The ‘Azirine/Oxazolone Approach’ to the Synthesis of Aib‐Pro Endothiopeptides

The reaction of methyl N‐(2,2‐dimethyl‐2H‐azirin‐3‐yl)‐L ‐prolinate ( 2a ) with thiobenzoic acid at room temperature gave the endothiopeptide Bz‐AibΨ[CS]‐Pro‐OMe ( 7 ) in high yield. In an analogous manner, (benzyloxy)carbonyl (Z)‐protected proline was transformed into the thioacid, which was reacted with 2a to give the endothiotripeptide Z‐Pro‐AibΨ[CS]‐Pro‐OMe ( 12 ). The corresponding thioacid of 7 was prepared in situ via saponification, formation of a mixed anhydride, and treatment with H₂S. A second reaction with 2a led to the endodithiotetrapeptide 9 , but extensive epimerization at Pro² was observed. Similarly, saponification of 12 and coupling with either 2a or H‐Phe‐OMe and 2‐(1H‐benzotriazol‐1‐yl)‐1,1,3,3‐tetramethyluronium tetrafluoroborate/1‐hydroxy‐1H‐benzotriazole (TBTU/HOBt) gave the corresponding endothiopeptides as mixtures of two epimers. The synthesis of the pure diastereoisomer BzΨ[CS]‐Aib‐Pro‐AibΨ[CS]‐N(Me)Ph ( 21 ) was achieved via isomerization of 7 to BzΨ[CS]‐Aib‐Pro‐OMe ( 16 ), transformation into the corresponding thioacid, and reaction with N,2,2‐trimethyl‐N‐phenyl‐2H‐azirin‐3‐amine ( 1a ). The structures of 12 and 21 were established by X‐ray crystallography.     

Reaction of γ‐Hydroxy‐N‐[1‐(dimethylcarbamoyl)ethyl]butanamides under the ‘Direct Amide Cyclization’ Conditions

The preparation of the title compounds was achieved via the ‘azirine/oxazolone method’ starting from the corresponding γ‐hydroxy acids. Upon subjecting the γ‐hydroxy‐N‐[1‐(dimethylcarbamoyl)ethyl]butanamides 4 to the so‐called ‘direct amide cyclization’ (DAC) conditions, chlorinated acids 11 or imino lactones 12 were obtained as the sole products instead of the expected cyclodepsipeptides A or their cyclodimers (Scheme 4). Variation of the substituents in 4 did not affect the outcome of the reaction and a mechanism for the formation of both products from the intermediate oxazolone 13 has been proposed. Under the acidic conditions of the DAC, the imino lactones are formed as their HCl salts 12 , which, in polar solvents or on silica gel, reacted further to give the chlorinated acids 11 . Stabilization of the imino lactones was achieved by increasing the substitution in the five‐membered ring, and their structure, in the form of the hydrochlorides, was established independently by X‐ray crystallography (Fig. 4). A derivative 15 of the imino lactone 12a was prepared by the reaction with the 2H‐azirin‐3‐amine 10a ; its structure was also established by an X‐ray crystal‐structure determination (Fig. 3). Furthermore, the structures of the ω‐chloro acids 11a and 11b were determined by X‐ray crystallography (Fig. 2).     

Synthesis of Poly‐Aib Oligopeptides and Aib‐Containing Peptides via the ‘Azirine/Oxazolone Method’, and Their Crystal Structures

The protected poly‐Aib oligopeptides Z‐(Aib)_n‐N(Me)Ph with n=2–6 were prepared according to the ‘azirine/oxazolone method’, i.e., by coupling amino or peptide acids with 2,2,N‐trimethyl‐N‐phenyl‐2H‐azirin‐3‐amine ( 1a ) as an Aib synthon (Scheme 2). Following the same concept, the segments Z‐(Aib)₃‐OH ( 9 ) and H‐L ‐Pro‐(Aib)₃‐N(Me)Ph ( 20 ) were synthesized, and their subsequent coupling with N,N′‐dicyclohexylcarbodiimide (DCC)/ZnCl₂ led to the protected heptapeptide Z‐(Aib)₃‐L ‐Pro‐(Aib)₃‐N(Me)Ph ( 21 ; Scheme 3). The crystal structures of the poly‐Aib oligopeptide amides were established by X‐ray crystallography confirming the 3₁₀‐helical conformation of Aib peptides.     

Synthesis and Conformational Analysis of Pentapeptides Containing Enantiomerically Pure 2,2‐Disubstituted Glycines

The synthesis and conformational analysis of model pentapeptides with the sequence Z‐Leu‐Aib‐Xaa‐Gln‐Valol is described. These peptides contain two 2,2‐disubstituted glycines (α,α‐disubstituted α‐amino acids), i.e., Aib (aminoisobutyric acid), and a series of unsymmetrically substituted, enantiomerically pure amino acids Xaa. These disubstituted amino acids were incorporated into the model peptides via the ‘azirine/oxazolone method’. Conformational analysis was performed in solution by means of NMR techniques and, in the solid state, by X‐ray crystallography. Both methods show that the backbones of these model peptides adopt helical conformations, as expected for 2,2‐disubstitued glycine‐containing peptides.     

Highly Constrained Linear Oligopeptides Containing Heterocyclic α‐Amino Carboxylic Acids

Two spiroheterocyclic 2H‐azirin‐3‐amines, 1f and 1g , were shown to be useful synthons for the dipeptides N‐(4‐aminotetrahydro‐2H‐pyran‐4‐yl)prolinate (Thp‐Pro) and the corresponding thiopyran derivative, Tht‐Pro, respectively. By coupling of 4‐bromobenzoic acid with 1f or 1g and saponification, followed by repeating the coupling and saponification steps, oligopeptides of type 4‐BrBz‐(Thp‐Pro)_n‐OMe and 4‐BrBz‐(Tht‐Pro)_n‐OMe were prepared, and their conformations were evaluated in solution by NMR techniques and in the crystalline state by X‐ray crystallography. All of these sterically highly congested oligopeptides adopt fairly rigid helical conformations. It is interesting to note that the hexapeptide with Thp forms a 3₁₀‐helix, whereas the Tht analog has a β‐bend ribbon spiral confirmation.     

Synthesis of Aib‐Pro Oligopeptides by Repeated Azirine Coupling with the Aib‐Pro Synthon

A new synthesis of (Aib‐Pro)_n oligopeptides (n=2, 3, and 4) via azirine coupling by using the dipeptide synthon methyl N‐(2,2‐dimethyl‐2H‐azirin‐3‐yl)‐L ‐prolinate ( 1b ; Fig. 1) is presented. The most important feature of the employed protocol is that no activation of the acid component is necessary, i.e., no additional reagents are required, and the coupling reaction is performed under mild conditions at room temperature. As an attempt to provide an answer to the question of the preferred conformation of the prepared molecules, we carried out experiments by using NMR techniques and X‐ray crystallography. For example, in the case of the hexapeptide 11 , it was possible to compare the conformations in the crystalline state and in solution. After the selective hydrolysis of the methyl ester p‐BrBz‐(Aib‐Pro)₄‐OMe ( 13 ) under basic conditions, the corresponding octapeptide acid was obtained, which was then converted into the octapeptide amide p‐BrBz‐(Aib‐Pro)₄‐NHC₆H₁₃ ( 15 ) by using standard coupling conditions and activating reagents (HOBt/TBTU/DIEA) of the peptide synthesis. The conformation of this compound, as well as those of the tetrapeptides 14 and 18 , was also established by X‐ray crystallography and in solution by NMR techniques. In the crystalline state, a β‐bend ribbon structure is the preferred conformation, and similar conformations are formed in solution.     

Synthesis of Enniatin‐like Cyclic Depsipeptides via ‘Direct Amide Cyclization’

The synthesis of several 18‐membered cyclodepsipeptides with an alternating sequence of α,α‐disubstituted α‐amino acids and α‐hydroxy acids (compounds 14a – 14e ) is described. The ring closure via macrolactonization was accomplished by treatment of a diluted suspension of the corresponding linear precursors 12a – 12e in toluene with HCl gas, i.e., the so‐called ‘direct amide cyclization’. The incorporation of the α,α‐disubstituted α‐amino acids was achieved via the ‘azirine/oxazolone method’ with 2H‐azirin‐3‐amines of type 6 and 9 as building blocks. The structure of the cyclic depsipeptide 14a was established by X‐ray crystallography.     

Synthesis of β3‐Homophenylalanine‐Derived Amino Acids and Peptides by Suzuki Coupling in Solution and on Solid Support

β‐Peptides and, to a certain extent, also mixed α,β‐peptides, are resistant to degradation by a variety of proteolytic enzymes that rapidly degrade natural α‐peptides. This is one of many characteristics that make β‐peptides an attractive class of compounds for drug‐discovery studies. On the other hand, modern organometallic reactions such as the Suzuki–Miyaura cross‐coupling have become standard tools in industry laboratories to derivatize side chains of α‐peptidic compounds to build up libraries of unnatural peptides. Combining both features, we prepared (4‐bromo)‐β³‐homophenylalanine derivatives 3 – 5 and 12 as precursors for Suzuki–Miyaura couplings. From these bromo compounds, we synthesized biaryl‐substituted β‐homoamino acids 6 , and analogs 13 and 15 of the anti‐AIDS drug Saquinavir.     

Solid‐Phase Synthesis of 2‐Alkenamides from Polystyrene‐Supported α‐Selenocarboxylic Acids

The polystyrene‐supported α‐selenoacetic acid and α‐selenopropionic acid were prepared and used for the synthesis of 2‐alkenamides from primary and secondary amines in good yields and high purities.     

Preparation of the β2‐Homoselenocysteine Derivatives Fmoc‐(S)‐β2hSec(PMB)‐OH and Boc‐(S)‐β2hSec(PMB)‐OH for Solution and Solid‐Phase Peptide Synthesis

Fmoc‐β²hSer(^tBu)‐OH was converted to Fmoc‐β²hSec(PMB)‐OH in five steps. To avoid elimination of HSeR, the selenyl group was introduced in the second last step (Fmoc‐β²hSer(Ts)‐OAll→Fmoc‐β²hSec(PMB)‐OAll). In a similar way, the N‐Boc‐protected compound was prepared. With the β²hSe‐derivatives, 21 β²‐amino‐acid building blocks with proteinogenic side chains are now available for peptide synthesis.     

Synthesis of α,β‐Unsaturated Ketones and Esters Using Polymer‐supported Selenium Bromide

Treatment of the polymer‐supported α‐phenylseleno ketones and esters prepared from polymer‐supported selenium bromide with ketone and ester enolates with hydrogen peroxide afford α,β‐unsaturated ketones and esters in good yields and high purities.     

5-雄烯-3β, 7α, 17β-三醇和5-雄烯-3β, 7β, 17β-三醇的合成

以5-雄烯二醇为原料,用微生物转化的方法合成了两个重要的神经甾体5-雄烯-3β, 7α, 17β-三醇和5-雄烯-3β, 7β, 17β-三醇。所用菌种总枝毛霉为我们自己筛选,并首次应用于5-雄烯-3β, 7α, 17β-三醇和5-雄烯-3β, 7β, 17β-三醇的合成中。     

Enantioselective Solid‐Phase Peptide Synthesis Using Traceless Chiral Coupling Reagents and Racemic Amino Acids

The enantioselective condensing reagent 4,6‐dimethoxy‐1,3,5‐triazine (DMT)/strychnine/BF$\rm{{_{4}^{-}}}$ was obtained by treatment of 2‐chloro‐4,6‐dimethoxy‐1,3,5‐triazine (CDMT) with strychnine tetrafluoroborate. The reagent was useful under typical conditions of solid‐phase peptide synthesis (SPPS) with enantiomerically homogeneous substrates. By SPPS, desired dipeptides were obtained in 84–94% yield using 4 equiv. of racemic Fmoc‐Ala, Fmoc‐Phe, and/or Fmoc‐Tyr for 1 equiv. of Wang resin loaded with Gly, Ala, Leu, Phe, Glu(^tBu), and/or Pro, respectively. For all three Fmoc‐protected amino acids, the configuration of the enantiomer preferred under SPPS conditions was independent of the structure of the acylated component and identical to that established in condensations proceeding in solution. In all cases, the enantiomer ratios L /D (er) were in a similar range, and varied from 9 : 92 to 2 : 98 for alanine, and from 90 : 10 to 100 : 0 for aromatic amino acids. The synthesis of Ac‐L ‐Lys(Ac)‐D ‐Ala‐D ‐Ala‐OH from racemic Fmoc‐Ala gave an L /D ratio of 10 : 90 for the esterification of Wang resin, and 0 : 100 for the formation of peptide bonds.     

Michael-Addition Reaction of Malononitrile with α,β-Unsaturated Cycloketones Catalyzed by KF／Al2O3

A series of KF/Al2O3 catalyzed Michael-addition reactions between malononitrile and α,β-unsaturated cycloketones in DMF solution were studied. At room temperature, 2-cyano-3-aryl-3-(1,2,3,4-tetrahydronaphthalen-1-one-2-yl) propionitrile derivatives were synthesized by the reaction between 2-arylmethylidene-1,2,3,4-tetra-hydronaphthalen-1-one and malononitrile. However, if the temperature was increased to 80℃, 2-amino-3-cyano-4-aryl-4H-benzo[h]chromene derivatives were obtained in high yields. When the α,β-unsaturated ketones were replaced by 2,6-biarylmethylidenecyclohexanone or 2,5-biarylmethylidenecyclopentanone, another series of 2-amino-3-cyano-4H-pyran derivatives was isolated successfully. The structures of the products were confirmed by X-ray diffraction analysis.     

Novel N‐(2,2‐Dimethyl‐2H‐azirin‐3‐yl)‐L‐prolinates as Aib‐Pro Synthons

The syntheses of phenacyl N‐(2,2‐dimethyl‐2H‐azirin‐3‐yl)‐L ‐prolinate and allyl N‐(2,2‐dimethyl‐2H‐azirin‐3‐yl)‐L ‐prolinate are reported. Reactions of these 2H‐azirin‐3‐amine derivatives with Z‐protected amino acids have shown them to be suitable synthons for the Aib‐Pro unit in peptide synthesis. After incorporation into the peptide by means of the ‘azirine/oxazolone method’, the C‐termini of the resulting peptides were deprotected selectively with Zn in AcOH or by a mild Pd⁰‐promoted procedure, respectively.     

Synthesis of a Derivative of the Peptaibol‐Antibiotic Trichovirin I 1B by Means of the ‘Azirine/Oxazolone Method’

According to the earlier published synthesis of the C‐terminal nonapeptide of Trichovirin I 1B, Z‐Ser(^tBu)‐Val‐Aib‐Pro‐Aib‐Leu‐Aib‐Pro‐Leuol ( 5 ), the complete tetradecapeptide Z‐Aib‐Asn(Trt)‐Leu‐Aib‐Pro‐Ser(^tBu)‐Val‐Aib‐Pro‐Aib‐Leu‐Aib‐Pro‐Leuol ( 11b ), a protected Trichovirin I 1B, has now been prepared by means of the ‘azirine/oxazolone method’. With the exception of the N‐terminal Aib(1), all Aib residues were introduced by the coupling of the corresponding amino or peptide acids with 2,2‐dimethyl‐2H‐azirine‐3‐(N‐methyl‐N‐phenylamine) ( 1a ) and methyl N‐(2,2‐dimethyl‐2H‐azirin‐3‐yl)‐L ‐prolinate ( 3a ) as the Aib and Aib‐Pro synthons, respectively. Single crystals of two segments, i.e., the N‐terminal hexapeptide Z‐Aib‐Asn(Trt)‐Leu‐Aib‐Pro‐Ser(^tBu)‐OMe ( 23 ) and the C‐terminal octapeptide Z‐Val‐Aib‐Pro‐Aib‐Leu‐Aib‐Pro‐Leuol ( 17 ), were obtained and their structures have been established by X‐ray crystallography. Following the same strategy, the C‐terminal nonapeptide of Trichovirin I 4A, Z‐Ala‐Val‐Aib‐Pro‐Aib‐Leu‐Aib‐Pro‐Leuol ( 26 ), was also synthesized and characterized by X‐ray crystallography.     

Experimental and Theoretical Studies on the Rearrangement of 2‐Oxoazepane α,α‐Amino Acids into 2′‐Oxopiperidine β2,3,3‐Amino Acids: An Example of Intramolecular Catalysis

Enantiopure β‐amino acids represent interesting scaffolds for peptidomimetics, foldamers and bioactive compounds. However, the synthesis of highly substituted analogues is still a major challenge. Herein, we describe the spontaneous rearrangement of 4‐carboxy‐2‐oxoazepane α,α‐amino acids to lead to 2′‐oxopiperidine‐containing β^2,3,3‐amino acids, upon basic or acid hydrolysis of the 2‐oxoazepane α,α‐amino acid ester. Under acidic conditions, a totally stereoselective synthetic route has been developed. The reordering process involved the spontaneous breakdown of an amide bond, which typically requires strong conditions, and the formation of a new bond leading to the six‐membered heterocycle. A quantum mechanical study was carried out to obtain insight into the remarkable ease of this rearrangement, which occurs at room temperature, either in solution or upon storage of the 4‐carboxylic acid substituted 2‐oxoazepane derivatives. This theoretical study suggests that the rearrangement process occurs through a concerted mechanism, in which the energy of the transition states can be lowered by the participation of a catalytic water molecule. Interestingly, it also suggested a role for the carboxylic acid at position 4 of the 2‐oxoazepane ring, which facilitates this rearrangement, participating directly in the intramolecular catalysis.     

Solid‐Phase Synthesis of Peptide Libraries Combining α‐Amino Acids with Inorganic and Organic Chromophores

The synthesis of two series of peptidic chains composed of bis(terpyridine)ruthenium(II) acceptor units and organic chromophores (coumarin, naphthalene, anthracene, fluorene) by stepwise solid‐phase peptide synthesis (SPPS) techniques is described. The first series of dyads comprises directly amide linked chromophores, while the second one possesses a glycine spacer between the two chromophores. All dyads were studied by UV/Vis and NMR spectroscopy, steady‐state luminescence, luminescence decay and electrochemistry, as well as by DFT calculations. The results of these studies indicate weak electronic coupling of the chromophores in the ground state. Absorpion spectra of all dyads are dominated by metal‐to‐ligand charge‐transfer (MLCT) bands around 500 nm. The bichromophoric systems, especially with coumarin as organic chromophore, display additional strong absorptions in the visible spectral region. All complexes are luminescent at room temperature (³MLCT). Efficient quenching of the fluorescence of the organic chromophore by the attached ruthenium complex is observed in all dyads. Excitation spectra indicate energy transfer from the organic dye to the ruthenium chromophore.