Polymerization of Conjugated Dienes Rubber

D must be derived from isomer A by thermal, conrotatory electrocyclic ring opening. 

Is the stereochemistry of the cycloaddition of B to D exo or endo Inspection of the relative positioning of the substituents at the four contiguous stereocenters shows it to be endo, as in E. 

Irradiation of ergosterol gives provitamin D2, a precursor of vitamin D2 (a deficiency of which causes softening of the bones, especially in children). Is the ring opening conrotatory or disrotatory (Caution The product is written in a more stable conformation than that obtained upon ling opening.) 

In Summary Conjugated dienes and hexatrienes are capable of (reversible) electrocyclic ring closures to cyclobutenes and 1,3-cyclohexadienes, respectively. The diene-cyclobutene system prefers thermal conrotatory and photochemical disrotatory modes. The triene-cyclohexadiene system reacts in the opposite way, proceeding through thermal disrotatory and photochemical comotatory rearrangements. The stereochemistry of such electrocychc reactions is governed by the Woodward-Hoffmann rules. 

Like simple alkenes (Section 12-15), conjugated dienes can be polymerized. The elasticity of the resulting materials has led to their use as synthetic rubbers. The biochemical pathway to natural rubber features an activated form of the five-carbon unit 2-methyl-l,3-butadiene (isoprene, see Section 4-7), which is an important building block in nature. 

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