D- and L-nucleosides and analogues thereof, including the 4'-thionucleoside series, are one of the most important biological and pharmaceutically active classes of compounds. A novel approach to their synthesis from chiral acyclic thioaminal, bearing the nucleobase, is described.
Synthesis of the highly biologically valuable cardenolide backbone was achieved via anionic polycyclization. Bromoketone 18, obtained from double-Michael cycloaddition between cyclohexenone 14 and gamma,delta-unsaturated beta-ketoester 16, was efficiently aldolized under reductive conditions. The highly functionalized tetracyclic compound 52 is an important synthetic intermediate that is potentially amenable to natural cardenolide total synthesis.
[reaction: see text] Acyclic diastereoselection is achieved for the formation of thioaminyl acetals. The highly intramolecular stereocontrolled S(N)2 displacement of the thioaminyls allows for the formation of cyclic nucleoside derivatives. This versatile approach may provide easy access to a large variety of N-glycosides.
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