温和なベンゾイル化試薬
トリフルオロメタンスルホン酸ベンゾイルは強力なベンゾイル化試薬であり，立体的に込み入ったヒドロキシ基とも反応することができます。この試薬は温和な条件で反応を行うことができ，使用する試薬の数も少ないことから，簡便なヒドロキシ基の保護基導入試薬として利用されています。
[1] “Benzoyl trifluoromethanesulfonate. A mild reagent for the benzoylation of sterically hindered hydroxyls”L. Brown, M. Koreeda, J. Org. Chem. 1984, 49, 3875. DOI: 10.1021/jo00195a001
[2] “Sequential Catalytic Asymmetric Heck−Iminium Ion Cyclization: Enantioselective Total Synthesis of the Strychnos Alkaloid Minfiensine”A. B. Dounay, L. E. Overman, A. D. Wrobleski, J. Am. Chem. Soc. 2005, 127, 10186. DOI: 10.1021/ja0533895
A catalytic asymmetric method for the chemical synthesis of alkaloids containing the 1,2,3,4-tetrahydro-9a,4a-(iminoethano)-9H-carbazole (1) moiety is reported and verified by the enantioselective total synthesis of (+)-minfiensine (4). The central step in this total synthesis is the sequential catalytic asymmetric Heck−N-acyliminium ion cyclization of dienyl carbamate triflate 10, prepared in six steps from 1,2-cyclohexanedione, to give enantiopure 3,4-dihydro-9a,4a-(iminoethano)-9H-carbazole (12) in 75% yield. Iminoethano-9H-carbazole 12 is transformed in six steps to dienyl iodide 17, which undergoes diastereoselective intramolecular Heck cyclization to form pentacyclic intermediate 18. In eight additional steps, this latter intermediate is transformed to (+)-minfiensine (4).
