石 煒, 段月姣, 闞洪柱, 楊琍蘋(píng), 胡文浩
(華東師范大學(xué) 化學(xué)系,上海 200062)
(S)-4-芐基-2-噁唑烷酮(3)是一種重要的手性輔基,在不對(duì)稱(chēng)合成中有廣泛的應(yīng)用[1~6]。有關(guān)3的合成已有很多報(bào)道,均是從天然的L-苯丙氨酸(1)出發(fā),先還原成氨基醇(2),再關(guān)環(huán)合成3。在以上合成方法中,所用的還原劑(LiAlH4[7], NaBH4/I2[8], NaBH4/H2SO4[9], LiBH4/Me3SiCl[10], BH3·Me2S[11])不是毒性大,有強(qiáng)腐蝕性,就是價(jià)格昂貴或者是分步進(jìn)行[12]。LiAlH4作為經(jīng)典的直接還原酸到醇的還原劑,在放大過(guò)程中的淬滅存在一定的危險(xiǎn)性。在關(guān)環(huán)反應(yīng)中,也存在產(chǎn)率較低,試劑毒性大、昂貴或反應(yīng)時(shí)間較長(zhǎng)等問(wèn)題[13~19]。
本文綜合以上文獻(xiàn)方法,對(duì)合成3的工藝條件進(jìn)行了改進(jìn)。用LiAlH4還原1為2,再在催化量的乙醇鈉存在下和碳酸二乙酯反應(yīng)合成3(Scheme 1)。在還原反應(yīng)中,通過(guò)滴加甲醇均相淬滅反應(yīng),控制了反應(yīng)放熱,大大降低了LiAlH4淬滅過(guò)程的危險(xiǎn)性。在環(huán)化反應(yīng)中優(yōu)化加料方式,在達(dá)到反應(yīng)溫度時(shí)滴加乙醇鈉溶液,很好地控制了反應(yīng)的過(guò)程。該方法具有試劑便宜易得、環(huán)境友好、反應(yīng)速率、操作簡(jiǎn)便、總收率(66%)較高等優(yōu)點(diǎn),適用于放大生產(chǎn),是一條合成手性助劑(S)-4-芐基-2-噁唑烷酮的新方法。
WRS-1A型數(shù)字熔點(diǎn)儀(溫度計(jì)未校正);Varian Inova 500型核磁共振儀(CDCl3為溶劑,TMS為內(nèi)標(biāo));Bruker Microtof Ⅱ型質(zhì)譜儀;高效液相色譜儀[HPLC,伊利特 P230 UV detector, Diamonsil C18 column,MeOH/H2O/TFA=70.0/30.0/0.1, 1 mL·min-1]。
Scheme1
1和LiAlH4,上海嘉辰化工有限公司;碳酸二乙酯和乙醇鈉,國(guó)藥集團(tuán)上海試劑有限責(zé)任公司;其余所用試劑均為化學(xué)純;THF含水量<0.05%。
(1)2的合成
在三頸瓶中加入1 200 g(1.21 mol)和THF 1 L,冰水浴冷卻,攪拌下滴加LiAlH467g(1.75 mol)的無(wú)水THF(1.5 L)溶液(<15 ℃),滴畢,于室溫反應(yīng)3 h。滴加甲醇110 mL淬滅反應(yīng)(有H2放出,控制滴加速度使氣泡放出不要太過(guò)劇烈,<30 ℃)。緩慢加水67 g和20%NaOH溶液350 mL,攪拌30 min。抽濾,濾餅用二氯甲烷(3×67 mL)洗滌,合并有機(jī)層,用無(wú)水硫酸鈉干燥,濃縮,殘余物用甲苯重結(jié)晶(濾餅用少量冷的甲丁醚洗滌)得淺黃色固體2 145 g,產(chǎn)率80%, m.p.92.3 ℃~93.7 ℃(88.5 ℃~91.0 ℃[13]);1H NMRδ: 7.22~7.36(m, 5H), 3.65~3.68(dd,J=4.0 Hz, 1H), 3.40~3.43(dd,J=7.2 Hz, 1H), 3.14~3.17(m, 1H), 2.81~2.84(dd,J=5.2 Hz, 1H), 2.53~2.58(dd,J=8.7 Hz, 1H), 1.92(broad s, 3H);13C NMRδ: 138.66, 129.18, 128.55, 126.39, 66.37, 54.14, 40.96; HR-MS: Calcd for C9H14NO3(MH+) 152.107 0, found 152.108 5。
(2)3的合成
在三頸瓶中加入2 145 g(0.96 mol)和碳酸二乙酯500 mL,攪拌,升溫至120 ℃~130 ℃;當(dāng)有碳酸二乙酯開(kāi)始蒸出時(shí),緩慢滴加20%乙醇鈉的乙醇溶液(金屬鈉6.5 g,乙醇33 mL),滴畢,繼續(xù)反應(yīng)30 min。冷卻至室溫,減壓蒸出碳酸二乙酯(可回收),殘余物加入乙酸乙酯600 mL和水600 mL,分液,水相用乙酸乙酯(150 mL)萃取,合并有機(jī)相,用無(wú)水Na2SO4干燥;脫溶,殘余物加入乙酸乙酯300 mL和石油醚600 mL,攪拌2 h后析出大量白色沉淀;抽濾,濾餅干燥得白色固體3 140 g,產(chǎn)率82%, m.p.88.7 ℃~89.6 ℃(84.5 ℃~86.5 ℃[13]);1H NMRδ: 7.18~7.36(m, 5H), 6.20(s, 1H), 4.41~4.44(t,J=8.2 Hz, 1H), 4.08~4.17(m, 2H), 2.90~2.95(dd,J=6.8 Hz, 1H), 2.84~2.88(dd,J=6.4 Hz, 1H);13C NMRδ: 159.63, 135.93, 129.04, 128.95, 127.20, 69.55, 53.77, 41.34; HR-MS: Calcd for C10H11NO2Na(MNa+) 200.068 2, found 200.070 1。
本文以LiAlH4為還原劑,而LiAlH4還原最主要的問(wèn)題是在反應(yīng)淬滅的時(shí)候容易沖料以及反應(yīng)后處理產(chǎn)物抽濾困難。傳統(tǒng)的方法是直接用水淬滅,會(huì)生成大量固體,使攪拌不能充分進(jìn)行,體系不均勻而造成淬滅反應(yīng)延遲而引起反應(yīng)沖料。本文先以甲醇淬滅反應(yīng),這一過(guò)程不會(huì)生成固體,反應(yīng)體系為溶液狀態(tài),反應(yīng)充分,使得H2的釋放能夠控制,而且最后生成的粒狀固體抽濾十分容易。另外,由于原料在THF中溶解性不好,本文改變了傳統(tǒng)的向LiAlH4的THF溶液中加入原料的方式,采用向1的THF懸濁液中滴加LiAlH4的THF溶液的方式,使操作更簡(jiǎn)便。
(1) 反應(yīng)溫度
反應(yīng)溫度對(duì)合成3影響較大,如果是先加料再升溫,反應(yīng)很難進(jìn)行,文獻(xiàn)方法為先將油浴預(yù)熱至125 ℃[14]或135 ℃[13],這使工業(yè)放大的可操作性大大降低。本文嘗試改變反應(yīng)的加料順序:先將2和碳酸二乙酯的反應(yīng)液加熱至130 ℃后再滴加乙醇鈉的乙醇溶液,反應(yīng)在30 min時(shí)轉(zhuǎn)化率達(dá)到90.5%(HPLC),大大提高了反應(yīng)的可操作性,并縮短了反應(yīng)時(shí)間。
(2) 反應(yīng)時(shí)間
反應(yīng)條件同1.2(2),考察反應(yīng)時(shí)間對(duì)3轉(zhuǎn)化率的影響,結(jié)果見(jiàn)表1。由表1可以看出,反應(yīng)30 min時(shí)轉(zhuǎn)化90.5%為最高轉(zhuǎn)化率,延長(zhǎng)反應(yīng)時(shí)間轉(zhuǎn)化率反而降低。
表 1 反應(yīng)時(shí)間對(duì)3轉(zhuǎn)化率的影響*Table 1 Effect of reaction time on conversion of 3
*反應(yīng)條件同1.2(2)
由此可見(jiàn),合成3的最佳反應(yīng)條件為,2和碳酸二乙酯的反應(yīng)液加熱至130 ℃后滴加乙醇鈉的乙醇溶液,滴畢,反應(yīng)30 min, 2的轉(zhuǎn)化率達(dá)90.5%。
[1] Joel Slade, David Parker, Michael Girgis,etal. A practical enantioselective synthesis of a novel peptide deformylase inhibitor[J].Organic Process Research & Development,2006,10:78-93.
[2] Jun Liu, Jef K De Brabander. A concise total synthesis of saliniketal B[J].J Am Chem Soc,2009,131:12562-12563.
[3] Yingpeng Su, Yanfen Xu, Junjie Han,etal. Total synthesis of (-)-bitungolide F[J].J Org Chem,2009,74:2743-2749.
[4] Michael T Crimmins, Danielle L Jacobs. Asymmetric total synthesis of pyranicin[J].Org Lett,2009,11:2695-2698.
[5] Luiz C Dias, Airton G Salles Jr. Total synthesis of pteridic acids A and B[J].J Org Chem,2009,74:5584-5589.
[6] Michael T Crimmins, Kyle A Emmitte. Total synthesis of (+)-laurencin:An asymmetric alkylation——ring-closing metathesis approach to medium ring ethers[J].Org Letters,1999,1:2029-2032.
[7] Jay R Luly, Joseph F Dellaria, Jacob J Plattner,etal. A synthesis of protected aminoalkyl epoxides fromα-amino acids[J].J Org Chem,1987,52:1487-1492.
[8] Marc J McKennon, A I Meyers, Karlheinz Drauz,etal. A convenient reduction of amino acids and their derivatives[J].J Org Chemistry,1993,58:3568-3571.
[9] Abiko A, Masamune S. An improved,convenient procedure for reduction of amino acids to aminoalcohols:Use of NaBH4-H2SO4[J].Tetrahedron Lett,1992,33:5517-5518.
[10] Giannis A, Sandhoff K. LiBH4(NaBH4)/Me3SiCl,an unusually strong and versatile reducing agent[J].Angew Chem Int Ed,1989,28:218.
[11] Smith G A, Gawley R E. Reduction ofα-amino acids:L-valinol[J].Org Synth,1985,63:136.
[12] Hashimoto S, Komeshima N, Yamada S,etal. Asymmetric michael reaction via chiralα,β-unsaturated aldimines[J].Tetrahedron Lett,1977,18:2907-2908.
[13] Gage, James R, Evans David A. (S)-4-(phenylmethyl)-2-oxazolidinone[J].Org Synth,1990,68:77-82.
[14] Kathleen Rein, Marta Goicoechea-Pappas, Tarakeshwar V,etal. Chiral dipole-stabilized anions: experiment and theory in benzylic and allylic systems.Stereoselective deprotonations,pyramidal inversions,and stereoselective alkylations of lithiated(tetrahydroisoquinolyl) oxazolines[J].J Am Chem Soc,1989,111:2211-2217.
[15] Hans-Joachim Knolker, Tobias Braxmeier. Synthesis of chirai oxazolidin-2-ones and imidazolidin-2-ones via DMAP-catalyzed isocyanation of amines with di-tert-butyl dicarbonate[J].Tetrahedron Lett,1998,39:9407-9410.
[16] Correa Arlene, Denis Jean-Noeel, Greene Andrew E. A safe,simple,one-pot preparation ofN-derivatizedβ-amino alcohols and oxazolidinones from amino acids[J].Synthetic Communications,1991,21:1-9.
[17] Tararov Vitali I, Kuznetzov Nikolai Yu, Bakhmutov Vladimir I,etal. Remarkable dependence of the regioselectivity of free radical additions to 3-cinnamoyloxazolidin-2-ones on the stability of the intermediate adduct-radical,electrophilicity of the adding radicals and the conditions for their generation[J].Journal of the Chemical Society,Perkin Transactions 1:Organic and Bio-Organic Chemistry,1997,20:3101-3106.
[18] Lanchi Vo, James Ciula, Owen W Gooding. Chemical development on the chiral auxiliary (S)-4-(phenylmethyl)-2-oxazolidinone utilizing automated synthesis and DoE[J].Organic Process Reseach & Development,2003,7:514-520.
[19] Kazuhiko Orito, Mamoru Miyazawa, Takatoshi Nakamura,etal. Pd(OAc)2-catalyzed carbonylation of amines[J].J Org Chem,2006,71:5951-5958.