Acid-Catalyzed Condensations of Heterocyclic Compounds and Aldehydes

4 Acid-Catalyzed Condensations of Heterocyclic Compounds and Aldehydes 

Although not formally classified as calixarenes, compounds closely related in architecture to the calix[4jarenes have been prepared by the condensation of furans, thiophenes, and pyrroles with aldehydes and ketones. Furan undergoes acid-catalyzed condensation with aldehydes and ketones to give 15 76-79), as shown in Fig. 9. 

The increase in yield of 15 a from ca 20% to over 40% in the presence of lithium perchlorate 77) has led to the suggestion of a template effect. However, recent experiments78) indicate that the higher yields correlate not with the metal ion but with the acidity of the reaction medium. In similar fashion, thiophene and pyrrole undergo acid-catalyzed condensations with acetone to yield 16 m and 17 81). Benz-aldehyde also condenses with pyrrole 82but the initially formed compound loses six hydrogens to form the planar tetraphenylporphin 18. 

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