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).     

Synthesis of β‐Hexa‐ and β‐Heptapeptides Containing Novel β2,3‐Amino Acids with Two Serine or Two Cysteine Side Chains – CD‐ and NMR‐Spectroscopic Evidence for 314‐Helical Secondary Structures in Water

Two representatives of a new type of β‐amino acids, carrying two functionalized side chains, one in the 2‐ and one in the 3‐position, have been prepared stereoselectively: a β‐Ser derivative with an additional CH₂OH group in the 2‐position (for β‐peptides with better water solubility; Scheme 2) and a β‐HCys derivative with an additional CH₂SBn group in the 2‐position (for disulfide formation and metal complexation with the derived β‐peptides; Scheme 3). Also, a simple method for the preparation of α‐methylidene‐β‐amino acids is presented (see Boc‐2‐methylidene‐β‐HLeu‐OH, 8 in Scheme 3). The two amino acids with two serine or two cysteine side chains are incorporated into a β‐hexa‐ and two β‐heptapeptides ( 18 and 23/24 , resp.), which carry up to four CH₂OH groups. Disulfide formation with the β‐peptides carrying two CH₂SH groups generates very stable 1,2‐dithiane rings in the centre of the β‐heptapeptides, and a cyclohexane analog was also prepared (cf. 27 in Scheme 6). The CD spectra in H₂O clearly indicate the presence of 3₁₄‐helical structures of those β‐peptides ( 18 , 23 , 24 , 27b ) having the `right' configurations at all stereogenic centers (Fig. 2). NMR Measurements (Tables 1 and 2, and Fig. 4) in aqueous solution of one of the new β‐peptides ( 24 ) are interpreted on the assumption that the predominant secondary structure is the 3₁₄‐helix, a conformation that has been found to be typical for β‐peptides in MeOH or pyridine solution, according to our previous NMR investigations.     

Preparation of Protected β2‐ and β3‐Homocysteine, β2‐ and β3‐Homohistidine,and β2‐Homoserine for Solid‐Phase Syntheses

The Ser, Cys, and His side chains play decisive roles in the syntheses, structures, and functions of proteins and enzymes. For our structural and biomedical investigations of β‐peptides consisting of amino acids with proteinogenic side chains, we needed to have reliable preparative access to the title compounds. The two β³‐homoamino acid derivatives were obtained by Arndt–Eistert methodology from Boc‐His(Ts)‐OH and Fmoc‐Cys(PMB)‐OH (Schemes 2–4), with the side‐chain functional groups' reactivities requiring special precautions. The β²‐homoamino acids were prepared with the help of the chiral oxazolidinone auxiliary DIOZ by diastereoselective aldol additions of suitable Ti‐enolates to formaldehyde (generated in situ from trioxane) and subsequent functional‐group manipulations. These include OH→O^tBu etherification (for β²hSer; Schemes 5 and 6), OH→STrt replacement (for β²hCys; Scheme 7), and CH₂OH→CH₂N₃→CH₂NH₂ transformations (for β²hHis; Schemes 9–11). Including protection/deprotection/re‐protection reactions, it takes up to ten steps to obtain the enantiomerically pure target compounds from commercial precursors. Unsuccessful approaches, pitfalls, and optimization procedures are also discussed. The final products and the intermediate compounds are fully characterized by retention times (t_R), melting points, optical rotations, HPLC on chiral columns, IR, ¹H‐ and ¹³C‐NMR spectroscopy, mass spectrometry, elemental analyses, and (in some cases) by X‐ray crystal‐structure analysis.     

Synthesis and CD Spectra in MeCN,MeOH, and H2O of γ‐Oligopeptides with Hydroxy Groups on the Backbone,Preliminary Communication

γ⁴‐Tripeptides and γ⁴‐hexapeptides, 1 – 4 , with OH groups in the 2‐ or 3‐position on each residue have been prepared. The corresponding 2‐hydroxy amino acids were obtained by Si‐nitronate (3+2) cycloadditions to the acryloyl derivative of Oppolzer's sultam and Raney‐Ni reduction of the resulting 1,2‐oxazolidines (Scheme 1). The 3‐hydroxy amino acid derivatives were prepared by chain elongation via Claisen condensation of Boc‐Ala‐OH, Boc‐Val‐OH, and Boc‐Leu‐OH, and NaBH₄ reduction of the methyl 4‐amino 3‐oxo carboxylates formed (Scheme 2). The N‐Boc hydroxy amino acids were coupled in solution to give the γ‐peptides. CD Spectra of the new types of γ‐peptides were recorded and compared with those of simple γ²‐, γ³‐, γ⁴‐, and γ^2,3,4‐peptides (Figs. 3, 4, and 5). An intense Cotton effect at ca. 200 nm ([Θ]=−2⋅10⁵ deg⋅cm²⋅dmol⁻¹) indicates that the hexapeptide built of (3R,4S)‐4‐amino‐3‐hydroxy acids (with the side chains of Val, Ala, Leu) folds to a secondary structure so far unknown. The stability of peptides from β‐ and γ‐amino acids, which carry heteroatoms on their backbones is discussed (Fig. 1). Positions on the γ‐peptidic 2.6₁₄ helix are identified at which non‐H‐atoms are `allowed' (Fig. 2).     

Preparation of poly(alkylenebenzimidazole) by ruthenium complex catalyzed polycondensation of 3,3′‐diaminobenzidine with α,ω‐diols

The reactions of 3,3′‐diaminobenzidine with 1,12‐dodecanediol in 1 : 1–1:3 molar ratios in the presence of RuCl₂(PPh₃)₃ catalyst give poly(alkylenebenzimidazole), [ (CH₂)₁₁ O (CH₂)₁₁ Im / (CH₂)₁₀ Im ]_n (Im: 5,5′‐dibenzimidazole‐2,2′‐diyl) (I_a‐I_d) in 71–92% yields. The relative ratio between the [(CH₂)₁₁ O (CH₂)₁₁ Im ] unit (A) and the [‐ (CH₂)₁₀ Im ] unit (B) in the polymer chain varies depending on the ratio of the substrates used. The polymer I_a obtained from the 1 : 3 reaction contains these structural units in a 98 : 2 ratio. The polymers are soluble in polar solvents such as DMF (N,N‐dimethylformamide), DMSO (dimethyl sulfoxide), and NMP (N‐methyl‐2‐pyrrolidone) and have molecular weights M_n (M_w) of 4,200–4,800 (4,800–6,500) by GPC (polystyrene standard). The polymerization of the diol and 3,3′‐diaminobenzidine in higher molar ratios leads to partial cross‐linking of the resulting polymers I_e and I_f via condensation of imidazole NH group with CH₂OH group. Similar reactions of 3,3′‐diaminobenzidine with α,ω‐diols, HO(CH₂)_mOH (m = 4–10), in a 1 : 3 molar ratio give the polymers containing [ (CH₂)_m−1 O (CH₂) _m−1 Im ] and [ (CH₂) _m−2 Im ] units with partial cross‐linked structures. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1383–1392, 1999     

Enantioselective Preparation of β2‐Amino Acids with Aspartate,Glutamate, Asparagine,and Glutamine Side Chains

(S)‐β²‐Homoamino acids with the side chains of Asp, Glu, Asn, and Gln have been prepared and suitably protected (N‐Fmoc, CO₂^tBu, CONHTrt) for solid‐phase peptide syntheses. The key steps of the syntheses are: N‐acylation of 5,5‐diphenyl‐4‐isopropyl‐1,3‐oxazolidin‐2‐one (DIOZ) with succinic and glutaric anhydrides (Scheme 2), alkylation of the corresponding Li‐enolates with benzyl iodoacetate and Curtius degradation (Scheme 4), and removal of the chiral auxiliary (Scheme 5). In addition, numerous functional‐group manipulations (CO₂H?CO₂^tBu, CO₂Bn?CO₂H, CbzNH→FmocNH, CO₂H→CO₂NH₂→CONHTrt; Schemes 2, 4, 5, and 6) were necessary, in order to arrive at the four target structures. The configurational assignments were confirmed by X‐ray crystal‐structure determinations (Scheme 2 and Fig. 3). The enantiomeric purities of a β²hAsn and of a β²hGln derivative were determined by HPLC on a Chiralcel column to be 99.7 : 0.3 and >99 : 1, respectively (Fig. 4). Notably, it took up to twelve steps to prepare a suitably protected trifunctional product with a single stereogenic center (overall yield of 10% from DIOZ and succinic anhydride)!     

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.     

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.     

Asymmetric Synthesis of β2‐homo‐tert‐Leucine via Radical Addition to Enantiopure N‐Fumaroylhexahydrobenzooxazolidin‐2‐one

β‐Amino acids are key structural elements in unnatural peptides, peptidomimetics, and many other physiologically active compounds. In view of their importance, we have developed an efficient synthetic route that provides highly enantiomerically enriched (R)‐ and (S)‐H‐β²‐h^tLeu‐OH via highly diastereo‐ and regioselective addition of tert‐butyl radical to enantiomerically pure N‐fumaroyloxazolidinones, followed by removal of the chiral auxiliary, Curtius rearrangement, ester hydrolysis, and catalytic hydrogenolysis.     

Oxidative Fragmentations of the 5α‐ and 5β‐Hydroxy‐B‐norcholestan‐ 3β‐yl Acetates

Oxidations of 5α‐hydroxy‐B‐norcholestan‐3β‐yl acetate ( 8 ) with Pb(OAc)₄ under thermal or photolytic conditions or in the presence of iodine afforded only complex mixtures of compounds. However, the HgO/I₂ version of the hypoiodite reaction gave as the primary products the stereoisomeric (Z)‐ and (E)‐1(10)‐unsaturated 5,10‐seco B‐nor‐derivatives 10 and 11 , and the stereoisomeric (5R,10R)‐ and (5S,10S)‐acetals 14 and 15 (Scheme 4). Further reaction of these compounds under conditions of their formation afforded, in addition, the A‐nor 1,5‐cyclization products 13 and 16 (from 10 ) and 12 (from 11 ) (see also Scheme 6) and the 6‐iodo‐5,6‐secolactones 17 and 19 (from 14 and 15 , resp.) and 4‐iodo‐4,5‐secolactone 18 (from 15 ) (see also Scheme 7). Oxidations of 5β‐hydroxy‐B‐norcholestan‐3β‐yl acetate ( 9 ) with both hypoiodite‐forming reagents (Pb(OAc)₄/I₂ and HgO/I₂) proceeded similarly to the HgO/I₂ reaction of the corresponding 5α‐hydroxy analogue 8 . Photolytic Pb(OAc)₄ oxidation of 9 afforded, in addition to the (Z)‐ and (E)‐5,10‐seco 1(10)‐unsaturated ketones 10 and 11 , their isomeric 5,10‐seco 10(19)‐unsaturated ketone 22 , the acetal 5‐acetate 21 , and 5β,19‐epoxy derivative 23 (Scheme 9). Exceptionally, in the thermal Pb(OAc)₄ oxidation of 9 , the 5,10‐seco ketones 10, 11 , and 22 were not formed, the only reaction being the stereoselective formation of the 5,10‐ethers with the β‐oriented epoxy bridge, i.e. the (10R)‐enol ether 20 and (5S,10R)‐acetal 5‐acetate 21 (Scheme 8). Possible mechanistic interpretations of the above transformations are discussed.     

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 Structural Characterization of Folded Peptides Containing the Conformationally Constrained β‐Amino Acid Residue β2,2Ac6c

Backbone alkylation has been shown to result in a dramatic reduction in the conformational space that is sterically accessible to α‐amino acid residues in peptides. By extension, the presence of geminal dialkyl substituents at backbone atoms also restricts available conformational space for β and γ residues. Five peptides containing the achiral β^2,2‐disubstituted β‐amino acid residue, 1‐(aminomethyl)cyclohexanecarboxylic acid (β^2,2Ac₆c), have been structurally characterized in crystals by X‐ray diffraction. The tripeptide Boc‐Aib‐β^2,2Ac₆c‐Aib‐OMe ( 1 ) adopts a novel fold stabilized by two intramolecular H‐bonds (C₁₁ and C₉) of opposite directionality. The tetrapeptide Boc‐[Aib‐β^2,2Ac₆c]₂‐OMe ( 2 ) and pentapeptide Boc‐[Aib‐β^2,2Ac₆c]₂‐Aib‐OMe ( 3 ) form short stretches of a hybrid αβ C₁₁ helix stabilized by two and three intramolecular H‐bonds, respectively. The structure of the dipeptide Boc‐Aib‐β^2,2Ac₆c‐OMe ( 5 ) does not reveal any intramolecular H‐bond. The aggregation pattern in the crystal provides an example of an extended conformation of the β^2,2Ac₆c residue, forming a ‘polar sheet’ like H‐bond. The protected derivative Ac‐β^2,2Ac₆c‐NHMe ( 4 ) adopts a locally folded gauche conformation about the C_β? C_α bonds (θ=?55.7°). Of the seven examples of β^2,2Ac₆c residues reported here, six adopt gauche conformations, a feature which promotes local folding when incorporated into peptides. A comparison between the conformational properties of β^2,2Ac₆c and β^3,3Ac₆c residues, in peptides, is presented. Backbone torsional parameters of H‐bonded αβ/βα turns are derived from the structures presented in this study and earlier reports.     

Synthesis and CD Spectra of Fluoro‐ and Hydroxy‐Substituted β‐Peptides

β‐Amino acids 1 – 3 with OH and F substituents in the α‐position have been prepared (Scheme) from the natural (S)‐α‐amino acids alanine, valine, and leucine, and incorporated into β‐hexa‐ and β‐heptapeptides 4 – 12 . The peptide syntheses were performed according to a conventional solution strategy (Boc/Bn protection) with fragment coupling. The new β‐peptides with (series a ) and without (series b ) terminal protection were isolated in HPLC‐pure form and characterized by NMR spectroscopy and MALDI mass spectrometry. The chemical properties as well as the patterns of the CD spectra (Figs. 3–5) depend upon constitution (OH, F, F₂ substitution) and configuration (l or u) of the amino acid residues, upon the total number of OH and F substituents in the peptide chain, and upon the solvent used (H₂O, MeOH, CF₃CH₂OH, (CF₃)₂CHOH). No reliable clues regarding the structures can be obtained from these CD spectra. Only a full NMR analysis will be able to answer the questions: a) with which known secondary structures (Figs. 1 and 2) of β‐peptides are the OH and F derivatives compatible? b) Are new secondary structures enforced by the polar and/or H‐bonding backbone substituents? Furthermore, the β‐peptides described here will enable us to study changes in chemical, enzymatic, and metabolic stability, and in physiological properties caused by the heteroatoms.     

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.     

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.     

Preparation of β2‐Amino Acid Derivatives (β2hThr, β2hTrp, β2hMet, β2hPro, β2hLys,Pyrrolidine‐3‐carboxylic Acid) by Using DIOZ as Chiral Auxiliary

The title compounds were prepared from valine‐derived N‐acylated oxazolidin‐2‐ones, 1 – 3, 7, 9 , by highly diastereoselective (≥ 90%) Mannich reaction (→ 4 – 6 ; Scheme 1) or aldol addition (→ 8 and 10 ; Scheme 2) of the corresponding Ti‐ or B‐enolates as the key step. The superiority of the ‘5,5‐diphenyl‐4‐isopropyl‐1,3‐oxazolidin‐2‐one’ (DIOZ) was demonstrated, once more, in these reactions and in subsequent transformations leading to various t‐Bu‐, Boc‐, Fmoc‐, and Cbz‐protected β²‐homoamino acid derivatives 11 – 23 (Schemes 3–6). The use of ω‐bromo‐acyl‐oxazolidinones 1 – 3 as starting materials turned out to open access to a variety of enantiomerically pure trifunctional and cyclic carboxylic‐acid derivatives.     

Linear,Peptidase‐Resistant β2/β3‐Di‐ and α/β3‐Tetrapeptide Derivatives with Nanomolar Affinities to a Human Somatostatin Receptor,Preliminary Communication

N‐Acyl‐β²/β³‐dipeptide‐amide somatostatin analogs, 5 – 8 , with β²‐HTrp‐β³‐HLys ('natural' sequence) and β²‐HLys‐β³‐HTrp (retro‐sequence) have been synthesized (in solution). Depending on their relative configurations and on the nature of the terminal N‐acyl and terminal C‐amino group, the linear β‐dipeptide derivatives have affinities for the human receptor hsst 4, ranging from 250 to >10000 nanomolar (Fig. 3). Also, N‐Ac‐tetrapeptide amides 9 and 10 , which contain one α‐ and three β‐amino acid residues (N‐β‐α‐β‐β‐C), have been prepared (solid‐phase synthesis), with the natural (Phe, Trp, Lys, Thr) and the retro‐sequence (Thr, Lys, Trp, Phe) of side chains and with two different configurations, each, of the two central amino acid residues. The novel ‘mixed', linear α/β‐peptides have affinities for the hsst 4 receptor ranging from 23 to >10000 nanomolar (Fig. 4), and, like ‘pure' β‐peptides, they are completely stable to a series of proteolytic enzymes. Thus, the peptidic turn of the cyclic tetradecapeptide somatostatin (Fig. 1) can be mimicked by simple linear di‐ and tetrapeptides. The tendency of β‐dipeptides for forming hydrogen‐bonded rings is confirmed by calculations at the B3LYP/6‐31G(d,p) level (Fig. 2). The reported results open new avenues for the design of low‐molecular‐weight peptidic drugs.     

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.     

Isotopically Labelled and Unlabelled β‐Peptides with Geminal Dimethyl Substitution in 2‐Position of Each Residue: Synthesis and NMR Investigation in Solution and in the Solid State

The preparation of (S)‐β^2,2,3‐amino acids with two Me groups in the α‐position and the side chains of Ala, Val, and Leu in the β‐position (double methylation of Boc‐β‐HAla‐OMe, Boc‐β‐Val‐OMe, and Boc‐β‐Leu‐OMe, Scheme 2) is described. These β‐amino acids and unlabelled as well as specifically ¹³C‐ and ¹⁵N‐labelled 2,2‐dimethyl‐3‐amino acid (β^2,2‐HAib) derivatives have been coupled in solution (Schemes 1, 3 and 4) to give protected (N‐Boc, C‐OMe), partially protected (N‐Boc/C‐OH, N‐H/C‐OMe), and unprotected β^2,2‐ and β^2,2,3‐hexapeptides, and β^2,2‐ and β^2,2,3‐heptapeptides 1 – 7 . NMR Analyses in solution (Tables 1 and 2, and Figs. 2–4) and in the solid state (2D‐MAS NMR measurements of the fully labelled Boc‐(β^2,2‐HAib)₆‐OMe ([¹³C₃₀, ¹⁵N₆]‐ 1e ; Fig. 5), and TEDOR/REDOR NMR investigations of mixtures (Fig. 6) of the unlabelled Ac‐(β^2,2‐HAib)₇‐OMe ( 4 ) and of a labelled derivative ([¹³C₄,¹⁵N₂]‐ 5 ; Figs. 7–11, and 19), a molecular‐modeling study (Figs. 13–15), and a search in the Cambridge Crystallographic Data Base (Fig. 16) allow the following conclusions: i) there is no evidence for folding (helix or turn) or for aggregation to sheets of the geminally dimethyl substituted peptide chains in solution; ii) there are distinct conformational preferences of the individual β^2,2‐ and β^2,2,3‐amino acid residues: close to eclipsing around the C(O) C(Me₂(CHR)) bond (τ_1,2), almost perfect staggering around the C(2) C(3) ethane bond (τ_2,3), and antiperiplanar arrangement of H(C3) and H(N) (τ_3,N; Fig. 12) in the solid state; iii) the β^2,2‐peptides may be part of a turn structure with a ten‐membered H‐bonded ring; iv) the main structure present in the solid state of F₃CCO(β^2,2‐HAib)₇‐OMe is a nonfolded chain (>30 Å between the termini and >20 Å between the N‐terminus and the CH₂ group of residue 5) with all CO bonds in a parallel alignment (±10°). With these structural parameters, a simple modelling was performed producing three (maybe four) possible chain geometries: one fully extended, two with parallel peptide planes (with zick‐zack and crankshaft‐type arrangement of the peptide bonds), and (possibly) a fourth with meander‐like winding ( D – G in Figs. 17 and 18).     

Design,Synthesis, NMR‐Solution and X‐Ray Crystal Structure of N‐Acyl‐γ‐dipeptide Amides That Form a βII′‐Type Turn

Conformational analysis of γ‐amino acids with substituents in the 2‐position reveals that an N‐acyl‐γ‐dipeptide amide built of two enantiomeric residues of unlike configuration will form a 14‐membered H‐bonded ring, i.e., a γ‐peptidic turn (Figs. 1 – 3). The diastereoselective preparation of the required building blocks was achieved by alkylation of the doubly lithiated N‐Boc‐protected 4‐aminoalkanoates, which, in turn, are readily available from the corresponding (R)‐ or (S)‐α‐amino acids (Scheme 1). Coupling two such γ‐amino acid derivatives gave N‐acetyl and N‐[(tert‐butoxy)carbonyl] (Boc) dipeptide methyl amides ( 1 and 10 , resp.; Fig. 2, Scheme 2); both formed crystals suitable for X‐ray analysis, which confirmed the turn structures in the solid state (Fig. 4 and Table 4). NMR Analysis of the acetyl derivative 1 in CD₃OH, with full chemical‐shift and coupling assignments, and, including a 300‐ms ROESY measurement, revealed that the predicted turn structure is also present in solution (Fig. 5 and Tables 1 – 3). The results described here are yet another piece of evidence for the fact that more stable secondary structures are formed with a decreasing number of residues, and with increasing degree of predictability, as we go from α‐ to β‐ to γ‐peptides. Implications of the superimposable geometries of the actual turn segments (with amide bonds flanked by two quasi‐equatorial substituents) in α‐, β‐, and γ‐peptidic turns are discussed.