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10.01 Bicyclic 5-5 Systems: Two Heteroatoms 1:1 ˇ ´kova´ A. Krutosı University of St. Cyril and Methodius, Trnava, Slovakia T. Gracza Slovak University of Technology, Bratislava, Slovakia ª 2008 Elsevier Ltd. All rights reserved. 10.01.1 Introduction 2 10.01.2 Theoretical Methods 5 10.01.3 Experimental Structural Methods 5 10.01.3.1 X-Ray Diffraction Studies 5 10.01.3.2 Proton NMR Spectroscopy 6 10.01.3.3 Carbon-13 NMR Spectroscopy 6 10.01.3.4 Nitrogen-15 NMR Spectroscopy 7 10.01.3.5 Polarography 8 10.01.3.6 UV Spectroscopy 9 10.01.3.7 Photoelectron Spectrometry 9 10.01.4 10.01.4.1 10.01.5 10.01.5.1 Thermodynamic Aspects 11 1,4-Diheteropentalene Systems (1) 11 Electrophilic attack at carbon Electrophilic attack at nitrogen Electrophilic attack at sulfur Ring-opening reactions Cycloaddition reactions 11 13 14 15 15 1,5-Diheteropentalene Systems (2) 16 10.01.5.2.1 10.01.5.2.2 10.01.5.3 16 19 20 Electrophilic attack at carbon Enzyme catalyzed attack at carbon Electrophilic attack at nitrogen Electrophilic attack at sulfur Addition and cycloaddition reactions 20 20 20 21 21 2,5-Diheteropentalene Systems (4) 10.01.5.4.1 10.01.5.4.2 10.01.6 Addition and cycloaddition reactions Radical reactions 1,6-Diheteropentalene Systems (3) 10.01.5.3.1 10.01.5.3.2 10.01.5.3.3 10.01.5.3.4 10.01.5.3.5 10.01.5.4 9 Reactivity of Fully Conjugated Rings 10.01.5.1.1 10.01.5.1.2 10.01.5.1.3 10.01.5.1.4 10.01.5.1.5 10.01.5.2 9 Aromaticity 21 Cycloaddition reactions Other Reactions 22 23 Reactivity of Nonconjugated Rings 23 10.01.6.1 1,4-Diheteropentalene Systems 23 10.01.6.2 1,5-Diheteropentalene Systems 24 10.01.6.3 1,6-Diheteropentalene Systems 25 10.01.6.4 2,5-Diheteropentalene Systems 25 10.01.7 10.01.7.1 Reactivity of Substituents Attached to Ring Carbon Atoms 1,4-Diheteropentalene Systems 25 25 1 2 Bicyclic 5-5 Systems: Two Heteroatoms 1:1 10.01.7.1.1 10.01.7.1.2 10.01.7.2 1,6-Diheteropentalene Systems 10.01.7.2.1 10.01.7.2.2 10.01.7.2.3 10.01.8 10.01.8.1 10.01.9 10.01.9.1 1,4-Diheteropentalene Systems Ring Syntheses Syntheses of 1,4-Diheteropentalene Systems (1) Synthesis by construction of the second heterocyclic ring onto an existing heterocycle Syntheses of 1,6-Diheteropentalene Systems (3) 10.01.9.3.1 10.01.9.4 Synthesis by construction of the second heterocyclic ring onto an existing heterocycle Syntheses of 1,5-Diheteropentalene Systems (2) 10.01.9.2.1 10.01.9.3 Carboxylic acids and derivatives Aldehydes Halogen substituents Reactivity of Substituents Attached to Ring Heteroatoms 10.01.9.1.1 10.01.9.2 Carboxylic acids and derivatives Aldehydes Synthesis by construction of the second heterocyclic ring onto an existing heterocycle Syntheses of 2,5-Diheteropentalene Systems (4) 10.01.9.4.1 10.01.9.4.2 Synthesis by construction of the second heterocyclic ring onto an existing heterocycle Other systems 25 30 32 32 33 36 37 37 39 39 39 42 42 50 50 52 52 55 10.01.10 Ring Syntheses by Transformati