Pyrazole ring enamines

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... 

The first disconnection to be considered is breaking all the C-X bonds in the ring. This is an obvious strategy for a pyrazole since one C-N bond is part of an enamine and the other is an imine 23. Both functional groups are made from amines and carbonyl compounds. Here hydrazine is the diamine that emerges from an easily made 1,3-dicarbonyl compound 24. 

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