Organocatalytic Construction of Polycyclic Compounds

Significance

The catalytic asymmetric aldol lactonization of keto acids accessing bi- and tricyclic β-lactones is reported. The catalytic species, ­homobenzotetramisole (HMBT) 1, in combination with Hünig’s base, tosyl chloride as coupling reagent, and LiCl as a Lewis acid additive, provided the corresponding products in high yields and enantiopurities. Interestingly, the addition of LiCl proved to be crucial to obtain high yields, whereas the enantioselectivities showed somewhat decreased values under these conditions. This observation is rationalized by possible Li chelations or completely racemic pathways via acid chloride intermediates.