Abstract
A series of diazo ketoimides prepared from (1H-indol-3-yl)acetyl chloride and alkyl 2-diazo-3-(3-substituted-2-oxopiperidin-3-yl)-3-oxopropanoates were treated with rhodium(II) acetate. Attack of the imido carbonyl oxygen at the resultant rhodium carbenoid center produced a transient push-pull carbonyl ylide dipole which underwent an intramolecular dipolar cycloaddition across the indole π-bond. In most cases, the resulting cycloadduct is the consequence of endo-cycloaddition with respect to the dipole and this is fully in accord with the lowest energy transition state. Interestingly, when a tert-butyl acetate substituent is located at the ring juncture of the starting diazo ketoimide, the exo-cycloadduct was the exclusive product obtained. In this case, the bulky tert-butyl ester functionality blocks the endo-approach thereby resulting in the cycloaddition taking place from the less congested exo-face.
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