Reversal of absolute stereochemistry of the pyrrolo[2,1-b]quinazoline alkaloids vasicine, vasicinone, vasicinol and vasicinolone

• Published in: Tetrahedron: asymmetry Vol. 7; no. 1; pp. 25 - 28
• Main Authors: Joshi, Balawant S., Newton, M.Gary, Lee, Doo Won, Barber, Angela D., Pelletier, S.William
• Format: Journal Article
• Language: English
• Published: OXFORD Elsevier Ltd 1996; Elsevier
• Subjects: Chemistry; Chemistry, Inorganic & Nuclear; Chemistry, Organic; Chemistry, Physical; Physical Sciences; Science & Technology; STRUCTURE REFINEMENT; CHEMICAL-SHIFTS; ADHATODA-VASICA
• ISSN: 0957-4166; 1362-511X
• DOI: 10.1016/0957-4166(95)00412-2

The previously assigned 3 R configuration of (−)-vasicinone has been reversed and this pyrrolo[2,1-b]quinazoline-9-one has been shown to have the 3 S-configuration ( 3) on the basis of an X-ray diffraction study of (+)-vasicinone hydrobromide. Likewise, the 3 R stereochemistry assigned earlier to (−)-vasicine (peganine) ( 1) on the basis of an X-ray analysis of its hydrochloride has also been reversed by reinvestigation of the X-ray diffraction analysis of the hydrobromide. The absolute stereochemistry of the alkaloids (+)-vasicinol ( 2) and vasicinolone ( 5) which have been inter-related, should also have the 3 S-configuration. A study of the 1H nmr spectroscopy of (−)-vasicine by the use of Mosher's method using MTPA [α-methoxy-α-(trifluoromethyl)phenylacetic acid] esters indicated an exception to this model for establishing the absolute configuration. The absolute stereochemistry of (−)-vasicine ( 1) and (−)-vasicinone ( 3) have been shown to have the 3 S configuration on the basis of x-ray analysis of the alkaloid hydrobromides. Vasicinol and vasicinolone which have been interrelated should also have the 3 S configuration.