A Rearrangement of 3-Pyrazolines as a Missing Link

The thermal conversion of 4-isoxazolines to 4-oxazolines involves the transposition of two ring members. The ring-contraction and ring-expansion sequence in the reaction 2→5 has been previously clarified. The low N−N bond energy should favor an analogous conversion of 3-pyrazolines 6 to 4-imidazolines 7; the first example of such a transformation is reported here. In the yellow 16, the 3-pyrazoline is part of a pyrazolo[5,1-a]isoquinoline system. Daylight induces a ring contraction, which affords the 2-isoquinolylaziridine derivative 21. The latter is converted at 65° to the tricyclic 4-imidazoline 26 by a sequence of electrocyclic aziridine ring-opening and 1,5-electrocyclization of a C=N-conjugated azomethine ylide 25.