Catalyst-Controlled Stereoselective O-Glycosylation: Pd(0) vs Pd(II)

Abstract

Stereoselective construction of various O-glycosidic bonds was first achieved by different palladium sources using 3,4-O-carbonate galactal as the donor to reach yields up to 95% under mild conditions. With Pd­(II) catalyst coordination of this glycal donor from the β-face directed by carbonate group, hard nucleophiles (aliphatic alcohols) gave β-glycosides and α-glycosides were obtained from soft nucleophiles (phenols). In contrast, with the Pd(0) catalyst coordinating the donor from the β-face due to steric effect, both hard and soft acceptors could only generate β-glycosides via hydrogen-bond-mediated aglycone delivery.

2-O-Benzyloxycarbonyl protected glycosyl donors: a revival of carbonate-mediated anchimeric assistance for diastereoselective glycosylation

“…glycosylated natural products) is the stereoselective formation of glycosidic linkages. [1][2][3][4] The synthesis of 1,2-trans glycosides (e.g. b-glucosides) applying neighboring participating protective groups is well established, [5][6][7] but still problematic considering the incompatibility of many natural products with reaction conditions usually applied for the removal of the most common participating groups, i.e.…”

“…The 2-O-picolinyl moiety, an arming participating group, 19 and the 3-(2 0 -benzyloxyphenyl)-3,3-dimethylpropanoyl group 20 can be cleaved by palladium-catalyzed hydrogenation, while the cyanopivaloyl group can be removed by reduction followed by mild base treatment. 21 Further studies include the investigation of new strategies to influence the diastereoselectivity of a glycosylation reaction by applying a linker effect, 22 remote protecting groups, 23 specific promotor-systems, 24 catalysts, 4,25 or steric effects. 26 However, all these methods have a limited scope in terms of diastereoselectivity, reactivity, and versatility of glycosylation and/or deprotection.…”

Part I. Palladium-catalysed C-glycosylations. Part II. Synthesis of indolizines via intramolecular C-N bond formation

“…Stannylated glycals can be applied in Stille reaction (Scheme 1.11). 33,34 This type of glycosylation proceeds via oxidative addition of organic halide to palladium(0), following which transmetallation with stannylated glycals and subsequent reductive elimination occurs to yield the desired product. Aryl halides, alkenyl halides and glycal halide were successfully applied 33 and interestingly, a desulfurative variant using aryl sulfonyl chloride was also reported.…”

“…Aryl halides, alkenyl halides and glycal halide were successfully applied 33 and interestingly, a desulfurative variant using aryl sulfonyl chloride was also reported. 34…”

“…Hence, we selected the galactal with a cyclic carbonate moiety 3.1a as our model glycosyl donor based on our recent report of conducting glycosylation at room temperature. 34 Next, in order to effect the electron transfer to our allylic cation generated, an appropriate electron donor should be chosen. Initial attempts of utilising potassium allyltrifluoroborate and potassium benzyltrifluoroborate salts failed to yield the desired product.…”