Aminomethylation/hydrogenolysis as an alternative to direct methylation of metalated isoquinolines – a novel total synthesis of the alkaloid 7-hydroxy-6-methoxy-1-methylisoquinoline

Abstract

Highly-substituted isoquinolines are important scaffolds in syntheses of natural products and in drug development and hence, effective synthetic approaches are required. Here we present a novel method for the introduction of a methyl group at C1 of isoquinolines. This is exemplified by a new total synthesis of the alkaloid 7-hydroxy-6-methoxy-1-methylisoquinoline. Direct metalation of 7-benzyloxy-6-methoxyisoquinoline with Knochel-Hauser base, followed by cuprate-mediated methylation gives the target alkaloid directly, but separation from the educt is cumbersome. Quenching the metalated intermediate with Eschenmoser's reagent gives an easy to clean tertiary benzylamine, which, after quaternization with iodomethane, is easily converted into the desired 1-methylisoquinoline by hydrogenolysis of both the benzylamine and benzyl ether groups. 130

Functionalization of 4-bromobenzo[c][2,7]naphthyridine via regioselective direct ring metalation. A novel approach to analogues of pyridoacridine alkaloids

“…Inspired by reports of Knochel’s [19–20] and our group [21–23] on direct metalation of substituted isoquinolines at C-1 and in continuation of our recent work on the synthesis of aromatic oxoaporphine, oxoisoaporphine and pyridoacridine alkaloids [17–18 21–22] using direct ring metalations of heterocycles with the hindered amide base TMPMgCl∙LiCl as crucial step, the Knochel–Hauser base was again the metalation reagent of our choice.…”

The ethoxycarbonyl group as both activating and protective group in N-acyl-Pictet–Spengler reactions using methoxystyrenes. A short approach to racemic 1-benzyltetrahydroisoquinoline alkaloids

“…In continuation of our recent work on the chemistry and pharmacology of benzylisoquinolines and related compounds [6][7][8][9][10][11][12][13][14] we investigated truncated analogues of the bisbenzylisoquino-line alkaloid tetrandrine as blockers of the calcium channel twopore channel 2 (TPC2), and identified 1-benzyl-1,2,3,4-tetrahydroisoquinolines bearing phenoxy and benzyloxy substituents (SG-005, SG-094; for structures of bioactive compounds mentioned in this text, see Figure S1 in Supporting Information File 1) on both aromatic rings as potent blockers with promising antitumor activity [15]. In this work we took advantage of the hitherto less explored N-acyl-Pictet-Spengler reaction and related chemistry based on the seminal work of Speckamp [16], where an N-acyl residue at the arylethylamine building block leads to enhanced cyclization rates due to the acid-mediated formation of highly electrophilic N-acyliminium intermediates [17].…”