Abstract
Several different heterocyclic aldehydes, derived from pyrazole, pyrimidine, pyridine, indole and thiazole, were converted to polyheterocyclic compounds containing four to seven rings. The key steps in the sequence were a Knoevenagel condensation of the aldehyde and a heterocyclic carbonyl compound, such as pyrazolone and isoxazolone, followed by an intramolecular hetero Diels–Alder reaction. Most final products were isolated with high yield and diastereoselecivity. The isoxazolo fused cycloadducts formed interesting spiro-adducts upon heating. The cis nature of the bridging hydrogens of the heterocycles was evidenced by X-ray diffraction analysis. † † Crystallographic data (excluding structure factors) ending pages for the structures in this paper, have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication numbers CCDC 169436, CCDC 169437 and CCDC 169438. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44(0)-1223-336033 or e-mail: deposit@ccdc.cam.ac.uk).
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