卡宾的综述(carbene,1999)
Looking for Stable Carbenes: The Difficulty in Starting Anew ANTHONY J.ARDUENGO,III
Institut fu¨r Anorganische und Analytische Chemie,
der Technischen Universita¨t Carolo-Wilhelmina,
D-38106Braunschweig,Germany
Received May10,1999
Introduction
A decade ago we initiated research,the goal of which was isolation of a stable carbene.Our success has helped to catalyze a resurgence of interest in readily available and easily handled carbenes.Research on stable(nucleophilic) carbenes is again a popular theme worldwide.Efforts in the general area of stable carbenes now focus not only on chemistry of the carbenes themselves but also on their applications to other chemical systems,where their chemical properties create technical opportunities that are unavailable with other functional groups.The quest for isolable carbenes is a long saga whose origin can be traced back to the first half of the1800s.A recently published history of this quest provides an important backdrop for the research described in this Account.1It is the intent of this Account to delineate the events and environment that led to the report from DuPont laboratories of the first isolation of a stable crystalline carbene.Getting Started
Although the isolation of an imidazol-2-ylidene did not occur until1990,our interest in key compounds necessary for its synthesis originated in the1970s.In an effort to find exceptionally stable carbenium ions that would make suitable substituents for dipole-stablized thiocarbonyl ylides,our attention was drawn to imidazole-2-thiones.2,3 Imidazole-2-thiones are commonly prepared by reaction of an imidazolium ion with sulfur in the presence of methanolic potassium carbonate(eq1).4-6In this reaction, imidazol-2-ylidenes(carbenes)formed by deprotonation of the imidazolium cations were presumed to be inter-mediates.
The imidazole-2-thione synthesis is remarkably insen-sitive to varied conditions that allow different substituents to be accommodated on the imidazole ring.7The reaction can even be run under dry air;however,moisture is not well tolerated.This reaction system simply does not behave as if unstable or delicate intermediates are in-volved,and no olefin byproducts are observed from dimerization of the putative carbene intermediates. Attention to History
Even though carbenes could reasonably be expected to be intermediates in imidazole-2-thione syntheses and there was some anecdotal evidence to suggest that these intermediates may possess unusual stability,pursuit of these particular carbenes did not seem warranted based on other literature precedents.H.-W.Wanzlick had rec-ognized earlier that the electron-rich imidazole nucleus should be capable of stabilizing a carbene center at the 2-position,between the two nitrogens.8Much of Wan-zlick’s work actually centered on the saturated imidazoline
Anthony Arduengo was born in Tampa,Florida,in1952.He studied chemistry at
the Georgia Institute of Technology,receiving his B.S.in1974and Ph.D.in1976.
After beginning his career with DuPont,he moved to University of Illinois-Urbana
as a member of the Organic Faculty.From1984to1998,he worked as a member
of the research staff at DuPont,advancing to Research Leader/Research Fellow.
Professor Arduengo currently holds the Saxon Chair in Organic Chemistry at the
University of Alabama(Tuscaloosa)and is adjunct professor of chemistry at the
Technical University in Braunschweig,Germany.His research interests include
unusual bonding arrangements,main group element chemistry,and various
aspects of applied chemistry.
10.1021/ar980126p CCC:$18.00?1999American Chemical Society VOL.32,NO.11,1999/ACCOUNTS OF CHEMICAL RESEARCH913 Published on Web08/21/1999
ring(2),in which the double bond between carbons4and 5is absent(eq2).9-20There are,however,reports by Wanzlick of his work involving unsaturated imidazole analogues21,22and a closely related triazole system.23 Nonetheless,no carbenes were reported as isolable in any of these cases.
Wanzlick’s work on these carbene systems was influ-enced by earlier false claims of the isolation of carbenes.1 Also playing an important role was the fact that Wanzlick’s academic mentor(Doktorvater)was Helmuth Scheibler, for whom he held a great admiration and who had also been active in searching for stable carbenes(dialkoxycar-benes)some years earlier.24Additionally,Wanzlick’s work occurred at a time when the discipline of organic chem-istry,because of advances in technology,was still coming to terms with differences between what a molecule is(in its ground state)and how it reacts.
Before the advent of modern analytical methods, traditional organic chemistry was a science that was rich with the descriptive chemistry of how and with what a molecule or substance reacts.Chemists learned batteries of reactions that could be used to classify a substance by the functional groups that it contained.Thus,associated with every organic functional grouping are expectations of how and with what that functional group should react. With respect to carbene chemistry,the reader is referred to an important Account of“carbenic reactivity”in connection with cyclopropanation.25The idea that the reactive state of a molecule and its isolated ground state (unperturbed by external reagents)might differ was not a useful concept in organic chemistry because it could not be tested.The identity of a molecule was determined primarily by how it reacted with other compounds.Only in the last half of the20th century did advances in technology enable chemists to probe a molecule’s struc-ture with a minimum of perturbation to its ground state. Wanzlick,and Scheibler before him,both relied primarily on reactivity to characterize their compounds.The one common analytical method that was applied by both Wanzlick and Scheibler was molecular weight determi-nation or estimation.In both cases,results were at best inconclusive and at worst misleading.
Although Wanzlick did uncover a great deal of reactivity that seemed to be characteristic of nucleophilic carbenes, the carbenes themselves were never isolated.During the course of Wanzlick’s investigation of imidazolin-2-ylidenes (2),an important article was published by D.M.Lemal and co-workers.26Lemal’s work indicated that chemistry similar to that which had been attributed by Wanzlick to carbenes was also observed if one began with the olefin that was essentially the dimer of an imidazolin-2-ylidene (3).Scrambling experiments allowed Lemal to show that, under his reaction conditions,carbenes were not inter-mediates,but rather carbene-like reactions could proceed directly from olefins.Although carbene-like reactivity can be observed from olefins(carbene dimers),this does not exclude the intermediacy of carbenes in Wanzlick’s ex-periments.It is also noteworthy that dimers of imidazol-2-ylidenes(1)are observed only when the nitrogens are restricted to nonplanar geometries27or C4and C5bear powerful electron-withdrawing groups.28Within Wan-zlick’s research group,imidazolin-2-ylidenes were referred to as“das doppelte Lottchen”,suggesting a mischievous duality between two carbenes and their dimer.29There has recently been a preliminary report that there may,indeed, exist an uncatalyzed equilibrium between imidazolin-2-ylidenes and their dimers,but further work is necessary to place these results in perspective against earlier re-ports.30
Temporary Diversions
With this volume of prior literature and the checkered history of stable carbenes as a backdrop,our interest in imidazol-2-ylidenes waned.However,curiosity about imidazol-2-thiones continued.We completed work to improve the imidazol-2-thione synthesis,and we showed that selenones and tellurones could be synthesized by similar procedures.6Our interest in carbenes temporarily shifted to highly electrophilic carbenes such as2,3,4,5-tetrakis(trifluoromethyl)cyclopentadien-1-ylidene(4).The chemistry of these highly electron deficient carbenes allowed the synthesis and isolation of a number of unusual compounds such as carbonyl,halonium,and nitrilium ylides.31
Getting Back into the Game
In1988,a commercial need arose for catalysts in various polymer cross-linking applications.7,32-35The imidazol-2-thiones seemed well suited to these applications because they are very powerful nucleophiles but are not strong bases.Other potential uses were also developed that could help drive commercialization of these thiones.36,37Existing syntheses of the imidazol-2-thiones were not well suited for large-scale manufacturing processes because of mate-rial costs,yield,separation,and byproduct problems.A more convenient synthesis of imidazol-2-thione was developed,7along with a new one-step synthesis of imidazolium ions(eq4)that allowed the production of substituted imidazolium ions that were not accessible by conventional routes.38This new imidazolium ion synthesis would later prove essential for our reentry into the quest for stable carbenes.
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As progress toward commercialization of imidazol-2-thiones continued,we ran reactions with a very wide range of substituents that again seemed to underscore the extreme stability and tolerance of carbene intermediates in these reactions.Pilot-plant tests were not always conducted with the same attention to detail that one can achieve when working on a small laboratory scale.Ad-ventitious water and air were sometimes present in the larger plant reactions.Nonetheless,the ease with which these reactions succeeded served to bolster our belief that the carbene intermediates were remarkably stable.Our industrial research environment at that time held in-depth scientific investigation and understanding as essential factors for the success of businesses that depend on technology and its applications.These factors combined to renew our interests in isolating stable carbenes from these reactions.
Novel imidazolium ions that were available from our new syntheses allowed us to attempt the generation of imidazol-2-ylidenes(1)that were not accessible to Wan-zlick.This new technology made it possible for us to do more than simply repeat Wanzlick’s chemistry,and there-fore our hopes were high that these new experiments would lead to the actual isolation of stable carbenes.Thus, using a combination of steric and electronic effects,we decided to investigate the chemistry of imidazol-2-ylidenes bearing large substituents on nitrogen in hopes of isolating a stable carbene.
New Results
Our first attempt to isolate a stable carbene was through use of1-adamantyl substituents on the imidazole nitro-gens(eq5).Anticipating that the desired carbenes would be marginally stable at best,for a base we chose sodium hydride with catalytic dimethyl sulfoxide(to generate the dimsyl anion)so that the byproducts(sodium chloride and hydrogen)could be easily separated from the carbene in a tetrahydrofuran(thf)reaction medium.
The deprotonation of imidazolium ion5proceeded smoothly,as monitored by hydrogen gas evolution to give carbene6.39Imidazol-2-ylidene6proved to be a remark-ably stable carbene both in solution and in the solid state.
A solution of this carbene in thf-d8,sealed under a few atmospheres of CO,has shown no decomposition or change after7years at room temperature.Carbene6 crystallizes from many solvents,readily forming very large crystals(up to several millimeters on a side),and it does not decompose at its melting point of240°C.40 The crystal structure of carbene6revealed features characteristic of a singlet carbene.39Particularly notewor-thy was the valence angle of102.2°at the carbene center. Although the structure of a singlet carbene had not been previously determined,numerous calculations were and have become available to predict the https://www.wendangku.net/doc/0f7524221.html,mon among all the theoretical models of singlet carbenes is a small valence angle of about102°at the carbene center.41-49 Thus,structural features of imidazol-2-ylidenes reflect those widely predicted for singlet carbenes.
Comparison of the structure of carbene6with that of its precursor carbenium(imidazolium)ion5was also instructive.The C-N bonds to the carbene center actually lengthen relative to those of the carbenium ion.Although it was tempting to attribute this increase in bond length to a decrease inπ-delocalization or pπ-pπinteraction in the carbene relative to the carbenium ion,it is important to remember that there are also substantial changes in hybridization at the carbene center that influence bond distances.These hybridization changes strongly affect the σ-system.As more s-orbital character is used to stabilize the in-plane lone pair of electrons at the carbene center, the N-Cσ-bonds take on more p-character at the carbene center.This would be expected to lead to an increase in N-C2distances on the basis ofσ-effects alone.Because σ-bonds are energetically more important thatπ-bonds, theseσ-effects are likely to be important contributors to structural changes.In fact,if one considers1A1:CH2and the methyl cation,there are analogous changes in the C-H distances for this pair of molecules for which π-interactions cannot play a role.50Thus,structural changes that occur in an imidazol-2-ylidene upon protonation (decrease in C2-N distances and increase in the N-C-N angle)appear to be typical for protonation of all singlet carbenes.
After a detailed consideration of the structure of carbene6and a preliminary investigation of some of its chemistry,our attention was drawn back to Wanzlick’s work.If carbene6was so stable,why had not Wanzlick isolated either of the two imidazol-2-ylidenes that he had studied?21,22Wanzlick’s two imidazol-2-ylidenes(7and8) both bore phenyl substituents on nitrogen.These phenyl substituents may have been unfortunate choices,either because ofπ-orσ-effects.We decided to synthesize aryl-substituted imidazol-2-ylidenes that would allow separa-tion of possibleπ-andσ-effects.To this end,p-tolyl-(9a) and mesityl-substituted(9b)imidazol-2-ylidenes were prepared.
Both aryl carbenes9a and9b were isolated as stable crystalline materials.51These aryl-substituted carbenes melt with decomposition around150°C,but they can be VOL.32,NO.11,1999/ACCOUNTS OF CHEMICAL RESEARCH915
isolated and manipulated with ease under a dry nitrogen atmosphere at room temperature.Thus,there appeared to be no specialσ-orπ-effects for aryl substituents.The X-ray structures of these carbenes revealed a central imidazol-2-ylidene ring that was very similar to that in6. It was also possible to show that mildly electron-accepting aryl groups are well tolerated as substituents on nitrogens of imidazol-2-ylidenes.This latter point was illustrated by the isolation of the1,3-bis(p-chlorophenyl)imidazol-2-ylidene.51The isolation of9a,b and1,3-bis(p-chlorophen-yl)imidazol-2-ylidene raised another question about the need for steric stabilization in the imidazol-2-ylidenes. When turned on edge,as in9b,these aryl substituents offer a steric effect in the plane of the carbene center that is similar to a that of simple methyl group.Was there a need for large bulky groups on the nitrogens?
The isolation of both10and11nicely illustrated that small substituents are also acceptable.51The1,3-dimeth-ylimidazol-2-ylidene,10,is a moderately stable oil at room temperature,but tetramethylimidazol-2-ylidene(11)is a stable crystalline material at room temperature under nitrogen(it is pyrophoric in moist air).The X-ray crystal structure of11reveals geometric features very similar to those of all the earlier imidazol-2-ylidenes.The geometry of the central imidazole ring is hardly affected by the variety of substituents that we have studied.The crystal structure of11packs the molecules along2-fold axes of a C2/c space group,reducing the number of crystallo-graphically unique atoms in this small molecule to11 (including hydrogens).This fortunate packing situation allowed a detailed structural investigation using both X-ray and neutron diffraction techniques to produce a high-quality experimental electron density map of the molecule.
In contrast to what could be accomplished at the time of Wanzlick’s original work,we were able to apply a number of exacting experimental techniques in order to characterize the ground state of the stable imidazol-2-ylidenes.These methods included an electron density mapping by a combination of electron and neutron diffraction techniques,52photoelectron spectroscopy,53and nuclear magnetic resonance(NMR)chemical shift ani-sotropy determination.54At the same time,a number of theoretical papers appeared which presented models for the bonding and structure in imidazol-2-ylidenes and diaminocarbenes in general.45-49,51Although a variety of methods have been employed,with semantic issues aside, the agreement among these representations is good.The availability of accurate experimental data on imidazol-2-ylidenes has served as a grounding point for the various theoretical models.Unperturbed by external reagents,the ground-state electron distribution in imidazol-2-ylidenes bears a remarkable similarity to those in some of the more classical1A1carbenes,such as CF2.For singlet CF2and an imidazol-2-ylidene model with only hydrogen substituents on the ring,a TCSCF wave function showed very similar mixing coefficients and electron populations at the two carbene centers.48
The experimentally determined electron distribution in carbene11-d12is quite interesting and provided us with a great deal of information about this simple imidazol-2-ylidene.52These data may be viewed in several ways, and some of these representations are presented in Figures1and2.The valence electron component of the experimental electron density is depicted in Figure1. Figure1a shows electron density around the carbene center itself.The illustrated plane is perpendicular to the molecular plane and incorporates the molecular2-fold axis.In this cross section,the in-plane lone pair of electrons is visible to the right,and electron density in theπ-direction is visible above and below the carbon. Clearly,the high in-plane electron density falls off to a minimum in theπ-direction,consistent with the expected electron distribution for a singlet carbene.Figure1b provides a view of a plane of electron density in the π-system70pm above the molecular plane.The electron density due to the nitrogen pπlone pairs of electrons is prominent.Theπ-component of the C4d C5double bond is evident,but there are no other distinctiveπ-features. The illustration(Figure1c)of the in-plane electron density reveals the high electronegativity of nitrogen relative to those of carbon and hydrogen(deuterium).The electron density of the in-plane lone pair of electrons at the carbene center is readily visible to the right,just outside the ring.
Another common view of such experimental electron density is found in so-called“deformation density”draw-ings.Deformation density is the difference between the observed electron distribution and that predicted for a “promolecule”made up of only spherically symmetric, noninteracting atoms located in their molecular positions. Deformation density is thus a representation of the electron distribution that is deformed into bonding re-gions between atoms.Such a representation makes it easier to see electron density that is responsible for bonding in a molecule.However,the problem of choosing
916ACCOUNTS OF CHEMICAL RESEARCH/VOL.32,NO.11,1999
a suitable reference “promolecule”has been noted previ-ously,and one should not overinterpret these drawings.55-57Nonetheless,deformation density drawings are instructive.Figure 2depicts the deformation density at the carbene in the same plane presented in Figure 1a.The carbene lone pair density is to the right of the carbon,and the p-density is in the plane of the drawing above and below the carbon.There is clearly a large surplus of electron density in the direction of the lone pair relative to a spherically symmetric isolated carbon atom.On the other hand,there is a deficit of electron density in the direction of the out-of-plane carbon p π-orbital,as illustrated by the negative density regions (red and yellow)above and below the carbon.The deficit in electron density in the p-direction at the carbene center is striking,but one needs to remember that this deficit is relative to the two-thirds of an electron that would be placed in this region from the spherically symmetric carbon atom in the promol-ecule.The actual amount of electron density that one should attribute to the carbene p π-orbital is difficult to quantify,because such an analysis is dependent on the basis set used to describe the molecule.Qualitatively,about one-third of an electron can be attributed to this π-region of space at the carbene center.
Such a precise experimental determination of electron density as described above,although highly informative,is not routinely practical.Measurement of NMR chemical shift anisotropy at the carbene center is more easily accomplished and provides information that also confirms the highly anisotropic nature of the electron distribution around the carbene center.For imidazol-2-ylidene 11,the NMR chemical shielding tensor at the carbene center has been observed experimentally and theoretically modeled for comparison with the singlet carbenes 1:CH 2and 1:CF 2.
54
FIGURE 1.Valence electron density determined for 1,3,4,5-tetra-methylimidazol-2-ylidene.(a)Carbene lone pair electron density perpendicular to the molecular plane.The C lone pair density lies to the right of the carbon.(b)π-electron density 70pm above the molecular plane.The nitrogen lone pairs and C -C double bond are obvious.(c)In-plane electron density.The framework σ-bonds and in-plane C lone pair are
evident.
FIGURE 2.Deformation density profile at the carbene center in 11.
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The most characteristic component of the chemical shielding tensor in a singlet carbene is theσ11component that relates the high density of the carbene in-plane lone pair with the formally unoccupied pπ-orbital at the carbene center.This tensor component is illustrated in Figure3and is orthogonal to the planes of the contoured density diagrams in Figures1a and2.This component is highly deshielding and is primarily responsible for the characteristically large downfield shift observed for car-bene centers.Experimentally,measurement of the13C chemical shift of a carbene is a highly reliable method for characterizing a carbene.The imidazol-2-ylidenes exhibit13C resonances for the carbene center from about δ210to220.39,51,58,59The saturated imidazolin-2-ylidenes and diaminocarbenes generally have further downfield-shifted resonances for the carbene center of aboutδ240-255,consistent with an even higher ansiotropy at the carbene center due to a lower population of the carbene pπ-orbital.60-63Various aminooxy-and aminothiocarbenes recently have been reported,and their chemical shifts range from about250to almost300ppm.64,65 After initial reports of the isolation of stable imidazol-2-ylidenes appeared,interest in stable nucleophilic car-benes spread rapidly.The principles that had been used to create stable carbenes were soon applied to ger-manylenes66and silylenes.67Synthetic methods such as the reduction of the corresponding thiones also appeared and provided access to new structures.58The triazole ring system,which had been studied earlier by Wanzlick,23 yielded its own contribution to a growing family of stable carbenes.59Amid this flurry of activity,our attention was again drawn back to Wanzlick’s work.The question this time was whether saturated imidazolin-2-ylidenes could actually be isolated.The motivation for us to pursue these diaminocarbenes was the observation that many simple, unhindered imidazol-2-ylidenes were so stable and easy to make,and although delocalization undoubtedly played some role in the stabilization of the imidazol-2-ylidenes, electron density studies did not reveal a highly delocalized π-system.Similarly,comparison of imidazolium ions with their corresponding imidazol-2-ylidenes showed a sub-stantial decrease in delocalization for carbenes compared to that for their obviously“aromatic”carbenium ions. Diaminocarbenes yielded to our efforts,and in1995we reported the isolation and structural characterization of 1,3-dimesitylimidazolin-2-ylidene(12)(eq6).60 We were not the only research group to have identified the diaminocarbenes as likely targets.Alder et al.reported the isolation of an acyclic diaminocarbene(bis(diisopro-pylamino)carbene)in1996.61More recent reports of diaminocarbenes have also appeared.62,63
The range of stable carbene structures was now quite large,but all such structures reported up to1996pos-sessed the common structural feature of nitrogen sub-stituents on the carbene center.68The chemistry of vitamin B1suggested that sulfur should be a likely candidate for stabilization of an adjacent carbene center.This observa-tion was not lost on Wanzlick,who had earlier studied a number of sulfur-containing carbenes.69-71After efforts to find the right substituent for the nitrogen so that we could take advantage of steric effects,we were able to isolate a stable thiazol-2-ylidene(13)(eq7).64Remarkably,not only the thiazolylidene could be isolated but its dimer(olefin) as well(eq8).
Thiazol-2-ylidenes and their dimers provided first examples of a heterocyclic carbene for which the corre-sponding dimer could also be isolated.Indeed,there is a Lewis acid-catalyzed equilibrium between the carbene and dimer that can be observed under appropriate condi-tions.64
There has always been an interest in the chemistry of heterosubstituted carbenes.Even before the isolation of stable analogues of these carbenes,there were early reports of reactivity derived from in situ-generated car-benes.Additional interest in this reactivity has surfaced now that these carbenes are easily handled,“bottleable”materials.Some of this chemistry has been previously reviewed,72and additional reviews of this rapidly develop-ing area will appear shortly.73Detailed comment on the reactivity derived from stable heterocyclic carbenes is
https://www.wendangku.net/doc/0f7524221.html,ponents of the chemical shielding tensor at the carbene center in11.
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beyond the scope of this Account.We note,however,that there is an interplay between the study of reactivity of stable carbenes and insights into sources of stability for these compounds.One such case arose during a system-atic study of the reactivity of imidazol-2-ylidenes and main group14halides.We discovered that carbon tetrachloride will actually chlorinate the4-and5-positions of imidazol-2-ylidenes(eq9).74The resulting4,5-dichloroimidazol-2-ylidenes possess remarkable stability and can even be handled in moist air without decomposition.
A range of stable carbenes can be made with electron-withdrawing groups in the4-and5-positions of the imidazole ring.Even1,3-dimethyl-4,5-bis(trifluoromethyl)-imidazol-2-ylidene has been synthesized and character-ized.75
The range of stable nucleophilic carbenes continues to grow rapidly to this day.Reports have now appeared from other laboratories of the isolation of aminooxycarbenes,65 and as discussed above,spectroscopic properties of these new additions fit nicely into trends that emerged for13C NMR spectra of carbenes.Even the parent imidazol-2-ylidene ring system bearing hydrogens on the nitrogens has now been characterized(although not isolated in a bottle).76,77We finally decided to examine one of the same molecules that had eluded Wanzlick earlier.We were able to isolate1,3,4,5-tetraphenylimidazol-2-ylidene and thus realize a dream that Wanzlick had for his research more than a quarter of a century earlier.78
Epilogue
In the end,we were fortunate that we challenged the convention against doing further research in an area that seemed to be overworked by our predecessors.Our current research efforts continue to focus on design and synthesis of new structures and incorporation of stable carbene fragments into molecules that are engineered to perform useful tasks.The satisfaction of a successful end to a quest is always the best reward for the effort made, particularly so when the quest appears to be somewhat risky at its outset.
The skill,insight,and enthusiasm of H.V.R.Dias,S.F.Gamper, D.Khasnis,M.Tamm,T.K.Prakasha,J.R.Goerlich,R.Krafczyk, and M.Unverzagt made our work with nucleophilic carbenes possible.The excellent technical assistance of H.A.Craig,M.Kline, and U.Sharma contributed enormously to successes in this area.
I am also grateful to the Alexander von Humboldt Stiftung for their support of a senior research prize that allowed completion of this text.Finally,I would like to dedicate this Account to the memory of my friend and colleague,Professor J.C.Martin.References
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_多肽的合成及其二级结构的研究_陈忠周
进展评述 β- 多肽的合成及其二级结构的研究陈忠周 李艳梅* 赵 刚 赵玉芬 (清华大学化学系 生命有机磷化学教育部重点实验室 北京 100084)陈忠周 男,25岁,博士生,从事生物有机化学研究。*联系人国家自然科学基金和曹光标高技术发展基金资助项目 1998-09-30收稿,1999-03-17修回摘 要 系统地介绍和评述了β-氨基酸和β-多肽的合成、以及β-多肽的二级结构与性质等研究领 域近年来的进展。β-多肽具有可预测和可再生的折叠形式、二级结构多样性、对酶稳定等优点,在药物及 催化剂领域中有很广阔的应用前景。 关键词 β-氨基酸 β-多肽 合成 二级结构 生物活性 Abstract Recent adv ances in the sy nthesis o f β-amino acids and β-peptides,a nd the inv estiga tio ns of the seco ndar y str uctur es and bio logical activ ity of β-peptides a re intro duced a nd review ed systema tica lly.β-Peptides,which hav e pr edicta ble and repro ducible folding pt terns,ma ny kinds of seco nda ry st ructures and specia l stability against peptidases,ar e being ev aluated a s a po tential so urce of new dr ug s and catalysts . Key words β-Amino acid,β-Peptide,Sy nthesis,Seco ndar y str uc ture,B io lo gica l activ ity 氨基与羧基之间相隔两个碳原子的氨基酸通常被称为β-氨基酸(β-aa )[1]。在碳-2或-3位置上具有侧链的β-氨基酸分别记为β2-Xxx 和β3-Xx x ,如在碳-2位置上有甲基侧链的β-氨基酸记为β2-Ala 。由β- 氨基酸构成的多肽称为β-多肽。对β-多肽的研究近年来之所以引起国际上如此广泛的关注,是由于它们具有与PN A 极其相似的特性:具有特定的空间结构和生物活性,但在生物体内却不易被酶所分解破坏,因此具有广阔的医药应用前景。 尽管自然界中β-氨基酸的含量大大少于α-氨基酸,但它们也是许多天然产物的重要组成部分。如天冬氨酸及其衍生物可被视为一种β-氨基酸;抗肿瘤药物紫杉醇[2]、β-环内酰胺抗生素(如卷曲霉素、紫霉素)[3]、从海洋生物中得到的大环肽[2]以及其它天然产物中均有β-氨基酸。β-氨基酸在天然产物中起着不可替代的作用。 在已知的天然聚合物中,蛋白质和RN A 是目前所知的仅有的两类具有紧密、确定构象的聚合物,它们特定的折叠方式使构成活性位点的官能团之间形成精确的空间取向关系从而形成活性位点,因而能够执行如催化和选择性识别等复杂的化学操作。由于蛋白质和RN A 具有高效性和高度专一性,这促使化学家考虑是否存在一种能够折叠成专一、紧密、确定构象而形成活性位点的新型聚合物,把蛋白质和RN A 的功能应用到普通的化学合成中。因此具有确定和可预测折叠倾向的分子骨架将是设计这些专一性分子器件的基础。而β-多肽具有以下特点:有螺旋、β-折叠、反转、管状堆积等多种二级结构,并且不同的残基决定着不同的二级结构类型[4] 并可以被预测,对常见的蛋白 ·21·ht tp ://china .chemistry mag .o rg 化学通报 1999年第11期DOI:10.14159/https://www.wendangku.net/doc/0f7524221.html, k i .0441-3776.1999.11.006
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