摘要/Abstract

报道了一种钯催化噁唑酮和烯丙醇的不对称烯丙基化反应合成相应的三苯胺核心非天然氨基酸化合物的方法.反应收率高达95%，对映选择性过量值最高为97%ee.该反应操作简单，条件温和，原子经济性好，水为唯一副产物.
关键词: 不对称烯丙基化, 非天然氨基酸, 三苯胺, 原子经济性
Allylic alkylation first pioneered by Tsuji in 1965 and, later adapted by Trost in 1973 with the introduction of phosphine ligands represents one of the straightforward and powerful synthetic tool for new carbon-carbon formation, especially the direct asymmetric allylic alkylation (AAA) has been widely utilized in the synthesis of natural products and pharmaceutical molecules. Conventionally, AAA reactions involve activated allylic alcohol derivatives, such as carbonates, amines, acetates, and halides, which require an equivalent strong base to react with the acidic by-product and inevitably results in stoichiometric waste. From the viewpoint of environmental and atom economy, the direct use of allylic alcohol instead its derivatives is much more practical by virtue of only water being formed as a byproduct. However, one of the challenges existed in such transformations is the poor reactivity of allylic alcohol. In 2006, a breakthrough was first made by Trost group, by using stoichiometric amounts of borane as the critical promoter in the direct AAA reaction of indoles with allylic alcohols. Afterwards, List, Gong, and Zhang reported independently the significant achievements applying aldehyde, pyrazol-5-ones, and ketones as nucleophiles, respectively. In 2004, our group enclosed the Brønsted acid accelerated Pd-catalyzed direct asymmetric allylic alkylation of azlactones with simple allylic alcohols. On the other hand, triphenylamine (TPA) as a strong electron-donating and oxidative stable molecule has been extensively utilized in the new organic electroluminescent materials, special dye synthesis and organic solar cells. Considering the impressive fluorescence emission ability of TPA and basing on these pioneering works, we reasoned that the direct connection of the TPA substructure with amino acid molecules could give rise to the fluorescence emission compounds. Thus, we report here the first installation of the non-natural amino acid derivatives bearing TPA core skeleton via Pd-catalyzed direct AAA reaction and the desired products were obtained with excellent yields (68%~95%) and enantioselectivities (90%~97% ee). The optimized reaction condition is as following:To a dried Schlenk tube were added activated 5 Å MS (100 mg), Pd₂(dba)₃ (4.0 mol%), L3 (10.0 mol%), solvent toluene (1.0 mL), and was stirred at 60℃ for 20 min. Then the reaction mixture was cooled down to room temperature, azlactones 1 (0.2 mmol), allylic alcohol 2 (0.3 mmol) and benzoic acid (10.0 mol%) in toluene (1.0 mL) was added and continue to stir at 60℃ for 20 h until the reaction was complete (monitored by TLC). The solvent was then removed under vacuum and the residue was purified by flash chromatography on silica gel to afford the desired product.
Key words: asymmetric allylic alkylation, non-natural amino acid, triphenylamine, atom economy


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