Synthesis and structure verification of an analogue of kuanoniamine A

Synthesis of 9-phenyl-7H-benzothiazolo[4,5,6-ij][2,7]naphthyridin-7-one 11, an analogue of kuanoniamine A 8, is described. The synthesis involves a he tero Diels-Alder reaction of crotonaldehyde dimethylhydrazone with 4,7-diox o-2-phenylbenzothiazole 18a or with 6-bromo-4,7-dioxo-2-phenylbenzothiazole 18b followed by annelation of the appropriate adduct. Reaction with 18a pr oduced two sets of regioisomers-the thiazoloquinolinediones 19a,b, and the dimethylamino dioxobenzothiazoles 23a,b. The structure of 23b was determine d by single-crystal X-ray structure analysis. Verification of the other str uctures, and methods used to improve the Diels-Alder reaction are described .