Anhydro compounds ring scission

The aziridine ring is more stable than the oxirane ring in alkaline solution, as demonstrated by the low reactivity in attempts to accomplish isomerization of the hydroxyepimines to amino epoxides in alkaline media at room temperature, which contrasts with the rapid epoxide migration (see Sect. V,2). Isomerization of hydroxyepimines occurs only at high temperatures, and leads finally to the formation of amino derivatives of 1,6-anhydrohexoses.379,740 For example, when 166 is heated in 5% potassium hydroxide, 2-amino-l,6-anhydro-2-deoxy-/3-D-mannopyranose (168) is formed as the main product this can be explained by transient formation of 2-amino-l,6 3,4-dianhydro-2-deoxy-/3-D-altropyranose (167), and its subsequent, diaxial hydrolysis.379 Compound 167 is probably in equilibrium with epimine 166. Acid hydrolysis of the aziridine ring in 153 also follows a diaxial mechanism, without scission of the 1,6-anhydride bond, to give 4-amino-l,6-anhydro-4-deoxy-)8-D-mannopyranose756 (177). 

---