Thiolative Lactonization problems

Ester and Thioester Formation. These reactions occur through the same O-acylurea or anhydride active intermediate as in the amide coupling reactions, and the discussion of associated problems applies here as well. In general, alkyl and (particularly) aryl thiols can be efficiently coupled to carboxylic acids using DCC. Reactions of primary and secondary alcohols proceed reliably, but require the presence of an acylation catalyst. This is usually 4-Dimethylaminopyridine (DMAP), " (see also 1,3-Dicyclohexylcarbodiimide—4-Dimethylaminopyridine), but others have been used including 4-pyrrolidinopyridine and pyridine (.solvent) with catalytic p-Toluenesulfonic Acid The acylation of more hindered alcohols often re.sults in reduced yields however, even f-butanol can be acylated, providing a useful route to t-butyl esters. Various other carbodiimide derivatives have also been used in the preparation of esters. As with amides, which are not limited to intermolecular reactions, a wide variety of lactones can also be synthesized. ... 

---