Application of Chiral Cationic Catalysts to Several Classical Syntheses of Racemic Natural Products Transforms Them into Highly Enantioselective Pathways

This paper describes the application of chiral oxazaborolidinium cations of type 2 to various
enantioselective Diels−Alder reactions that have served as early key steps for the syntheses of complex
natural products. In the original syntheses these Diels−Alder reactions produced racemic adducts and led
to racemic target molecules unless a separation of enantiomers by classical resolution was employed. By
use of chiral catalysts of type 2, chiral products were obtained directly from Diels−Alder reactions of achiral
components in excellent yield and enantioselectivity and with the mechanistically predicted absolute
configuration. As a result, a number of classical syntheses could be converted to enantioselective versions,
including (1) cortisone/cortisol (Merck/Sarett), (2) dendrobine (Kende), (3) vitamin B12 (Eschenmoser), (4)
myrocin C (Chu-Moyer/Danishefsky), (5) coriolin and hirsutene (Mehta), (6) dendrobatid 251F (Aubé), (7)
silphinene (Ito), and (8) nicandrenone core (Stoltz/Corey).