“…Following exposure to base, ring contraction would provide compounds of the type 25 , which is formally the product of a DA cycloaddition with the cyclopropene acid 26 . 14 In the forward direction, monobromoketene 15 was generated in situ , and underwent [2+2] cycloaddition with ethoxyacetylene 27 to provide the strained vinylogous ester 29 . Following some early setbacks, it was found that DIBAL-H cleanly reduces the ketone to allylic alcohol 30 , which could be unraveled in the presence of a small amount of BF 3 •OEt 2 to furnish 4-bromocyclobutenone 23 as a solution in CDCl 3 .…”