A Complete Switch of the Directional Selectivity in the Annulation of 2‐Hydroxybenzaldehydes with Alkynes

Abstract

Controlling reaction selectivity is an eternal pursuit for chemists working in chemical synthesis. As part of this endeavor, our group has been exploring the possibility of constructing different natural product skeletons from the same simple starting materials by using different catalytic systems. In our previous work, an isoflavanone skeleton was obtained from the annulation of a salicylaldehyde and an alkyne when a gold catalyst was employed. In this paper, it is shown that a coumarin skeleton can be efficiently obtained through an annulation reaction with the same starting materials, that is, terminal alkynes and salicylaldehydes, by simply switching to a rhodium catalyst. A plausible reaction mechanism is proposed for this new annulation based on isotopic substitution experiments.

Brønsted acid catalyzed annulations of ketene dithioacetals: synthesis of 3-aryl coumarins and indenes

“…This approach is complementary to the transition-metal-catalyzed methods in the literature 19 and avoids harsh conditions 20 or the use of stoichiometric amounts of oxidants. 21 Our initial studies with phenylene dithiolane 1a and salicylaldehyde (2a) are outlined in Table 1. Employing 20 mol% PTSA in chloroform solvent at 50 °C provided the product in 73% yield (in 24 h).…”

“…This approach is complementary to the transition-metal-catalyzed methods in the literature 19 and avoids harsh conditions 20 or the use of stoichiometric amounts of oxidants. 21…”