cis-3,4-Dichlorocyclobutene as a Versatile Synthon in Organic Synthesis. Rapid Entry into Complex Polycyclic Systems with Remarkably Stereospecific Reactions We thank Drs. D. H. Huang and G. Siuzdak for their assistance with NMR spectroscopy and mass spectrometry, respectively. This work was financially supported by the National Institutes of Health (USA) and The Skaggs Institute for Chemical Biology, a postdoctoral fellowship from the Skaggs Institute for Research (to V.G.), and grants from Abbott, Amgen, ArrayBiopharma, Boehringer-Ingelheim, Glaxo, Hoffmann-La Roche, DuPont, Merck, Novartis, Pfizer, and Schering Plough.