IntroductionThe synthesis of enantiomerically pure molecules is one of the main tasks in modern synthetic chemistry. 1 Mild and simple methodologies associated with readily available starting materials are desired. In this context, sugars as raw materials 2 and free radical technologies as a method for carbon-carbon bond formation 3 are now well established choices for organic chemists. The advantages and the success of these techniques have been particularly evident in the preparation of enantiomerically pure and densely functionalized carbocycles from carbohydrates. 4My group has been working to develop new stereoselective vias for the synthesis of polyfunctionalized carbocycles from readily available carbohydrates. Particular emphasis has been devoted to the use of free radical based technologies. Moreover, we have assumed that intramolecular ring closures are well suited to the rapid assembling of chiral and complex radical precursors. Two strategies have been devised to achieve these goals: a) monosaccharide protection, opening, functionalization positioning the radical donor and the radical acceptor, and finally radical cyclization (Scheme 1, via A); b) the sugar is protected and manipulated to introduce branched chains containing the radical donor and the radical acceptor, with the anomeric position "locked" in an O-glycosylated form. In this approach an annulated sugar results around the sugar template (in pyranose or furanose form) (Scheme 1, via B). In this account we report a summary of results obtained by using the second protocol. Treatment of the radical precursor with tributyltin hydride and AIBN as promotor, at appropriate concentration and addition rate, in toluene as solvent, are the usual experimental conditions. Free radical methodology allows the conservation of the stereochemical integrity at the different stereocenters in the radical precursor. This is an important point in enantiomerically pure polyhydroxylated compounds as sugars. Another critical aspect is the potential high degree of the extent of the qualitative and quantitative asymmetric induction when new stereocenters are formed during ring closure. In this project we have learned how to modulate sugars in order to provide a wide variety of radical precursors and how to obtain efficient yields, with high diastereoselection, of the resulting carbocycles.Scheme 1 BackgroundAt the outset of this project, a plethora of reports concerning the synthesis of annulated pyranoses was available in the literature, and will not be described here in detail. The reader is referred to the exhaustive Abstract: Tributyltin hydride plus AIBN mediated free radical cyclization of chiral precursors derived from O-glycosylated carbohydrates is an efficient method for the asymmetric synthesis of carbocycles (cyclitols, aminocyclitols and pseudosugars) of biological and pharmacological interest. Starting from simple aldoses and after a series of trivial synthetic steps for hydroxyl protection and functionalization, it is possible to prepare the required ra...

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[reactions: see text] Readily available sulfinyl and sulfonyl tetrahydrofuran methanol derivatives have been transformed efficiently into a variety of substituted tetrahydrofuryl alcohols by treatment with (PhSe)2 in the presence of an excess of NaBH4. Alternatively, oxirane cleavage with MgI2 produces the related ketones, amenable to stereocontrolled reduction. This reductive cleavage methodology has been applied to short formal syntheses of trans-Kumausyne and Kumausallene.

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