Hydrofurans in the synthesis of heterocycles — A review

Abstract

The use of hydrofurans in the synthesis of heterocyclic compounds is reviewed.Hydrofuran derivatives have been widely used as reagents. This results from theft great reactivity in combination with the possibility of obtaining them from cheap sources --furfural, furan, and tetrahydrofurfuryl alcohol [1]. Recently satisfactory methods have been developed based on hydrofurans for the synthesis of many 5-and 6-membered nitrogencontaining heterocycles, furans, thiophenes, tropinones, monosaccharides, and a number of condensed systems.The extensive material on the use of hydrofurans in heterocyclic synthesis has not yet been reviewed. We have decided to give a survey of methods and not a complete bibliography on the use of these compounds, which could not be contained within a single paper. For example the Clauson-Kass method for the synthesis of pyrroles alone has been cited as a routine procedure in tens of publications and patents. The literature on 2,5-dialkoxyhydrofurans is cited in a volume in which N. l~l'ming has presented an additional review of these compounds [2]. DIHYDROFURANSMost reactions of dihydrofurans which lead to the formation of other heterocycles can be divided into two large groups: a) various types of cycloaddition to the double bond with the formation of stable adducts which then undergo further conversions; b) nucleophilic scission of the C-O bond with recyclization (intramolecular recyclization is possible if a substituent on the furan ring contains a nucleophilic group in the chain). The unsubstituted adduct Ha (R = R 1 = R 2 = H) is used in the synthesis of isoindole [5] and porphyrins '[6]. The authors of the latter paper boiled compound Ha and its oxide (ILl) with benzyl carbamate in glacial acetic acid for many hours to give pyrroles which were converted further into the corresponding porphyrins (IV and V).