Add time:07/24/2019         Source:sciencedirect.com

Contrary to usual observation, both venenatine (cas 1055-75-0) (1a) and alstovenine (1b) give initially only a single oxindote derivative (3) instead of a pair. This oxindole displays unusual resistance to isomerisation in both acidic and basic media and is unique in its total insensitivity to pyridine. The unprecedented instability of its isomer, formed only in acidic media under very stringent conditions, foiled all attempts at isolating it in the pure state. However, by conformational analysis and from a comparison of the chemical shifts of the aromatic OMe of the pure more stable isomer and of the isomers in mixture, recorded in CDCl3 and CF3CO2H, the more stable isomer was assigned the allo B (5b) and the other allo A (5a) stereochemistry. The uncommon stereochemistry basicity relationship has been rationalised.

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