Ring closure double

Erythronolide B, the biosynthetic progenitor of the erythromycin antibiotics, was synthesized for the first time, using as a key step a new method for macrolactone ring closure (double activation) which had been devised specifically for this problem. Retrosynthetic simplification included the clearance of the stereocenters at carbons 10 and 11 and the disconnection of the 9,10-bond, leading to precursors A and B. Cyclic stereocontrol and especially the Baeyer-Villiger and halolactonization transforms played a major role in the retrosynthetic simplification of B which was synthesized starting from 2,4,6-trimethylphenol. 

Behinpour, K., Hopkins, A., Williams, A. Macrolide Ring-Closure - Double Activation Mechanism. Tetrahedron Lett. 1981,22, 275-278. 

N-Condensed heterocyclics from hydroxycarboxylic acid amides via Bischler-Napieralski ring closure Double ring closure... 

---