Abstract
A novel general method for the synthesis of oxindoles, namely the 'azirine/oxindole ring enlargement via amidinium-intermediates' has been established: the reaction of 2H-azirin-3-amines 1 with BF3.OEt2 in THF solution at -78° leads to 1,3,3-trialkyl-2-amino-3H-indolium tetrafluoroborates 14 in good yields (Scheme 5). Treatment of aqueous solutions of 14 at 0° with aqueous NaOH (30%) and extraction with CH2Cl2 gives oily substances that are either hydrates of 1,3,3-trialkyl-2-dihydroindol-2-imines 15 or the corresponding indolium hydroxides. These products are transformed to the corresponding 1,3,3-trialkyl-2,3-dihydroindol-2-ones 17 in modest yields upon refluxing in H2O/THF. Reaction of 14 with Ac2O in pyridine at ca. 23° for 16 h followed by aqueous workup and chromatographic separation leads to mixtures of N-(1,3,3-trialkyl-2,3-dihydro-indol-2-yliden)acetamides 16 and oxindoles 17 (Scheme 6). Hydrolysis of 16 with aqueous HCl under reflux for 1 - 2 h gives oxindoles 17 in a good yield. Several oxindoles, spiro-oxindoles, and 5-substituted oxindoles were synthesized by means of the reactions mentioned above.
Mekhael, Maged K G