Practical Phase-Transfer Synthesis of N-(Ethoxycarbonyl)Phthalimide and Its Analogues

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• The observed stability of 1 depends on its structure, namely for both nitro-substituted derivatives (1 d and e) their half-life at r.t. is about 2 months, 1 a decomposes in ca. 5% during this time, while dichloro-derivative 1 g does not show any decomposition when stored for 5 months. Moreover, 1 f undergoes spontaneous retro Diels-Alder reaction with the half-life of 5 months at r.t.
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• The successful preparation of 1 a required potassium phthalimide being essentially free from the hydrolytic impurities. When this reagent, but of the other batch (aged) was used, unexpectedly ethyl 2-cyanobenzoate was obtained in 60% yield. Since in the IR spectrum of potassium phthalimide of this batch we detected potassium phthalamate present as an impurity, we believe that the observed cyanoester formation is due to the known simultaneous dehydration and esterification of the amic acid salt.9 For confirmation, water was added to pure potassium phthalimide and the reaction with ethyl chloroformate gave again ethyl 2-cyanobenzoate in 60% yield. On the other hand, the high yield of standard N-alkylation product obtained from potassium phthalimide of this batch suggests that only up to 20% of this compound was hydrolyzed, so the cyanoester formed may also result from the alcoholysis of phthalimide initiated by the impurity.
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