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2. Palladium-Catalyzed Intramolecular Cyclization Reaction Abell and co-workers recently developed a new synthetic method for trisubstituted imidazoles, 2-substituted 1-benzyl-4- methylimidazoles 2, based on the palladium-catalyzed intramolecular amino-Heck reaction of amidoximes 1 (Scheme 1).[15] With the success of the catalytic imidazole synthesis by the Pd-catalyzed intramolecular cyclization reaction, which employed relatively simple amidoximes, Abell and co-workers applied this transformation to the substrates prepared from amino acids to introduce an imidazole moiety as a stable ester isostere at the C terminus of peptidomimetics[23] (Scheme 2). 3. Palladium-Catalyzed Multicomponent Coupling Reaction Siamaki and Arndtsen succeeded in developing a direct synthesis of imidazoles by the palladium-catalyzed multicomponent coupling reaction (MCR) starting from two imines 3 and 5 and acid chlo- ACHTUNGTRENUNGrides 4 under CO atmosphere (Scheme 4).[16] In the early stages of the investigation, Siamaki and Arndtsen suffered low yields of the desired imidazole due to the formation of undesired a-sulfonylamide 7 and amide 8 (Scheme 5). 4. d10（10為上標(biāo)）-Metal-Catalyzed Cycloaddition Reaction As shown in Scheme 7, only two examples were reported. The reaction of isocyanides 9a and 9b afforded the corresponding imidazoles 10 a and 10b in excellent yields. During the course of our research on the copper-catalyzed synthesis of pyrroles starting from isocyanides and acetylenes conjugated with an electron- withdrawing group (EWG),[31] we encountered homodimerization of ethyl isocyanoacetate (12 a, EWG=CO2Et; Scheme 9) with a catalytic amount of Cu2O and 1,10-phen- ACHTUNGTRENUNGanthroline (phen) to produce 1-(ethoxycarbonylmethyl)-4- (ethoxycarbonyl)imidazole (10 c, R=Et; Scheme 7) in 49% yield. 5. Lewis AcidCatalyzedMulticomp onent Coupling Reaction In this context, Sharma et al. investigated a Lewis acid catalyzed synthesis of imidazoles. They surveyed the three-component coupling reaction (TCR) of benzil, aldehydes 14, and NH4OAc in the presence of a catalytic amount of ZrCl4 and observed a dramatic acceleration in imidazole formation (Scheme 11).[19] The ZrCl4-catalyzed four-component coupling reaction (FCR) was further examined with benzil, benzaldehyde, primary amine 16, and NH4OAc to synthesize the fully substituted imidazoles 17 (Scheme 12). 6. Thiazolium-CatalyzedThree-Com ponent Coupling Reaction Frantz et al. developed the one-pot synthesis of imidazoles through the synthesis of a-ketoamides (N) via the thiazolium- catalyzed addition of aldehydes 19 to acylimines generated from a-sulfonylamides 18[39] and subsequent treatment with amines 20 (Scheme 13).[20] |
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2.鈀催化分子間成環(huán)反應(yīng)。 Abell和其同事最近在鈀催化脒肟類分子內(nèi)氨基Heck反應(yīng)的基礎(chǔ)上研制了一種新的合成三取代咪唑（2-取代 1-芐基-4-甲基咪唑）的方法。（圖1） 隨著用相對簡單的脒肟類鈀催化分子內(nèi)成環(huán)反應(yīng)催化合成咪唑的成功，Abell和其同事把這種反應(yīng)應(yīng)用于氨基酸制成的襯底以引入咪唑基元并用其作為肽鏈C端的穩(wěn)定酯類異構(gòu)體。（圖2） 3.鈀催化多組分偶聯(lián)反應(yīng) Siamaki和Arndtsen使用鈀催化多組分偶聯(lián)反應(yīng)（MCR）成功用一步法合成了咪唑。反應(yīng)在CO氣氛下使用兩份亞胺和 XXX 和酰氯反應(yīng)。（圖4） 在研究的早期階段，Siamaki和Arndtsen經(jīng)歷了極低的咪唑產(chǎn)率，這主要是因為產(chǎn)生了α-磺酰胺和酰胺。（圖5） 4.d10金屬催化環(huán)加成反應(yīng)。 如表7所示，只有兩個實例見諸報導(dǎo)。9a和9b的異腈反應(yīng)為10a和10b中相應(yīng)咪唑的生成提供了極好的產(chǎn)率。 在我們對于 反應(yīng)物為異腈和與吸電子基團(tuán)共軛的炔烴的 銅催化合成吡咯的研究過程中，我們遭遇了異腈基乙酸乙酯(12a, EWG=CO2Et; 圖 9)的同二聚作用，當(dāng)時使用了催化劑量的Cu2O和1,10-鄰二氮雜菲（鄰二氮菲）來制備1-乙酸乙酯基-4-乙酯基咪唑(10 c, R=Et; 圖7) ，產(chǎn)率為49%。 5.路易斯酸催化多組分偶聯(lián)反應(yīng) 在這種情況下，Sharma等人研究了咪唑的路易斯酸催化合成。他們進(jìn)行了四氯化鋯催化二苯乙二酮、乙醛、醋酸銨三組分偶聯(lián)反應(yīng)（TCR）并觀察到咪唑的的加速形成。 ZrCl4催化四組分偶聯(lián)反應(yīng)（FCR）被進(jìn)一步用二苯乙二酮、苯甲醛、伯胺、醋酸銨等組分合成全取代咪唑。（圖12） 6.噻唑催化三組分偶聯(lián)反應(yīng) Frantz等人開發(fā)了咪唑的一鍋法合成。它經(jīng)過兩步：1. 噻唑催化乙醛和由α-磺酰胺生成的酰亞胺反應(yīng)生成α-酮酰胺。2. 所得產(chǎn)物與胺反應(yīng)。（圖13） |
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