Inter- and Intramolecular Hetero-Diels-Alder Reactions, Part 48.¹ De-Novo Synthesis of Enantiopure Carbohydrates: Preparation of Ethyl β-D- and β-L-Mannopyranosides by an Asymmetrically Induced Hetero Diels-Alder Reaction

The synthesis of dihydropyrans 4 and 5 based on the asymmetrically 1,6-induced intermolecular hetero Diels-Alder reaction of the heterodiene 2 [(4S)-4-tert-Butyl-3-[(E)-4′-benzyloxy-2′-oxo-3′-butenoyl]-oxazolidin-2-one] with (Z)-1-acetoxy-2-ethoxyethene 3 in the presence of a Lewis acid is described. A reversal of facial differentiation could be achieved by variation of the Lewis acid. Thus cycloaddition of 2 and 3 preferentially yields 4 in the presence of Me₂AICl and 5 using TMS-OTf. Simple transformations of 4 and 5, respectively afforded the desired ethyl mannopyranosides 12 and ent-12 diastereoselectively and in good yield.