Dehydrogenation of Diels Alder addition products

While looking into the chemistry of potassium sorbate I came across some paper that describe the Diels Alder type condensation of sorbic acid with maleic anhydride (1) for instance to yield the partial anhydride of 4-methyl-2,5-cyclohexadiene 1,2,3-tricarboxylic acid, this apparently tends to decarboxylate to give 2,3-anhydride. If this could be dehydrogenated to an aromatic compound it would give 3-methylphthalic anhydride.



Similarly 2,3-dimethyl-1,3-butadiene (available from pinacol) condenses to give 4,5-dimethyl-1,4-cyclohexadiene-1,2-dicarboxylic acid which on dehydrogenation would give 4,5-dimethylphthalic anhydride. The condensations are well established but I can't find any examples of the dehydrogenation or oxidation of the compounds to phthalic acids.

Does any one have any ideas as to the sort of reagents might bring this about. Sulphur at high temperatures was once used but the conditions are a bit brutal. Would say, Pd on charcoal under vacuum, work (reverse hydrogenation)? Or maybe an oxidizing agent such as iodine?

I did find a few Indian papers that reported phthalic acid derivatives but they used acetylenic derivatives and aryl substituted crotonate derivatives.

1) Craig and Shipman; JACS, v74; is5; p2905, [1952]

The chemistry of sorbic acid is a long standing question in my mind too! It is a fairly cheap and OTC available diene and it would be interesting to make something useful/unusual/pleasant smelling compound from it.

As for the dehydrogenation my first bet 'd have been sulfur. I read sometime somewhere that dehydrogenations with sulfur are not always too brute force, depending on the substrate - and your definition on brutality.

My second (similarly old-school) bet 'd have been distillation from zinc powder. (Although with an anhydride it may not be usable, but as I remember it was used e.g. in the structure determination of koniine. Distillation of koniine (2-propyl-piperidine) gave 2-propil-pyridine and its oxidation product 2-picolinic acid was a known compound.)

Your suggested "reverse hydrogenation" may also be a good method and is surely less stressful to the substrate - unless there's an unexpected quirk. Which is so many times the case in organic chemistry.

I collected some sorbic acid papers during the past years. I was mostly interested in the Diels-Alder reaction so I probably have the same articles what you have too.

Even making a simple ester of this acid is a challenge because as I remember the diene is sensitive to acid so a Fischer esterification won't work. (But I may be entirely wrong regarding the reaction, it was long ago when I studied this compound.)

That 3-hexenoic acid is an interesting compound though. I'd like to read about your adventures with its synthesis and all!

Quote: Originally posted by Boffis

While looking into the chemistry of potassium sorbate I came across some paper that describe the Diels Alder type condensation of sorbic acid with maleic anhydride (1) for instance to yield the partial anhydride of 4-methyl-2,5-cyclohexadiene 1,2,3-tricarboxylic acid, this apparently tends to decarboxylate to give 2,3-anhydride. If this could be dehydrogenated to an aromatic compound it would give 3-methylphthalic anhydride. 1) Craig and Shipman; JACS, v74; is5; p2905, [1952]