Thiourea mediates cooperative glycosidation through hydrogen bonding. N,N'-Diarylthiourea as cocatalyst enforces an SN2-type acid-catalyzed glycosidation even at room temperature (see scheme; Bn=benzyl). From O-(α-glycosyl) trichloroacetimidates as glycosyl donors and various acceptors, β-glycosides are preferentially or exclusively obtained.
The combination of PPh3/I2 has been shown to be effective for conversion of a range of carboxylic acids to 2°, 3°, and Weinreb amides. Simplification of the procedure was possible with the use of polymer-supported PPh3/I2. Weinreb amides produced via the use of polymer-supported PPh3 could be filtered through a short silica gel plug and used in further transformations. Thus, use of polymer-supported PPh3 offers potential applicability to diversity-oriented reactions. Formal total syntheses of apocynin and pratosine, as well as syntheses of anhydrolychorinone and hippadine, have been achieved via the use of this amide-forming method. An attempt has been made to gain insight into this reaction.
The glycosylation of O-glycosyl trichloroacetimidate donors using a synergistic catalytic system of electron-deficient pyridinium salts/aryl thiourea derivatives at room temperature is demonstrated. The acidity of the adduct formed by the 1,2-addition of alcohol to the electron-deficient pyridinium salt is increased in the presence of an aryl thiourea derivative as an hydrogen-bonding cocatalyst. This transformation occurs under mild reaction conditions with a wide range of O-glycosyl trichloroacetimidate donors and glycosyl acceptors to afford the corresponding O-glycosides in moderate to good yields with predictable selectivity. In addition, the optimized method is also utilized for the regioselective O-glycosylation by using a partially protected acceptor.
Adduct formation between alcohols as glycosyl acceptors and phenylsilicon trifluoride (PhSiF3) as catalyst permits acid‐base‐atalyzed glycosidations with O‐glycosyl trichloroacetimidates as glycosyl donors. In this way, from various glycosyl donors and acceptors 1,2‐trans‐ and some 1,2‐cis‐glycosides could be obtained with high anomeric selectivity. A preference for an intramolecular bimolecular nucleophilic substitution (SN2‐type) reaction course with concomitant donor and acceptor activation is supported by the results.
An
efficient and operationally simple gold(III)-catalyzed glycosylation
protocol was developed using newly synthesized benchtop stable phenylpropiolate
glycosyl (PPG) donors. Gold(III)-catalyzed activation of PPGs proceeds
well with various carbohydrate and noncarbohydrate-based glycosyl
acceptors and leads to their corresponding O/N-glycosides in good to excellent yields with regeneration
of reusable and easily separable phenylpropiolic acid. Differentially
protected PPGs reacted well under the optimized reaction conditions.
In particular, good anomeric selectivity was observed with mannosyl
and rhamnosyl PPG donors. A preliminary mechanistic study reveals
that the presence of a triple bond adjacent to the ester group is
essential for activation, and PPG-based donor shows higher reactivity
than analogous acetate and benzoate donors.
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