title: Efficient construction of pyrano[3,2-a]carbazoles: application to a biomimetic total synthesis of cyclized monoterpenoid pyrano[3,2-a]carbazole alkaloids.
abstract: We have developed a highly efficient route to 2-hydroxy-3-methylcarbazole (1) via a palladium-catalyzed construction of the carbazole skeleton. Using 1 as relay compound, different methods for annulations of pyran rings by reaction with terpenoid building blocks have been tested. The Lewis acid promoted reaction of 1 with prenal (21) opened up an efficient route to girinimbine (3) and the corresponding reaction with citral (25) afforded mahanimbine (5). Oxidation of compounds 3 and 5 provided murrayacine (4) and murrayacinine (6). Following the biogenetic proposal, mahanimbine (5) has been exploited for efficient biomimetic syntheses of the cyclized monoterpenoid pyrano[3,2-a]carbazole alkaloids cyclomahanimbine (7), mahanimbidine (8) and bicyclomahanimbine (9). The interconversions of 5, 7, 8 and 9 are described and mechanistic implications are discussed. Structural assignments are unambiguously verified by X-ray crystal structure determinations. Moreover, cyclomahanimbine (7) was transformed into murrayazolinine (10) and exozoline (11).
authors.affiliation: Department of Chemistry, Technische Universität Dresden, Bergstrasse 66, 01069 Dresden (Germany), Fax: (+49) 351-463-37030.
authors.forename: Ronny
authors.initials: R
authors.lastname: Hesse
authors.forename: Konstanze K
authors.initials: KK
authors.lastname: Gruner
authors.forename: Olga
authors.initials: O
authors.lastname: Kataeva
authors.forename: Arndt W
authors.initials: AW
authors.lastname: Schmidt
authors.forename: Hans-Joachim
authors.initials: HJ
authors.lastname: Knölker
mesh_terms: D032701:Biomimetics; D002227:Carbazoles / Q000138:chemical synthesis* / Q000737:chemistry*; D018360:Crystallography, X-Ray; D003500:Cyclization; D058116:Lewis Acids / Q000737:chemistry*; D015394:Molecular Structure; D039821:Monoterpenes / Q000138:chemical synthesis* / Q000737:chemistry; D010084:Oxidation-Reduction; D010165:Palladium / Q000737:chemistry; D011714:Pyrans / Q000138:chemical synthesis* / Q000737:chemistry
chemical_list: D002227:Carbazoles; D058116:Lewis Acids; D039821:Monoterpenes; D011714:Pyrans; D010165:Palladium
keywords: CH bond activation; alkaloids; crystal-structure determination; cyclization; natural products; palladium