Quantum chemical studies cyclizations

Quantum chemical studies of cyclizations of enediynes and enyneallenes have been reviewed.180 The intermediates are computationally tractable as a result of the unrestricted broken spin symmetry (UBS) approach using GGA functionals for the description of open-shell biradicals. The intermediacy of biradicals in Cope-type rearrangements, to which the Bergman and Myers-Saito reactions belong, are shown to be predictable using a very simple rule biradicals are likely to be intermediates if they are stabilized either by allyl resonance or by aromaticity. 

As can be seen from this comparison, the resulting values are affected by the choice of the critical structure and on going from X(n/4) to X(-7t/4), the systematic shift of the dominant similarity from the zwitterionic state Z + Z2 to the state Zj -Z2 is observed. We can thus see that the predictions for both types of critical structures differ and the problem thus appears which of the above two critical structures should be regarded as a true model of the transition state in forbidden reactions. Similarly as in the case of allowed reactions such a decision does not arise from the approach itself, but some external additional information is generally required. This usually represents no problem since the desired information can be obtained, as in the case of allowed reactions, from the simple qualitative considerations based on the least motion principle [80,81], or from the direct quantum chemical calculations.This is also the case with us here, where the desired information is provided by the quantum chemical study [63] of the thermally forbidden cyclization of the butadiene to cyclobutene. From this shufy it follows that the ground state of the transition state should correspond to the ground state of the cyclobutadiene which is the Zj - Z2 state. 

A cytochrome P450-catalyzed Friedel-Crafts reaction is proposed in the biosynthesis of viridicatumtoxin (225, Scheme 45). ° The transformation involves a spirocyclization of the geranyl-substituted substrate (226). This biosynthetic chemistry is notable as the first terpene cyclization catalyzed by a P450 enzyme (VrtK). It suggests a mechanism with oxidation to the allyl cation (227), with subsequent ring formation, hydride shift, and Friedel-Crafts cyclization steps. The proposed mechanism was further studied by quantum chemical calculations. 

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