Stereoselective S_E2‘ Additions of Enantioenriched Allylic Tin and Indium Reagents to Protected Threose and Erythrose Aldehydes:  A General Strategy for the Stereocontrolled Synthesis of Precursors to the Eight Diastereomeric Hexoses and Their Enantiomers

The enantioenriched (∼90−95% ee) α-alkoxy allylic stannanes (S)- and (R)-2.1 undergo in situ transmetallation with InCl₃ in EtOAc and subsequent S_E2‘ addition to aldehydes to afford anti adducts 3.2a−d stereospecifically with excellent diastereoselectivity (90:10−98:2). Additions to the protected threose and erythrose aldehydes 4.2 and 4.4 are reagent controlled, yielding the anti adducts 5.1−5.4 with high stereoselectivity. These adducts are potential precursors of differentially protected l-talose, d-allose, l-glucose, and d-mannose.