α-N-Acetylmannosamine (ManNAc) Synthesis via Rhodium(II)-Catalyzed Oxidative Cyclization of Glucal 3-Carbamates

Abstract

[reaction: see text] Glucal 3-carbamates 1 and 7 underwent oxidative cyclization with iodobenzene diacetate or iodosobenzene in the presence of Rh2(OAc)4, providing mannosamine 2-N,3-O-oxazolidinones. With iodosobenzene, incorporation of 4-penten-1-ol provided a readily separable anomeric mixture of n-pentenyl glycosides, with the anomers exhibiting pronounced differences in reactivity as glycosyl donors. N-acylation of the sugar oxazolidinones led to alpha-selective glycosyl donors for the elaboration of various 2-mannosamine frameworks. Alternatively, the anomeric n-pentenyl glycosides of N-Cbz 2-mannosamine oxazolidinones were converted separately to oxazolidinone-opened derivatives 28alpha and 28beta. These served as stereoconvergent glycosyl donors, and the alpha-linked products were readily advanced to a variety of N-acetylmannosamine (ManNAc) frameworks, using an intramolecular O-->N acetyl transfer as the final step.

Nitrene Chemistry in Organic Synthesis: Still in Its Infancy?

“…Angewandte penten-1-ol as the glycosyl acceptor, leads to a 1.3:1 anomeric mixture of readily separable glycosides 4 accompanied by the dihydropyranone by-product 5 (Scheme 4). [30] The formation of 5 is explained by the ipso activation of the pseudo-axial electron-rich C3 À H bond, mediated by the generated rhodium nitrene. [31] The Rojas group, in addition, has nicely demonstrated the influence of the protecting groups at positions 4 and 6 on the stereo-and chemoselectivity of the reaction.…”

Nitren‐Chemie in der organischen Synthese: noch in den Kinderschuhen?

“…Neben Rhodium(II)‐Komplexen wird die Amidoglycosylierung auch durch Kupfersalze wie [Cu(MeCN) 4 ]PF 6 katalysiert, allerdings mit geringerer Effizienz (38–49 % Ausbeute im Vergleich zu 47–75 % Ausbeute mit Rh II ‐Katalysatoren). Dagegen bewirken die gleichen Bedingungen mit dem analogen Glucal‐3‐carbamat 3 unter Verwendung von 4‐Penten‐1‐ol als Glycosylakzeptor die Bildung einer 1.3:1‐Anomerenmischung aus den leicht trennbaren Glycosiden 4 sowie des Dihydropyranon‐Nebenprodukts 5 (Schema ) 30…”

Recent trends in the synthesis of O-glycosides of 2-amino-2-deoxysugars

“…30,218,[227][228][229][230] Tetraacetate 101 can be easily prepared from either unprotected glucosamine 226 or from 2-amino tetraacetate 75a. 218 The latter approach is preferred if the synthesis of 101 in anomerically pure b-form is required (Scheme 19).…”