Pergamon Tetrahedron Letters 39 (1998) 1961-1964 TETRAHEDRON LETTERS Synthesis of Meso-Substituted Core Modified Expanded Porphyrins; Effect of Acid Catalysts on the Cyclization Aiagar Srinivasan, Sumeet Mahajan, Simi K. Pushpan, Murugaeson Ravikumar and Tavarekere K. Chandrashekar*. Departmentof Chemistry,Indian Instituteof Technology, Kanpur208 016, INDIA. Received 17 November 1997; revised 22 December 1997; accepted 9 January 1998 Abstract: [3+2] condensation between modified tripyrromethane and bithiophene or bifurandiol leadsto the formation of 18n, 22n, 26~ macrocyclesunderLewis acid conditions while only 22n macrocycleis formed underprotic acid conditions. © 1998ElsevierScienceLtd. All rightsreserved. Expanded porphyrins have lately emerged as attractive target molecules because of their potential biomedical application in photodynamic therapy and as anion receptors) '2Synthetic efforts to date have resulted in characterization of diverse systems containing five to ten pyrrolic units covalently joined through methine bridges? Core modification by replacement of one or more pyrrolic units by other heterocycles such as furan and thiophene leads to a new class of expanded porphyrins having unusual optical, electrochemical and magnetic properties due to altered electronic structure. 3 A perusal of literature reveals only a few scattered reports on the synthesis of core modified expanded porphyrins probably due to non-availability of methods to prepare the heterocyclic precursors and their inherent instability relative to the pyrrolic counterpart.4 Herein, we wish to describe an easy and facile method for the synthesis ofmeso substituted core modified expanded porphyrins in which a [3+2] MacDonald type condensation between the appropriate precursors leads to the formation of expected 22n macrocycle in addition to the unexpected 26n and 18x macrocycles. We targeted the synthesis of la and lb by an acid catalysed condensation between modified tripyrrane 2a or 2b and bithiophene diol 3 [scheme]. This strategy was based on the method used for the synthesis ofmono and dithiaporphyrins by Ulman and coworkers) This method makes use of bithiophene diol 3 instead of condensation resulted diformylbipyrrole which is commonly used for [3+2] or [4+2] condensations. Thus, the in the formation of 4a or 4b and 5 in addition to the expected la or lb. The product distribution and the yields were found to be dependent on the nature of the Lewis acid or protic acid used and their concentration. Use of BF:~.OEt 2 or SnCI4 as a Lewis acid resulted in the formation of all the three products and the yields were dependent on the concentration of the Lewis acid used. However, in TFA or p-TsOH, exclusively la or lb is formed in moderately good yields. 7 Table summarizes the conditions of the reaction along with the isolated yields. 0040-4039/98/$19.00 © 1998 Elsevier Science Ltd. All rights reserved. PII: S0040-4039(98)00105-1