RESUMEN
A series of novel tricyclic quinazolinone-iminosugars 1 (a-c) were synthesized from the benzyl protected sugars through three steps. Firstly, the benzyl protected sugar (aldehyde) 5 reacted with o-aminobenzamide by the iodine-induced oxidative condensation to afford the corresponding aldo-quizanolinone 6. Secondly, through the intramolecular cyclization of the unprotected OH and the amide NH in 6, the tricyclic compounds 7 and 8 were constructed by the key Mitsunobu reaction. Finally, removal of the benzyl group gave the target tricyclic quinazolinone-iminosugars 1. The protocol was effective for the preparation of the tricyclic iminosugars in satisfactory yield. Interestingly, an unusual C-2 epimerization was observed with d-mannose and d-ribose compounds under the conditions of the Mitsunobu reaction that generated the products having the trans configuration at the C-2 and C-3 positions. Unfortunately, such tricyclic quinazolinone-iminosugars showed no inhibitory effects on the tested five glycosidases.