In 2020, the Diao group showcased a diastereoselective synthesis of aryl/heteroaryl-C-glycosides via the cross-coupling of DHP-derived glycosyl esters with aryl bromides (Scheme 5).[15] This method involves the initial oxidation of DHP-derived glycosyl ester with excited photocatalyst 4CzIPN, followed by deprotonation, to generate radical 49 (Scheme 5-A). Fragmentation of this radical, driven by the formation of Hantzsch pyridine 50 , then enables the formation of glycosycarbonyl radical 51 . The subsequent ejection of CO2leads to the formation of a glycosyl radical 52 , which was then captured by aryl-Ni(II) species 54 to form an essential Ni(III) intermediate 53 . This is followed by reductive elimination to yield aryl C-glycoside 46 and the concomitant formation of Ni(I) species 47 . The authors suggested that the initial formation of active Ni(0) species from a Ni(II) precatalyst could be accomplished by the reduced photocatalyst. The reaction tolerated a range of aryl bromides when using D-mannofuranosyl esters as coupling partners, but oxidizable substrates such as para-dimethylaminophenyl bromide (46d ) and 3-bromo furan (46h )/pyrrole (46i ) were ineffective. The furanoses, including D-xylofuranose (46q ), D-glucofuranose (46r ), D-galactofuranose (46s ), and D-arabinofuranose (46t ), were all suitable substrates, and all of these reactions favored the generation of 1,2-trans products due to the steric hindrance caused by adjacent substituents. This notion was further supported by a moderate 1,2-trans selectivity when 2-deoxyl-D-ribose (46u ), lacking a C2 substituent. The pyranoses were also competent coupling partners in the developed reaction and the observed diastereoselectivity can be attributed to the counterbalance between the kinetic anomeric effect and steric repulsion generated by adjacent substituents. For example, excellent 1,2-trans selectivity was observed in the case of D-mannopyranose (46v–46x )due to the presence of these two effects, which both favor α-attack. Poor selectivity was observed in the reactions of D-glucosyl (46ab, 46ac ) and D-galactosyl esters (46ad ), as the steric hindrance at C2 favors β-attack, though the transition state for the α-attack could be stabilized by the anomeric effect, thereby neutralizing these two effects. Interestingly, a much better α-selectivity was observed for per-Bn-protected 2-deoxy-D-glucose (46y ) than per-Bn-protected D-glucose (46ab ), despite there being no further discussion of this in the report.
In 2019, the Niu group reported a nickel-catalyzed Suzuki-Miyaura cross-coupling of 1-sulfonyl glycals with aryl boronic acids
and their derivatives to prepare aryl C-glycals (Scheme 6).[16] The developed methods proceed under mild conditions and can accommodate a broad spectrum of organoboron nucleophiles and 1-sulfonyl glycals (50a–50r , Scheme 6). Notably, the synthetic value of the transformation was demonstrated by the successful synthesis of ipragliflozin 51a , 2-deoxy ipragliflozin 51b , and glycosyl tryptophan derivative50t (Scheme 7-A).
Scheme 6 Nickel-catalyzed Suki-Miyaura cross-coupling