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.”