Furans isonitriles

Pyrrolidines and pyrrolidinones can be synthesized on insoluble supports by intramolecular carbometallation of alkenes (Entry 1, Table 15.4) and by Ugi reaction of support-bound isonitriles with amines and 4-oxo acids (Entry 4, Table 15.4). In Entry 5 in Table 15.4, the pyrrolidinone ring is formed by intramolecular Diels-Alder reaction of a furan with a fumaric acid derivative. 2-Pyrrolidinones have also been prepared by treatment of resin-bound O-mesitylenesulfonyl cyclobutanone oximes with... 

As illustrations of the variety of methods available, the three processes below show (i) the addition of isonitrile anions to a,P-unsaturated nitro-compounds, with loss of nitrous acid to bring about aromatisation, (ii) the interaction of thioglycolates with 1,3-dicarbonyl-compounds, for the synthesis of thiophene 2-esters, and (iii) the cycloaddition/cycloreversion preparation of furans from oxazoles. 

Carbonyl protection and deprotection is also important. Yoshihisa Kobayashi of the University of California, San Diego has devised (/. Org. Chem. 2007, 72, 3913) the isonitrile 15. Usually, the product 16 after Ugi condensation would he very difficult to hydrolyze. In the case of 16, mild acid effected cyclization to the acyl indole 17, which was easy to hydrolyze. In a different approach, Francesco Naso of the Universita di Bari has shown (Chem. Commun. 2007, 3756) that acid chlorides such as 18 condensed with 19 to give the furan 20. Such fiirans are easily oxidized, liberating the starting acid. 

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