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- Title
Glycosylidene Carbenes. Part 6. Synthesis of alkyl and fluoroalkyl glycosides.
- Authors
Briner, Karin; Vasella, Andrea
- Abstract
The syntheses of glycosides from the diazirine 1 and a range of alcohols under thermal and/or photolytic conditions are described. Yields and diastereoselectivities depend upon the p K HA values of the alcohols, the solvent, and the reaction temperature. The glycosidation of weakly acidic alcohols (MeOH, EtOH, i-PrOH, and t-BuOH, 1 equiv. each) in CH2Cl2 at room temperature leads to the glycosides 2-5 in yields between 60 and 34% ( Scheme 1 and Table 1). At −70 to −60°, yields are markedly higher. In CH2Cl2, diastereoselectivities are very low. In THF, at −70 to −60°, however, glycosidation of i-PrOH leads to α- D-/β- D- 4 in a ratio of 8:92. More strongly acidic alcohols, such as CF3CH2OH, (CF3)2 CHOH, and (CF3)2C(Me)OH, and the highly fluorinated long-chain alcohols CF3(CF2)5(CH2)2OH ( 11) and CHF2(CF2)9CH2OH ( 13) react (CH2Cl2, r.t.) in yields between 73 and 85% and lead mainly to the β- D-glucosides β- D- 6 to β- D- 8, β- D- 12, and β- D- 14 (d.e. 14-68%). Yields and diastereoselectivities are markedly improved, when toluene, dioxane, 1,2-dimetoxyethane, or THF are used, as examined for the glycosidation of (CF3)2C(Me)OH, yielding (1,2-dimethoxyethane, 25°) 80% of α- D-/ β- D- 8 in a ratio of 2:98 (d.e. 96%; Table 4). In EtCN, (CF3)2C(Me)OH yields up to 55% of the imidate 10. Glycosidation of di- O-isopropylideneglucose 15 leads to 16 (CH2Cl2, r.t.; 65%, α- D/ β- D = 33:67). That glycosidation occurs by initial protonation of the intermediate glycosylidene carbene is evidenced, for strongly acidic alcohols, by the formation of 10, derived from the attack of (CF3)2MeCO− on an intermediate nitrilium ion ( Scheme 4), and for weakly acidic alcohols, by the formation of α- D- 9 and β- D- 9, derived by attack of i-PrO− on intermediate tetrahydrofuranylium ions. A working hypothesis is presented ( Scheme 3). The diastereoselectivities are rationalized on the basis of a protonation in the σ plane of the intermediate carbene, the stabilization of the thereby generated ion pair by interaction with the BnOC(2) group, with the solvent, and/or with the alcohol, and the final nucleophilic attack by RO− in the π plane of the (solvated) oxonium ion.
- Publication
Helvetica Chimica Acta, 1992, Vol 75, Issue 2, p621
- ISSN
0018-019X
- Publication type
Article
- DOI
10.1002/hlca.19920750218