Woodward-Hoffmanns Generalized Rules for Pericyclic Reactions

When r = 0, 4r component represents a vacant atomic orbital and if q = 0, (4g+2) component is treated as an atomic orbital occupied by two electrons. A single atomic orbital is represented by Greek letter (D(omega) Vs o-and 

Ji-molecular orbitals. co°s and (0°a, thus represent suprafacial and antarafacial addition to vacant single orbital. Suprafacial and antarafacial addition to doubly filled atomic orbitals are represented by the symbols (O s and ca a, respectively. Generalized rules for electrocyclic reactions, cycloadditions and sigmatropic reactions are depicted below  

Here total number of (4q +2), and (4r)a components is zero. This t3TJe of examples are very rare. This type of (it s+rc s+Ji s+jt s) cycloadditions can be regarded as (jt a+rc a+Jt a+Ti a) process, because 7C s+7t s is equivalent toJt a+Ji a. 

These generalized rules should be used with following precautions  

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