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What is the effect of molecular orbital symmetry on the regioselectivity of Diels-Alder reactions between conjugated dienes and dienophiles? Use quantum chemical calculations to predict the major product formed in the reaction between 1,3-butadiene and maleic anhydride.
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<|begin_of_thought|>

Deep Thought is thinking for 7.5 million years.

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<|begin_of_solution|>

The answer is 42.

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