SN1‐Type Substitution Reactions of N‐Protected β‐Hydroxytyrosine Esters: Stereoselective Synthesis of β‐Aryl and β‐Alkyltyrosines

The title compounds were prepared by aldol reaction of anisaldehyde and the respective N,N‐dibenzyl glycinates. Deprotection of the nitrogen atom with Pearlman’s catalyst delivered the unprotected β‐hydroxytyrosine esters, which were further N‐protected as N,N‐phthaloyl (Phth) and N‐fluorenylmethylcarbonyloxy (Fmoc) derivatives. The Friedel–Crafts reaction with various arenes was studied employing these alcohols as electrophiles. It turned out that the facial diastereoselectivitiy depends on the nitrogen protecting group and on the ester group. The unprotected substrates (NH₂) gave preferentially syn‐products but the anti‐selectivity increased when going from NHFmoc over NPhth to NBn₂. If the ester substituent was varied the syn‐preference increased in the order Me <Et <iPr. The reactions were shown to be fully stereoconvergent and proceeded under kinetic product control. A model is suggested to explain the facial diastereoselectivity based on a conformationally locked benzylic cation intermediate. The reactions are preparatively useful for the N‐unprotected isopropyl ester, which gave Friedel–Crafts alkylation products with good syn‐selectivity (anti/syn=21:79 to 7:93), and for the N,N‐dibenzyl‐protected methyl ester, which led preferentially to anti‐products (anti/syn=80:20 to >95:5). Upon acetylation of the latter compound to the respective acetate, Bi(OTf)₃‐catalyzed alkylation reactions became possible, in which silyl enol ethers served as nucleophiles. The respective alkylation products were obtained in high yield and with excellent anti‐selectivitiy (anti/syn≥95:5).     

Lewis Acid‐Mediated Addition of Amino Acid‐Substituted α‐Allylsilanes to Aromatic Acetals

Unnatural amino acids extend the pharmacological formulator's toolkit. Strategies to prepare unnatural amino acid derivatives using Lewis acid‐activated allylsilane reactions are few. In this regard, we examined the utility of allylsilanes bearing an amino acid substituent in the reaction. Diastereoselective addition of methyl 2‐(N‐PG‐amino)‐3‐(trimethylsilyl)pent‐4‐enoate and methyl (E)‐2‐(N‐PG‐amino)‐3‐(trimethylsilyl)hex‐4‐enoate (PG=protecting group), 2 and 13 , respectively, to aromatic acetals in the presence of Lewis acids is described. Of those examined, TiCl₄ was found to be the most effective Lewis acid for promoting the addition. At least 1 equiv. of TiCl₄ was required to achieve high yields, whereas 2 equiv. of BF₃?OEt₂ were required for comparable outcomes. Excellent selectivity (>99% syn/anti) and high yield (up to 89%) were obtained with halo‐substituted aromatic acetals, while more electron‐rich electrophiles led to both lower yields and diastereoselectivities.     

Differentiation of Boc‐protected α,δ‐/δ,α‐ and β,δ‐/δ,β‐hybrid peptide positional isomers by electrospray ionization tandem mass spectrometry

Two new series of Boc‐N‐α,δ‐/δ,α‐ and β,δ‐/δ,β‐hybrid peptides containing repeats of L ‐Ala‐δ⁵‐Caa/δ⁵‐Caa‐L ‐Ala and β³‐Caa‐δ⁵‐Caa/δ⁵‐Caa‐β³‐Caa (L ‐Ala = L ‐alanine, Caa = C‐linked carbo amino acid derived from D ‐xylose) have been differentiated by both positive and negative ion electrospray ionization (ESI) ion trap tandem mass spectrometry (MS/MS). MSⁿ spectra of protonated isomeric peptides produce characteristic fragmentation involving the peptide backbone, the Boc‐group, and the side chain. The dipeptide positional isomers are differentiated by the collision‐induced dissociation (CID) of the protonated peptides. The loss of 2‐methylprop‐1‐ene is more pronounced for Boc‐NH‐L ‐Ala‐δ‐Caa‐OCH₃ (1), whereas it is totally absent for its positional isomer Boc‐NH‐δ‐Caa‐L ‐Ala‐OCH₃ (7), instead it shows significant loss of t‐butanol. On the other hand, second isomeric pair shows significant loss of t‐butanol and loss of acetone for Boc‐NH‐δ‐Caa‐β‐Caa‐OCH₃ (18), whereas these are insignificant for its positional isomer Boc‐NH‐β‐Caa‐δ‐Caa‐OCH₃ (13). The tetra‐ and hexapeptide positional isomers also show significant differences in MS² and MS³ CID spectra. It is observed that ‘b’ ions are abundant when oxazolone structures are formed through five‐membered cyclic transition state and cyclization process for larger ‘b’ ions led to its insignificant abundance. However, b₁⁺ ion is formed in case of δ,α‐dipeptide that may have a six‐membered substituted piperidone ion structure. Furthermore, ESI negative ion MS/MS has also been found to be useful for differentiating these isomeric peptide acids. Thus, the results of MS/MS of pairs of di‐, tetra‐, and hexapeptide positional isomers provide peptide sequencing information and distinguish the positional isomers. Copyright © 2010 John Wiley & Sons, Ltd.     

The Cyclo‐β‐Tetrapeptide (β‐HPhe‐β‐HThr‐β‐HLys‐β‐HTrp): Synthesis,NMR Structure in Methanol Solution,and Affinity for Human Somatostatin Receptors

The known solid‐state structure (Fig. 1, top) of cyclo(β‐HAla)₄ was used to model the structure of the title compound 1 as a prospective somatostatin mimic (Fig. 1, bottom). The synthesis started with the N‐protected natural amino acids Boc‐Phe‐OH, Boc‐Trp‐OH, Boc‐Lys(2‐Cl‐Z)‐OH, and Boc‐Thr(OBn)‐OH, which were homologated to the corresponding β‐amino‐acid derivatives (Scheme 1) and coupled to the β‐tetrapeptide Boc‐β‐HTrp‐β‐HPhe‐β‐HThr(OBn)‐β‐HLys(2‐Cl‐Z)‐OMe ( 16 ); the (N‐Me)‐β‐HThr‐(N‐Me)‐β‐HPhe analog 17 was also prepared. C‐ and N‐terminal deprotection and cyclization through the pentafluorophenyl ester gave the insoluble β‐tetrapeptide with protected Thr and Lys side chains ( 18 ). Solubilization and debenzylation could only be effected in LiCl‐containing THF (ca. 10% yield; with ca. 55% recovery). HPLC Purification provided a sample of the title compound 1 , the structure of which, as determined by NMR‐spectroscopy (Fig. 2, left) was drastically different from the `theoretical' model (Fig. 1). There is a transannular H‐bond dividing the macrocyclic 16‐membered ring, thus forming a ten‐ and a twelve‐membered H‐bonded ring, the former mimicking, or actually being superimposable on, an α‐peptidic so‐called β‐turn. Still, the four side chains occupy equatorial positions on the ring, as planned, albeit with somewhat different geometry as compared to the `original'. The cyclo‐β‐tetrapeptide has micromolar affinities to the human somatostatin receptors (hsst 1 – 5). Thus, we have demonstrated for the first time that it is possible to mimic a natural peptide hormone with a small β‐peptide. Furthermore, we have discovered a simple way to construct the ubiquitous β‐turn motif with β‐peptides (which are known to be stable to mammalian peptidases).     

Determination of Enantiomer Purity of β‐ and γ‐Amino Acids by NMR Analysis of Diastereoisomeric Palladium Complexes

Like α‐amino acids, β‐ and γ‐amino acids form spirobicyclic complexes (see 2 and 3 ) by reaction with the chiral di‐μ‐chlorobis{2‐[1‐dimethylamino‐ϰN)‐ethyl]phenyl‐ϰC}dipalladium complexes 1 under basic conditions (Scheme 1 and X‐ray structures in Fig. 1). The diastereoisomeric complexes formed with mixtures of enantiomers of either the amino acids or the dichloro‐dipalladium complexes give rise to marked chemical‐shift differences in the ¹H‐ and ¹³C‐NMR spectra (Figs. 2 – 4) to allow determination of the enantiomer purities. A simple procedure is described by which β‐ and γ‐amino acids (which may be generated in situ from Boc‐ or Fmoc‐protected precursors) are converted to the Pd complexes and subjected to NMR measurements. The effects of solvent, temperature, and variation of the aryl group in the chiral derivatizing Pd reagent are described (Figs. 4 and 5). The methyl esters of β‐amino acids can also be employed, forming diastereoisomeric chloro[(amino‐ϰN)aryl‐ϰC][(amino‐ϰN)alkanoate]palladium complexes 6 for determining enantiomer ratios (Scheme 6). The new method has great scope, as demonstrated for β²‐, β³‐, β^2,3‐, β^2,2,3‐, γ²‐, γ³‐, γ⁴‐, and γ^2,3,4‐amino acid derivatives.     

Preparation of β2‐Homotryptophan Derivatives for β‐Peptide Synthesis

In view of the prominent role of the 1H‐indol‐3‐yl side chain of tryptophan in peptides and proteins, it is important to have the appropriately protected homologs H‐β² HTrp OH and H‐β³ HTrp OH (Fig.) available for incorporation in β‐peptides. The β²‐HTrp building block is especially important, because β²‐amino acid residues cause β‐peptide chains to fold to the unusual 12/10 helix or to a hairpin turn. The preparation of Fmoc and Z β²‐HTrp(Boc) OH by Curtius degradation (Scheme 1) of a succinic acid derivative is described (Schemes 2–4). To this end, the (S)‐4‐isopropyl‐3‐[(N‐Boc‐indol‐3‐yl)propionyl]‐1,3‐oxazolidin‐2‐one enolate is alkylated with Br CH₂CO₂Bn (Scheme 3). Subsequent hydrogenolysis, Curtius degradation, and removal of the Evans auxiliary group gives the desired derivatives of (R)‐H β²‐HTrp OH (Scheme 4). Since the (R)‐form of the auxiliary is also available, access to (S)‐β²‐HTrp‐containing β‐peptides is provided as well.     

Facile regiospecific syntheses of N‐α,N‐1(τ)‐dialkyl‐l‐histidines

Two diverse methodologies describe the first synthesis of suitably protected N‐α,N‐1(τ)‐dialkyl‐Lhistidine derivatives. Synthesis of suitably protected N‐α,N‐1(τ)‐dialkyl‐L‐histidines 7‐9 containing different alkyl groups at the N‐α and N‐1(τ) positions was achieved in four steps starting from L‐histidine methyl ester. Whereas, in the one‐step alternate route N‐α‐Boc‐L‐histidine methyl ester upon direct and simultaneous N‐α and N‐1(τ) alkylation with various alkyl halides in the presence of sodium hydride in DMF easily afforded N‐α,N‐1(τ)‐dialkyl‐L‐histidines 14 containing identical alkyl group at the N‐α and N‐1(τ) positions in high yields. Both procedures allowed facile entry to methyl and other higher alkyl groups at the N‐α‐position of the histidine ring     

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.     

Stereoselective Palladium‐Catalyzed Functionalization of Homoallylic Alcohols: A Convenient Synthesis of Di‐ and Trisubstituted Isoxazolidines and β‐Amino‐δ‐Hydroxy Esters

Enantiopure, Boc‐protected alkoxyamines 12 and 13 , derived from the readily available homoallylic alcohols 4 via a reaction that involves either inversion or retention of configuration, undergo a diastereoselective Pd‐catalyzed ring‐closing carbonylative amidation to produce isoxazolidines 16/17 (≤50:1 diastereoisomer ratio (d.r.)) that can be readily converted into the N‐Boc‐protected esters of β‐amino‐δ‐hydroxy acids and their γ‐substituted homologues 37 . The key carbonylative cyclization proceeds through an unusual syn addition of the palladium and the nitrogen nucleophile across the C?C bond ( 19 → 21 ), as revealed by the reaction of 15 , which afforded isoxazolidine 18 with high diastereoselectivity.     

Four oxoindole‐linked α‐alkoxy‐β‐amino acid derivatives

Four structures of oxoindolyl α‐hydroxy‐β‐amino acid derivatives, namely, methyl 2‐{3‐[(tert‐butoxycarbonyl)amino]‐1‐methyl‐2‐oxoindolin‐3‐yl}‐2‐methoxy‐2‐phenylacetate, C₂₄H₂₈N₂O₆, (I), methyl 2‐{3‐[(tert‐butoxycarbonyl)amino]‐1‐methyl‐2‐oxoindolin‐3‐yl}‐2‐ethoxy‐2‐phenylacetate, C₂₅H₃₀N₂O₆, (II), methyl 2‐{3‐[(tert‐butoxycarbonyl)amino]‐1‐methyl‐2‐oxoindolin‐3‐yl}‐2‐[(4‐methoxybenzyl)oxy]‐2‐phenylacetate, C₃₁H₃₄N₂O₇, (III), and methyl 2‐[(anthracen‐9‐yl)methoxy]‐2‐{3‐[(tert‐butoxycarbonyl)amino]‐1‐methyl‐2‐oxoindolin‐3‐yl}‐2‐phenylacetate, C₃₈H₃₆N₂O₆, (IV), have been determined. The diastereoselectivity of the chemical reaction involving α‐diazoesters and isatin imines in the presence of benzyl alcohol is confirmed through the relative configuration of the two stereogenic centres. In esters (I) and (III), the amide group adopts an anti conformation, whereas the conformation is syn in esters (II) and (IV). Nevertheless, the amide group forms intramolecular N—H...O hydrogen bonds with the ester and ether O atoms in all four structures. The ether‐linked substituents are in the extended conformation in all four structures. Ester (II) is dominated by intermolecular N—H...O hydrogen‐bond interactions. In contrast, the remaining three structures are sustained by C—H...O hydrogen‐bond interactions.     

β‐Hairpins Generated from Hybrid Peptide Sequences Containing both α‐ and β‐Amino Acids

The incorporation of the β‐amino acid residues into specific positions in the strands and β‐turn segments of peptide hairpins is being systematically explored. The presence of an additional torsion variable about the C(α) C(β) bond (θ) enhances the conformational repertoire in β‐residues. The conformational analysis of three designed peptide hairpins composed of α/β‐hybrid segments is described: Boc‐Leu‐Val‐Val‐^DPro‐β Phe ‐Leu‐Val‐Val‐OMe ( 1 ), Boc‐Leu‐Val‐β Val ‐^DPro‐Gly‐β Leu ‐Val‐Val‐OMe ( 2 ), and Boc‐Leu‐Val‐β Phe ‐Val‐^DPro‐Gly‐Leu‐β Phe ‐Val‐Val‐OMe ( 3 ). 500‐MHz ¹H‐NMR Analysis supports a preponderance of β‐hairpin conformation in solution for all three peptides, with critical cross‐strand NOEs providing evidence for the proposed structures. The crystal structure of peptide 2 reveals a β‐hairpin conformation with two β‐residues occupying facing, non‐H‐bonded positions in antiparallel β‐strands. Notably, βVal(3) adopts a gauche conformation about the C(α) C(β) bond (θ=+65°) without disturbing cross‐strand H‐bonding. The crystal structure of 2 , together with previously published crystal structures of peptides 3 and Boc‐β Phe ‐β Phe ‐^DPro‐Gly‐β Phe ‐β Phe ‐OMe, provide an opportunity to visualize the packing of peptide sheets with local ‘polar segments' formed as a consequence of reversal peptide‐bond orientation. The available structural evidence for hairpins suggests that β‐residues can be accommodated into nucleating turn segments and into both the H‐bonding and non‐H‐bonding positions on the strands.     

cis‐trans Peptide‐Bond Isomerization in α‐Methylproline Derivatives

α‐Methyl‐L ‐proline is an α‐substituted analog of proline that has been previously employed to constrain prolyl peptide bonds in a trans conformation. Here, we revisit the cis‐trans prolyl peptide bond equilibrium in derivatives of α‐methyl‐L ‐proline, such as N‐Boc‐protected α‐methyl‐L ‐proline and the hexapeptide H‐Ala‐Tyr‐αMePro‐Tyr‐Asp‐Val‐OH. In Boc‐α‐methyl‐L ‐proline, we found that both cis and trans conformers were populated, whereas, in the short peptide, only the trans conformer was detected. The energy barrier for the cis‐trans isomerization in Boc‐α‐methyl‐L ‐proline was determined by line‐shape analysis of NMR spectra obtained at different temperatures and found to be 1.24 kcal/mol (at 298 K) higher than the corresponding value for Boc‐L ‐proline. These findings further illuminate the conformationally constraining properties of α‐methyl‐L ‐proline.     

Effective Methods for the Synthesis of N‐Methyl β‐Amino Acids from All Twenty Common α‐Amino Acids Using 1,3‐Oxazolidin‐5‐ones and 1,3‐Oxazinan‐6‐ones

N‐Methyl β‐amino acids are generally required for application in the synthesis of potentially bioactive modified peptides and other oligomers. Previous work highlighted the reductive cleavage of 1,3‐oxazolidin‐5‐ones to synthesise N‐methyl α‐amino acids. Starting from α‐amino acids, two approaches were used to prepare the corresponding N‐methyl β‐amino acids. First, α‐amino acids were converted to N‐methyl α‐amino acids by the so‐called ‘1,3‐oxazolidin‐5‐one strategy’, and these were then homologated by the Arndt–Eistert procedure to afford N‐protected N‐methyl β‐amino acids derived from the 20 common α‐amino acids. These compounds were prepared in yields of 23–57% (relative to N‐methyl α‐amino acid). In a second approach, twelve N‐protected α‐amino acids could be directly homologated by the Arndt–Eistert procedure, and the resulting β‐amino acids were converted to the 1,3‐oxazinan‐6‐ones in 30–45% yield. Finally, reductive cleavage afforded the desired N‐methyl β‐amino acids in 41–63% yield. One sterically congested β‐amino acid, 3‐methyl‐3‐aminobutanoic acid, did give a high yield (95%) of the 1,3‐oxazinan‐6‐one ( 65 ), and subsequent reductive cleavage gave the corresponding AIBN‐derived N‐methyl β‐amino acid 61 in 71% yield (Scheme 2). Thus, our protocols allow the ready preparation of all N‐methyl β‐amino acids derived from the 20 proteinogenic α‐amino acids.     

Preparation of the β3‐Homoselenocysteine Derivatives Fmoc‐β3hSec(PMB)‐OH and Boc‐β3hSec(PMB)‐OH for Solution and Solid‐Phase‐Peptide Synthesis and Selenoligation

The title compounds, 4 and 7 , have been prepared from the corresponding α‐amino acid derivative selenocystine ( 1 ) by the following sequence of steps: cleavage of the Se? Se bond with NaBH₄, p‐methoxybenzyl (PMB) protection of the SeH group, Fmoc or Boc protection at the N‐atom and Arndt–Eistert homologation (Schemes 1 and 2). A β³‐heptapeptide 8 with an N‐terminal β³‐hSec(PMB) residue was synthesized on Rink amide AM resin and deprotected (‘in air’) to give the corresponding diselenide 9 , which, in turn, was coupled with a β³‐tetrapeptide thiol ester 10 by a seleno‐ligation. The product β³‐undecapeptide was identified as its diselenide and its mixed selenosulfide with thiophenol (Scheme 3). The differences between α‐ and β‐Sec derivatives are discussed.     

N‐tert‐butoxycarbonyl‐N‐substituted hydrazines in SNAr displacements. Synthetic pathways to N‐1‐substituted anthrapyrazoles,aza‐anthrapyrazoles and aza‐benzothiopyranoindazoles

The synthesis of several N‐tert‐butoxycarbonyl(Boc)‐protected‐N‐substituted hydrazines has been accomplished. The use of these protected hydrazines in S_NAr substitutions leads to products in which the most nucleophilic nitrogen displaces the leaving group. Treatment of these compounds with trifluoroacetic acid readily removes the Boc‐protecting group and the intermediates readily undergo cyclizations to yield N‐1‐substituted aza‐benzothiopyranoindazoles, anthrapyrazoles and aza‐anthrapyrazoles. Side chain buildup was employed in the synthesis of several aza‐anthrapyrazoles.     

Synthesis,CD Spectra,and Enzymatic Stability of β2‐Oligoazapeptides Prepared from (S)‐2‐Hydrazino Carboxylic Acids Carrying the Side Chains of Val,Ala, and Leu

β‐Peptides offer the unique possibility to incorporate additional heteroatoms into the peptidic backbone (Figs. 1 and 2). We report here the synthesis and spectroscopic investigations of β²‐peptide analogs consisting of (S)‐3‐aza‐β‐amino acids carrying the side chains of Val, Ala, and Leu. The hydrazino carboxylic acids were prepared by a known method: Boc amidation of the corresponding N‐benzyl‐L ‐α‐amino acids with an oxaziridine (Scheme 1). Couplings and fragment coupling of the 3‐benzylaza‐β²‐amino acids and a corresponding tripeptide (N‐Boc/C‐OMe strategy) with common peptide‐coupling reagents in solution led to β²‐di, β²‐tri‐, and β²‐hexaazapeptide derivatives, which could be N‐debenzylated ( 4 – 9 ; Schemes 2–4). The new compounds were identified by optical rotation, and IR, ¹H‐ and ¹³C‐NMR, and CD spectroscopy (Figs. 4 and 5) and high‐resolution mass spectrometry, and, in one case, by X‐ray crystallography (Fig. 3). In spite of extensive measurements under various conditions (temperatures, solvents), it was not possible to determine the secondary structure of the β²‐azapeptides by NMR spectroscopy (overlapping and broad signals, fast exchange between the two types of NH protons!). The CD spectra of the N‐Boc and C‐OMe terminally protected hexapeptide analog 9 in MeOH and in H₂O (at different pH) might arise from a (P)‐3₁₄‐helical structure. The N‐Boc‐β²‐tri and N‐Boc‐β²‐hexaazapeptide esters, 7 and 9 , were shown to be stable for 48 h against the following peptidases: pronase, proteinase K, chymotrypsin, trypsin, carboxypeptidase A, and 20S proteasome.     

Synthesis of the Anticodon Hairpin tRNAfMet Containing N‐{[9‐(β‐D‐Ribofuranosyl)‐9H‐purin‐6‐yl]carbamoyl}‐L‐threonine (=N6‐{{[(1S,2R)‐1‐Carboxy‐2‐hydroxypropyl]amino}carbonyl}adenosine,t6A)

As part of our studies on the structure of yeast ^tRNA_f^Met, we investigated the incorporation of N‐{[9‐(β‐D ‐ribofuranosyl)‐9H‐purin‐6‐yl]carbamoyl}‐L ‐threonine (t⁶A) in the loop of a RNA 17‐mer hairpin. The carboxylic function of the L ‐threonine moiety of t⁶A was protected with a 2‐(4‐nitrophenyl)ethyl group, and a (tert‐butyl)dimethylsilyl group was used for the protection of its secondary OH group. The 2′‐OH function of the standard ribonucleotide building blocks was protected with a [(triisopropylsilyl)oxy]methyl group. Removal of the base‐labile protecting groups of the final RNA with 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) and then with MeNH₂ was done under carefully controlled conditions to prevent hydrolysis of the carbamate function, leading to loss of the L ‐threonine moiety.     

Preparation of (S,S)‐Fmoc‐β2hIle‐OH, (S)‐Fmoc‐β2hMet‐OH,and (S)‐Fmoc‐β2hTyr(tBu)‐OH for Solid‐Phase Syntheses of β2‐ and β2/β3‐Peptides

The preparation of three new N‐Fmoc‐protected (Fmoc=[(9H‐fluoren‐9‐yl)methoxy]carbonyl) β²‐homoamino acids with proteinogenic side chains (from Ile, Tyr, and Met) is described, the key step being a diastereoselective amidomethylation of the corresponding Ti‐enolates of 3‐acyl‐4‐isopropyl‐5,5‐diphenyloxazolidin‐2‐ones with CbzNHCH₂OMe/TiCl₄ (Cbz=(benzyloxy)carbonyl) in yields of 60–70% and with diastereoselectivities of >90%. Removal of the chiral auxiliary with LiOH or NaOH gives the N‐Cbz‐protected β‐amino acids, which were subjected to an N‐Cbz/N‐Fmoc (Fmoc=[(9H‐fluoren‐9‐yl)methoxy]carbonyl) protective‐group exchange. The method is suitable for large‐scale preparation of Fmoc‐β²hXaa‐OH for solid‐phase syntheses of β‐peptides. The Fmoc‐amino acids and all compounds leading to them have been fully characterized by melting points, optical rotations, IR, ¹H‐ and ¹³C‐NMR, and mass spectra, as well as by elemental analyses.     

Fluorine in Peptides: The Synthesis of α‐Fluoro‐β‐Amino Dipeptides by Direct Deoxofluorination/Rearrangement of N‐Seryl Dipeptides

This article describes the stereo‐ and regioselectivity of the deoxofluorination of N‐terminal dipeptides bearing a serine residue to generate, after rearrangement, α‐fluoro‐β‐amine‐terminated dipeptides. The ratio of the rearranged α‐fluorinated regioisomer is increased, relative to the non‐rearranged β‐fluoro isomer, with N‐alkylated amides. Otherwise, an intramolecular H‐bond between the free amine and the amide NH suppresses formation of the key aziridinium intermediate required for α‐fluorination. N‐Methyl and N‐allyl amides give exclusively α‐fluorination products. Subsequent deprotection of the N‐allyl amide to give a α‐fluoro‐β‐amino dipeptide product is demonstrated.     

Synthesis of β‐Peptides with β‐Helices from New C‐Linked Carbo‐β‐Amino Acids: Study on the Impact of Carbohydrate Side Chains

The design and synthesis of β‐peptides from new C‐linked carbo‐β‐amino acids (β‐Caa) presented here, provides an opportunity to understand the impact of carbohydrate side chains on the formation and stability of helical structures. The β‐amino acids, Boc‐(S)‐β‐Caa_(g)‐OMe 1 and Boc‐(R)‐β‐Caa_(g)‐OMe 2 , having a D ‐galactopyranoside side chain were prepared from D ‐galactose. Similarly, the homo C‐linked carbo‐β‐amino acids (β‐hCaa); Boc‐(S)‐β‐hCaa_(x)‐OMe 3 and Boc‐(R)‐β‐hCaa_(x)‐OMe 4 , were prepared from D ‐glucose. The peptides derived from the above monomers were investigated by NMR, CD, and MD studies. The β‐peptides, especially the shorter ones obtained from the epimeric (at the amine stereocenter C_β) 1 and 2 by the concept of alternating chirality, showed a much smaller propensity to form 10/12‐helices. This substantial destabilization of the helix could be attributed to the bulkier D ‐galactopyranoside side chain. Our efforts to prepare peptides with alternating 3 and 4 were unsuccessful. However, the β‐peptides derived from alternating geometrically heterochiral (at C_β) 4 and Boc‐(R)‐β‐Caa_(x)‐OMe 5 (D ‐xylose side chain) display robust right‐handed 10/12‐helices, while the mixed peptides with alternating 4 and Boc‐β‐hGly‐OMe 6 (β‐homoglycine), resulted in left‐handed β‐helices. These observations show a distinct influence of the side chains on helix formation as well as their stability.