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Journal of the American Chemical Society, Vol.138, No.11, 3926-3932, 2016
Total Synthesis of Mannopeptimycins alpha and beta
The mannopeptimycins are a class of glycopeptide natural products with unusual structures and potent antibiotic activity against a range of Gram-positive multidrug-resistant bacteria. Their cyclic hexapeptide core features a pair of unprecedented beta-hydroxyenduracididines (L- and D-beta hEnd), an O-glycosylated D-Tyr carrying an alpha-linked dimannose, and a beta-methylated Phe residue. The D-beta hEnd unit also carries an alpha-linked mannopyranose at the most hindered N of its cyclic guanidine ring. Herein, we report the first total synthesis of mannopeptimycin alpha and beta with fully elaborated N- and O-linked sugars. Critically, a gold-catalyzed N-glycosylation of a D-beta hEnd substrate with a mannosyl ortho-alkynylbnzoate donor enabled the synthesis of the most challenging N-Man-D-beta hEnd unit with excellent efficiency and stereoselectivity. The L-beta MePhe unit was prepared using a Pd-catalyzed C-H arylation method. The L-beta hEnd, D-Tyr(di-Man), and L-beta MePhe units were prepared in gram quantities. A convergent assembly of the cyclic peptide scaffold and a single global hydrogenolysis deprotection operation provided mannopeptimycin alpha and beta.