Cyclobutene, electrocyclic ring opening

The first example illustrates how a 1,4-dehydroaromatic system with cyclohexane ring having two double bonds may be also disconnected according to a retro-Diels-Alder to give a diene and an acetylene as the dienophile [25]. The second example makes clear that even an aromatic double bond may be -in some instances-involved in a retrosynthetic pericyclic disconnection [26]. In the synthetic direction, the polycyclisation involves a conrotatory electrocyclic cyclobutene ring opening, (16 15) followed by an intramolecular Diels-Alder addition (see Scheme... 

This electrocyclic cyclobutene ring opening also gives the product as a single stereoisomer. 

The parent compound has notyetbeen synthesized [11]. However, substituted thiepins are stable and accessible by various routes. For instance, 3-amino-substituted thiophenes and activated alkynes undergo a (2 + 2)-cycloaddition followed by electrocyclic cyclobutene ring-opening (cf. p. 95) to give the thiepin 1 ... 

In a lengthy theoretical paper, Rondan and Houk considered the available data, described ab initio calculations and discussed earlier explanations concerning the stereochemical aspects of the ring openings of substituted cyclobutenes.These authors came to the following conclusions. The stereochemistry of the thermal electrocyclic conrotatory ring opening of 3- and 4-substituted cyclobutenes is controlled by... 

In addition, Swenton and cowm kers have reported that the dianion derivatives of certain pyrimidine-acetylene photoadducts undergo an electrocyclic reaction which may involve an anion-accelerated cyclobutene ring opening see R. N. Comber, J. S. Swenton and A. J. Wexler, /. Am. Chem. Soc., 1979,101, 5411. 

Sol 14. The compound I undergoes sequential cyclobutene ring opening to give cyclohexenofused [12]annulene (V), which then undergoes thermal Mobius TT-bond shifts to substimted di-trans isomer VI. The compound VI then forms the cage compound VIII in two steps thermal electrocyclization followed by [4 + 2] cycloaddition. 

There are several general classes of pericyclic reactions for which orbital symmetry factors determine both the stereochemistry and relative reactivity. The first class that we will consider are electrocyclic reactions. An electrocyclic reaction is defined as the formation of a single bond between the ends of a linear conjugated system of n electrons and the reverse process. An example is the thermal ring opening of cyclobutenes to butadienes ... 

This compound is less stable than 5 and reverts to benzene with a half-life of about 2 days at 25°C, with AH = 23 kcal/mol. The observed kinetic stability of Dewar benzene is surprisingly high when one considers that its conversion to benzene is exothermic by 71 kcal/mol. The stability of Dewar benzene is intimately related to the orbital symmetry requirements for concerted electrocyclic transformations. The concerted thermal pathway should be conrotatory, since the reaction is the ring opening of a cyclobutene and therefore leads not to benzene, but to a highly strained Z,Z, -cyclohexatriene. A disrotatory process, which would lead directly to benzene, is forbidden. ... 

The cyclohexadiene-hexatriene system seems to be less complicated than the cyclobutene-butadiene system. Cyclohexadiene undergoes photochemical electrocyclic ring opening ... 

Compound 6 is a pivotal intermediate in Schreiber s synthesis. It was hoped that the conspicuous and strained bridgehead cyclobutene substructure in 6 would undergo a conrotatory electrocyclic ring opening upon thermolysis to give an isomeric 1,3-diene (8, Scheme 1). In the event, when a solution of cyclobutene 6 in toluene is confined to a sealed tube and heated to 180°C for 12 h, a stereoisomeric mixture of 1,3-dienes 7 and 8 is produced in an excellent yield of 95% (7 8 ca. 5 1). Finally, irradiation of the 5 1 mixture of cis-7 and trans-8, or of each independently, establishes a photostationary state in which the desired trans isomer 8 predominates (8 7 ca. 10 1). 

Table 13-1. Computed reaction barriers and isomer stabilities [kcal/mol] for the electrocyclic ring opening of cyclobutene (relative to cyclobutene 1, including zero-point vibrational contributions). Except for G2, the results were obtained using the 6-311+G(d,p) basis set.

The nucleophilic attack with 2-700 and 2-701 can proceed in a syn or anti manner to provide either 2-702 or 2-703, or both [357]. If2-703 is formed, it follows a charge-driven conrotatory opening of the cyclobutene ring with generation of the coiled 1,3,5,7-octatetraene 2-704. This intermediate is capable of a rapid helical equilibration [357] and a regioselective 8jt electrocyclic ring closure to give 2-705 [358]. 

The forerunner in the Co-catalyzed [2+2+2] cycloaddition domino processes was that identified by Vollhardt and colleagues [273], with their excellent synthesis of steroids. Reaction of 6/4-1 with [CpCo(CO)2] gave compound 6/4-3 with an aromatic ring B via the intermediate 6/4-2. In this process, trimerization of the three alkyne moieties first takes place, and this is followed by an electrocyclic ring opening of the formed cyclobutene to give o-quinodimethane. This then undergoes a Diels-Alder reaction to provide the steroid 6/4-3 (Scheme 6/4.1). 

We are also carrying out calculations 40) for the electrocyclic ring opening of cyclobutene (27) and bicyclobutane (28) to 1,3-butadiene (29). While.this work is not yet complete, we have established that 27 opens preferentially by... 

We break the C4-C6 bond, and we form C3-C8 and C4-C9. The formation of the latter two bonds and the fact that we re forming a cyclobutanone suggests a [2+2] cycloaddition between a ketene at C3=C4=0 and the C8=C9 n bond. We can generate the requisite C3=C4 n bond by electrocyclic ring opening of the cyclobutene ring in the S.M. 

Spin-coupled theory has been used to smdy the changes that occur in the electronic wavefunction as a system moves along the intrinsic reaction coordinate for the case of the conrotatory and disrotatory pathways in the electrocyclization of cyclobutene to c/x-butadiene. Against intuitive expectations, conrotatory opening of cyclobutenes was found to be promoted by pressure. Ab initio MO and density functional calculations have indicated that the ring opening of the cyclobutene... 

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