Best reagent to convert 4-chloro-4H-pyran into an aromatic compound

Question

My question:

My attempt: (I am not really good at mechanisms, so a little help would be good. Thank-you)

I do not know what kind of reactions these will give.

Answer 1

In order to choose the best reagent to convert the compound to an aromatic one, let's ask the question of what is preventing this compound from being aromatic? At carbon-4, the pi-system is disrupted by an sp³-hybridized atom. If carbon-4 could be converted to sp² by some reaction, then the compound could become aromatic. At C-4, we see a chloride. We know that chlorides are good leaving groups, so let's look at the product of that step.

After chloride leaves, there's a cyclic, fully conjugated pi-system containing six electrons (two for each double bond, two from the oxygen, and zero from the carbocation). Drawing a resonance structure shows that this is an oxygen analog of benzene (known as pyrylium ion).

What kind of reagent would promote this process, i.e., a halide leaving? A Lewis acid, such as $\ce{AlCl3}$ could promote this in two ways. First, it could interact with the chloride in the starting material, building up positive charge on the chloride, making it a better leaving group. Secondly, the Lewis acid can coordinate the chloride on the product side, preventing it from attacking the pyrylium ion.