Pyrazoline diastereomerization

Pyrolysis at 190° of the resulting diastereomeric A -pyrazolines (8) and (11) leads to elimination of nitrogen and formation of the cis- and tmns-cydo-propanecarboxylates (9) and (12), respectively. Thermal decomposition of the A -pyrazoline (13) affords methyl tiglate (14) in addition to the cyclopropane derivative (15) in a ratio 2 1, while A -pyrazolines such as (3) give only 0L,[i- or, y-unsaturated esters, and no cyclopropane derivatives. 

The (ri" -diene tricarbonyliron)-substituted diazocarbonyl compounds 25 have been found to undergo 1,3-dipolar cycloaddition with methyl acrylate in high yield, but with little or no diastereoselectivity (56). Nevertheless, the facile chromatographic separation of the diastereomeric products 26a,b and 27a,b (Scheme 8.8), permits the synthesis of pure enantiomers when optically active diazo compounds (25) [enantiomeric excess (ee) >96%] are employed. When the reaction of 25 (R = C02Et) with methyl acrylate was carried out at 70 °C, cyclopropanes instead of A -pyrazolines were formed. The enantiomerically pure... 

The synthesis and relative stability of 3,5-diacyl-4,5-dihydro-l//-pyrazoles prepared by dipolar cycloaddition of enones and a-diazoketones has been published <2004JOC9085>. 3-Acyl-4-aryl-2-pyrazolines have been synthesized by the reaction of a,/3-unsaturated ketones with diazomethane <1996IJB1091>. Ethyl diazoacetate added to 1,3-diarylpropenones in a regioselective fashion to give the intermediate 4,5-dihydto-3//-pyrazole derivative 1,3-hydride shift in the latter led to the formation of the isomeric ethyl 4-aryl-5-aroyl-4,5-dihydro-l//-pyrazole-3-carboxylate and ethyl 4-aryl-3-aroyl-4,5-dihydro-l/7-pyrazole-5-carboxylate in a ratio of 5 1 <2001RJ01517>. 1,3-Dipolar cycloaddition of 2-diazopropane with diarylideneacetones afforded diastereomeric bis-A -pyrazolines <1999T449>. 

This chromium complex yields a mixture of the diastereomeric pyrazolines 6.83 and 6.84 in a ratio 92 8. The latter isomer corresponds to the pyrazoline formed with the uncomplexed methyl 1,2-dihydro-2-methylnaphthalene-3-carboxylate (6.80, R = methyl), as major product. The corresponding chromium complex of the methyl... 

In terms of both regio- and diastereoselectivity metal alkoxycarbenes are superior to their isolobal ester analogs as demonstrated by the cycloaddition of the (-)-8-phenylmenthol-derived chromium carbene 44 with nitrilimine 41 (Scheme 11.17). Whereas complex 44 affords a 55% yield of A -pyrazoline ester 45a/b in a 92 8 mixture of diastereomers, its cinnamate congener 47 produces a 38 62 ratio of regioisomers 45 and 46, although in quantitative yield, as diastereomeric mixtures (d.r. = 32 68 for 45a/b and 71 29for 46a/b). 

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