A new strategy toward indole alkaloids involving an intramolecular cycloaddition/rearrangement cascade.
The intramolecular Diels-Alder reaction between an amidofuran moiety tethered onto an indole component was examined as a strategy for the synthesis of Aspidosperma alkaloids. Furanyl carbamate 23 was acylated using the mixed anhydride 26 to provide amidofuran 22 in 68% yield. Further N-acylation of this indole furnished 27 in 88% yield. Cyclization precursors were prepared by removing the carbamate moiety followed by N-alkylation with the appropriate alkyl halides. Large substituent groups on the amido nitrogen atom causes the reactive s-trans conformation of the amidofuran to be more highly populated, thereby facilitating the Diels-Alder cycloaddition. The reaction requires the presence of an electron-withdrawing substituent on the indole nitrogen in order for the cycloaddition to proceed. Treatment of N-allyl-bromoenamide 48 with n-Bu(3)SnH/AIBN preferentially led to the 6-endo trig cyclization product 50, with the best yield (91%) being obtained under high dilution conditions. The initially generated cyclohexenyl radical derived from 48 produces the pentacyclic heterocycle 50 by either a direct 6-endo trig cyclization or, alternatively, by a vinyl radical rearrangement pathway.
Padwa
Albert
A
Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA. chemap@emory.edu
Brodney
Michael A
MA
Lynch
Stephen M
SM
Rashatasakhon
Paitoon
P
Wang
Qiu
Q
Zhang
Hongjun
H
D000470:Alkaloids / Q000138:chemical synthesis / Q000737:chemistry*; D030009:Aspidosperma / Q000737:chemistry; D003500:Cyclization; D007211:Indoles / Q000737:chemistry*; D015394:Molecular Structure
D000470:Alkaloids; D007211:Indoles