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Efficient Construction of Pyrano[3,2‐ a ]carbazoles: Application to a Biomimetic Total Synthesis of Cyclized Monoterpenoid Pyrano[3,2‐ a ]carbazole Alkaloids
Author(s) -
Hesse Ronny,
Gruner Konstanze K.,
Kataeva Olga,
Schmidt Arndt W.,
Knölker HansJoachim
Publication year - 2013
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201301792
Subject(s) - carbazole , chemistry , pyran , citral , lewis acids and bases , catalysis , organic chemistry , stereochemistry , combinatorial chemistry , chromatography , essential oil
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 ).