Nitrogen versus Oxygen Group Protection in Hydroxypropylbenzimidazoles

Abstract

In order to convert 1′H-benzimidazol-2′-ylpropanols into aryl ethers using Mitsunobu coupling, it was necessary to protect the benzimidazole nitrogen in the starting alcohols. Selective protection at nitrogen was achieved through N-benzyl derivatives, but attempts to protect the nitrogen directly through tert-butoxycarbonyl, acetyl, trityl, or tetrahydropyranyl derivatives were complicated either by selective reactions at oxygen or by the formation of bis-protected compounds. Transformations of some oxygen-protected derivatives are discussed, and in particular the conversion of the acetates of 1′H-benzimidazol-2′-ylpropanols to N-tetrahydropyranyl derivatives is described. Mitsunobu coupling involving the N-benzyl and N-tetrahydropyranyl derivatives and methyl 4-hydroxybenzoate were achieved, and thus afforded synthetic routes to the desired propylbenzimidazole aryl ethers.