Short Total Synthesis of the Marine Alkaloid Subarine

The marine benzo[c][2,7]naphthyridine alkaloid subarine is prepared in 4 steps, starting from commercially available 1,10-phenanthroline, via oxidative cleavage to the bipyridine-dicarboxylate, conversion to the mono(2-iodoanilide), and radical cyclization. The alkaloid does not show any significant antimicrobial or cytotoxic activity.


Introduction
Tunicates (ascidians) are sessile marine organisms which have intensely been investigated for the occurrence of low molecular, bioactive natural products [1].Among the numerous types of compounds isolated from tunicates, the pyridoacridone type alkaloids [2,3] have attended greatest interest, due to their significant cytotoxic activities [1,4,5].
Our work was aimed at a more convenient approach to the alkaloid, in order to evaluate the biological activities, and to gain further evidence on structure-activity relationships in this class of natural products.
The final step of the total synthesis of subarine (2) required an intramolecular biaryl coupling starting from 2-haloanilides 6a/b.It seemed most convenient to perform this reaction in a palladium-catalyzed coupling reaction.Numerous examples of intramolecular Pd-catalyzed biaryl syntheses starting from haloarenes have been published in literature, including examples starting from halogenated benzanilides [16,17].In our hands, 2-haloanilides 6a/b completely failed to cyclize to subarine (2) under Pd(II) catalysis in the presence of diverse phosphine ligands (triphenylphosphine, tri(o-tolyl)phosphine, tri(n-butyl)phosphine, 1,3-bis(diphenylphosphino)propane), and bases like silver, potassium and sodium carbonates.Besides unreacted starting material only varying amounts of dehalogenated anilide 6c could be isolated.The structure of 6c was confirmed by synthesis from aniline and diester 5 under Weinreb conditions, as described above.Sci Pharm.2009; 77; 1-7.
Finally, we investigated cyclization of 6a under radical conditions.Following the protocol described by Ganguly et al. [18], reaction of iodoanilide 6a with tributyltin hydride (Bu 3 SnH) and a catalytic amount of azobisisobutyronitrile (AIBN) in refluxing benzene gave alkaloid subarine (2) in poor yield (7%). Once again, dehalogenation to give anilide 6c was the dominating reaction.The 1 H and 13 C NMR data of synthetic subarine are in perfect accordance with those published for the natural product [11].Despite the poor yield in the last step of the synthesis, this new approach wins over by the small number of reaction steps.Sufficient amounts of subarine (2) could be obtained for investigation of the antimicrobial and cytotoxic activities.
In an in vitro screening for antimicrobial activities against a number of gram-positive, gramnegative bacteria and yeasts, and for cytotoxic activity against HL 60 cells in an MTT assay [19], subarine did not show significant activity.Thus, the intact polycyclic ring system of the pyridoacridone alkaloids seems to be indispensable for antimicrobial and cytotoxic activity, seco-analogues like subarine are default of significant activity.

Experimental
NMR spectra were recorded on JEOL Eclipse plus NMR workstations (Jeol GSX 400 or JNMR GX 500 instrument) at 500 MHz or 400 MHz for 1 H MNR and 125 MHz or 100 MHz for 13 C NMR. Spectra were calibrated using residual undeuterated solvent (HCCl 3 , 1 H 7.25 ppm, 13 C 77.00 ppm).Mass Spectra were recorded on a Hewlett Packard 5989 A MSengine.Elemental analyses were performed on a CHN-Analyser Rapid (Heraeus).IR spectra were obtained on a Perkin Elmer Paragon 1000 spectrometer.Melting points were determined on a Büchi 540 apparatus.Flash column chromatography (FCC) was performed using Kieslegel 60, 230-400 mesh (Merck).