Ajmaline group, synthesis

Aldehyde 198 served as a key intermediate in a synthesis of the alkaloid ajmaline. The. Mannich aminomethylation transform triggers disconnection of two bonds in 198 to form dialdehyde 199, which by connective transform application can be converted to cyclopentene 200.58,59 The reduction in functional group reactivity and in structural complexity are both apparent by comparison of 198 and 200. 

Ajmaline (14) was chosen as a starting material because its total synthesis and absolute configuration were already established (14-16), and furthermore, the equilibrium isomer 17 from ajmaline chemically corresponds to the hypothetical intermediate 13. Ajmaline (14) was first converted into the hydrazone derivative by treatment with A,)V-dimethylhydrazine and a catalytic amount of sulfuric acid. After protection of the resulting secondary amine with methyl carbamate, the hydrazone was hydrolyzed with cop-per(II) chloride in aq. tetrahydrofuran (pH 7) to afford the aldehyde 19 in 75% yield. The hydroxy group in compound 19 was protected with the methoxyethoxymethyl (MEM) ether, and then bromine was selectively... 

This intermediate was used to prepare several indole alkaloids of the ajmaline-sarpagine group [333]. The method has also been used to synthesize examples of the fumitremorgin group [334]. Lewis has summarized application of these and other methods to alkaloid synthesis [335]. 

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