Combinatorial Biomimetic Chemistry: Parallel Synthesis of a Small Library of β‐Hairpin Mimetics Based on Loop III from Human Platelet‐Derived Growth Factor B

Combinatorial diversity in hypervariable β‐hairpin loops is exploited by the immune system to select binding sites on antibodies for a wide variety of different protein antigens. In a first step towards mimicking this strategy in vitro, for the selection of novel protein ligands, an approach is described here for the parallel synthesis of small libraries of conformationally defined β‐hairpin protein epitope mimetics. Starting from a protruding hairpin loop in platelet‐derived growth factor B (PDGF‐B), 8 and 12 residues were first transplanted from the protein to a D ‐Pro‐L ‐Pro template, to afford the cyclic peptide‐loop mimetics 1 and 2 , respectively. NMR and MD studies in aqueous solution show that both mimetics populate conformations which closely mimic the β‐hairpin in the crystal structure of the native protein (Fig. 5). Based on 1 as a scaffold, a library of 24 mimetics was synthesized in which the four residues at the tip of the loop (VRKK) were held constant, and flanking residues at positions 1, 2, 7, and 8 in the hairpin were varied (Fig. 7). The library was prepared by parallel synthesis in a two‐stage solid‐phase assembly/solution‐phase cyclization process. The products were analyzed by MS, NMR, and CD. 2D‐NOESY revealed for most library members characteristic long‐range NOEs that show that the hairpin conformation is stably maintained. The results suggest that this approach may be useful for the synthesis of much larger libraries of peptide and protein mimetics based on a β‐hairpin scaffold.     

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.     

Stabilization of a β-Hairpin Conformation in a Cyclic Peptide Using the Templating Effect of a Heterochiral Diproline Unit

A straightforward and effective method of stabilizing a β-hairpin conformation in a cyclic protein loop mimetic is described, which exploits the templating effect of a heterochiral D -Pro-L -Pro dipeptide unit. A twelve-residue β-hairpin loop was grafted from the extracellular interferon γ receptor onto the heterochiral D -Pro-L -Pro dipeptide template to afford a fourteen-residue cyclic peptide. The residues directly attached to the D -Pro-L -Pro template are shown by NMR spectroscopy to structurally mimic corresponding residues in adjacent antiparallel β-strands in the receptor. MD Simulations with and without time-averaged distance restraints support this view and indicate that the tip of the loop is more flexible, as inferred also for the receptor protein from crystallographic data. The templating effect of the heterochiral diproline unit also promotes efficient backbone cyclization of the fourteen-residue linear peptide precursor, suggesting that a wide variety of related protein loop mimetics incorporating the D -Pro-L -Pro template might be readily accessible.     

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 the Spermidine Alkaloids (−)‐(2R,3R)‐ and (−)‐(2R,3S)‐3‐Hydroxycelacinnine: Macrocyclization with Oxirane‐Ring Opening and Inversion via Cyclic Sulfamidates

The two epimers (?)‐ 1a and (?)‐ 1b of the macrocyclic lactam alkaloid 3‐hydroxycelacinnine with the (2R,3R) and (2R,3S) absolute configurations, respectively, were synthesized by an alternative route involving macrocyclization with the regio‐ and stereoselective oxirane‐ring opening by the terminal amino group (Schemes 2 and 6). Properly N‐protected chiral trans‐oxirane precursors provided (2R,3R)‐macrocycles after a one‐pot deprotection‐macrocyclization step under moderate dilution (0.005–0.01M ). The best yields (65–85%) were achieved with trifluoroacetyl protection. Macrocyclization of the corresponding cis‐oxiranes was unsuccessful for steric reasons. Inversion at OH? C(3) via nucleophilic displacement of the cyclic sulfamidate derivative with NaNO₂ led to (2R,3S)‐macrocycles. The synthesized (?)‐(2R,3S)‐3‐hydroxycelacinnine ((?)‐ 1b ) was identical to the natural alkaloid.     

Synthesis and α‐Mannosidase Inhibitory Evaluation of (2R,3R,4S)‐ and (2S,3R,4S)‐2‐(Aminomethyl)pyrrolidine‐3,4‐diol Derivatives

The synthesis of 46 derivatives of (2R,3R,4S)‐2‐(aminomethyl)pyrrolidine‐3,4‐diol is reported (Scheme 1 and Fig. 3), and their inhibitory activities toward α‐mannosidases from jack bean (B) and almonds (A) are evaluated (Table). The most‐potent inhibitors are (2R,3R,4S)‐2‐{[([1,1′‐biphenyl]‐4‐ylmethyl)amino]methyl}pyrrolidine‐3,4‐diol ( 3fs ; IC₅₀(B)=5 μM , K_i=2.5 μM ) and (2R,3R,4S)‐2‐{[(1R)‐2,3‐dihydro‐1H‐inden‐1‐ylamino]methyl}pyrrolidine‐3,4‐diol ( 3fu ; IC₅₀(B)=17 μM , K_i=2.3 μM ). (2S,3R,4S)‐2‐(Aminomethyl)pyrrolidine‐3,4‐diol ( 6 , R?H) and the three 2‐(N‐alkylamino)methyl derivatives 6fh, 6fs , and 6f are prepared (Scheme 2) and found to inhibit also α‐mannosidases from jack bean and almonds (Table). The best inhibitor of these series is (2S,3R,4S)‐2‐{[(2‐thienylmethyl)amino]methyl}pyrrolidine‐3,4‐diol ( 6o ; IC₅₀(B)=105 μM , K_i=40 μM ). As expected (see Fig. 4), diamines 3 with the configuration of α‐D ‐mannosides are better inhibitors of α‐mannosidases than their stereoisomers 6 with the configuration of β‐D ‐mannosides. The results show that an aromatic ring (benzyl, [1,1′‐biphenyl]‐4‐yl, 2‐thienyl) is essential for good inhibitory activity. If the C‐chain that separates the aromatic system from the 2‐(aminomethyl) substituent is longer than a methano group, the inhibitory activity decreases significantly (see Fig. 7). This study shows also that α‐mannosidases from jack bean and from almonds do not recognize substrate mimics that are bulky around the O‐glycosidic bond of the corresponding α‐D ‐mannopyranosides. These observations should be very useful in the design of better α‐mannosidase inhibitors.     

cyclo(β‐Asp‐β3‐hVal‐β3‐hLys) – Solid‐Phase Synthesis and Solution Structure of a Water Soluble β‐Tripeptide. Preliminary Communication

The H₂O‐soluble cyclic β³‐tripeptide cyclo(β‐Asp‐β³‐hVal‐β³‐hLys) ( 4 ) was obtained by on‐resin cyclization of the side‐chain‐anchored β‐peptide 3 (Scheme). In aqueous solution, 4 adopts a structure with uniformly oriented amide bonds and all side chains in lateral positions (Fig. 3).     

New Open‐Chain and Cyclic Tetrapeptides,Consisting of α‐, β2‐, and β3‐Amino‐Acid Residues,as Somatostatin Mimics – A Survey

Cyclo‐β‐tetrapeptides are known to adopt a conformation with an intramolecular transannular hydrogen bond in solution. Analysis of this structure reveals that incorporation of a β²‐amino‐acid residue should lead to mimics of ‘α‐peptidic β‐turns’ (cf. A, B, C ). It is also known that short‐chain mixed β/α‐peptides with appropriate side chains can be used to mimic interactions between α‐peptidic hairpin turns and G protein‐coupled receptors. Based on these facts, we have now prepared a number of cyclic and open‐chain tetrapeptides, 7 – 20 , consisting of α‐, β²‐, and β³‐amino‐acid residues, which bear the side chains of Trp and Lys, and possess backbone configurations such that they should be capable of mimicking somatostatin in its affinity for the human SRIF receptors (hsst_1–5). All peptides were prepared by solid‐phase coupling by the Fmoc strategy. For the cyclic peptides, the three‐dimensional orthogonal methodology (Scheme 3) was employed with best success. The new compounds were characterized by high‐resolution mass spectrometry, NMR and CD spectroscopy, and, in five cases, by a full NMR‐solution‐structure determination (in MeOH or H₂O; Fig. 4). The affinities of the new compounds for the receptors hsst_1–5 were determined by competition with [¹²⁵I]LTT‐SRIF₂₈ or [¹²⁵I] [Tyr¹⁰]‐CST₁₄. In Table 1, the data are listed, together with corresponding values of all β‐ and γ‐peptidic somatostatin/Sandostatin^® mimics measured previously by our groups. Submicromolar affinities have been achieved for most of the human SRIF receptors hsst_1–5. Especially high, specific binding affinities for receptor hsst₄ (which is highly expressed in lung and brain tissue, although still of unknown function!) was observed with some of the β‐peptidic mimics. In view of the fact that numerous peptide‐activated G protein‐coupled receptors (GPCRs) recognize ligands with turn structure (Table 2), the results reported herein are relevant far beyond the realm of somatostatin: many other peptide GPCRs should be ‘reached’ with β‐ and γ‐peptidic mimics as well, and these compounds are proteolytically and metabolically stable, and do not need to be cell‐penetrating for this purpose (Fig. 5).     

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.     

Synthese von (R)-β, β-Carotin-2-ol und (2R, 2′R)-β, β-Carotin-2,2′-diol

Synthesis of (R)-β, β-Caroten-2-ol and (2R, 2′R)-β, β-Carotene-2,2′-diol Starting from geraniol, the two carotenoids (R)-β, β-caroten-2-ol ( 1 ) and (2R, 2′R)-β, β-carotene-2,2′-diol ( 3 ) were synthesized. The optically active cyclic building block was obtained by an acid-catalysed cyclisation of the epoxide (R)- 4 . The enantiomeric excess of the product was > 95 %.     

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.     

NMR‐Solution Structures of Fluoro‐Substituted β‐Peptides: A 314‐Helix and a Hairpin Turn. The First Case of a 90° OC?C?F Dihedral Angle in an α‐Fluoro‐Amide Group

To further study the preference of the antiperiplanar (ap) conformation in α‐fluoro‐amide groups, two β‐peptides, 1 and 2 , containing a (2‐F)‐β³hAla and a (2‐F)‐β²hPhe residue, have been synthesized. Their NMR‐solution structures in CD₃OH were determined and compared with those of non‐F‐substituted analogs, 3 and 4a . While we have found in a previous investigation (Helv. Chim. Acta 2005 , 88, 266) that a stereospecifically introduced F‐substituent in the central position of a β‐heptapeptide is capable of ‘breaking’ the 3₁₄‐helical structure by enforcing the F? C? C?O ap‐conformation, we could now demonstrate that the same procedure leads to a structure with the unfavorable ca. 90° F? C? C?O dihedral angle, enforced by the 3₁₄‐helical folding in a β‐tridecapeptide (cf. 1 ; Fig. 4). This is interpreted as a consequence of cooperative folding in the longer β‐peptide. A F‐substituent placed in the turn section of a β‐peptidic hairpin turn was shown to be in an ap‐arrangement with respect to the neighboring C?O bond (cf. 2 ; Fig. 7). Analysis of the non‐F‐substituted β‐tetrapeptides (with helix‐preventing configurations of the two central β²/β³‐amino acid residues) provides unusually tight hairpin structural clusters (cf. 3 and 4a ; Figs. 8 and 9). The skeleton of the β‐tetrapeptide H‐(R)β³hVal‐(R)β²hVal‐(R)β³hAla‐(S)β³hPhe‐OH ( 4a ) is proposed as a novel, very simple backbone structure for mimicking α‐peptidic hairpin turns.     

β‐Peptidic Secondary Structures Fortified and Enforced by Zn2+ Complexation – On the Way to β‐Peptidic Zinc Fingers?

The correlation between β²‐, β³‐, and β^2,3‐amino acid‐residue configuration and stability of helix and hairpin‐turn secondary structures of peptides consisting of homologated proteinogenic amino acids is analyzed (Figs. 1–3). To test the power of Zn²⁺ ions in fortifying and/or enforcing secondary structures of β‐peptides, a β‐decapeptide, 1 , four β‐octapeptides, 2 – 5 , and a β‐hexadecapeptide, 10 , have been devised and synthesized. The design was such that the peptides would a) fold to a 14‐helix ( 1 and 3 ) or a hairpin turn ( 2 and 4 ), or form neither of these two secondary structures (i.e., 5 ), and b) carry the side chains of cysteine and histidine in positions, which will allow Zn²⁺ ions to use their extraordinary affinity for RS^? and the imidazole N‐atoms for stabilizing or destabilizing the intrinsic secondary structures of the peptides. The β‐hexadecapeptide 10 was designed to a) fold to a turn, to which a 14‐helical structure is attached through a β‐dipeptide spacer, and b) contain two cysteine and two histidine side chains for Zn complexation, in order to possibly mimic a Zn‐finger motif. While CD spectra (Figs. 6–8 and 17) and ESI mass spectra (Figs. 9 and 18) are compatible with the expected effects of Zn²⁺ ions in all cases, it was shown by detailed NMR analyses of three of the peptides, i.e., 2, 3, 5 , in the absence and presence of ZnCl₂, that i) β‐peptide 2 forms a hairpin turn in H₂O, even without Zn complexation to the terminal β³hHis and β³hCys side chains (Fig. 11), ii) β‐peptide 3 , which is present as a 14‐helix in MeOH, is forced to a hairpin‐turn structure by Zn complexation in H₂O (Fig. 12), and iii) β‐peptide 5 is poorly ordered in CD₃OH (Fig. 13) and in H₂O (Fig. 14), with far‐remote β³hCys and β³hHis residues, and has a distorted turn structure in the presence of Zn²⁺ ions in H₂O, with proximate terminal Cys and His side chains (Fig. 15).     

(1R,8R,11R)‐3,3,11‐Tri­methyl‐6,6‐ethyl­ene­dioxy­bi­cyclo­[6.3.0]­undecan‐2‐one and (1R,2R,8R,11R)‐3,3,11‐tri­methyl‐6,6‐ethylenedioxybicyclo[6.3.0]undecan‐2‐ol

The reduction of (1R,8R,11R)‐3,3,11‐tri­methyl‐6,6‐ethyl­ene­dioxy­bi­cyclo­[6.3.0]­undecan‐2‐one, C₁₆H₂₆O₃, (I), gave exclusively an alcohol, C₁₆H₂₈O₃, (II). The stereochemistry of the hydroxyl group in (II) was shown as R. The conformation around the eight‐membered carbocycle in (I) differs markedly from that in (II).     

Hydrolysis Reaction of N-Phosphoryl-α-, β- and γ-amino Acids Studied by HPLC

The hydrolysis reactions of N-（O,O＇diisopropyl）phosphoryl-L-α-alanine （DIPP-L-α-Ala）, N-（O,O＇diisopropyl）- phosphoryl-D-α-alanine （DIPP-D-α-Ala）, N-（O,O＇-diisopropyl）phosphoryl-β-alanine （DIPP-β-Ala） and N-（O,O＇-diisopropyl）phosphoryl-γ-amino butyric acid （DIPP-γ-Aba）, were studied by HPLC and their hydrolysis reaction kinetic equations were obtained. Under acid conditions, the reaction rate of DIPP-L-α-Ala was close to that of DIPP-D-α-Ala and the same rule was true between DIPP-β-Ala and DIPP-γ-Aba. Meantime, the reaction rate of DIPP-L/D-α-Ala was as 10 times as that of DIPP-β-Ala or DIPP-γ-Aba. Under basic conditions, the hydrolysis reactions of DIPP-β-Ala and DIPP-γ-Aba almost did not take place and the reaction rate of DIPP-L/D-α-Ala was about 1/10 of that under acid conditions. Moreover, theoretical calculation further illuminated the differences of the hydrolysis rate from the view of energy. The results would provide some helpful clues to why nature chose a-amino acids but not other kinds of analogs as protein backbones.     

Tolterodinium (+)‐(2R,3R)‐hydrogen tartrate

In the crystal structure of (R)‐N,N‐diisopropyl‐3‐(2‐hydroxy‐5‐methyl­phenyl)‐3‐phenyl­propyl­aminium (2R,3R)‐hydrogen tartrate, C₂₂H₃₂NO⁺·C₄H₅O₆⁻, the hydrogen tartrate anions are linked by O—H⋯O hydrogen bonds to form helical chains built from (9) rings. These chains are linked by the tolterodine molecules via N—H⋯O and O—H⋯O hydrogen bonds to form separate sheets parallel to the (101) plane.     

Synthesis and NMR Analysis in Solution of Oligo(3‐hydroxyalkanoic acid) Derivatives with the Side Chains of Alanine,Valine, and Leucine (β‐Depsides): Coming Full Circle from PHB to β‐Peptides to PHB

Oligomers of 3‐hydroxyalkanoic acids that contain two, three, and six residues with and without O‐terminal (tBu)Ph₂Si and C‐terminal PhCH₂ protection have been synthesized in such a way that the side chains on the oligoester backbone were those of the proteinogenic amino acids Ala (Me), Val (CHMe₂), and Leu (CH₂CHMe₂). The enantiomerically pure 3‐hydroxyalkanoates were obtained by Noyori hydrogenation of the corresponding 3‐oxo‐alkanoates with [Ru((R)‐binap)Cl₂](binap=2,2′bis(diphenylphosphanyl)‐1,1′‐binaphthalene)/H₂ (Scheme 1), and the coupling was achieved under the conditions (pyridine/(COCl)₂, CH₂Cl₂, −78°) previously employed for the synthesis of various oligo(3‐hydroxybutanoic acids) (Schemes 2 and 3). The Cotton effects in the CD spectra of the new oligoesters provided no hints about chiral conformation (cf. a helix) in MeOH, MeCN, octan‐1‐ol, or CF₃CH₂OH solutions (Figs. 1 and 2). Detailed NMR investigations in CDCl₃ solution (Figs. 3–6, and Tables 1–5) of the hexa(3‐hydroxyalkanoic acid) with the side chains of Val (HC), Ala (HB), Leu (HH), Val, Ala, Leu (from O‐ to C‐terminus; 3 ) gave, on the NMR time‐scale, no evidence for the presence of any significant amount of a 2₁‐ or a 3₁‐helical conformation, comparable to those identified in stretched fibers of poly[(R)‐3‐hydroxybutanoic acid], or in lamellar crystallites and in single crystals of linear and cyclic oligo[(R)‐3‐hydroxybutanoic acids], or in the corresponding β‐peptide(s) (the oligo(3‐aminoalkanoic acid) analogs; 1 – 3 ). Thus, the extremely high flexibility (averaged or ‘random‐coil' conformation) of the polyester chain (CO−O rotational barrier ca. 13 kcal/mol; no hydrogen bonding), as compared to polyamide chains (CO−NH barrier ca. 18 kcal/mol; hydrogen bonding) has been demonstrated once again. The possible importance of this structural flexibility, which goes along with amphiphilic properties, for the role of PHB in biology, in evolution, and in prebiotic chemistry is discussed. Structural similarities of natural potassium‐channeling proteins and complexes of oligo(3‐hydroxybutanoates) with Na⁺, K⁺, or Ba²⁺ are alluded to (Figs. 7–9).     

Enantioselective Synthesis of β‐Amino Acids,Part 13

Inexpensive acryloyl chloride was converted in 91% overall yield to two derivatives of β‐alanine, (R,R,R)‐ 6 and (R,R,S)‐ 6 , containing two chiral auxiliaries. C‐Alkylation of (R,R,R)‐ and (R,R,S)‐ 6 via a dianion derivative, was performed by direct metallation with 2.2 equiv. of lithium hexamethyldisilazane (LHMDS) in THF at ?78°. C‐Alkylation of (R,R,S)‐ 6 ‐Li₂ (‘matched' pair of chiral auxiliaries) afforded the mono‐alkylated products 8 – 11 in 29–96% yield and 54–95% stereoselectivity. Employment of LiCl as an additive generally increased stereoselectivities, whereas the effect of HMPA as a cosolvent was erratic. Chemical correlation of the major diastereoisomer from the alkylation reactions with (S)‐α‐alkyl‐β‐alanine ( 12 – 15 ) showed that addition of the electrophile preferentially takes place on the enolate's Si‐face. This conclusion is also supported by molecular‐modeling studies (ab initio HF/3‐21G), which indicate that the lowest‐energy conformation for (R,R,S)‐ 6 ‐Li₂ presents the more sterically hindered Re‐face of the enolate. The theoretical studies also predict a determining role for N? Li? O chelation in (R,R,S)‐ 6 ‐Li₂, giving rise to an interesting ‘ion‐triplet' configuration for the dilithium dianion.     

The absolute configuration of (2S,4S)‐ and (2R,4R)‐2‐tert‐butyl‐4‐methyl‐3‐(4‐tolyl­sulfon­yl)‐1,3‐oxazolidine‐4‐carbaldehyde

The title enanti­omorphic compounds, C₁₆H₂₃NO₄S, have been obtained in an enanti­omerically pure form by crystallization from a diastereomeric mixture either of (2S,4S)‐ and (2R,4S)‐ or of (2R,4R)‐ and (2S,4R)‐2‐tert‐butyl‐4‐methyl‐3‐(4‐tolyl­sulfon­yl)‐1,3‐oxazolidine‐4‐carbaldehyde. These mixtures were prepared by an aziridination rearrangement process starting with (S)‐ or (R)‐2‐tert‐butyl‐5‐methyl‐4H‐1,3‐dioxine. The crystal structures indicate an envelope conformation of the oxazolidine moiety for both compounds.     

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).