Abstract
A new family of enantiomerically pure pyrazoles with a variety of substitutions on a key stereogenic center was synthesized from (R)-(+)-pulegone by a straightforward, large-scale route involving initial construction of the pyrazole ring via formylation/dehydration with hydrazine followed by ozonolysis to yield a readily functionalized ketone (5). Alkylation of 5 with a variety of Grignard reagents, dehydration, hydrogenation, and recrystallization afforded the set of new chiral pyrazoles (7a−c). Cis and/or trans diastereomers of one of these pyrazoles having a phenyl substituent (7a) were elaborated into enantiopure, multidentate C1-symmetric bis(pyrazolyl)diphenylborate (Tl[cis-Ph2Bppm]), C2-symmetric bis(pyrazolyl)methane (cis- and trans-Xpm), and C3-symmetric tris(pyrazolyl)phosphine oxide (cis- and trans-OPpm) and tris(pyrazolyl)hydroborate (K[trans-Tppm]) ligands. Interestingly, epimerization of the benzylic stereogenic center occurred during the synthesis of K[trans-Tppm], as determined by comparis...
Published Version
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