A flexible Pinner preparation of orthoesters: the model case of trimethylorthobenzoate

摘要

In the absence of additional solvents{,} a novel procedure was implemented for the synthesis of trimethylorthoesters through the Pinner reaction. At 5 [degree]C{,} the reaction of both aliphatic and aromatic nitriles (RCN; R = Et{,} Bu{,} Ph) with a moderate excess of MeOH and gaseous HCl gave the corresponding imidate hydrochlorides [RC([double bond{,} length as m-dash]NH)OR[prime or minute][middle dot]HCl] in excellent yields (>90%). At 25-65 [degree]C{,} the methanolysis of alkyl imidate salts provided trimethylortho-propionate and valerate{,} while only traces of trimethylorthobenzoate (TMOB) were observed. However{,} the aromatic hydrochloride could be readily converted into the hydrogenphosphate salt [PhC([double bond{,} length as m-dash]NH)OR[prime or minute][middle dot]H3PO4] which{,} in turn{,} underwent a selective (>80%) reaction with MeOH to produce TMOB in a 62% isolated yield. This allowed for an unprecedented Pinner-type synthesis of TMOB starting from benzonitrile{,} rather than from the highly toxic trichloromethylbenzene. Overall{,} remarkable improvements in safety and process intensification were achieved.