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hydrastine
Hydrastine is an alkaloid which was discovered in 1851 by Alfred P. Durand.〔.〕 Hydrolysis of hydrastine yields hydrastinine, which was patented by Bayer as a haemostatic drug during the 1910s. It is present in ''Hydrastis canadensis'' (thus the name) and other plants of the Ranunculaceae family.
==Total synthesis==
The first attempt for the total synthesis of hydrastine was reported by Sir Robert Robinson and co-workers〔Hope, E.; Pyman, F. L.; Remfry, F. G. P.; Robinson, R., J. Chem. Soc. 1931, 236.〕 in 1931. Following studies,〔Haworth, R. D.; Pinder, A. R.; Robinson, R., Nature 1950, 165, 529.〕〔Haworth, R. D.; Pinder, A. R., J. Chem. Soc. 1950, 1776.〕 which were mostly troublesome for the synthesis of the key lactonic amide intermediate (structure 4 in Figure). The major breakthrough was achieved in 1981 when J. R. Falck and co-workers〔Falck, J. R.; Manna, S., Tetrahedron Lett. 1981, 22, 619.〕 reported a four-step total synthesis of hydrastine from simple starting materials. The key step in the Flack synthesis was using a Passerini reaction to construct the lactonic amide intermediate 4.
:Falck’s total synthesis of hydrastine, the mechanism of the Passerini reaction for synthesis of the key intermediate is also illustrated
Starting from a simple phenylbromide variant 1, alkylation reaction with lithium methylisocyanide gives the isocyanide intermediate 2. Reacting isocyanide intermediate 2 with opianic acid 3 initiated the intramolecular Passerini reaction to give the key lactonic amide intermediate 4. The tetrahydro-isoquinolin ring was formed by first a ring-closure reaction under dehydration conditions using POCl3 and then a catalyzed hydrogenation using PtO2 as the catalyst. Finally, hydrastine was synthesized by installing the N-methyl group via reductive amination reaction with formaldehyde.
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