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Predict: trans-hexatriene heated gives which cyclohexadiene stereoisomer? Step 1: 6 π electrons = 4n+2 system, thermal. Step 2: Woodward–Hoffmann: thermal 4n+2 → disrotatory closure. Step 3: Disrotatory: ends rotate opposite → trans substituents → trans product. Step 4: Orbital symmetry predicts stereochemistry — no mechanism needed!