Isolation and Structures of Citropone-A, -B, and -C from Citrus Plants, the First Examples of Naturally Occurring Homoacridone Alkaloids Containing a Seven-Membered Ring System

The first isolation of the unique homoacridone alkaloids, citropone-A (1), -B (2), and -C (3) from the root or stem bark of some cultivated Citrus plants is described. The structure of citropone-A (1) was established unequivocally by spectrometric and single-crystal X-ray analyses, while citropone-B, which has the same skeleton as 1, was assigned structure 2 by comparison of the 1H- and 13C-nuclear magnetic resonance spectra with those of 1. The structure of citropone-C was proposed as formula 3 on the basis of spectrometric analyses using H-C long-range two-dimensional correlation spectroscopy and heteronuclear multiple bond connectivity spectrometries.