Ribalinium salt

A full report has appeared of the isolation and characterization of the antimicrobial quaternary derivative pteleatinium salt (32). Dehydration of the chloride with concentrated sulphuric acid and recovery of the product from a basic solution gave a yellow compound, CigHigNOa, formulated as the 8-hydroxy-derivative (34), but by analogy with the compound obtained from ribalinium salt (33) the dehydration product of pteleatinium salt is more likely to be the zwitterion... 

Evidence for an intermediate epoxide was obtained by reacting the salt 31 in dimethylformamide with sodium hydride to give an optically inactive compound shown by NMR spectroscopy to be epoxide 37 racemization was assumed to occur via the zwitterion 36. Treatment of the epoxide with sodium hydroxide furnished the optically inactive diol 34 by a process thought to involve direct attack of hydroxide ion on the epoxide ring An alternative mechanism for this reaction is proposed later (see Section III, F), and other features of the epoxide route are also discussed. The diol 33 from ribalinium salt was also obtained as a mixture of the (-) and (+) enantiomers (ratio, 4.4 1), and a corresponding epoxide pathway may also be involved. 

Ribalinium salt was also isolated by Reisch et al. from Ruta graveolens... 

The structure of ribaline (45) was established by correlation with ribal-inium salt and with ribalinidine (46). Heating (+)-ribalinium salt (30) with pyridine afforded (+)-ribaline this reaction occurred with partial race-mization, and ( )-ribaline was obtained by crystallization. The furoquinolone-pyranoquinolone rearrangement (see Section III,F) was applied to ( )-ribaline which, with acetic anhydride in pyridine at 125°, furnished ribalinidine diacetate, yielding ribalinidine (43) on hydrolysis. Similarly, (+)-methylribaline was transformed into (+)-methylribalinidine acetate and then by hydrolysis into (-)-methylribalinidine (44). 

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