脯氨酸衍生催化剂的设计、合成及其在手性含氧杂环合成中的应用
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摘要
含氧杂环是一类有显著的生理活性的化合物，作为核心结构存在于大量的天然产物和药物有效成分中。手性含氧杂环的不对称合成对于天然产物化学和药物开发等领域有重要的意义。所以建立简单、高效、对映选择性和原子经济性地构建手性含氧杂环的方法学就显得十分必要。本论文的研究内容主要集中在羰基化合物的不对称碳-碳键形成反应以及以此为基础的手性含氧杂环的合成上。
首先以脂肪醛与硝基烯烃的不对称Michael加成反应为模型，我们设计并合成了一类脯氨酸衍生的手性双官能催化剂5a-d。通过增加催化剂分子中两个活性官能团的距离，有效地防止了惰性中间体的生成，催化效率得到明显提高。反应在仅1.5mol%的催化剂5d作用下即可顺利完成，以71-99%的产率得到加成产物γ-硝基醛。该催化剂也在反应中显示出了非常好的立体选择性，在优化的反应条件下反应dr值和ee值分别高达99:1和97%。
接下来上述构建碳碳键方法被成功地应于与手性苯并吡喃化合物的合成中。通过仲氨催化剂促进的脂肪醛与(E)-2-(2-硝基乙烯基)-苯酚之间的Michael加成/半缩醛化串联反应，以令人满意的产率和立体选择性生成了苯并吡喃衍生物2-羟基色烷11。通过简单的衍生化反应，产物11可以分别被转化成手性二氢香豆素、色烷和色烯等天然产物中常见的结构。在这个反应中，催化剂的选择对于反应的立体选择性有非常明显的影响。二苯基脯氨醇硅醚2能催化反应以优异的对映选择性得到产物12，但是较低的催化活性和非对映选择性限制了它的应用价值。相比之下我们自己开发的双官能催化剂5d有更好的催化活性，能以更好的产率和优秀的非对映选择性得到苯并吡喃化合物，然而反应的对映选择性不如前者理想。
在羰基化合物作为电子受体的加成反应中，脯氨酸衍生的小分子同样可以作为配体形成手性Lewis酸催化剂，控制反应的立体选择性。接下来的研究中，以脯氨酸衍生的手性二胺L1与二价铜盐的配合物为催化剂，邻炔基苯甲醛类化合物成功地通过Henry反应在分子中引入了立体中心；反应中间体手性β-硝基醇中的羟基对炔烃的分子内加成得到1H-异色烯19和1,3-二氢异苯并呋喃20。考虑到β-硝基醇在碱性环境、高温以及强的Lewis酸环境下的不稳定性，金(I)催化剂的选用十分关键，它成功避免了β-硝基醇的逆Henry反应、脱水和关环过程中ee值的损失。这个反应方法的建立提供了一种对映选择性合成手性1H-异色烯和1,3-二氢异苯并呋喃衍生物的有效途径。另外反应关环步骤中观察到的区域选择性和底物电子性质之间的关系也得到了进一步的验证与研究。通过对底物取代基的调整，可以选择性地合成1H-异色烯或1,3-二氢异苯并呋喃化合物。
手性双烯体与活泼的亚硝基化合物之间的不对称Diels-Alder反应也是合成手性含氧杂环的途径之一。双烯体上的取代基对于反应的区域选择性有明显的影响：相对于乙酰氧基，给电子能力较强的硅氧基OTIPS使得双烯体的电子分布更不对称，所以相应的化合物25a在反应中显示出了非常好的区域选择性和面选择性。通过该方法可以在4-取代手性环己烯酮的3,6位同时引入两个杂原子取代的手性中心。
Oxygen heterocycles are found in many natural products and synthetic molecules,exhibiting fascinating biological and pharmacological activities. The synthesis of this typeof compound is of great importance to the field of natural products chemistry andmedicinal chemistry. So, it is essential to establish some simple, atom economicmethodologies for the construction of oxa-heterocycles with high efficiency and highstereoselectivity. The topic of my thesis mainly focused on the deveploping of asymmetriccarbonyl involved C-C bond forming reactions and their applications in preparing oxygenheterocycles such as benzopyrans and benzofurans.
Based on enamine activation mode, several novel prolylprolinol catalysts5a-d havebeen synthesized for the C-C bond forming reactions. By rational design, the formation ofunreactive species hemiaminal between catalyst and aldehyde was avoided. So, this typeof catalysts showed high catalytic efficiency on promoting the direct addition ofunmodified aliphatic aldehydes to nitroalkenes. Among the catalysts surveyed, the leastbulky5d exhibited the best performance on both efficiency and stereoselectivity,providing the products with up to97%ee and up to99:1dr value with1.5-5mol%catalyst loading. Additionally, computational studies of transition state have beenconducted to explain the high diastereo-and enantioselectivity of5d.
Then this asymmetric Michael addition methodology had been used in the reactionbetween aliphatic aldehydes and (E)-2-(2-nitrovinyl)-phenols. By the followingspontaneous hemiactalization, this cascade was successfully applied in the synthesis ofbenzopyran frameworks. Interestingly, the diastereo-and enantioselectivities changedmarkedly when the reaction was mediated by different types of secondary-amine catalysts.The diphenylprolinol silyl ether2promoted the reaction with excellent enantioselectivities(up to99%ee) while suffered from poor reactivity and diastereoselectivities (2.8:1to10:1).(L,L)-prolylprolinol5d is also identified as an effective catalyst that showed highcatalytic activity, good enantioselectivities (up to89%ee) and excellentdiasereoselectivities (up to50:1dr). In addition, the chroman-2-ols11b yielded in the above reactions could be conveniently transformed to synthetically and biologicallysignificant chiral dihydrocoumarin, chroman and4H-chromene derivatives.
The Proline derivatives are also applicable as chiral ligands along with metal saltsfor the Lewis acid promoted carbonyl addition reactions. By combining the copper(II)catalyzed asymmetric Henry reaction of o-alkynylbenzaldehydes with subsequent gold(I)catalyzed cycloisomerization, optically active1H-isochromenes and1,3-dihydroiso-benzofurans were successfully synthesized in good overall yields with good to excellentenantioselectivities (up to98%). The employment of gold catalyst is vital to thecycloisomerization step because the intermediate β-nitroalcohols18are fairly unstable andprone to racemize under basic, thermo or strong acidic conditions. Various substrates wereinvestigated and a correlation between the regioselectivity and electronic nature of thesubstrates was studied. By tuning the electronic property of the substituents, manysubstrates could be selectively transformed to either1H-isochromene or1,3-dihydroisobenzofuran.
The hetero-Diels-Alder reaction between chiral dienes and highly reactive acylnitroso compouns is another method for construction of chiral oxygen containingheterocycles. The regioselectivity seems largely depended on the substituent on diene.Compared to OAc, the OTIPS group makes the dienen more electronicly asymmetric andleads to the excellent selectivity of the reaction between nitroso and OTIPS-diene25a.This reaction seccessifully introduced two hetero substituents on chiral ketene moleculessimultaneously in one step.
首先以脂肪醛与硝基烯烃的不对称Michael加成反应为模型，我们设计并合成了一类脯氨酸衍生的手性双官能催化剂5a-d。通过增加催化剂分子中两个活性官能团的距离，有效地防止了惰性中间体的生成，催化效率得到明显提高。反应在仅1.5mol%的催化剂5d作用下即可顺利完成，以71-99%的产率得到加成产物γ-硝基醛。该催化剂也在反应中显示出了非常好的立体选择性，在优化的反应条件下反应dr值和ee值分别高达99:1和97%。
接下来上述构建碳碳键方法被成功地应于与手性苯并吡喃化合物的合成中。通过仲氨催化剂促进的脂肪醛与(E)-2-(2-硝基乙烯基)-苯酚之间的Michael加成/半缩醛化串联反应，以令人满意的产率和立体选择性生成了苯并吡喃衍生物2-羟基色烷11。通过简单的衍生化反应，产物11可以分别被转化成手性二氢香豆素、色烷和色烯等天然产物中常见的结构。在这个反应中，催化剂的选择对于反应的立体选择性有非常明显的影响。二苯基脯氨醇硅醚2能催化反应以优异的对映选择性得到产物12，但是较低的催化活性和非对映选择性限制了它的应用价值。相比之下我们自己开发的双官能催化剂5d有更好的催化活性，能以更好的产率和优秀的非对映选择性得到苯并吡喃化合物，然而反应的对映选择性不如前者理想。
在羰基化合物作为电子受体的加成反应中，脯氨酸衍生的小分子同样可以作为配体形成手性Lewis酸催化剂，控制反应的立体选择性。接下来的研究中，以脯氨酸衍生的手性二胺L1与二价铜盐的配合物为催化剂，邻炔基苯甲醛类化合物成功地通过Henry反应在分子中引入了立体中心；反应中间体手性β-硝基醇中的羟基对炔烃的分子内加成得到1H-异色烯19和1,3-二氢异苯并呋喃20。考虑到β-硝基醇在碱性环境、高温以及强的Lewis酸环境下的不稳定性，金(I)催化剂的选用十分关键，它成功避免了β-硝基醇的逆Henry反应、脱水和关环过程中ee值的损失。这个反应方法的建立提供了一种对映选择性合成手性1H-异色烯和1,3-二氢异苯并呋喃衍生物的有效途径。另外反应关环步骤中观察到的区域选择性和底物电子性质之间的关系也得到了进一步的验证与研究。通过对底物取代基的调整，可以选择性地合成1H-异色烯或1,3-二氢异苯并呋喃化合物。
手性双烯体与活泼的亚硝基化合物之间的不对称Diels-Alder反应也是合成手性含氧杂环的途径之一。双烯体上的取代基对于反应的区域选择性有明显的影响：相对于乙酰氧基，给电子能力较强的硅氧基OTIPS使得双烯体的电子分布更不对称，所以相应的化合物25a在反应中显示出了非常好的区域选择性和面选择性。通过该方法可以在4-取代手性环己烯酮的3,6位同时引入两个杂原子取代的手性中心。
Oxygen heterocycles are found in many natural products and synthetic molecules,exhibiting fascinating biological and pharmacological activities. The synthesis of this typeof compound is of great importance to the field of natural products chemistry andmedicinal chemistry. So, it is essential to establish some simple, atom economicmethodologies for the construction of oxa-heterocycles with high efficiency and highstereoselectivity. The topic of my thesis mainly focused on the deveploping of asymmetriccarbonyl involved C-C bond forming reactions and their applications in preparing oxygenheterocycles such as benzopyrans and benzofurans.
Based on enamine activation mode, several novel prolylprolinol catalysts5a-d havebeen synthesized for the C-C bond forming reactions. By rational design, the formation ofunreactive species hemiaminal between catalyst and aldehyde was avoided. So, this typeof catalysts showed high catalytic efficiency on promoting the direct addition ofunmodified aliphatic aldehydes to nitroalkenes. Among the catalysts surveyed, the leastbulky5d exhibited the best performance on both efficiency and stereoselectivity,providing the products with up to97%ee and up to99:1dr value with1.5-5mol%catalyst loading. Additionally, computational studies of transition state have beenconducted to explain the high diastereo-and enantioselectivity of5d.
Then this asymmetric Michael addition methodology had been used in the reactionbetween aliphatic aldehydes and (E)-2-(2-nitrovinyl)-phenols. By the followingspontaneous hemiactalization, this cascade was successfully applied in the synthesis ofbenzopyran frameworks. Interestingly, the diastereo-and enantioselectivities changedmarkedly when the reaction was mediated by different types of secondary-amine catalysts.The diphenylprolinol silyl ether2promoted the reaction with excellent enantioselectivities(up to99%ee) while suffered from poor reactivity and diastereoselectivities (2.8:1to10:1).(L,L)-prolylprolinol5d is also identified as an effective catalyst that showed highcatalytic activity, good enantioselectivities (up to89%ee) and excellentdiasereoselectivities (up to50:1dr). In addition, the chroman-2-ols11b yielded in the above reactions could be conveniently transformed to synthetically and biologicallysignificant chiral dihydrocoumarin, chroman and4H-chromene derivatives.
The Proline derivatives are also applicable as chiral ligands along with metal saltsfor the Lewis acid promoted carbonyl addition reactions. By combining the copper(II)catalyzed asymmetric Henry reaction of o-alkynylbenzaldehydes with subsequent gold(I)catalyzed cycloisomerization, optically active1H-isochromenes and1,3-dihydroiso-benzofurans were successfully synthesized in good overall yields with good to excellentenantioselectivities (up to98%). The employment of gold catalyst is vital to thecycloisomerization step because the intermediate β-nitroalcohols18are fairly unstable andprone to racemize under basic, thermo or strong acidic conditions. Various substrates wereinvestigated and a correlation between the regioselectivity and electronic nature of thesubstrates was studied. By tuning the electronic property of the substituents, manysubstrates could be selectively transformed to either1H-isochromene or1,3-dihydroisobenzofuran.
The hetero-Diels-Alder reaction between chiral dienes and highly reactive acylnitroso compouns is another method for construction of chiral oxygen containingheterocycles. The regioselectivity seems largely depended on the substituent on diene.Compared to OAc, the OTIPS group makes the dienen more electronicly asymmetric andleads to the excellent selectivity of the reaction between nitroso and OTIPS-diene25a.This reaction seccessifully introduced two hetero substituents on chiral ketene moleculessimultaneously in one step.
引文
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