A little bit of retrosyntheticky goodness

In celebration of being done with synthetic homework assignments, I thought I’d present this morsel of retrosynthetic tastiness:It seems so simple, doesn’t it? Just a little half-inversion of stereochemistry. Just one little flip, and you’re good to go. Such a teaser.

Right about when I started getting desperate, I got the idea that if there were some way to convert the syn diol back into an alkene, there was surely a way to stereoselectively make an anti diol from the alkene. Surely…heh. Googling “glycols to alkenes”–quotes and all–gives exactly three results, the first of which is a VTOC page on reactions involving sulfur and phosphorus. Thankfully that was all I needed, as the Corey-Winter reaction suited the problem quite well. The reaction involves the formation of a thiocarbonate from the two adjacent OH’s and thiophosgene, followed by nucleophilic attack by trialkoxy phosphine on sulfur and the loss of S=P(OR)3 to form a heterocyclic carbene. The carbene undergoes decarboxylation to give an alkene.

At this point I was halfway home–the only thing left was making an anti diol out of the alkene. Google came through on this one as well, yielding an OC Portal page on the Prevost reaction. I’ve got a soft spot in my heart for anything that involves iodine, and this one involves the utterly hardcore formation of an iodonium ion across the alkene. Benzoate anion attacks it, and benzoate’s carbonyl oxygen kicks out the iodine as a leaving group from the other carbon. A second equivalent of benzoate anion comes in and displaces the newly made C-O bond, forcing the first and second benzoates into an anti relationship. Ester hydrolysis leads to the anti diol.

To quote Tim Meadows in Mean Girls: “I could not be happier this semester is ending.”

Advertisements

5 Comments

  1. I’m certain that with careful reagent control and cold temps you can selectively get the enantiopure 1,2-ketol by PCC or PDC, then a sterically hindered reducing method (LTBA or Et3SiH/TFA) should give you the syn diol

    Reply

  2. You had me at “careful” sir! In my hands I could probably bathe in the diketone before getting an acceptable amount of the ketol. That’s certainly a less expensive way of doing it though.

    Reply

  3. you may have a problem doing that corey winter reaction… since it is stereospecific, you would get a trans-cycloheptene. while these can be formed under photochemical condititons, they relax to cis alkenes rapidly… might make the barrier to elimination prohibitively high and make for one crappy reaction.. but luckily this is paper chem!

    btw nice blog, i don’t get why it is not more popular in the orgochemblogosphere

    Reply

  4. Good call. And thanks; I have no idea why I’m not more popular either 🙂 I bounce around somewhat between organic and p chem, but pretty soon I’ll be a die-hard o chemist, so we’ll see what happens from there.

    Reply

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s