Diastereoselective synthesis of the monosaccharide kedarosamine and incorporation in an analogue of the enediyne kedarcidin chromophore

Kedarcidin chromophore, as with most enediyne antitumor antibiotics, contains unusual monosaccharide moieties. Synthesis of one of these moieties, the 2,4,6-trideoxy-4-dimethylaminohexose kedarosamine, from D-threonine and incorporation into an analogue of kedarcidin chromophore (1) is described herein. Conversion of D-threonine into allyl ketone 7 and stereoselective reduction by using tetramethylammonium triacetoxyborohydride for intramolecular hydride delivery were key steps in the preparation of kedarosamine. A thioglycoside derivative of kedarosamine (12) was found to be less efficient as a glycosyl donor, whereas a 1-O-acetate (15) gave the desired alpha-glycoside exclusively in 60-80% yield when treated with borontrifluoride etherate. Use of a Cbz instead of a Fmoc protecting group for the C-4 amino group of kedarosamine was essential for the successful preparation of analogue 1. Finally, dimethylation of the amino group at C-4 of kedarosamine was found to require careful adjustment of the reaction conditions in order to avoid byproduct formation.