Syntheses, constitutions and properties of stentorin and isostentorin

The dissociation of hypericin (1) was investigated using atmospheric pressure ionization mass spectrometry of 80% ethanolic solutions under variation of their pH values. Thus, it could be unequivocally established that the species predominant within the pH region of 3±10 and 11±13 are the mono-and dideprotonated compounds 1 À and 1 2À . These results nicely corroborate the hitherto accumulated mostly spectrophotometric evidence for the ®rst deprotonation step occurring at a pK a value in the order of 2 and the second one at about 12. Moreover, this example demonstrates the potential of atmospheric pressure ionization mass spectrometry as a valuable tool to investigate dissociation equilibria.

Section: Resultssupporting

confidence: 66%

Concerning the Dissociation of Hypericin: An Atmospheric Pressure Ionization Mass Spectrometric Study

Ahrer

Falk

Tran

1998

“…Hypericin is a derivative of the octacyclic benzenoid hydrocarbon phenanthro [1,10,9,8-o,p,q, r,a,]perylene. Its tautomer Q(7, 14) possesses a conjugated %-electron system for which 24 Kekule Â formulas can be drawn, e.g.…”

Section: Methodsmentioning

confidence: 99%

On the Tautomers of Hypericin

Gutman

Marković

Solujić

et al. 1998

Hypericin has 16 theoretically conceivable tautomers (10 with and 6 without Kekule Â structural formulas). AM1 calculations show that two factors determine the relative stability of the tautomers. The dominant factor is the mode of conjugation of the %-electrons. A less pronounced, yet signi®cant, factor is the steric repulsion of the carbonyl and/or hydroxy groups in positions 3 and 4. Among tautomers with equal %-electron conjugation, the least stable are those having carbonyl groups in positions 3 and/or 4. The tautomer Q(7, 14), with carbonyl groups in positions 7 and 14, is the far most stable one; the next two tautomers ± Q(1, 7) and Q(7, 13) ± have by some 40 kJ/mol higher energies, followed by Q (3, 7), Q(8, 13), and Q(1, 6) with energies 60, 70 and 70 kJ/mol above the energy of Q(7, 14), etc. The energy values of the hypothetical tautomers without Kekule Â structures lie 150±200 kJ/mol above Q(7, 14); therefore, these are of no practical relevance.

“…Spectrophotometric titrations were carried out as described in Ref. [11] using 80% aqueous ethanol solutions and tetramethylammonium hydroxide or tri¯uoroacetic acid as titrants for pK a values above 0, and a series of aqueous H 2 SO 4 solutions with H 0 values ranging from 0 to À8 for pK a values below 0. For uorescence spectroscopy, 95% ethanol of fu Èr die Fluoreszenzspektroskopie grade (Merck), otherwise p.a.…”

Section: Methodsmentioning

confidence: 99%

“…This behavior is due to the destabilization of the formed phenolate ion by interaction with the closely spaced lone pair of the ether oxygen atom of the benzyloxy group. The enhanced acidity of this bay-hydroxyl group as compared to the peri-hydroxyl group is due to the delocalization inherent to its vinylogous carboxylic acid structure [4,11].…”

Section: Ionizationmentioning

confidence: 99%

“…Upon splitting off the last protecting group to formally yield hypericin (1) itself, stabilization of the bay-mono-deprotonated species was, in addition to the vinylogous carboxylic acid delocalization, dramatically enhanced by hydrogen bonding between the negative charge and the adjacent hydroxyl group [4,5,11]. The peri-deprotonation step was observed to be more or less unin¯uenced by the last formal deprotection step.…”

Section: Ionizationmentioning

confidence: 99%

See 1 more Smart Citation

The Dissociation and Tautomerization Equilibria of Hypericin: Alkyl Protected Hydroxyl Derivatives

Amer

Falk

Tran

1998

3, 3,6,8,and 1,6,8, were synthesized by alkylation and dealkylation procedures starting from hypericin. The pK a value correlation of these derivatives allowed the unequivocal assignment of the protonation and deprotonation pK a values of hypericin. Thus, for hypericin the pK a of about À6 was assigned to the C=O groups, that of about 2 to the deprotonation of one hydroxyl group in the bay-positions 3/4, and that of about 9 was found to be characteristic of the bay-peri-diphenolate ion. None of the changes in the spectra characteristic of changes in the tautomeric equilibria could be found for these derivatives. Thus, it was concluded that the undisturbed peripheral hydroxyl groups of hypericin have to be present to allow for tautomeric changes.Die Dissoziations-und Tautomeriegleichgewichte von Hypericin: Alkylgeschu È tzte Hydroxyderivate Zusammenfassung. Die 3-Benzyl-, 3,4-Dibenzyl-, 3,4-Dibenzyl-1,6,8,13-tetramethyl-und 1,6,8,13-Tetramethylhypericinderivate wurden mit Hilfe von Alkylierungs-und Dealkylierungsverfahren synthetisiert. Die pK a -Werte dieser Derivate erlaubten eine zweifelsfreie Zuordnung der Protonierungs-und Deprotonierungsschritte fu Èr Hypericin. Demzufolge ist der pK a Wert von ca. À6 fu Èr die Protonierung der Carbonylgruppe, jener von ca. 2 fu Èr die Deprotonierung einer bayHydroxylgruppe und jener bei etwa 9 fu Èr das bay-peri-Diphenolation charakteristisch. Keine der charakteristischen spektroskopischen A È nderungen fu Èr einen Wechsel im Tautomeriegleichgewicht von Hypericin konnte fu Èr diese Derivate belegt werden. Dementsprechend wurde geschlossen, daû intakte periphere Hydroxylgruppen im Hypericin die Voraussetzung fu Èr tautomere A È nderungen bilden.