Pyrazoles syntheses

The application described here shows how a classified data set of reactions producing pyrazole derivatives can be used to predict the correct regioisomer in a pyrazole synthesis before it is carried out practically in the laboratory. 

Til most cases, only one of the two regioisomers is preferentially formed. Wc will show here how reaction classification by a self-organizing neural network can be used for the prediction of the preferred regioisomer in a pyrazole synthesis. 

Any attempt in learning also needs information on situations where the event to be studied will not occur. This principle, if translated to learning on reactions, would require information on reactions that will not proceed. Unfortunately, such information is not contained in reaction databases, and therefore has to he generated. In our case, we also needed information on which regioisomer is not formed, or is formed only to a minor extent, in a pyrazole synthesis. 

H-Furo[3,4-e]pyrazole, 1,4-dihydro-4-oxo-l-substituted synthesis, 6, 989 Furo[2,3-d]pyrazoles synthesis, 6, 998 Furo[3,4-d]pyrazoles synthesis, 6, 998 Furo[2,3-d]pyridazine H NMR, 4, 984 synthesis, 4, 984... 

H,3H- Pyrrolo[l, 2-c]oxazole-l, 3-dione, 5,6,7,8-tetrahydro-IR spectra, 6, 978 [2.2](2,5)Pyrrolophane, N-aryl-rearrangements, 4, 209 Pyrrolophanes natural products, 7, 764 synthesis, 7, 771 Pyrrolophanes, N-aryl-synthesis, 7, 774 (2,4)Pyrrolophanes synthesis, 7, 771 Pyrrolo[3,4-c]pyran-4-ones synthesis, 4, 288 Pyrrolopyrans synthesis, 4, 525, 526 Pyrrolopyrazines synthesis, 4, 526 Pyrrolo[l, 2-a]pyrazines synthesis, 4, 516 Pyrrolo[2,3-6]pyrazines Mannich reaction, 4, 504 Vilsmeier reaction, 4, 505 Pyrrolo[3,4-c]pyrazole, 1,3a,6,6a-tetrahydro-structure, 6, 976 synthesis, 6, 1019 Pyrrolopyrazoles synthesis, 5, 164 Pyrrolo[l,2-6]pyrazoles synthesis, 6, 1002, 1006 Pyrrolo[3,4-c]pyrazoles reactions, 6, 1034 synthesis, 6, 989, 1043 Pyrrolo[3,4-c]pyrazolones synthesis, 6, 989 Pyrfolopyridazines synthesis, 4, 517 Pyrrolo[l, 2-6]pyridazines synthesis, 4, 297 6/7-Pyrrolo[2,3-d]pyridazines synthesis, 4, 291 2/f-Pyrrolo[3,4-d]pyridazines synthesis, 4, 291 6/7-Pyrrolo[3,4-d]pyridazines synthesis, 4, 291... 

K N O R R Pyrazole Synthesis Pyrazole synthesis from a -dicarbonyl compourxi arxi a hydrazine. 

The Knorr pyrazole synthesis has been extensively utilized in the preparation of a number of pyrazoles as metal chelators, photographic dyes, herbicides, and biologically active... 

Application of the Knorr pyrazole synthesis has also been demonstrated on solid support. ° To prepare trisubstituted pyrazoles, the diketone was linked to the solid support to make 57 using a linker with an amide bond. Alkylation of the diketone followed by condensation of the hydrazine with the resulting diketone gave the desired pyrazoles as mixtures of isomers. Subsequent cleavage of the amide bond linker then provided the pyrazole amides 59. ... 

Pyrazole synthesis was optimized, especially in terms of regioselectivity. 

Individual substitutions may not necessarily be true electrophilic aromatic substitution reactions. Usually it is assumed that they are, however, and with this assumption the furan nucleus can be compared with others. For tri-fluoroacetylation by trifluoroacetic anhydride at 75 C relative rates have been established, by means of competition experiments 149 thiophene, 1 selenophene, 6.5 furan, 1.4 x 102 2-methylfuran, 1.2 x 105 pyrrole, 5.3 x 107. While nitrogen is usually a better source of electrons for an incoming electrophile (as in pyrrole versus furan) there are exceptions. For example, the enamine 63 reacts with Eschenmoser s salt at the 5-position and not at the enamine grouping.150 Also amusing is an attempted Fischer indole synthesis in which a furan ring is near the reaction site and diverted the reaction into a pyrazole synthesis.151... 

Sakya, S. M. Knorr Pyrazole Synthesis In Name Reactions in Heterocyclic Chemistry, Eds, Li, J. J. Corey, E. J. Wiley Sons Hoboken, NJ, 2005, 292—300. (Review). 

Reaction between 1,4-ketones and primary amines (or ammonia) to give pyrroles. A variation of the Knorr pyrazole synthesis (page 331). 

Similarly, a pyrazole synthesis occurs on addition of diazoacetic ester to diacylimine thereby, the diazo group of the intermediate adduct is converted with triphenylphosphane into a phosphazine, which gives pyrazole as described above (68TL4371). 

Isoxazole and pyrazole synthesis While 1,3-diketones undergo condensation with hydroxylamine to produce isoxazoles, with hydrazine they yield corresponding pyrazoles. 

PECHMANN PYRAZOLE SYNTHESIS. Formation of pyrazoles from acetylenes and diazomethane. The analogous addition of diazoacetic esters to the triple bond yields pyrazolecarboxylic acid derivatives. 

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