Polycyclic aromatic compounds via radical cyclizations of benzannulated enyne-allenes derived from Ireland-Claisen rearrangement

A new synthetic sequence involving the use of Ireland-Claisen rearrangement of propargylic acetates to form the corresponding benzannulated enyne-allenes followed by Schmittel cyclization to generate benzofulvene biradicals for radical cyclizations leading to polycyclic aromatic compounds was established. Treatment of 9-fluorenone (8) with the lithium acetylide 9 followed by acetic anhydride produced the propargylic acetate 10. A sequence of reactions occurred after 10 was converted to the corresponding silyl ketene acetal 11. An initial Ireland-Claisen rearrangement produced the benzannulated enyne-allene 12, which then underwent a Schmittel cyclization reaction to generate the benzofulvene biradical 13. A subsequent intramolecular radical-radical coupling then produced the formal Diels-Alder adduct 14, which in turn underwent a prototropic rearrangement to give the silyl ester 15 and, after hydrolysis, the carboxylic acid 16 in 57% overall yield from 10 in a single operation. An intramolecular acylation reaction of 16 produced the ketone 17. The carboxylic acids 24-26 were likewise prepared from the diaryl ketones 18-20, respectively. However, the intramolecular [2 + 2] cycloaddition reaction of the benzannulated enyne-allene 33 having a tert-butyl group at the allenic terminus occurred preferentially, producing the 1H-cyclobut[a]indenyl acetic acid 35 as the predominant product.