Cyclization Bergman-like

Bergman-like cyclization pathways of the ene-diynes substructures in the substrates, which would involve radical intermediates, can be excluded as the typical interceptor for such radicals, the hydrogen atom donor 1,4-cyclohexadiene (44), also provided benzocyclobutene 45 and not products... 

A Bergman-like [99,100] cyclization has been employed to synthesize substi-tuted-PPPs 23 [101]. Using this strategy enediynes are cyclized and subsequently coupled upon thermal treatment, as shown in Scheme 28. The polymers obtained in this manner are 2,3-disubstituted and display number average molecular weights on the order of 1500-2500 g/mol. 

The cycloaromatization of enediynes, having a structure like 1, proceeds via formation of a benzenoid 1,4-diradical 2, and is commonly called the Bergman cyclization. It is a relatively recent reaction that has gained importance especially during the last decade. The unusual structural element of enediynes as 1 has been found in natural products (such as calicheamicine and esperamicine) which show a remarkable biological activity... 

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. 

Much like the sparse demonstrations of electronically activated metalloenediyne Bergman cyclization, only two examples of photochemically induced... 

Total synthesis of asperlicin C and E via condensation route by Bock et al. is well known [76-79,243]. The first total synthesis of asperlicin D via both cyclodehydration and intramolecular aza-Wittig routes has been fulfilled very recently by Al-Said et al. [244], and the iminophosphorane intermediates having secondary amide protons have been shown to provide a one-step entry to quinazolino[l,4]benzodiazepine ring system like asperlicin D (a positional isomer of asperlicin C). A facile route to N-benzylsclerotigenin has been reported also by this group [245]. On the other hand, total synthesis of circumdatin F (19) (and circumdatin C) via dehydrative cyclization (or ben-zoxazine) route has been also fulfilled by Bergman et aL [246]. 

Hastings CJ, Backlund MP, Bergman RG, Raymond KN (2011) Enzyme-like control of carbocation deprotonation regioselectivity in supramolecular catalysis of the nazarov cyclization. Angew Chem Int Ed 50(45) 10570-10573... 

Like the Bergman cyclization transforming enediynes to aromatic systems, the Myers-Saito cyclization of (Z)-l,2,4-heptatrien-6-ynes also proceeds via the formation of 3-a-didehydrotoluene diradicals 3.428. But in contrast to the Bergman cyclization, the acyclic enyne-allenes 3.444-3.446 react along the Myers-Saito route under conditions close to physiological ones (Scheme 3.7) [221,222]. Cyclic enyne-allenes react in a similar manner to their acyclic counterparts. The biological activity of neocarzinostatin [24, 27] is attributed to this reaction. 

Like metal ions, which change activation barriers and affect the kinetics of Bergman cyclizations, the organometallic reagents also activate enediynes. It was noted above that the Myers-Saito cyclization occurs at a much lower temperature than the Bergman cyclization of enediynes... 

Like the case of Bergman and Myers-Saito cyclizations, the geometry of the transition state of the C -C (Schmittel) cyclization of enyne-allenes is close to the reaction product but its electronic state is close to... 

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