What product would you expect to obtain from the photochemical cyclization of (2 E, 4 Z, 6 E)-2,

step 1 What product of our photochemical cyclization would you expect from those two compounds? There a

What product would you expect to obtain from the...

Key Concepts

Electrocyclic Reactions

Electrocyclic reactions are pericyclic processes in which a conjugated polyene undergoes a ring closure (or opening) by a cyclic reorganization of its ? electrons. The reaction converts a linear array of conjugated double bonds into a cyclic structure. The stereochemical outcome of these reactions is dictated by the manner—conrotatory or disrotatory—in which the terminal p?orbitals rotate during the process.

Woodward–Hoffmann Rules

The Woodward–Hoffmann rules provide a theoretical framework for predicting the stereochemistry of pericyclic reactions, including electrocyclic reactions. They state that the allowed mode of orbital rotation (conrotatory versus disrotatory) depends on the number of ? electrons involved and whether the reaction is induced thermally or photochemically. For a 6?-electron system, the reaction proceeds via a disrotatory pathway under thermal conditions and via a conrotatory pathway under photochemical conditions.

Stereochemistry in Photochemical Cyclizations

In photochemical electrocyclic reactions, the absorption of light alters the orbital symmetry, often reversing the allowed mode of rotation compared to thermal reactions. For a 6?-electron system, photochemical conditions favor a conrotatory motion of the terminal groups during cyclization. Thus, the stereochemical arrangement of substituents in the starting polyene (for example, the different E/Z configurations) directly affects the three-dimensional structure of the cyclized product.

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