7-Hydroxy-4-methylcoumarin can be synthesized from ethyl acetoacetate and resorcinol with iodine as a catalyst. What is the mechanism for this? From what I understand it follows the Pechmann reaction but I'm having trouble to understand how it works in this particular case.

Reaction scheme

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    $\begingroup$ @bon I'm on it ;) $\endgroup$ – orthocresol May 13 '16 at 15:51

There is a paper about this exact reaction, but in addition to the use of $\ce{I2}$ it mentions the use of microwaves. In the reaction, $\ce{I2}$ acts as a Lewis acid, catalyzing both the transesterificaiton and electrophilic aromatic substitution reactions. It can be seen acting as such in the first step of the mechanism:

$\hspace{2cm}$Step 1

Next, a proton transfer occurs:

Step 2

Next, the resorcinol derivative can act as a nucleophile, and ethanol is ejected, completing the transesterification reaction:

step 3

step 4

Next, $\ce{I2}$ coordinates to the keto carbonyl, activating the carbon towards electrophilic addition. Because this resorcinol derivative is so activated by the $\ce{-OH}$ and $\ce{-OOCR}$ groups, a weak Lewis acid like $\ce{I2}$ is sufficient to catalyze the reaction:

Step 5

$\hspace{1.5cm}$Step 6

$\hspace{1.8cm}$Step 7

$\hspace{1.6cm}$Step 8


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