A new route to exo-glycals 4 is described in which glycosyl sulfones 3 are subjected to the Meyers variant of the Ramberg−Bäcklund rearrangement. The conversion of sulfones derived from glucose, galactose, mannose, cellobiose, and ribose into di-, tri-, and tetra-substituted alkenes is re-

Synthetic applications of exo-glycals, derived from S-glycoside dioxides using the Meyers variant of the Ramberg−Bäcklund rearrangement, are described. These include a formal synthesis of a β-glycosidase inhibitor 12 and an efficient route to spirocyclic glucose derivatives 17 and [a] Scheme 2. exo-Glycals are versatile intermediates for the synthesis of more highly functionalised C-glycosides CHCl 3 )}. Sigmatropic rearrangements involving the enol ether function have recently been described: a Claisen rearrangement of 6-vinyl exo-glycals [12] and a ClaisenϪIreland rearrangement of 2-O-acyl exo-glycals [13] furnish cyclooctene and C-glycan derivatives, respectively. Exomethylenic sugars have also proved to be good substrates for ring-closing olefin metathesis: derivatives of 2a bearing an unsaturated side chain at position 2 have been shown to undergo ruthenium catalysed ring closure to give bicyclic glycosylidene compounds. [14] Postema and Calimente have also exploited this powerful carbonϪcarbon bond-forming process for the preparation of C1-substituted glycals [15a] and C-linked disaccharides. [15b] The phenyl-substituted exo-glycal derivative 2b is accessible in high yield and good stereoselectivity from the tandem halogenation-RambergϪBäcklund rearrangement of benzyl sulfone 1b (Scheme 1). We decided to explore the elaboration of this substrate (Scheme 3). When hydroboScheme 3. HydroborationϪoxidation of exo-glycal 2b to give the known C-glycoside 11Eur.

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Readily available S-glycoside dioxides were utilized in a Ramberg-Bäcklund rearrangement for the construction of C-linked disaccharides. This approach is ideally suited to analogue synthesis simply by variation of the alkylating agent, and is illustrated here by the synthesis of beta,beta-C-trehalose (see reaction scheme), a higher homologue of C-trehalose, and methyl C-gentiobioside. Bn=benzyl.