Electrocyclic transformations conrotatory process

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 stereochemistry of a thermally induced 10e electrocyclization (predicted to be disrotatory) has not been firmly established and the main synthetic application is found in the formation of azulenes and ring-fused azulenes as in the transformation (452) to (453). Thermolysis of (454) with spontaneous elimination of dimethylamine from intermediate (455) afforded the fused azulene structure (456). The chemistry of even higher order (12e to 20e") pericyclic processes has been recently reviewed. An example of an unusual sequence of pericyclic processes is the transformation of heptahendecafulvadiene (457) to the pentacyclic hydrocarbons (462) and (463) in a 2 1 ratio. The pathway for this transformation can be viewed as an initial conrotatory 20e electrocyclization followed by a cascade of 10e and 6e pericyclic processes. ... 

The photochemical transformation is a 6tt electron (4n + 2) electrocyclic ring opening. The selection rules predict a conrotatory process as illustrated ... 

By contrast, the retro electrocyclization by photochemical irradiation is well known. For example, the photochemical transformation (6e conrotatory ring opening) of provitamin D (5) to previtamin D (3) and then thermal isomerization (1,7-H shift) of the latter is a well-established sequence leading to vitamin D (4). It is a sequence involved in vitamin D biosynthesis and in the laboratory synthesis of vitamin D. Moreover, the process is used commercially. 

Photochemical electrocyclization of unsaturated enamides is a well-documented reaction of high synthetic value in the field of alcaloids [56]. The reaction can be viewed as a conrotatory photocyclization process leading to a dipolar intermediate which can be transformed into the final products either through a thermal, suprafacial, 1,5-sigmatropic shift or by an intermolecular protonation process. 

The red cyclotetrasilene 20 (puckered Si ring 2.36 A may arise via an intermediate disilyne R Si SiR (6), which may then transform via the tetrasilabutadiene R HSi=SiR -SiR =SiHR (6j ) into 20. Thus, the intermediate must be formed by a migration of two R groups associated with a relief of the steric crowding of 6, and must then react further by an electrocyclic process in a conrotatory sense with formation of 20. 

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