Diethoxyphosphorylmethylfurans in Curtius reaction

Abstract

Methyl (diethoxyphosphorylmethyl)furoates are selectively hydrolyzed at the carboxy group with the equimolar amount of potassium hydroxide in ethanol. The carboxylic acids obtained by means of the consecutive treatment with ethyl chloroformate in the presence of triethylamine and sodium azide are converted into furoyl azides. 3-Furoylazides and 5-(diethoxyphosphorylmethyl)-2-furoyl azide while heating to 110°C undergo Curtius rearrangement in the corresponding isocyanates which add methanol to form stable methyl urethanes. 4-(Diethoxyphosphorylmethyl)-5-methyl-2-furoyl azide also forms stable isocyanate, but while treating this substance with methanol the process does not stop on the formation of methyl urethane. Addition of the second methanol molecule takes place, and the subsequent nucleophilic substitution and elimination of ammonia lead to O-[4-(diethoxyphosphorylmethyl)furyl-2](methyl)carbonate. 3-(Diethoxyphosphorylmethyl)-2-furoyl azide forms polymer at 100°C in toluene, but while boiling it in the 1:1 toluene-methanol mixture formation of urethane is observed spectroscopically. The latter compound also undergoes conversion to phosphorylated furyl carbonate. By means of the 1H and 31P NMR spectroscopy the intermediate products formed in the course of transformations described were traced, and on this basis probable mechanisms of addition of methanol to isocyanate and rearrangement of urethane to carbonate were suggested.