Trans orientation, Zaitsev elimination

As mentioned before, allenes can be formed by prototropic rearrangement of alkynes or, if an appropriate hydrogen is present in the allylic position, by direct elimination. The bromovinyl ether yields, in a trans elimination, the alkyne ether (Scheme 39), but the other isomer, where trans elimination is not possible, gives both the alkynyl ether and the allenyl ether (Scheme 40). This corresponds to the problem of Hofmann and Zaitsev orientation in alkene synthesis. 

As shown in Solved Problem 7-3, the H and the Br must have trans-diaxial orientation for the E2 reaction to occur. In part (a), the cis isomer has the methyl in equatorial position, the NaOCH3 can remove a hydrogen from either C-2 or C-6, giving a mixture of alkenes where the most highly substituted isomer is the major product (Zaitsev). In part (b), the trans isomer has the methyl in the axial position at C-2, so no elimination can occur to C-2. The only possible elimination orientation is toward C-6. 

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