Facile Synthesis of 1,1′-Dialkylisoindigos through Deoxygenation Reaction of Isatins and Tris(diethylamino)phosphine

Abstract: A convenient one-step synthesis of symmetrical 1,1’-dialkyl derivatives of isoindigo is described. Key words carbenes, deoxygenation, heterocycles, regioselectivity, phosphorus. In the last years, isoindigo [1H,1’H-bis(indolin-3-yliden)-2,2’-dione] and especially it’s N- and N,N’-derivatives attract great attention not only as dyes, but as substances which possess a specific biological activity (anti-leukemia, antiproliferative, anti-inflammatory etc.). 1-4 Thus, 1-(β-D-glucopyranosyl)-isoindigo was found to be an anti-leukemia drug. 2-4 It has to be noted that the synthesis of 1-substituted isoindigo often is a multistep procedure. The best approach to the synthesis of isoindigo derivatives seems to be the dimerization of ketocarbenes generated from isatin derivatives. It has been assumed 5,6 that mixture of 3-(triphenylphosphoranyliden)-2,3-dihydro-1H-indol-2-one and isoindigo is formed with moderate yield by the reaction of triphenylphosphine with isatin and 1-methylisatin through intermediate carbene formation. However, generation of carbenes from heterocyclic -diketones under the action of trivalent phosphorus deriv-atives has been insufficiently studied.