Green protection of pyrazole, thermal isomerization and deprotection of tetrahydropyranylpyrazoles, and high-yield, one-pot synthesis of 3(5)-alkylpyrazoles

We report a new synthetic approach that opens up the possibility of large scale, one-pot pyrazole derivatization by a wide variety of functionalities, including alkyl, halogen, hydroxyl, amino, azido, carbonyl, and organo-element (e.g., B, Si, P) groups. The approach is illustrated by the highly efficient synthesis of fourteen 3(5)-alkylpyrazoles, including the novel isopentyl-and n-hexadecyl derivatives, as well as 1,6-bis(pyrazol-3(5)-yl) hexane. The value of the new approach lies in the discovery of a green (solvent-and catalyst-free, quantitative) protection of pyrazole, followed by a high-yield lithiation/alkylation/deprotection sequence in the same pot. For the first time, the corresponding N-tetrahydropyran-2-yl (THP) intermediates have been isolated and characterized. Thermal isomerization of the 5-alkyl-1-(THP) to the 3-alkyl-1-(THP) isomer is shown to be an advantageous, green alternative to the acid-catalyzed, sequential protecting-group switching methodology in pyrazole chemistry. The X-ray crystal structures of 1,6-bis(pyrazol-3(5)-yl) hexane and 5-n-hexadecyl-1-(tetrahydropyran-2-yl) pyrazole reveal supramolecular architectures that shine light on the remarkable affinity for water and unexpected insolubility in organic solvents of alkylene-bridged bis(pyrazoles). 3(5)-Alkylpyrazoles are obtained in high yield from pyrazole by a one-pot procedure.