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

1,3-Dipolar cycloaddition reaction of 2-diazopropane I with diarylideneacet ones 2 carried out at 0 degrees C led to a minor Delta(2)-pyrazoline monocy cloadduct 4 and two diastereoisomeric bicycloadducts 5 and 6. The same addi tion realised at -60 degrees C has enabled us to observe, beside bis-Delta( 2)-pyrazolines 5 and 6, the formation of unexpected Delta(3)-(1,3,4)oxadiaz oline derivatives 9 and 10. These two diastereoisomers result from the addi tion of 2-diazopropane on the carbonyl of unstable Delta(1)-pyrazoline inte rmediates 3' and 3 " with an unusual regiochemical way. Oxidation of 5 and 6 gave 3H-pyrazoles 7 and 8. (C) 1998 Elsevier Science Ltd. All rights rese rved.