Ethers, acid cleavage boron trifluoride complexes

In contrast to the reductive cleavage of 1-methylcyclohexene epoxide with LiAlH4 or, better, with LiEtjBH to produce 1-methylcyclohexanol, reduction of the epoxide with sodium cyanoborohydride in the presence of boron trifluoride etherate furnishes cE-2-methylcyclohexanol. In this case, complexation of the epoxide oxygen with the Lewis acid BF3 now directs hydride addition to the more substituted carbon, which can better sustain the induced partial positive charge. 

Medium and large rings can be made via cationic intermediates which are usually generated by treatment of suitable precursors with acid. Reactions of this class are perchloric acid catalyzed rearrangements of bicyclo[n.l.0]alkan-2-ols (n > 5), solvolysis of the corresponding esters, boron trifluoride-diethyl ether complex catalyzed cleavage of epoxides, and tri-fluoroacetic acid catalyzed reactions of 7-(methylsulfanyl)bicyclo[n.l.0]alkanes. 

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