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Stereocontrolled Nucleophilic Addition to Five-Membered Oxocarbenium Ions Directed by the Protecting Groups. Application to the Total Synthesis of (+)-Varitriol and of Two Diastereoisomers Thereof.

A stereodivergent C-glycosidation of carbohydrate-derived lactones can be mediated by the protecting groups and applied to the total synthesis of (+)-varitriol and of two diastereoisomers thereof, which represent an unprecedent use of the protecting groups in the synthesis of a naturally occurring compound. In particular, the stereoselective nucleophile attack for 2,3-trans-substituted five-membered ring oxocarbenium ions is strongly influenced by the presence of aromatic rings in the protecting groups. According to quantum chemical calculations, the stereoselectvity depends on the π-π interactions between the aromatic ring of the C-2 protecting group with the exocyclic triple bond and the oxocarbenium ion. These interactions account for the stabilization of the conformer in which the C-2 and C-3 substituents adopt pseudoaxial orientations. When protecting groups do not contain an aromatic ring, the sterochemical outcome is dictated by stereoelectronic factors established by the Woerpel's model. Based on these findings, a concise total synthesis of the natural product (+)-varitriol and of two diastereoisomers was acomplished.