An enantioselective ring expansion route leading to furanose and pyranose nucleosides featuring spirodiketopiperazines at the anomeric position

A study directed at the enantioselective synthesis of spirodiketopiperazine homologues of hydantocidin is described. Furanoid glycals, systems that are amenable to C-5 metalation in the presence of tert-butyllithium, are readily coupled to N-protected 2,3-azetidinediones provided that at least 1 equiv of BF3. OEt2 is present to curb enolization. The resulting 1:1 mixtures of carbinols undergo smooth ring expansion to spirocyclic keto amides when heated with pyridinium p-toluenesulfonate in benzene. 1,2-Acyl shifts operate exclusively. Since attempts to engage these products in Beckmann rearrangement proved singularly unsuccessful, recourse was alternatively made to new methodology based upon sequential Baeyer-Villiger oxidation and ammonolysis. The data show that the first of these steps occurs with exclusive migration of the quaternary carbon. Furthermore, nucleophilic attack by NH3 can be directed regioselectively to the anomeric region. If heating is supplied during acid-promoted cyclization to the spirodiketopiperazines, spiropyranose derivatives are produced in a complementary process. The central issue of this synthesis effort was the utilization of 4-phenylseleno-substituted furanoid glycals so as to ultimately enable introduction of the cis-diol functionality at C-3 and C-4 (hydantocidin numbering).