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The seeming difference in the topochemical [2 + 2] photoreactivity of 3-cyanocinnamic acid 1 and 4-cyanocinnamic acid 2 is caused by differing degrees of ease of molecular motion in the crystals and the unreactive 1 becomes reactive when heated to 130 °C.
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Dhurjati et al. (1991) studied this question.
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