Studies on Organic Sulfur Compounds. XII. Desulfuric Ring-Closure Reaction of o-Substituted Aromatic Thioallophanic Acid Esters

Abstract

Reaction of ethyl isothiocyanatoformate (II) with o-aminobenzenethiol (Ia) affords ethyl 4-(o-mercaptophenyl)-3-thioallophanate (IIIa), which is readily converted into ethyl 2-benzothiazolecarbamate (IVa), with desulfurization, by heating. Ethyl 4-(o-aminophenyl)-3-thioallophanate (IIIb) and ethyl 4-(o-hydroxyphenyl)-3-thioallophanate (IIIc), which do not afford any cyclization product in aprotic solvents, can also be cyclized to ethyl 2-benzimidazolecarbamate (IVb) and ethyl 2-benzoxazolecarbamate (IVc), respectively, by heating in strongly protic solvents. Heavy metal ions and/or hydrogen peroxide, which are known as soft acids, accelerate desulfuric ring-closure reactions in the presence of basic anions.