摘要：

O R H + SiMe3 OH R Lewis acid O CCHX2 R + SiMe3 1 OH RC O 2 R = Ph, X = OH 3 R = p-HOC6H4, X = OH 4 R = p-MeOC6H4, X = OH 5 R = BuO, X = OH 6 R = menthylO, X = OH 7 R = BuO, X2 = O 8 R = Ph, X2 = O solvent, room temp. Yb(OTf)3 9 R = Ph 10 R = p-HOC6H4 11 R = p-MeOC6H4 12 R = BuO 13 R = menthylO Yb(OTf)3 catalysed allylation of the hydrates of a-keto aldehydes and glyoxylates with allylsilane Yang Yang, Mingwen Wang and Dong Wang* Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P.R. China Yb(OTf)3 catalysed allylation reactions of the hydrates of a-keto aldehydes and glyoxylates with allyltrimethylsilane at room temperature give a-keto and a-ester homoallylic alcohols in good yield. Recently, lanthanide trifluoromethanesulfonates [lanthanide triflates, Ln(OTf)3] have attracted attention1 because of their catalytic ability in various Lewis acid promoted carbon–carbon bond forming reactions, including Diels–Alder reactions,2 Michael reactions3 and allylation reactions of carbonyl compounds by allyl metals, for example allyltributylstannane and tetraallylgermane.4 Kobayashi5 also reported the Ln(OTf)3 catalysed reaction of enol silyl ethers with carbonyl compounds (Mukaiyama aldol reaction) in aqueous media.Among the Lewis acid promoted C–C bond forming reactions, the allylation of carbonyl compounds with allylsilanes (Sakurai reaction,6 Scheme 1) is one of the most important reactions in organic synthesis.However, stoichiometric amounts of Lewis acid must be employed due to the low nucleophilicity of allylsilane and the tight binding of the produced homoallylic alkoxide to the Lewis acid. The reaction can be performed using sub-stoichiometric amounts of silylating reagent [Me3SiOTf, Me3SiI and Me3SiB(OTf)4],7 while the Ln(OTf)3 catalysed Sakurai reaction was not realized until quite recently.8 Compared with typical Lewis acids such as TiCl4, SnCl4, BF3 and AlCl3, Ln(OTf)3 possesses stronger Lewis acidity, and is water soluble or water-tolerant, air-stable and easy to handle, not requiring anhydrous treatment.On the basis of the tolerance of Ln(OTf)3 to active protons, we examined the ytterbium trifluoromethanesulfonate [Yb(OTf)3] catalysed allylation of a-keto aldehydes hydrates 2–4 and glyoxylate hydrates 5 and 6 with allyltrimethylsilane 1. We also explored the extent of asymmetric induction in the allylation reactions of chiral glyoxylate hydrate 6 in the presence of catalyst Yb(OTf)3.Oxidation of the substituted acetophenones with selenium dioxide gave the corresponding hydrates 2–4, while the (2)-menthyl glyoxylate monohydrate 6 was prepared according to Kornblum’s method,9 starting from (2)-menthol. The allylation reactions (Scheme 2) were carried out following a standard procedure. To a solution of the hydrate (0.25 mmol) and Yb(OTf)3 (0.025 mmol) in 2 ml of CH2Cl2, allyltrimethylsilane (0.3 mmol) was added, followed by stirring for a given time (Table 1) at room temperature.The reaction mixture was worked up with brine and extracted with Et2O. The crude product was purified by flash chromatography. The experimental results are listed in Table 1. Compounds with a-keto and a-ester homoallylic alcohol moieties have been used as precursors for a number of biologically active natural products such as antibiotics.10 Among the procedures currently available for the preparation of these compounds, allylation of a-keto aldehydes and glyoxylates using allyltrimethylsilane promoted by Lewis acids is a convenient method.However, the a-keto aldehyde and glyoxylates are often moisture sensitive and easily hydrated and polymerized. In practice, although the hydrates are more stable to air and moisutre, it is necessary to transform them to the corresponding a-keto aldehydes and glyoxylates before carrying out the Lewis acid promoted reaction, in order to avoid interaction between the Lewis acid and the hydroxy group.Under conventional Lewis acid conditions, hydroxy Scheme 1 Table 1 Yb(OTf)3 catalysed allylation of hydrates 2–6 and glyoxylates 7 and 8 at room temperature

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