Total synthesis of (−)-eudistomins with an oxathiazepine ring. Part 1. Formation of the oxathiazepine ring system

Formation of the oxathiazepine ring in eudistomins 1 was investigated. Thiazolidinyl-β-carboline 5 was successfully transformed into thiaindoloquinolizidine 7, but attempted oxidative transformation of 7 to 1 was not successful. The oxidative cyclization of 1-substituted-2 hydroxy-β-carboline 24 with NCS or the acid-catalyzed cyclization of the corresponding S-oxide 26 with TsOH gave oxathiazepine 25, which was readily converted to (+)-debromoeudistomin L (+)-1f. (−)-Debromoeudistomin L (−)-1f was prepared from N-hydroxytryptamine 11 and the D-cysteinal 30.