Scalable total synthesis of aflaquinolone I and confirmation of the absolute configuration

Abstract

The first total synthesis of aflaquinolone I (1), which represents the first example of natural occurring 3,4-dioxygenated 4-aryl-quinolin-2(1H)-one alkaloid with 3R, 4R configurations, was achieved in 9 and 4 steps by two parallel strategies, respectively. The key steps of the first approach included a base-catalyzed coupling reaction to forge α-hydroxyanilide and a diastereoselective aldol cyclization to construct the 3,4-dioxygenated-4-aryl-quinolin-2(1H)-one core. The second approach included a regioselective insertion of aryne into unsymmetric imide to build the N-glycolated 2-amino-benzophenone as a key step. In addition, the dehydrated product of aflaquinolone I, viridicatol (2), was successfully synthesized. We also achieved the chiral resolution of (±)-aflaquinolone I (1) and (±)-20, using chiral chromatography, and determined the absolute configuration of the enantiomers by electronic circular dichroism (ECD). The absolute configuration of aflaquinolone I was further confirmed. Interestingly, we found that the ECD spectra of aflaquinolone I and its related analogues were affected by the substituted groups on C-4 phenyl group and C-3 hydroxy group.