The Meloscine Group

Another alkaloid present, termed alkaloid-4, has a carbomethoxy group, removed by sodium methoxide treatment. Acid hydrolysis and decarboxylation gives meloscine (I) on the basis of this result and NMR-data alkaloid-4 may be ascribed the structure CXXXIX. 

Meloscandine, which accompanies the above alkaloids, may also be converted to meloscine (I) either by heating or base-catalyzed solvolysis. The empirical formula, C20H20N2O2, and an extra carbonyl absorption at 1748 cm i suggest a formyl substituent at C-3 (structure CXL) but the absence of a formyl proton in the NMR-spectrum casts doubt on the monomeric structure. 

Comparison of the ORD-curves of tetrahydromeloscine (CXXXIV) and tetrahydroepimeloscine (3-epi-CXXXIV) with those of aspido-spermidine derivatives (Section II,A 59) leads to the probable absolute configurations shown. Biogenetically this group of alkaloids may be derived from tabersonine-like bases by way of an intermediate such as CXLI (1, 77). 

The absolute configurations presented in Volume VIII must now be reversed. Molecular rotation measurements already indicated that the correct absolute configurations of aspidofractinine (CXIII) and pleio- 

These three alkaloids occur in two species of Pleiocarpa and Hunteria eburnea and were originally attributed the structures CLIII-CLV (32). Although NMR-evidence for the hydroxymethylene group attached to C-3 was not obtained the presence of this group was suspected from the production of formaldehyde on heating pleiocarpoline. Later evidence, 

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The meloscine group of alkaloids has been added to the list of those whose n.m.r. spectra have been thoroughly analysed by a French-American collabora-tion as example, the assignments for meloscine (190) are quoted. Details have been given,in a Russian paper, of the A-ray determination of the structureof pseudokopsinine. 

The diamagnetic anisotropic effect of the carbomethoxyl group indicated it to be a and the B/C ring junction trans. Because (+)-scandine gave rise to (-t-)-meloscine (238), it was assigned the structure and absolute stereochemistry shown in 241 (110). 

The formation of the tricyclic 9a-arylhydrolilolidine ring system is exemplified in the conversion of (154) to (155).114 Wolff ring contraction of the ketonic ring of (155) and further functional group modifications provided the Melodinus alkaloid (i)-meloscine. (Scheme 55). 

In a previous discussion of the genus Melodinus (Volume XI, p. 242), the isolation and structure elucidation of meloscine (238) and 16-epimeloscine (239) were discussed. The Hoffmann-La Roche group has subsequently published a full presentation of the structure elucidation of these novel alkaloids (110). 

The interval since the last review was written has seen the isolation of a number of new alkaloids but of few fundamentally new skeletal types. Meloscine (I) (1) and related alkaloids represent a completely novel group, whereas aspidodasycarpine (II) (2) is a new variation of the akuammigine type. 

Mukai and coworkers reported the concise total synthesis of Melodinus alkaloid ( )-meloscine (40) [Route (a), Fig. 13], in which the final key transformation involved an alkene RCM of the triene precursor (42) in the presence of Hoveyda-Grubbs II catalyst [55], The RCM between the A(-allyl group and the top-oriented vinyl moiety exclusively occurred to generate the desired diastereomer (40) in almost quantitative yield. The extremely high diastereoselectivity could be rationalized on the basis of ring strain to favor one conformer. Shortly after this work, Curran et al. disclosed another approach to the total syntheses of ( )-epimeloscine (41) and ( )-meloscine (40) applying the same RCM protocol for the final ring formation [Route (b), Fig. 13] [56],... 

Total Synthesis of Meloscine Meloscine 189, a member of the Melodinus alkaloids group, is noted for containing a six-membered quinolone ring within a mono-terpenoid Aspidosperma carbon skeleton. 

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