Stereoselective synthesis of sugar-fused (or 1,2-annulated) isochromans and isochromanones by using oxa-Pictet-Spengler reaction

Diastereoselective synthesis of glycopyrans 1,2-annulated with dioxazinanes from 1,2-anhydrosugars and N-substituted nitrones

1,2-Annulated pyranose sugars fused with six membered rings have emerged as an important class of carbohydrates with wide biological and synthetic utility. We now describe zinc chloride catalyzed one pot diastereoselective synthesis of sugar fused dioxazinanes from 1,2-anhydro sugars and N-substituted aromatic nitrones. Various aromatic nitrones with different substituents undergo the reaction smoothly. The developed strategy works well with both ester and ether protection on the sugar and proceeds under mild reaction conditions. The mechanism seems to involve activation of the anhydrosugar by ZnCl₂ for nucleophilic attack by the nitrone followed by cyclization.

Enantioselective synthesis of indanone spiro-isochromanone derivatives via a dinuclear zinc-catalyzed Michael/transesterification tandem reaction

An enantioselective Michael/transesterification tandem reaction of α-hydroxy indanones with ortho-ester chalcones was realized using dinuclear zinc catalysts. A series of enantiomerically pure spiro[indanone-2,3'-isochromane-1-one] derivatives were obtained in good yields with excellent stereoselectivities (up to >20 : 1 dr, up to >99% ee). This protocol could be conducted on a gram scale without affecting its stereoselectivities. In addition, the absolute stereochemistry of the products was determined by X-ray crystallographic analysis of 3ac, and a positive nonlinear effect was observed. Finally, a possible catalytic cycle was proposed to explain the origin of the enantioselectivity.

Stereoselective synthesis of substituted 1,2-annulated sugars by domino double-Michael addition reaction

A simple, highly stereoselective one-pot methodology for the synthesis of novel 1,2-annulated sugars comprising of oxa-oxa and oxa-carbasugar fused skeletons from 2-nitrogalactal and a sugar-derived enone, obtained from 2-formylgalactal, promoted by KO^tBu and CH₃ONa respectively is described. Both processes rely on a domino double-Michael addition reaction resulting in the formation of three stereocenters in a single pot, including a quaternary center.

Palladium catalyzed synthesis of sugar-fused indolines via C(sp2)-H/NH activation

A simple Pd(OAc)₂ catalyzed strategy for the synthesis of sugar-fused indolines from 2-N-oxalylamido-2-deoxy-C-aryl glycosides is reported by utilizing N-oxalylamido group as an auxiliary via C(sp²)-H/NH Activation. The reaction is successfully applied on glucose as well as galactose derived differently substituted 2-N-oxalylamido-2-deoxy-C-aryl glycosides to give sugar-fused indolines in moderate to good yields. The utility of this strategy in the synthesis of sugar-fused indoles is also described.