Carbohydrates exist as glycol-conjugates and interact with various biomolecules in cells. However, the exact functions of carbohydrates remain poorly understood, because they are known to readily undergo hydrolytic cleavage by metabolic enzymes. C-glycoside analogues have been of particular interest as carbohydrate mimics, because the replacement of the oxygen atom by a methylene group renders them stable against metabolic enzymes [1]. C-glycoside analogues have been shown to adopt similar conformations to native O-glycosides and also to exhibit similar or even more potent biological activities [2]. However, it is generally complicated to synthesize these analogues because of the highly oxidized structure and the dense sp³ carbon centers. To supply a variety of biologically useful C-glycosides, efficient synthetic method was required.
In this study, we aimed to synthesize the typical (1,6)-CH₂-glycoside analogues. We thought that “direct coupling” between a C1-sp³-donor and an acceptor with additional carbon atom at C6 would be the one of the powerful method [3,4]. This time, we focused on the atom-transfer radical coupling reaction [5] between the xanthate donor 1 and the acceptor 2 with a terminal olefin (Scheme 1). The glycosyl radical species 3 generated from stable donor 1 by treatment with a peroxide reagent added to alkene 2. Following xanthates transfer from donor 1 to transient secondary radical on the 6-position of the acceptor resulted in C-glycoside 4. The xanthate group of 4 was easily removed by means of a simple reduction process to give CH₂-linked (1,6)-disaccharide 5. The radical coupling reaction proceeded to form the α-isomer exclusively, when we employed the glycosyl donor 1 with appropriate protecting group in order to restrict it ⁴C₁ conformation. This methodology was successfully applicable to synthesize the CH₂-linked isomaltose analogue 6 by employing the appropriate glucose acceptor. Details of the synthesis of both donor 1 and acceptor 2, and their radical coupling reaction will be disclosed in this presentation [6].
Scheme 1. Radical coupling with xanthate donor 1 and acceptor 2
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- Representative synthesis of a variety of CH₂-linked disaccharides with C1-sp² hybridized donor; Werz, D. B. et al. Angew. Chemie - Int. Ed. 2013, 52, 2985.
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