A facile synthesis of bicyclo[3.2.1]octa-2,6-dienes (3), R1 = alkyl, and 5-(aryl-methylene)norbornenes (4), R1 = aryl, is possible by cycloaddition of allenyl cations (2) to cyclopentadiene. (2) was generated from propargyl halides (1) with catalytic amounts of ZnCl2 in ether/CH2Cl2.
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Mayr et al. (1980) studied this question.
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