Stereo- and chemoselectivity in 1,3-dipolar cycloaddition reaction of 2- diazopropane with diarylidenacetones

Article abstract of DOI:10.1016/S0040-4020(98)01056-4

1,3-Dipolar cycloaddition reaction of 2-diazopropane 1 with diarylidenencetones 2 carried out at 0°C led to a minor Δ²-pyrazoline monocycloadduct 4 and two diastereoisomeric bicycloadducts 5 and 6. The same addition realised at -60°C has enabled us to observe, beside bis-Δ²- pyrazolines 5 and 6, the formation of unexpected Δ³(1,3,4)oxadiazoline derivatives 9 and 10. These two diastereoisomers result from the addition of 2-diazopropane on the carbonyl of unstable Δ¹-pyrazoline intermediates 3' and 3'' with an unusual regiochemical way. Oxidation of 5 and 6 gave 3H- pyrazoles 7 and 8.