Synthesis of spiro[1,4-benzothiazine-2,2′-pyrans] starting from methyl β-d-arabino-2-hexulopyranosonate

Methyl beta-D-arabino-2-hexulopyranosonate 1 has via the novel glycosyl donor 3 been transformed into the thiophenyl glycosides 4 and 5. Catalytic hydrogenation of the nitro compound 4 in alkaline solution led to spontaneous cyclization and deprotection to form the cyclic hydroxamic acid 7. The related lactams 8 and 9 were obtained from amine 5. The spiro[1,4-benzothiazine-2,2'-pyrans] 7-9 are the first representatives of a novel class of heterocycles structurally related to bioactive natural products. As shown by the values for J(3',4') and J(4',5') the glycosides 4, 5 and 6 adopt a C-5(2) conformation of the pyranoid ring whereas the 1,4-benzothiazine system in 7-9 forces a conformational change into the C-2(5) conformation.