Designed Beta‐Turn Mimic Based on the Allylic‐Strain Concept: Evaluation of Structural and Biological Features by Incorporation into a Cyclic RGD Peptide (Cyclo(‐L‐arginylglycyl‐L‐α‐aspartyl‐))

The (3R,5S,6E,8S,10R)‐11‐amino‐3,5,8,10‐tetramethylundec‐6‐enoic acid (ATUA; 1 ), which was designed as a βII′‐turn mimic according to the concepts of allylic strain and 2,4‐dimethylpentane units, was incorporated into a cyclic RGD peptide. The three‐dimensional structure of cyclo(‐RGD‐ATUA‐) (=cyclo(‐Arg‐Gly‐Asp‐ATUA‐)) 4 in H₂O was determined by NMR techniques, distance geometry calculations and molecular‐dynamics simulations. The RGD sequence of 4 shows high conformational flexibility but some preference for an extended conformation. The structural features of the RGD sequence of 4 were compared with the RGD moiety of cyclo(‐RGDfV‐) (=cyclo(‐Arg‐Gly‐Asp‐D ‐Phe‐Val‐)). In contrast to cyclo(‐RGDfV‐), which is a highly active αvβ3 antagonist and selective against αIIbβ3, cyclo(‐RGD‐ATUA‐) shows a lower activity and selectivity. The structure of the ATUA residue in the cyclic peptide resembles a βII′‐turn‐like conformation. Its middle part, adjacent to the C?C bond, strongly prefers the designed and desired structure.     

Synthesis of Z‐Protected Aib‐ and Phe(2Me)‐Containing Pentapeptides and Their Crystal Structures

A series of pentapeptide derivatives containing α,α‐disubstituted α‐amino acids have been prepared by a combination of the ‘azirine/oxazolone method’ and segment condensations. X‐Ray crystal‐structure determinations of the molecular structures confirmed the presence of helical conformations stabilized by β‐turns of type III or III′. Pentapeptides containing (R)‐Phe(2Me) form a right‐handed helix, whereas those containing (S)‐Phe(2Me) adopt a left‐handed helical structure.     

A Novel 2H‐Azirin‐3‐amine as a Dipeptide (Aib‐Hyp) Synthon

The synthesis of methyl (2S,4R)‐4‐(benzyloxy)‐N‐(2,2‐dimethyl‐2H‐azirin‐3‐yl)prolinate ( 10 ), a novel 2H‐azirin‐3‐amine (`3‐amino‐2H‐azirine'), is described (Scheme 1). The reaction of methyl (2S,4R)‐N‐(2‐methylpropanoyl)‐4‐(benzyloxy)prolinate ( 7 ) with Lawesson reagent gave methyl (2S,4R)‐4‐(benzyloxy)‐N‐[2‐(methylthio)propanoyl]prolinate ( 8 ) and consecutive treatment with COCl₂, 1,4‐diazabicyclo[2.2.2]octane (DABCO), and NaN₃ led to 10 . The use of 10 as a building block of the dipeptide Aib‐Hyp (Aib=2‐aminoisobutyric acid, Hyp=(2S,4R)‐4‐hydroxyproline) is demonstrated by the syntheses of several model peptides (Scheme 2 and Table). The benzyl protecting group of the 4‐OH function in Hyp in the model peptides has been removed in good yields.     

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.     

Conformation of Peptides Containing a Chiral α‐Ethylated α,α‐Disubstituted α‐Amino Acid: (S)‐α‐Ethylleucine (=(2S)‐2‐Amino‐2‐ethyl‐4‐methylpentanoic Acid) within Sequences of Dimethylglycine and Diethylglycine Residues

An optically active (S)‐α‐ethylleucine ((S)‐αEtLeu) as a chiral α‐ethylated α,α‐disubstituted α‐amino acid was synthesized by means of a chiral acetal auxiliary of (R,R)‐cyclohexane‐1,2‐diol. The chiral α‐ethylated α,α‐disubstituted amino acid (S)‐αEtLeu was introduced into the peptides constructed from 2‐aminoisobutyric acid (=dimethylglycine, Aib), and also into the peptide prepared from diethylglycine (Deg). The X‐ray crystallographic analysis revealed that both right‐handed (P) and left‐handed (M) 3₁₀‐helical structures exist in the solid state of CF₃CO‐(Aib)₂‐[(S)‐αEtLeu]‐(Aib)₂‐OEt ( 14 ) and CF₃CO‐[(S)‐αEtLeu]‐(Deg)₄‐OEt ( 18 ), respectively. The IR, CD, and ¹H‐NMR spectra indicated that the dominant conformation of pentapeptides 14 and CF₃CO‐[(S)‐αEtLeu]‐(Aib)₄‐OEt ( 16 ) in solution is a 3₁₀‐helical structure, and that of 18 in solution is a planar C₅ conformation. The conformation of peptides was also studied by molecular‐mechanics calculations.     

Solid‐state conformations of linear depsipeptide amides with an alternating sequence of α,α‐disubstituted α‐amino acid and α‐hydroxy acid

Depsipeptides and cyclodepsipeptides are analogues of the corresponding peptides in which one or more amide groups are replaced by ester functions. Reports of crystal structures of linear depsipeptides are rare. The crystal structures and conformational analyses of four depsipeptides with an alternating sequence of an α,α‐disubstituted α‐amino acid and an α‐hydroxy acid are reported. The molecules in the linear hexadepsipeptide amide in (S)‐Pms‐Acp‐(S)‐Pms‐Acp‐(S)‐Pms‐Acp‐NMe₂ acetonitrile solvate, C₄₇H₅₈N₄O₉·C₂H₃N, ( 3b ), as well as in the related linear tetradepsipeptide amide (S)‐Pms‐Aib‐(S)‐Pms‐Aib‐NMe₂, C₂₈H₃₇N₃O₆, ( 5a ), the diastereoisomeric mixture (S,R)‐Pms‐Acp‐(R,S)‐Pms‐Acp‐NMe₂/(R,S)‐Pms‐Acp‐(R,S)‐Pms‐Acp‐NMe₂ (1:1), C₃₂H₄₁N₃O₆, ( 5b ), and (R,S)‐Mns‐Acp‐(S,R)‐Mns‐Acp‐NMe₂, C₃₀H₃₇N₃O₆, ( 5c ) (Pms is phenyllactic acid, Acp is 1‐aminocyclopentanecarboxylic acid and Mns is mandelic acid), generally adopt a β‐turn conformation in the solid state, which is stabilized by intramolecular N—H…O hydrogen bonds. Whereas β‐turns of type I (or I′) are formed in the cases of ( 3b ), ( 5a ) and ( 5b ), which contain phenyllactic acid, the torsion angles for ( 5c ), which incorporates mandelic acid, indicate a β‐turn in between type I and type III. Intermolecular N—H…O and O—H…O hydrogen bonds link the molecules of ( 3a ) and ( 5b ) into extended chains, and those of ( 5a ) and ( 5c ) into two‐dimensional networks.     

Asymmetric Mannich‐Type Synthesis of N‐Phosphinyl α‐Aminophosphonic Acid Monoesters

A convenient diastereoselective synthesis of diisopropyl (2R,3R)‐3‐{{{(R/S)‐aryl[(diethoxyphosphinyl)amino]methyl}hydroxyphosphinyl}oxy}‐2‐hydroxybutanedioate through Mannich‐type reactions is reported. The reactions take place under mild conditions in good yields, and this makes it possible to introduce various substituents at the α‐position to the P‐atom of α‐aminophosphonates. The chiral diisopropyl (4R,5R)‐2‐chloro‐1,3,2‐dioxaphospholane‐4,5‐dicarboxylate ( 3 ) was found to be a good phosphonylating agent in this stereoselective reaction.     

Synthesis of Cyclohexapeptides Containing Pro and Aib Residues

Cyclization reactions on hexapeptides containing several α‐aminoisobutyric acid (=2‐amino‐2‐methylpropanoic acid; Aib) residues and the turn‐promoting glycine (Gly) and proline (Pro) residues were investigated. Eight linear hexapeptides were synthesized, and their cyclization was attempted with various coupling reagents. The macrolactamization step proved to be difficult since only three hexapeptides could be cyclized. Two of these latter peptides were the linear precursors of the same cyclic hexapeptide, cyclo(Aib‐Aib‐Phe‐Pro‐Aib‐Gly) ( 1 ). Surprisingly, they gave 1 in almost the same yield. Thus, 1 was obtained in 35% yield upon ring closure at the Phe/Pro site by using DEPBT as the coupling reagent, whereas the cyclization at the Aib/Phe site led to 1 in 28 and 34% yield by using PyAOP and DEPC, respectively (DEPBT=3‐[(diethoxyphosphoryl)oxy]‐1,2,3‐benzotriazin‐4(3H)‐one, PyAOP=(1H‐7‐azabenzotriazol‐1‐yloxy)tripyrrolidin‐1‐ylphosphonium hexafluorophosphate, DEPC=diethyl phosphorocyanidate). Another cyclic hexapeptide, cyclo(Aib‐Aib‐Gly‐Aib‐Pro‐Gly) ( 2 ) was prepared in 34% yield when DEPC was used in the cyclization step. The solid‐state conformation of 1 was established by X‐ray crystallography.     

Diastereoselective Electrophilic Amination of Chiral 1‐Benzoyl‐2,3,5,6‐tetrahydro‐3‐methyl‐2‐(1‐methylethyl)pyrimidin‐4(1H)‐one for the Asymmetric Syntheses of α‐Substituted α,β‐Diaminopropanoic Acids

The chiral compounds (R)‐ and (S)‐1‐benzoyl‐2,3,5,6‐tetrahydro‐3‐methyl‐2‐(1‐methylethyl)pyrimidin‐4(1H)‐one ((R)‐ and (S)‐ 1 ), derived from (R)‐ and (S)‐asparagine, respectively, were used as convenient starting materials for the preparation of the enantiomerically pure α‐alkylated (alkyl=Me, Et, Bn) α,β‐diamino acids (R)‐ and (S)‐ 11 – 13 . The chiral lithium enolates of (R)‐ and (S)‐ 1 were first alkylated, and the resulting diasteroisomeric products 5 – 7 were aminated with ‘di(tert‐butyl) azodicarboxylate’ (DBAD), giving rise to the diastereoisomerically pure (≥98%) compounds 8 – 10 . The target compounds (R)‐ and (S)‐ 11 – 13 could then be obtained in good yields and high purities by a hydrolysis/hydrogenolysis/hydrolysis sequence.     

Chiral differentiation of some cyclic β‐amino acids by kinetic and fixed ligand methods

Differentiation of β ‐amino acid enantiomers with two chiral centres was investigated by kinetic method with trimeric metal‐bound complexes. Four enantiomeric pairs of β ‐amino acids were studied: cis‐(1R,2S)‐, cis‐(1S,2R)‐, trans‐(1R,2R)‐ and trans‐(1S,2S)‐2‐aminocyclopentanecarboxylic acids (cyclopentane β ‐amino acids), and cis‐(1R,2S)‐, cis‐(1S,2R)‐, trans‐(1R,2R)‐, and trans‐(1S,2S)‐2‐aminocyclohexanecarboxylic acids (cyclohexane β ‐amino acids). The results showed that the choice of metal ion (Cu²⁺, Ni²⁺) and chiral reference compound (α‐ and β ‐amino acids) had an effect on the enantioselectivity. Especially, aromaticity of the reference compound was noted to enhance the enantioselectivity. The fixed‐ligand kinetic method, a modification of the kinetic method, was then applied to the same β ‐amino acids, with dipeptides used as fixed ligands. With this method, dipeptide containing an aromatic side chain enhanced the enantioselectivity. Copyright © 2009 John Wiley & Sons, Ltd.     

Stereoselective Preparation of 3‐Amino‐2‐fluoro Carboxylic Acid Derivatives,and Their Incorporation in Tetrahydropyrimidin‐4(1H)‐ones,and in Open‐Chain and Cyclic β‐Peptides

The preparation of (2S,3S)‐ and (2R,3S)‐2‐fluoro and of (3S)‐2,2‐difluoro‐3‐amino carboxylic acid derivatives, 1 – 3 , from alanine, valine, leucine, threonine, and β³h‐alanine (Schemes 1 and 2, Table) is described. The stereochemical course of (diethylamino)sulfur trifluoride (DAST) reactions with N,N‐dibenzyl‐2‐amino‐3‐hydroxy and 3‐amino‐2‐hydroxy carboxylic acid esters is discussed (Fig. 1). The fluoro‐β‐amino acid residues have been incorporated into pyrimidinones ( 11 – 13 ; Fig. 2) and into cyclic β‐tri‐ and β‐tetrapeptides 17 – 19 and 21 – 23 (Scheme 3) with rigid skeletons, so that reliable structural data (bond lengths, bond angles, and Karplus parameters) can be obtained. β‐Hexapeptides Boc[(2S)‐β³hXaa(αF)]₆OBn and Boc[β³hXaa(α,αF₂)]₆‐OBn, 24 – 26 , with the side chains of Ala, Val, and Leu, have been synthesized (Scheme 4), and their CD spectra (Fig. 3) are discussed. Most compounds and many intermediates are fully characterized by IR‐ and ¹H‐, ¹³C‐ and ¹⁹F‐NMR spectroscopy, by MS spectrometry, and by elemental analyses, [α]_D and melting‐point values.     

The ‘Azirine/Oxazolone Method’ on Solid Phase: Introduction of Various α,α‐Disubstituted α‐Amino Acids

Peptides containing various α,α‐disubstituted α‐amino acids, such as α‐aminoisobutyric acid (Aib), 1‐aminocyclopentane‐1‐carboxylic acid, α‐methylphenylalanine, and 3‐amino‐3,4,5,6‐tetrahydro‐2H‐pyran‐3‐carboxylic acid have been synthesized from the N‐ to the C‐terminus by the ‘azirine/oxazolone method’ under solid‐phase conditions. In this convenient method for the synthesis of sterically demanding peptides on solid‐phase, 2H‐azirin‐3‐amines are used to introduce the α,α‐disubstituted α‐amino acids without the need for additional reagents. Furthermore, the synthesis of poly(Aib) sequences has been explored.     

Chiral Heterospirocyclic 2H‐Azirin‐3‐amines as Synthons for 3‐Amino‐2,3,4,5‐tetrahydrofuran‐3‐carboxylic Acid and Their Use in Peptide Synthesis

The heterospirocyclic N‐methyl‐N‐phenyl‐5‐oxa‐1‐azaspiro[2.4]hept‐1‐e n‐2‐amine (6 ) and N‐(5‐oxa‐1‐azaspiro[2.4]hept‐1‐en‐2‐yl)‐(S)‐proline methyl ester ( 7 ) were synthesized from the corresponding heterocyclic thiocarboxamides 12 and 10 , respectively, by consecutive treatment with COCl₂, 1,4‐diazabicyclo[2.2.2]octane, and NaN₃ (Schemes 1 and 2). The reaction of these 2H‐azirin‐3‐amines with thiobenzoic and benzoic acid gave the racemic benzamides 13 and 14 , and the diastereoisomeric mixtures of the N‐benzoyl dipeptides 15 and 16 , respectively (Scheme 3). The latter were separated chromatographically. The configurations and solid‐state conformations of all six benzamides were determined by X‐ray crystallography. With the aim of examining the use of the new synthons in peptide synthesis, the reactions of 7 with Z‐Leu‐Aib‐OH to yield a tetrapeptide 17 (Scheme 4), and of 6 with Z‐Ala‐OH to give a dipeptide 18 (Scheme 5) were performed. The resulting diastereoisomers were separated by means of MPLC or HPLC. NMR Studies of the solvent dependence of the chemical shifts of the NH resonances indicate the presence of an intramolecular H‐bond in 17 . The dipeptides (S,R)‐ 18 and (S,S)‐ 18 were deprotected at the N‐terminus and were converted to the crystalline derivatives (S,R)‐ 19 and (S,S)‐ 19 , respectively, by reaction with 4‐bromobenzoyl chloride (Scheme 5). Selective hydrolysis of (S,R)‐ 18 and (S,S)‐ 18 gave the dipeptide acids (R,S)‐ 20 and (S,S)‐ 20 , respectively. Coupling of a diastereoisomeric mixture of 20 with H‐Phe‐O^tBu led to the tripeptides 21 (Scheme 5). X‐Ray crystal‐structure determinations of (S,R)‐ 19 and (S,S)‐ 19 allowed the determination of the absolute configurations of all diastereoisomers isolated in this series.     

The achiral tetrapeptide Z‐Aib‐Aib‐Aib‐Gly‐OtBu

The title achiral peptide N‐benzyloxycarbonyl‐α‐aminoisobutyryl‐α‐aminoisobutyryl‐α‐aminoisobutyrylglycine tert‐butyl ester or Z‐Aib‐Aib‐Aib‐Gly‐OtBu (Aib is α‐aminoisobutyric acid, Z is benzyloxycarbonyl, Gly is glycine and OtBu indicates the tert‐butyl ester), C₂₆H₄₀N₄O₇, is partly hydrated (0.075H₂O) and has two different conformations which together constitute the asymmetric unit. Both molecules form incipient 3₁₀‐helices. They differ in the relative orientation of the N‐terminal protection group and at the C‐terminus. There are two 4→1 intramolecular hydrogen bonds.     

Diastereoselective addition of trimethylsilyl cyanide to chiral O‐, S‐ and N‐heterocyclic aldimines

Systematic investigation of asymmetric trimethylsilylcyanation of heterocyclic azomethines has been realized. The addition of trimethylsilyl cyanide to optically active furan, thiophene and pyridine aldimines, derived from (R)‐ and (S)‐1‐phenylethylamine, was studied in the presence of Lewis acids, and a series of the corresponding α‐amino nitriles was obtained in fair to good yields (up to 91%). Unsaturated nitriles were also formed from pyridine imines. The sense of asymmetric induction and the degree of diastereoselectivity in the synthesis of α‐amino nitriles were determined by means of ¹H NMR. The stereochemical outcome is a result of the same sense of asymmetric induction: Re face attack to the (S)‐imines and Si face addition to the (R)‐imines took place. The (R,R)‐ (up to 81%) or (S,S)‐ (up to 87%) α‐amino nitriles predominated in the products obtained from the all furan, thiophene and pyridine (R)‐ or (S)‐imines respectively. Copyright © 2002 John Wiley & Sons, Ltd.     

The Propensity of α‐Aminoisobutyric Acid (=2‐Methylalanine; Aib) to Induce Helical Secondary Structure in an α‐Heptapeptide: A Computational Study

We present a molecular‐dynamics simulation study of an α‐heptapeptide containing an α‐aminoisobutyric acid (=2‐methylalanine; Aib) residue, Val¹‐Ala²‐Leu³‐Aib⁴‐Ile⁵‐Met⁶‐Phe⁷, and a quantum‐mechanical (QM) study of simplified models to investigate the propensity of the Aib residue to induce 3₁₀/α‐helical conformation. For comparison, we have also performed simulations of three analogues of the peptide with the Aib residue being replaced by L ‐Ala, D ‐Ala, and Gly, respectively, which provide information on the subtitution effect at C(α) (two Me groups for Aib, one for L ‐Ala and D ‐Ala, and zero for Gly). Our simulations suggest that, in MeOH, the heptapeptide hardly folds into canonical helical conformations, but appears to populate multiple conformations, i.e., C₇ and 3₁₀‐helical ones, which is in agreement with results from the QM calculations and NMR experiments. The populations of these conformations depend on the polarity of the solvent. Our study confirms that a short peptide, though with the presence of an Aib residue in the middle of the chain, does not have to fold to an α‐helical secondary structure. To generate a helical conformation for a linear peptide, several Aib residues should be present in the peptide, either sequentially or alternatively, to enhance the propensity of Aib‐containing peptides towards the helical conformation. A correction of a few of the published NMR data is reported.     

Me‐BIPAM for the Synthesis of Optically Active 3‐Aryl‐3‐hydroxy‐2‐oxindoles by Ruthenium‐catalyzed Addition of Arylboronic Acids to Isatins

A chiral O‐linked C₂‐symmetric bidentate phosphoramidite (Me‐BIPAM) was found to be efficient for the ruthenium‐catalyzed addition of arylboronic acids to isatins. Asymmetric synthesis of 3‐aryl‐3‐hydroxy‐2‐oxindoles by 1,2‐addition of arylboronic acids to isatins was carried out in the presence of [RuCl₂(PPh₃)₃]/(R,R)‐Me‐BIPAM and KF, resulting in an enantioselectivity as high as 90 % ee. It was found that the reaction with N‐protected isatins proceeds with high yields and good enantioselectivities. The best protective groups on the nitrogen atom were different depending on the substituents on the aromatic ring. The use of a N‐benzyl group resulted in excellent enantioselectivities in many substrates compared with other groups.     

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.     

The Stereoselective Synthesis of 2‐Aryl‐2‐hydroxybutanoic Acid via Menthyl Chiral Auxiliaries

In the presence of titanium(IV) tetraethoxide ((EtO)₄Ti), menthyl arylglyoxylates are prepared by transesterification of ethyl arylglyoxylates and natural (−)‐(1R,2S,5R)‐menthol. Using menthyl as a chiral auxiliary, the corresponding novel (R)‐menthyl 2‐aryl‐2‐hydroxybutanoates are synthesized by the addition of Et₂Zn with menthyl arylglyoxylates. The structures of the products are characterized by IR and ¹H‐ and ¹³C‐NMR spectroscopy, mass spectrometry, and elemental analysis. The diastereoselectivities are analyzed by HPLC. The addition reactions are completed with good yields and high diastereoisomeric excess (de up to 95%), and, after hydrolysis, the (R)‐2‐aryl‐2‐hydroxybutanoic acids are obtained with high optical purities.     

The Effect of Fluoro Substitution upon the β‐Hairpin Fold of a β‐Tetrapeptide in Methanol

The importance of β‐peptides lies in their ability to mimic the conformational behavior of α‐peptides, even with a much shorter chain length, and in their resistance to proteases. To investigate the effect of substitution of β‐peptides on their dominant fold, we have carried out a molecular‐dynamics (MD) simulation study of two tetrapeptides, Ac‐(2R,3S)‐β^2,3hVal(αMe)‐(2S)‐β²hPhe‐(R)‐β³hLys‐(2R,3S)‐β^2,3‐Ala(αMe)‐NH₂, differing in the substitution at the C_α of Phe2 (pepF with F, and pepH with H). Three simulations, unrestrained (UNRES), using ³J‐coupling biasing with local elevation in combination with either instantaneous (INS) or time‐averaging (AVE) NOE distance restraining, were carried out for each peptide. In the unrestrained simulations, we find three (pepF) and two (pepH) NOE distance bound violations of maximally 0.22 nm that involve the terminal residues. The restrained simulations match both the NOE distance bounds and ³J‐values derived from experiment. The fluorinated peptide shows a slightly larger conformational variability than the non‐fluorinated one.