ortho-thioquinones, new acceptors for the stereoselective synthesis of aryl 2-deoxy-O-glycosides

alpha-Hydroxynaphthylthiophthalimide (1) is a suitable precursor of the rea ctive ortho-thioquinone 2, which can be generated in situ and trapped by gl ycals. The reaction is an inverse electron-demand [4+2] cycloaddition that occurs in a totally regioselective and highly stereoselective way. A series of differently substituted glycals (3, 5-12, 21, 22, 38) as well as variou s ortho-thioquinones (25-27) are successfully used as electron-rich dienoph iles and electron-poor dienes, respectively, with chloroform, dimethylforma mide, and dimethyl sulfoxide as solvents. The stereochemistry of substituen ts on the dienophile strongly influences the selectivity of the cycloadditi on, which becomes totally stereoselective when galactals 8-10 or arabinals 21 and 22 are employed as dienophiles. Among the heterodienes tested, the a -naphthol derivative 25 and the tyrosine derivative 26 were successfully us ed to prepare the naphthyl-alpha-O-rhamnoside 32 alpha and the tyrosine-alp ha-O-glucoside 34, respectively. Cycloadducts 4 alpha, 17, 24, and 33 alpha were successfully desulfurized, affording the corresponding aryl 2-deoxy-a lpha-O-glycosides 39 and 40-42.