The steric course of some reactions of 2,3-dihalotetrahydropyrans with sodium methoxide and with methyl- and phenyl-magnesium bromides has been reinvestigated. Whereas the methanolysis of the cis- and trans-dichlorides occurs with practically complete inversion, it has been confirmed that their reactions, as well as those of the dibromides, with methyl-magnesium bromide are non-stereospecific, yielding mixtures of trans- and cis-3-halo-2-methyl-tetrahydropyrans in ratios that are independent of the configurations of the starting dihalides. It has been further established that Grignard reagents cause equilibration of cis- and trans-dihalide mixtures in a reaction that is much faster than the Grignard coupling step.
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