C-Glycosylation of tri-O-benzyl-2-deoxy-D-glucose:: synthesis of naphthyl-substituted 3,6-dioxabicyclo[3.2.2]nonanes

The syntheses of naphthol 7, naphthol 8, naphthol 11 and naphthol 12 are described, starting from juglone 13. C-Glycosylation of naphthol 8 with benzyl-protected glycosyl donor 10 using trimethylsilyl trifluoromethanesulfonate and silver perchlorate or boron trifluoride-diethyl ether affords rearranged product 36 in which the glycosyl donor has undergone an unusual 1,6-hydride shift. Use of the corresponding naphthol 12 as the glycosyl acceptor under the same conditions affords the expected C-glycoside 34. Use of the naphthol 7 and naphthol 11 affords predominantly rearranged products 35 and 37 respectively, albeit in much lower yield than the reactions using the corresponding bromonaphthols. The study described herein establishes that introduction of an acetyl group to C-3, as in C-glycosylnaphthoquinone 4, as required for conversion to analogues of medermycin 1 such as 3, necessitates that the C-glycosylation step be effected before regioselective introduction of the acetyl group.