Natural product synthesis four-membered ring formation

Chelidonine, a representative benzo[c]phenanthridine alkaloid (1,2), was synthesized as the first application of this reaction in natural product synthesis (145). Initial thermal opening of the four-membered ring in 299 led to the formation of a transient -o-quinodimethane (301), which has a dienamide structure in the diene part and is then trapped by the suitably positioned multiple bond in the same molecule. They applied this intramolecular reaction to the acetylenic cyclobutene 300 for the synthesis of ( )-chelidonine (Scheme 110). 

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