1. Synthesis of Heterocyclic Rings 7 (Cycloaddition Reactions)

2. A cycloaddition is a pericyclic chemical reaction, in which "two or more unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity." [1] pericyclicchemical reaction [1] a pericyclic reaction is a type of organic reaction wherein the transition state of the molecule has a cyclic geometry,organic reactiontransition state

3. µ-electrons Reaction (Geometry) Cycloaddition 2+2 Chelotropic 1 + 2 = 3 2+2 2+ 2 = 4 2+4 Chelotropic 2 + 4 = 5 2+4 1,3-dipolar cycloaddition 2+ 3 = 5 2+4 Diels-Alder 2+4 = 6

4. 1,3-Dipolar Cycloaddition Reactions

5. Resonance Structures of 1,3-Dipoles Each molecule has at least one resonance structure which indicates separation of opposite charges in 1,3- relationship.

6. Mechanism of Cycloaddition: 1,3-Dipolar cycloaddition reactions were found to be stereoselective.

7. Most of them are regioselective.

8. Orbital picture of 1,3-dipoles and dipolarophiles

9. Concerted mechanism: one step mechanism in which two bonds are formed at the same time (Huisgen)

11. Biradical mechanism (Stepwise mechanism) (Firestone)

12. Regiochemistry: orbitals of the same size interact

13. Generation of 1,3-Dipoles Nitrile oxides

14. Nitrile Sulfides

15. Nitrile Imides (Nitrilimines)