Synthesis of orthogonally protected lanthionines

Synthetic approaches to the lantibiotics, a family of thioether-bridged antimicrobial peptides, require flexible synthetic routes to a variety of orthogonally protected derivatives of lanthionine 1. The most direct approaches to lanthionine involve the reaction of cysteine with an alanyl beta-cation equivalent. Several possibilities exist for the alanyl beta-cation equivalent, including direct activation of serine under Mitsunobu conditions: however, the low reactivity of sulfur nucleophiles in the Mitsunobu reaction has previously precluded its use in the synthesis of the lantibiotics. We report here a new approach to the synthesis of protected lanthionine, using a novel variant of the Mitsunobu reaction in which catalytic zinc tartrate is used to enhance the nucleophilicity of the thiol. In the course of these studies, we have also demonstrated that the synthesis of lanthionine from trityl-protected beta-iodoalanines is prone to rearrangement, via an aziridine, to give predominantly trityl-protected alpha-iodo-beta-alanines, and hence norlanthionines, as the major products.     

‘Bis-ornithine’ (2,2-bis(aminopropyl)glycine): a new tetravalent template for assembling different functional peptides

Synthesis of bis-ornithine, a new C^α,α-disubstituted α-amino acid bearing orthogonally protected α and δ amine groups is reported. Bis-ornithine (bisOrn) and dipeptides containing bis-ornithine have been synthesized in solution up to multigram scale. As a first example, one of these compounds Boc-βAla-bisOrn(Alloc)₂-OH has been attached to a solid support and used as template for the synthesis of a symmetrical assemblage of peptides.     

The synthesis of active and stable diaminopimelate analogues of the lantibiotic peptide lactocin S

Lantibiotic peptides are potent antimicrobial compounds produced by Gram-positive bacteria. They can be used in food preservation, and some also show potential for clinical applications. Unfortunately, some of these peptides can be susceptible to inactivation by oxidation of the sulfur-containing amino acid lanthionine, limiting their use. Here we describe the synthesis and testing of diaminopimelate analogues of the lantibiotic lactocin S. These analogues were designed to improve the oxidative stability of the peptide by replacing the sulfur in lanthionine with a methylene unit. Lanthionine was systematically replaced with diaminopimelate during solid-phase peptide synthesis to produce several analogues. One analogue, A-DAP lactocin S, was found to retain full biological activity in addition to displaying increased stability. This is the first time a synthetic lanthionine ring analogue of a lantibiotic has retained natural activity levels. This methodology is potentially very promising for use in producing more stable, medically relevant lantibiotics.     

Synthesis of an Orthogonally Protected Polyhydroxylated Cyclopentene from l‐Sorbose

The use of l ‐sorbose in the synthesis of functionalized cyclopentene derivatives was accomplished. These cyclopentene derivatives are related to those found in naturally occurring jatrophane frameworks and in other bioactive compounds. The formation of allyl α‐l ‐sorbopyranoside was a key synthetic step. Regioselective introduction of protecting groups was followed by the hydrolysis of the allyl glycoside to furnish a fully protected acyclic l ‐sorbose derivative. This acyclic intermediate was subsequently used to give an orthogonally protected polyhydroxylated cyclopentene, which has potential for further synthesis of bioactive compounds. The protected cyclopentene itself showed a clear cytotoxic activity when tested against a panel of human cancer cell lines (HT29, LS174T, SW620, A549, and HeLa cells).     

Versatile and stereoselective syntheses of orthogonally protected beta-methylcysteine and beta-methyllanthionine

[reaction: see text] Lantibiotics are a class of lanthionine (and/or beta-methyllanthionine)-containing peptides with antibioitic activity against Gram-positive bacteria. As part of our research effort directed toward the synthesis and mechanistic study of the lantibiotic peptide mersacidin (1), we report stereoselective syntheses of orthogonally protected beta-methylcysteine (beta-MeCys) and beta-methyllanthionine (beta-MeLan), two key nonnatural amino acid components of the mersacidin architecture.     

Multi-detachable resin supports for solid phase fragment synthesis

Two multi-detachable resin supports, each containing orthogonally removable benzyl and photolabile esters were prepared and applied to the synthesis of Leu-enkephalin and angiotensin. These resins, called Pop and Pon, could be cleaved by acid to give the free peptide, by nucleophiles to give protected peptides, and by photolysis to give the protected peptide still esterified to an oxymethylphenylacetic acid handle. Peptides of the latter type were readily attached to an aminomethyl-resin for further peptide elongation.     

Orthogonally protected cyclo-beta-tetrapeptides as solid-supported scaffolds for the synthesis of glycoclusters

Two novel peptide scaffolds, viz. cyclo[(N(alpha)-Alloc)Dpr-beta-Ala-(N(alpha)-Fmoc)Dpr-beta-Ala] (1) and cyclo[(N(alpha)-Alloc)Dpr-alpha-azido-beta-aminopropanoyl-(N(alpha)-Fmoc)Dpr-beta-Ala] (2), composed of orthogonally protected 2,3-diaminopropanoyl (Dpr) and beta-alanyl residues, have been described. Fmoc chemistry on a backbone amide linker derivatized resin has been used for the chain assembly. Selective removal of the 4-methyltrityl (Mtt) and 1-methyl-1-phenylethyl protections (PhiPr) exposes the beta-amino and carboxyl terminus, respectively, and on-resin cyclization then gives the desired orthogonally protected cyclo-beta-tetrapeptides (1 and 2). The alpha-amino groups, bearing the Fmoc and Alloc protections and the azide mask, allow stepwise orthogonal derivatization of these solid-supported cyclo-beta-tetrapeptide cores (1 and 2). This has been demonstrated by attachments of various sugar units [viz., acetyl- or toluoyl-protected carboxymethyl alpha-d-glycopyranosides (13-15) and methyl 6-O-(4-nitrophenoxycarbonyl)-alpha-d-glycopyranosides (22-24)] to obtain diverse di- and trivalent glycoclusters (33-42). Acidolytic release (TFA) from the support, followed by conventional NaOMe-catalyzed transesterification (33-40) or hydrazine-induced acyl substitution in DMF (41 and 42), gives the fully deprotected clusters (43-52) as final products.     

Ribosylation of adenosine: an orthogonally protected building block for the synthesis of ADP-ribosyl oligomers

A method to ribosylate adenosine on the 2' hydroxyl function in an α-selective fashion and in good yield is presented. Protective groups chosen for the acceptor and donor used in this glycosylation not only direct α-selectivity but also allow the construction of a fully orthogonally protected building block for the future assembly of oligo-ADP-ribosylated peptides and proteins.     

Chemical synthesis and biological activity of analogues of the lantibiotic epilancin 15X

Lantibiotics are a large family of antibacterial peptide natural products containing multiple post-translational modifications, including the thioether structures lanthionine and methyllanthionine. Efforts to probe structure-activity relationships and engineer improved pharmacological properties have driven the development of new methods to produce non-natural analogues of these compounds. In this study, solid-supported chemical synthesis was used to produce analogues of the potent lantibiotic epilancin 15X, in order to assess the importance of several N-terminal post-translational modifications for biological activity. Surprisingly, substitution of these moieties, including the unusual N-terminal D-lactyl moiety, resulted in relatively small changes in the antimicrobial activity and pore-forming ability of the peptides.     

An expedient synthesis of orthogonally protected lysinoalanine from Aloc-protected Garner’s aldehyde

An expedient synthesis of orthogonally protected lysinoalanine has been developed. We have prepared a novel Garner’s aldehyde derivative bearing an Aloc group; reductive amination of this aldehyde with Fmoc-Lys-OPMB gave the lysinoalanine skeleton. This was then transformed into an orthogonally protected lysinoalanine derivative suitable for the synthesis of side-chain bridged cyclic peptides by solid phase peptide synthesis methods.     

Totally stereoselective synthesis of 1,3-disaccharides through Diels-Alder reactions

A nonclassical, totally stereoselective synthesis of orthogonally protected 1,3-disaccharides is reported. Enantiomerically pure beta-keto-delta-lactones, efficiently obtained from glucal and galactal, are transformed into electron-poor heterodienes and chemo-, regio-, and stereoselectively cycloadded to glycals as electron-rich dienophiles, to directly afford 2-thiodisaccharides. The reductive desulfurization of the latter smoothly gave the corresponding 2,2'-dideoxydisaccharides.     

Stereoselective synthesis of methyl 7-dihydro-trioxacarcinoside B

A stereoselective synthesis of 7-dihydro-triocacarcinose B, a branched octose from the quinocyclines, has been achieved. The biocatalytic resolution of a Baylis-Hillman adduct and a subsequent ring-closing metathesis were used to assemble the molecular framework. Subsequent key steps were a highly stereoselective epoxidation and a regio- and stereoselective opening of the epoxide by allyl alcohol and HClO4 to introduce the C(3)-OH group in protected form. The 7-dihydro-triocacarcinose B could be converted into the corresponding 1,7-anhydrosugar.     

Parallel synthesis of 19-membered ring macro-heterocycles via intramolecular thioether formation

Starting from resin-bound orthogonally protected lysine, the generation of 19-membered ring macro-heterocycles via intramolecular thioether formation is described. The on resin cyclization occurred by the coupling of p-fluoro-m-nitro benzoic acid or bromo acetic acid followed by intramolecular substitution S_NAr or S_N2 displacement of the fluoro and bromo groups, respectively. The described approaches present versatile synthetic routes toward the synthesis of libraries of macro-heterocycles in an attempt to establish lead drug candidates. The desired cyclic products were obtained in good yields and good purities.     

A Scalable Approach to Obtaining Orthogonally Protected β-d-Idopyranosides

A practical method to obtain orthogonally protected d-idopyranose from d-galactose has been developed, which is the first method to enable synthesis of the challenging β-d-idopyranoside linkage. The method relies on a key double inversion at O-2 and O-3 in an easily prepared d-galactose derivative, which proceeds regio- and stereoselectively through a 2,3-anhydrotalopyranoside; reaction using a selection of alkoxides affords exclusively the 3-O-alkylidopyranoside, which can be used to generate an orthogonally protected monosaccharide. The process is scalable and requires minimal purification, so it could be used to produce building blocks to aid in the synthesis of various β-idopyranose-containing oligosaccharide targets to further probe their biological functions.     

An Orthogonally Protected alpha,alpha-bis(aminomethyl)-beta-alanine building block for the construction of glycoconjugates on a solid support

Synthetic glycoclusters are extensively used as mimetics of naturally occurring, multivalent carbohydrate ligands in various glycobiological applications. Their preparation, however, is far from trivial, and it still is a limiting factor in the study of carbohydrate binding. We herein report the synthesis of an orthogonally protected building block, N-Alloc-N'-Boc-N' '-Fmoc-alpha,alpha-bis(aminomethyl)-beta-alanine (1), and its use in the preparation of triantennary peptide glycoclusters (21-24) on a solid support. The assembly of the clusters involves removal of the amino protections of the solid-supported branching unit 1 in the order Fmoc, Boc, and Alloc, and subsequent coupling of peracetylated O-(glycopyranosyl)-N-Fmoc-L-serine pentafluorophenyl esters (galactose, glucose, mannose, and ribose) to each amino group exposed.     

Practical and Efficient Synthesis of Orthogonally Protected α‐2,3‐Diaminopropionic Acid (2,3‐Dap), 2,4‐Diaminobutanoic Acid (2,4‐Dab), and their N‐Methylated Derivatives

The synthesis of orthogonally protected Fmoc‐Dap/Dab (Boc/Z/Alloc)‐OH starting from Fmoc‐Asp/Glu has been described. The salient features of our synthetic strategy involved formation of Fmoc‐Asp/Glu‐5‐oxazolidinone acids, conversion of acid function to acyl azides, Curtius rearrangement, and hydrolysis of the oxazolidinone group.     

Solid-phase synthesis of asymmetrically branched sequence-defined poly/oligo(amidoamines)

We present for the first time the synthesis of asymmetrically branched sequence-defined poly/oligo(amidoamines) (PAAs) using solid-phase synthesis with the capability of introducing diversity at the side chains. We introduce two new versatile (diethylenetriamine) building blocks for solid-phase synthesis bearing Fmoc/Boc and Fmoc/Alloc protecting groups expanding recently used Fmoc/Boc protecting group strategy for linear PAAs to an Fmoc/Alloc/Boc strategy. This allows for orthogonal on-resin cleavage of Fmoc and Alloc protecting groups during solid-phase synthesis of PAAs with backbones differing in chain length and sequence. With these structures we then demonstrate the potential for generating asymmetrical branching by automated multiple on-resin cleavage of Alloc protecting groups as well as the introduction of side chains varying in length and number. Such systems have high potential as nonviral vectors for gene delivery and will allow for more detailed studies on the correlation between the degree of branching of PAAs and their resulting biological properties.     

Stereo- and regiocontrolled synthesis of highly functionalized cyclopentanes with multiple chiral centers

Abstract

The synthesis of some highly substituted three-dimensional cyclopentanes with multiple chiral centers and with high regiochemical and stereochemical diversity has been accomplished starting from cyclopentadiene-derived aminocyclopentenecarboxylic acids. The small-molecular design consisted of stereo- and regiocontrolled functionalization of the starting cyclopentene β- and γ-amino acids through oxirane formation/oxirane opening and afforded regio- and diastereoisomers of orthogonally protected aminocyclopentanecarboxylates.     

Engineering dehydro amino acids and thioethers into peptides using lacticin 481 synthetase

Lantibiotics are peptide antimicrobials containing the thioether-bridged amino acids lanthionine (Lan) and methyllanthionine (MeLan) and often the dehydrated residues dehydroalanine (Dha) and dehydrobutyrine (Dhb). While biologically advantageous, the incorporation of these residues into peptides is synthetically daunting, and their production in vivo is limited to peptides containing proteinogenic amino acids. The lacticin 481 synthetase LctM offers versatile control over the installation of dehydro amino acids and thioether rings into peptides. In vitro processing of semisynthetic substrates unrelated to the prelacticin 481 peptide demonstrated the broad substrate tolerance of LctM. Furthermore, a chemoenzymatic strategy was employed to generate novel thioether linkages by cyclization of peptidic substrates containing the nonproteinogenic cysteine analogs homocysteine and beta-homocysteine. These findings are promising with respect to the utility of LctM toward preparation of conformationally constrained peptide therapeutics.     

Enantioselective synthesis of unsaturated amino acids using p-methoxybenzylamine as an ammonia equivalent

A versatile, non-alkylative enantioselective synthesis of unsaturated α-amino acids based on the Sharpless asymmetric epoxidation has been developed. Enantiomerically enriched trans epoxy alcohols bearing unsaturated substituents were prepared and submitted to regio- and stereospecific ring-opening with p-methoxybenzylamine as a nucleophile, leading to anti-3-(p-methoxybenzylamino)-1,2-diols which were further protected by reaction with Boc₂O. The 1,2-diol fragment was then oxidatively cleaved by a sequential treatment with sodium periodate and sodium chlorite to afford the corresponding amino acid. Using this methodology, doubly N-protected (p-methoxybenzyl and Boc) allyl glycine, 3-butenyl glycine and 4-pentenyl glycine have been prepared in three synthetic steps from the corresponding allyl alcohols. As a demonstration of the orthogonal nature of the nitrogen protection, both protecting groups have been selectively removed from the fully protected amino ester.