electrons from an electron rich source to an electron poor sink. Rather, electrons are redistributed around a cyclic transition state. Thus, electrons can be pushed in either of two directions for a pericyclic reaction. For some pericyclic reactions, however, there is a definite polarization of charge at the transition state due to asynchronicity (bond formation and breaking do not occur to a uniform extent at the transition state). Thus, one direction may be preferred over another, although arguably, both depictions are still formally correct. In the case of the Diels-Alder reaction shown below, resonance arguments make clear the direction of polarization. In more complex situations, however, detailed computations may be needed to determine the direction and extent of polarization. Pseudopericyclic processes Closely related to pericyclic processes are reactions that are pseudopericyclic. Although a pseudopericyclic reaction proceeds through a cyclic transition state, two of the orbitals involved are constrained to be orthogonal and cannot interact. Perhaps the most famous example is the hydroboration of an olefin. Although this appears to be a 4-electron Hückel topology forbidden group transfer process, the empty p orbital and sp2 hybridized B–H bond are orthogonal and do not interact. Hence, the Woodward-Hoffmann rules do not apply. (The fact that hydroboration is believed to proceed through initial π complexation may also be relevant.) In biochemistry Pericyclic reactions also occur in several biological processes: Claisen rearrangement of chorismate to prephenate in almost all prototrophic organisms [1,5]-sigmatropic shift in the transformation of precorrin-8x to hydrogenobyrinic acid non-enzymatic, photochemical electrocyclic ring opening and a (1,7) sigmatropic hydride shift in vitamin D synthesis Isochorismate pyruvate lyase catalyzed conversion of isochorismate into salicylate and pyruvate See also Coarctate reaction Woodward–Hoffmann rules References Rearrangement reactions Reaction mechanisms