N‐Pyridylureas as Masked Isocyanates Fort the Late‐Stage Diversification of Pyridine‐N‐Oxides

Abstract

A new method for the synthesis of pyridine‐ and quinoline‐N‐oxides containing a ureido or carbamoyl fragment in position 2 has been developed. This method concluded in the oxidation of relatively readily available substituted N,N‐dimethyl‐N′‐(pyridine‐2‐yl)ureas to the corresponding N‐oxides, followed by modification of the dimethylureide moiety. Being heated to 90 °C in the presence of amines in a solution of DMF or chlorobenzene, N‐oxide of N,N‐dimethyl‐N′‐(pyridine‐2‐yl)urea undergoes a transamination reaction without loss of the N‐oxide function. This produces N‐oxides of N‐alkyl/aryl‐N’‐(pyridine‐2‐yl)ureas in 24–99 % yields, (about 30 examples). Values of the NH protons chemical shifts are a good criterion for the selectivity of the reaction. The reaction is also applicable to N‐oxides of ureas substituted in the pyridine ring and to the quinoline congener. N‐Oxides of pyridine/quinoline‐containing carbamates can be similarly prepared by heating N‐oxides of N,N‐dimethyl‐N′‐(pyridine‐2‐yl)/(quinoline‐2‐yl)‐urea in an appropriate alcohol solution at 120 °C.