Two stereoisomeric pentacyclic oxindole alkaloids from Uncaria tomentosa: uncarine C and uncarine E

The chloroform solvate of uncarine C (pteropodine), (1'S,3R,4'aS,5'aS,10'aS )-1,2,5',5' a,7',8',10',10' a-octahydro-1'-methyl-2-oxospiro[3H-indole-3,6' (4'aH)-[1H]pyrano[3,4-f]-indolizine]-4'-carboxylic acid methyl ester, C21H 24N2O4. CHCl3,has an absolute configuration with the spiro C atom in the R configuration. Its epimer at the spiro C atom, uncarine E (isopteropodine), (1'S,3S,4'aS,5'aS,10'aS)-1,2,5',5' a,7',8',10',10' a-octahydro-1'-methyl-2 -oxospiro[3H-indole-3,6'(4'aH)-[1H]pyrano[3,4-f]indolizine]-4'-carboxylic a cid methyl ester, C21H24N2O4, has Z/ = 3, with no solvent. Both form interm olecular hydrogen bonds involving only the oxindole, with N . . .O distance s in the range 2.759 (4)-2.894 (5) Angstrom.