Phosphine-Mediated [4 + 3] Annulation of Diynoates and 2-Arylidene Indane-1,3-diones: Access of Indeno[1,2- b ]oxepin-4-ylidenes and Beyond.

A novel unprecedented triphenylphosphine-mediated [4 + 3] annulation reaction of 2-benzylidene indane-1,3-diones and -diynoates through initial phosphine α-addition was discovered and found to result in biologically interesting indeno[1,2- b ]oxepin-4-ylidenes in up to 75% yield. The seven-membered separable Z and E isomeric oxepins were confirmed using single-crystal X-ray diffraction.