The present invention has an object to provide a phosphoramidite activator that improves a solubility of 5-benzylthio-1H-tetrazole in acetonitrile and does not reduce the yield and a purity of a target product.
The present invention relates to a phosphoramidite activator containing (i) at least one compound that is selected from the group consisting of piperidine, pyrrolidine, N-alkylpiperidine, and N-alkylpyrrolidine, (ii) 5-benzylthio-1H-tetrazole, and (iii) acetonitrile; and a method for activating a phosphoramidite and a method for synthesizing a phosphate ester or a thiophosphate ester, both methods using the activator.