Enantioselective Construction of New Chiral Cyclic Scaffolds Using [2 + 2 + 2] Cycloaddition (Report‪)‬

INTRODUCTION The transition-metal-catalyzed [2 + 2 + 2] cycloaddition of alkynes is the most atom-economical and facile protocol for the construction of a benzene ring. Yamazaki is a pioneer in this field and reported the cobalt complex-mediated trimerization of diphenylacetylene [1]. Vollhardt reported a catalytic reaction using cobalt carbonyl complex: this reaction offered a new route for the preparation of substituted benzenes by alkyne trimerization [2]. As for an enantioselective [2 + 2 + 2] cycloaddition of alkynes, Mori reported the chiral nickel complex-catalyzed intermolecular reaction of triynes with acetylene: in this reaction, an asymmetric carbon atom was generated at the benzylic position of the obtained benzene ring by enantiotopic group selection [3]. Stary reported the chiral nickel complex-catalyzed intramolecular reaction of triynes, which induced helical chirality [4]. Even though these were pioneering and fascinating works, enantioselectivity was not sufficiently high. In 2004, three groups including ours independently reported enantioselective [2 + 2 + 2] cycloaddition using chiral cobalt, iridium, and rhodium catalysts, respectively, for the induction of axial chirality [5]. Since then, various types of enantioselective [2 + 2 + 2] cycloaddition have been comprehensively studied by us and other groups [6]. This manuscript discloses two approaches based on the [2 + 2 + 2] cycloaddition of triynes for the construction of new chiral scaffolds.