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CHAKRABARTY ET AL.
irradiation, which, however, required considerably longer
periods.
The cleavage of amides to amines, a common step in many organic
syntheses, is usually carried out by treatment with mineral acid or caustic
alkali, often requiring prolonged periods of heating.1–4 Several other
reagents have also been reported for special cases, e.g. heating with hydra-
zine hydrate in case of N-acetylglucosaminides,5 electrolysis of benzamides
in presence of tetramethylammonium chloride,6 Meerwein’s reagent for
acetamidodeoxysugars,7 DIBAL for the reductive debenzoylation during a
synthesis of quinine and quinidine,8 polyhydrogen fluoride for N-debenzoy-
lation of cytosines and adenines,9 fusion with phenol for N-debenzoylation
during peptide synthesis,10 etc. But harsh reaction conditions (e.g. sealed
tube, 100ꢀ), long reaction time (e.g. 20 h), wide ranging yields (8–99%) and
lack of generality are the principal limitations of these methods.
Consequently and even otherwise, there is always a need for the develop-
ment of new, milder reagents which would not require the use of particularly
any acid or alkali.
While trying to develop a new synthesis of pyrrolocarbazoles, a
recently reviewed class of bioactive heterocycles,11 employing the Barton-
Zard pyrrole synthesis,12 N-acetyl-3-nitrocarbazole (1a) was treated with
ethyl isocyanoacetate (EIA) in presence of DBU in tetrahydrofuran for
20 h. The sole product, isolated in quantitative yield, was identified as
3-nitrocarbazole. It was a clear case of N-deacetylation, in which EIA
was not likely to have played any role. The reaction was, therefore,
repeated without EIA, when 3-nitrocarbazole was again isolated as the
only product in quantitative yield in 7h. These results suggested that
DBU was responsible for the observed N-deacetylation, which prompted
us to check the generality of this property of DBU on a few carbazoles
and indoles.
Since the reaction of 1a in tetrahydrofuran at room temperature still
required rather long time (7h) for completion, it was repeated using aceto-
nitrile and methanol separately at room temperature and under reflux. The
cleavages were found to be equally slow at room temperature in all three
solvents and practically independent of the stoichiometry of DBU used. But
the reactions were quite fast in refluxing tetrahydrofuran and acetonitrile
and much faster in refluxing methanol, being directly related to the concen-
tration of DBU. N-Acetylcarbazole (1b), for example, required 3, 2.5 and
2 h, respectively for complete N-deacetylation using 1, 2 and 5 equivalents of
DBU. In all subsequent experiments, two equivalents of DBU were, there-
fore, used as a compromise amount.
Chakrabarty, Manas
