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enter image description here (A) to (B) is simply reduction, tosylation followed by intramolecular reaction and removal of tosylate. However, I am not able to guess how dimerization will take place, seems similar to Rauhut–Currier reaction, but can't guess. Any hint?

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    $\begingroup$ TsOH does not form a tosylate. That's likely just the workup for the borohydride reduction. $\endgroup$ – Zhe May 1 '20 at 15:31
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    $\begingroup$ My hint for you is that you can determine the which of A through D is mostly likely to be the product (B) based on several factors. Then working backwards, you can figure out what (A) is. $\endgroup$ – Zhe May 1 '20 at 15:38
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    $\begingroup$ That's kind of like how you can (sort of) alkylate stuff using alkyl chlorides RCl, but you can't alkylate stuff using alcohols ROH. $\endgroup$ – orthocresol May 1 '20 at 16:58
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    $\begingroup$ No tosic acid is not like tosyl chloride, in the same way methyl alcohol is not like methyl chloride. I have a putative mechanism, but I thought I'd let you try to figure it out for yourself first. $\endgroup$ – Zhe May 1 '20 at 16:58
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    $\begingroup$ ... I think I'll stop jumping in here.... $\endgroup$ – orthocresol May 1 '20 at 16:58
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The dimer of methyl vinyl ketone (1) is not shown in the choices in the question. The dimer is A shown below. It is a "tail-to-tail" dimer. Product B is an intramolecular ketal as a probable mixture of epimers. Compare with this paper.

B. P. Mundy, et al., J. Org. Chem., 1978, 43, 2347.

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  • $\begingroup$ Don't have access to the paper... And I still can't get the mechanism for dimerization.... $\endgroup$ – Zenix May 1 '20 at 17:41
  • $\begingroup$ You can usually predict Diels-Alder reactions by using ionic charges. This one is probably radical in nature. Whoever made up the structure (B) fell into this trap. $\endgroup$ – user55119 May 1 '20 at 17:45

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