds (126 and 127 [76AHC(S1), pp. 377,379). 

For N-hydroxy-1,2,4-triazole, the zwitterionic structure 128 was suggested [89AX(C)782,96CHEC-II(4)127]. Whereas N -substituted 3-hydroxy-1,2,5-triazoles prefer the hydroxy form 129, 2-hydroxy-1,3,4-triazoles exist as 0X0 forms 130 (Scheme 49) [76AHC(S1), pp. 380,383,388,389],... 

Iminoimidazolin-4-ones 227b are the only tautomers detected for 4-hydroxy-2-aminoimidazoles (R = H, Ph R = H) (91KGS62). Similar tautomeric forms are also characteristic of l-methyJ-2-imino-5-benzylideneim-idazole-4-one 228 (91KGS62) and 3-amino-l,2,4-triazol-5-ones 229 (R = R = H, COMe) (Scheme 78) [76AHC(S1), p. 482]. 

We do not discuss in detail the cases of tautomerism of heterocycles embedded in supramolecular structures, such as crown ethers, cryptands, and heterophanes, because such tautomerism is similar in most aspects to that displayed by the analogous monocyclic heterocycles. We concentrate here on modifications that can be induced by the macrocyclic cavity. Tire so-called proton-ionizable crown ethers have been discussed in several comprehensive reviews by Bradshaw et al. [90H665 96CSC(1)35 97ACR338, 97JIP221J. Tire compounds considered include tautomerizable compounds such as 4(5)-substituted imidazoles 1///4//-1,2,4-triazoles 3-hydroxy-pyridines and 4-pyridones. 

Hydroxy-1,2,3-triazoles are known, but their detailed structure apparently has not as yet been investigated with respect to theit existence in hydroxy or oxo forms. Suitably substituted hydroxy-1,2,3-triazoles (120) can undergo another type of tautomerism to give the... 

MP2/6-31-l-G calculations in the gas phase indicate that 2//-1,2,3-triazole is about -5 kcal/mol more stable than the H isomer [92JOC3698]. The energy differences between 1-hydroxy-l,2,3-triazole 56a and its 2H (56b) and 3// (56c) tautomers were investigated up to the MP4(SDTQ)/6-31 - -G level. The 1 -hydroxy form 56a is the preferred tautomer in the gas phase, but owing to the strong polarity of the V-oxide 3// tautomer 56c, this is the most stable structure in solution (Scheme 37) [92JOC3698]. 

In [l,2,4]triazolo[4,3-a]pyrazine (174) bromination took place at the 5-position rather than in the triazole ring (77JOC4197). It was not possible to convert the 3-hydroxy derivative into the 3-chloro analogue (68JHC485). The isomeric [1,5-a] compound (175) was also brominated at C-5 (74TL4539), whereas its 7-oxide gave the 8-chloro derivative under Meisenheimer conditions [80JCS(P1)506]. 

The pK values for azolediazonium ions (Scheme 12-4) refer to the heterolysis of the NH bond, not to the addition of a hydroxy group. Therefore, these heteroaromatic diazo components may react either as a cation (as shown in Scheme 12-4) or as the zwitterion (after loss of the NH proton). Diener and Zollinger (1986) investigated the relative reactivities of these two equilibrium forms (Scheme 12-5) in the azo coupling reaction of l,3,4-triazole-2-diazonium ion with the tri-basic anion of 2-naphthol-3,6-disulfonic acid. 

---