The invention has its object to provide a method of lactonization for a compound of the general formula (1) in which the dimer formation reaction can be drastically inhibited. The invention provides a process for producing a compound of the general formula (2) comprising lactonizing a compound of the general formula (1) under conditions such that the solubility of the compound of the general formula (1) and/or the compound of the general formula (2) is not more than 0. 5 w/w % to thereby give a compound of the general formula (2) with a dimer content of not more than 0.3 mo1e % .
