Diversity-Oriented Synthesis of Spiropentadiene Pyrazolones and 1H-Oxepino[2,3-c]pyrazoles from Doubly Conjugated Pyrazolones via Intramolecular Wittig Reaction

An efficient method for the diversity-oriented synthesis of spiropentadiene pyrazolones and 1H-oxepino [2,3-c]pyrazoles is reported. The methodology attributes O-acylation of phosphorus zwitterions which were formed by a tandem phospha1,6-addition of PBu3 to alpha,beta,gamma,delta-unsaturated pyrazolones, further generating betaine intermediates that preferentially resulted in the aforementioned cyclic products in a diversity-oriented manner. The mechanistic investigations revealed that formation of the betaines is the key step to provide the products via an intramolecular Wittig reaction or an unprecedented delta-C-acylation/cyclization/Wittig reaction.