We've been asked for the mechanism for the synthesis of the following difuran from the starting materials shown below:

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The ytterbium complex will act as a Lewis acid and activate the oxygen of the benzaldehyde. However, I don't know what the next step in the mechanism would be.

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    $\begingroup$ Electrophilic aromatic substitution? $\endgroup$ – Zhe Oct 25 '16 at 21:31

This is actually a series of two electrophilic aromatic substitutions. The ytterbium stabilizes the benzyl cation enough for it to attack position 5 of the 2-methylfuran. After the new ytterbium complex dissociates away, the resulting cation can then attack another 2-methylfuran, and the final product is formed.


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