Abstract
Attempts to effect direct C-glycosylation of naphthols 6, 7, and 8 using glucosyl donors 9 and 10 were unsuccessful. O-Glycosides 11, 12 and 15 were obtained under Mitsunobu conditions, however, these failed to undergo rearrangement to the C-glycosides 13, 14 and 16, respectively. Successful Cglycosylation of naphthols 7 and 8 was realized using the more reactive 2- deoxyglucosyl acetate donor 18 with trimethylsilyl triflate and silver perchlorate as the Lewis acid promoters. Use of acetonitrile as solvent formed the C-glycosides 20 and 22 in preference to the corresponding O-glycosides 19 and 21, respectively.
Highlights
The pyranonaphthoquinone family of antibiotics have attracted considerable synthetic interest[1] due to their proposed ability to act as bioreductive alkylating agents via quinone methide intermediates
Our approach to the synthesis of several of the simpler members of the pyranonaphthoquinone antibiotics has focused on the addition of a silyloxyfuran to a naphthoquinone followed by oxidative rearrangement of the resultant furonaphthofuran adduct.[2]
We report our model studies on the C-glycosylation of naphthols 6,7 and 8 using glucosyl donors 9,10 and 2-deoxyglucosyl donors 17,18
Summary
The pyranonaphthoquinone family of antibiotics have attracted considerable synthetic interest[1] due to their proposed ability to act as bioreductive alkylating agents via quinone methide intermediates. Our approach to the synthesis of several of the simpler members of the pyranonaphthoquinone antibiotics has focused on the addition of a silyloxyfuran to a naphthoquinone followed by oxidative rearrangement of the resultant furonaphthofuran adduct.[2] Recently our attention has been directed towards the synthesis of some of the more complex pyranonaphthoquinone antibiotics which contain C-glycoside moieties as typified by medermycin 13 and mederrhodin 2.4 To date only one lengthy synthesis of medermycin 1 has been reported[5] in which the pyranonaphthalene skeleton was assembled by addition of a Cglycosyl-sulfonylphthalide to an enone. Given that the naphthoquinone functionality is installed via oxidation of an oxygenated naphthalene precursor, a crucial step for the synthesis of glucosyl pyranonaphthoquinones 3 and 4 is a successful method to effect C-glycosylation of appropriate naphthol precursors. We report our model studies on the C-glycosylation of naphthols 6,7 and 8 using glucosyl donors 9,10 and 2-deoxyglucosyl donors 17,18
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