Total synthesis of the strychnos alkaloid (+)-minfiensine: tandem enantioselective intramolecular Heck-iminium ion cyclization

A 1,2,3,4-tetrahydro-9a,4a-(iminoethano)-9H-carbazole (4) is a central structural feature of the Strychnos alkaloid minfiensine (1) and akuammiline alkaloids such as vincorine (5) and echitamine (6). A cascade catalytic asymmetric Heck-iminium cyclization was developed that rapidly provides 3,4-dihydro-9a,4a-(iminoethano)-9H-carbazoles in high enantiomeric purity. Two sequences were developed for advancing 3,4-dihydro-9a,4a-(iminoethano)-9H-carbazole 27 to (+)-minfiensine. In our first-generation approach, a reductive Heck cyclization was employed to form the fifth ring of (+)-minfiensine. In a second more concise total synthesis, an intramolecular palladium-catalyzed ketone enolate vinyl iodide coupling was employed to construct the final ring of (+)-minfiensine. This second-generation total synthesis of enantiopure (+)-minfiensine was accomplished in 6.5% overall yield and 15 steps from 1,2-cyclohexanedione and anisidine 13. A distinctive feature of this sequence is the use of palladium-catalyzed reactions to form all carbon-carbon bonds in the transformation of these simple precursors to (+)-minfiensine.

Fast and Regioselective Heck Couplings with N-Acyl-N-vinylamine Derivatives

Highly regioselective Heck couplings of aryl triflates with N-acyl-N-vinylamines lacking an N-alkyl substituent were achieved with reaction times of approximately 1 h in yields ranging from 62 to 98% using 1.5 mol % of Pd(2)(dba)(3), 3 mol % of DPPF, and diethylisopropylamine in dioxane. The efficiency of these cross-couplings were studied with several N-vinylamides and an example each of an N-vinylcarbamate and an N-vinylurea. The Heck coupling products easily underwent acidic hydrolysis to the corresponding aryl methyl ketone or in situ hydrogenation in the presence of (Ph(3)P)(3)RhCl under a hydrogen atmosphere to provide the N-acyl derivatives of pharmaceutically relevant benzylic amines. The coupling of a vinyl triflate and more interestingly a vinyl tosylate to N-vinyl acetamide was also studied affording a 2-acylamino-1,3-butadiene with the same high regioselectivity in preference for the alpha-isomer. This result suggests that Heck couplings of electron-rich alkenes with vinyl tosylates also follow a cationic pathway.