Pyrazoles

An atom efficient, green synthesis of 2-pyrazoline derivatives under solvent-free conditions using grinding technique

“…Amongst five-membered heterocycles, pyrazolines represent a class of compounds of great importance in heterocyclic chemistry (23,24). These compounds have intrinsic biological activities and constitute the structural feature of many bioactive compounds.…”

Recent advances in the synthesis of polysubstituted 3-pyrazolidinones

“…[40][41][42]44,47,[72][73][74][75][76][77] The 1 H-NMR spectroscopic data of the pyrazolidinones 4 and 25-33 and azomethine imines 24 revealed some interesting structural features of these compounds. In solution, these pyrazolidinone derivatives can equilibrate between two envelope conformers A and C via the planar conformer B (Scheme 21).…”

“…In the last decade, our studies on [3+2] cycloadditions of (1Z,4R*,5R*)-1-arylmethylidene-4-benzoylamino-3-oxo-5-phenyltetrahydropyrazol-1-ium-2-ides 41 to various dipolarophiles has revealed the general reactivity and selectivity of these cycloadditions, 9 as well as their applicability in high-throughput synthesis. 72,73 Stereocontrol can be illustrated by cycloadditions of 41 to methyl methacrylate (Scheme 10). The stereoselectivity of the cycloadditions of dipoles 41a-e to methyl methacrylate was in agreement with the stereochemistry observed by related cycloadditions 9,72,73,75 and could be explained in the following way: an ortho-unsubstituted 1′-aryl group in dipoles 41a-c can rotate around the C(1′)-Ar bond, thus shielding equally both faces of the dipole 41′a-c. Consequently, the phenyl ring at position 5 is the stereodirecting group, which hinders the (1′Si)-face of the (Z)-dipole 41′.…”

“…72,73 Stereocontrol can be illustrated by cycloadditions of 41 to methyl methacrylate (Scheme 10). The stereoselectivity of the cycloadditions of dipoles 41a-e to methyl methacrylate was in agreement with the stereochemistry observed by related cycloadditions 9,72,73,75 and could be explained in the following way: an ortho-unsubstituted 1′-aryl group in dipoles 41a-c can rotate around the C(1′)-Ar bond, thus shielding equally both faces of the dipole 41′a-c. Consequently, the phenyl ring at position 5 is the stereodirecting group, which hinders the (1′Si)-face of the (Z)-dipole 41′. Formation of the major (1R*,3R*,5R*,6R*)-isomers 42a-c is explainable by preferential endo-attack of methyl methacrylate from the less hindered (1′Re)-face of dipole 41′a-c via the proposed transition state TS1.…”

“…74 Selectivity of these cycloadditions was somewhat lower than expected on the basis of results with analogous reactions. 9,38,[72][73][74][75] Nevertheless, isolation of multiple isomers may also be advantageous, providing more stereochemical diversity of the 6-amino-7-oxotetrahydropyrazolo[1,2-a]-pyrazole-1(or 2)-carboxylic acid scaffold (c.f. .…”