Singlet, planar spin-equilibrated

The 5-methoxy derivatives of 3b or 3c show a clear spin correlation effect. The singlet reaction gives different product ratios for exo- and endo-54, whereas the triplet reaction gives the same ratio 79/21 for both stereoisomers. From this result an equilibrated pyramidal or planar diradical was inferred as an intermediate 9°). 

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