α-Selective glycosylations using glycosyl N-(ortho-methoxyphenyl)trifluoroacetimidates

Substrate specific closed-loop optimization of carbohydrate protective group chemistry using Bayesian optimization and transfer learning

A new way of performing reaction optimization within carbohydrate chemistry is presented. This is done by performing closed-loop optimization of regioselective benzoylation of unprotected glycosides using Bayesian optimization. Both 6-O-monobenzoylations and 3,6-O-dibenzoylations of three different monosaccharides are optimized. A novel transfer learning approach, where data from previous optimizations of different substrates is used to speed up the optimizations, has also been developed. The optimal conditions found by the Bayesian optimization algorithm provide new insight into substrate specificity, as the conditions found are significantly different. In most cases, the optimal conditions include Et₃N and benzoic anhydride, a new reagent combination for these reactions, discovered by the algorithm, demonstrating the power of this concept to widen the chemical space. Further, the developed procedures include ambient conditions and short reaction times.

Ferrocenium complex aided O-glycosylation of glycosyl halides

A new strategy for the activation of glycosyl halide donors to be utilized in glycosylation reactions is presented, utilizing the ferrocenium (Fc) complexes [FcB(OH)₂]SbF₆ and FcBF₄ as promoters. The scope of the new system has been investigated using glycosyl chloride and glycosyl fluoride donors in combination with common glycosyl acceptors, such as protected glucose. The corresponding glycosylation products were formed in 95 to 10% isolated yields with α/β ratios ranging from 1/1 to β only (2 to 14 h reaction time at room temperature, 40 to 100% ferrocenium promoter load).

Ferrocenium complexes as a new, tunable platform for O-glycosylation reactions are introduced.

Synthesis of isobemisiose, neosartose, and fischerose: three α-1,6-linked trehalose-based oligosaccharides identified from Neosartorya fischeri

Three complex α-1,6-linked trehalose-based oligosaccharides with unique preservation properties, isobemisiose, neosartose, and fischerose, were recently identified from the extreme stress-tolerant ascospores of Neosartorya fischeri. Herein, we report the first concise, scalable, and iterative chemical synthesis of these oligosaccharides from a differentially protected thioglycoside donor and a selectively protected, asymmetric trehalose acceptor. This work constitutes an improved synthesis of isobemisiose, and is also the first reported synthesis of neosartose, a tetrasaccharide, and fischerose, a pentasaccharide, in good yield. Importantly, in-depth studies of biological function are enabled by this synthetic platform.

The first concise and scalable chemical synthesis of three complex α-1,6-linked trehalose-based oligosaccharides, isobemisiose, neosartose, and fischerose, are reported for the first time.