Stereoselectivity in the Epoxidation of Carbohydrate-Based Oxepines

Abstract

The facial selectivity in the DMDO epoxidation of carbohydrate-based oxepines derived from glucose, galactose, and mannose has been determined by product analysis and density functional theory (DFT, B3LYP/6-31+G**//B3LYP/6-31G*) calculations. Oxepines 3 and 4, derived from d-galactose and d-mannose, largely favor alpha- over beta-epoxidation. The results reported here, along with selectivities in the DMDO-mediated epoxidation of d-xylose-based oxepine 1 and d-glucose-based oxepines 2 and 5 reported earlier, support a model in which electronic effects, guided by the stereochemistry of the oxygens on the oxepine ring, largely determine the stereoselectivity of epoxidation. Other contributing factors included conformational issues in the oxepine's transition state relative to the reactant, the asynchronicity in bond formation of the epoxide, and the overall steric bulk on the alpha- and beta-faces of the oxepine. Considered together, these factors should generally predict facial selectivity in the DMDO-epoxidation of cyclic enol ethers.

Expanding the Scope of Aminosugars: Synthesis of 2‐Amino Septanosyl Glycoconjugates Using Septanosyl Fluoride Donors

“…Product septanosides exclusively of the a-configuration were obtained in good yields (69-88 %). Attempted reaction with (S)-phenyl b-2,3,4-tri-Obenzyl glucoside (24) under the established conditions failed to provide the desired disaccharide, both at RT and when heating 1,2-dichloroethane (DCE) to reflux. [16] Overall, this route provided facile access to 2-amino septanosyl derivatives using simple alcohols as co-solvents, but it suffered from two major limitations.…”

“…Formation of a fused oxazolidinone, 35, through a mechanism depicted in Scheme 7, seemed likely. Oxocarbenium ion formation (e.g., 34) for the mannose-configured systems is known to be facile, [24,25] and examples of oxazolidinones arising from similarly functionalized pyranose examples are known. [26] Also, the structure of the proposed product was consistent with the NMR spectra and MS data.…”

“…NMR analysis confirmed the presence of the Fmoc group and only four benzyl signals, which raised the possibility of intramolecular attack at C1 of oxocarbenium 36 by the C5 benzyloxy group as observed before by us in other examples. [24,28] Reaction through the C5 benzyloxy group is probably the dominant pathway. We propose that formation of the oxazoladinone in 35 is unique and arises only because the C5 pathway is shut down in this system.…”

Tandem cyclization and acetalization of a homologated D-glucose delivers D-glycero-D-guloseptanosides

“…6,7,8,9,10 Carbohydrate based oxepines can be epoxidized to give 1,2-anhydrosugars such as 4 in a manner akin to glycals. 11 Addition of an appropriate nucleophile then gives the corresponding glycoside. If a thiol is used, the resulting thioseptanoside may be used for subsequent glycosylation reactions.…”

A mild and efficient method for the selective cleavage of primary p-methoxybenzyl protecting group of saccharides by Co2(CO)8–Me2PhSiH–CO system

“…Mannosyl substrates 6a and 7a (entries 4 and 5) gave poorer yields when treated with 0.5 equiv Co 2 (CO) 8 , but higher yields were obtained for 7a when the amount of Co 2 (CO) 8 was increased to 1.0 equiv, which indicated the 2-axial-OPMB group can greatly influence the reactivity of the 6-O-PMB group. 18,19 Moreover, no reaction occurred for substrate 11a (entry 9) when the equivalent of Co 2 (CO) 8 was increased to 1.5 and 11a was recovered in almost quantitative yields. This result indicated that 4,6-O-benzylidine group was stable under this condition.…”