Precautions in the construction of Correlations Diagrams

Whether a cycloaddition reaction is allowed, depends upon the symmetry properties of HOMO of one reactant and LVMO of other under the reaction condition. A favourable bonding interaction takes place when wave signs of interacting atomic orbitals of HOMO and LVMO are same. K + JCs cycloaddition 

In case one ethylene moiety is allowed to react with another is suprafacial-antarafacial manner (on the opposite side of olefin) k +k, the reaction becomes symmetry allowed in ground state. 

This approach was developed by M.J.S. Dewar and too leads to similar conclusion about pericyclic reactions as above two methods without taking into account symmetry of molecular orbitals. 

for disrotatory process, 0 node. 6 electrons, aromatic (Hiickd sjrstem) thermally allowed 

for conrotatory process. 1 node, 6 electrons, antiaromatic (Mobius system) hv allowed. 

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