Chemoselective acid-catalyzed [4 + 2]-cycloaddition reactions of -quinone methides and styrenes/stilbenes/cinnamates.

ortho-Quinone methides (o-QMs) generated from the corresponding benzyl acetate precursors chemoselectively underwent the formal [4 + 2]-cycloadditions with the olefin of styrene, stilbene, or cinnamate derivatives by using different transition metal salts or Brønsted acids. Such selectivity was obtained when these olefins either separately acted as the dienophiles or were simultaneously present on the same dienophiles. Complete selectivity was also achieved between the stilbene olefin and acetylene to furnish the key chroman intermediate for the subsequent ring-closing metathesis (RCM), affording the corresponding tetracyclic 5H-dihydronaphtho[1,2-c]chromene.