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1、12A pericyclic reaction is characterized as a change in bonding relationships that takes place as a continuous,concerted reorganization of electrons.The term concerted specifies that there is one single transition state and therefore no intermediates are involved in the process.To maintain continuou
2、s electron flow,pericyclic reactions occur through cyclic transition states.More precisely:The cyclic transition state must correspond to an arrangementof the participating orbitals which has to maintain a bonding interaction between the reaction components throughout the course of the reaction34567
3、8910111213141516171819202122232425262728The simplest approach(Supra/Supra)is forbidden under thermal activation.The less obvious approach(Antara/Supra)is allowed thermally but geometrically rather congested.It is believed to occur in some very specific cases(e.g.ketenes)where the steric congestion is reduced.2930313233343536373839404142434445