Hydrazine hydrate: A new reagent for Fmoc group removal in solid phase peptide synthesis

In solid-phase peptide synthesis using the Fmoc/tBu strategy (SPPS-Fmoc/tBu), an orthogonal protection scheme of amino acids is used; specifically, the alpha-amine group is protected by the 9-fluorenylmethyloxycarbonyl (Fmoc) group, which is removed by weak bases, while side chains are protected by groups that are acid labile. We demonstrated that hydrazine hydrate is an efficient reagent for eliminating the Fmoc group in SPPS-Fmoc/tBu. First, experimental conditions were established for Fmoc group removal from Fmoc-Val-OH in solution. It was determined that the Fmoc group was completely removed with 16% hydrazine hydrate in DMF after 60?min at rt. Second, SPPS-Fmoc/tBu using hydrazine hydrate for Fmoc group removal was standardized. The Fmoc group removal was completed using 16% hydrazine hydrate in DMF for 10?min at rt (twice). When the reaction of Fmoc group removal was microwave-assisted, the reaction only required 30?s to efficiently remove the Fmoc group in SPPS-Fmoc/tBu. The method reported here can be routinely used, and it is equivalent to conventional SPPS-Fmoc/tBu methodologies where 4-methylpiperidine or piperidine is used.     

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.     

Synthesis of branched oligonucleotides with three different sequences using an oxidatively removable tritylthio group

We synthesized a three-way branched oligodeoxynucleotide (ODN) 30-mer using a new branch unit with acid-labile DMTr and oxidatively cleavable TrS groups as orthogonal protecting groups. The branched ODN was successfully synthesized using 5-[3,5-bis(trifluoromethyl)phenyl]-1H-tetrazole and (2R,8aS)-(+)-(camphorylsulfonyl)oxaziridine as the activator of phosphoramidite units and the oxidizing reagent, respectively. We also found that the TrS group was orthogonal to the Lev, TBDMS, and Fmoc groups. These results indicate the possibility of the synthesis of more complex four- and five-way branched ODNs by the combined use of DMTr, TrS, Lev, TBDMS, and Fmoc groups.     

Improvement of proline enantioselective stationary phases by replacing the 9-fluorenylmethoxycarbonyl group

The role of 9-fluorenylmethoxycarbonyl (Fmoc) in previously reported proline enantioselective stationary phases was investigated. Seven stationary phases in which the Fmoc group was replaced by other groups were prepared and evaluated in normal phase mode. The Fmoc group proved nonessential for the broad enantioselectivity observed, as the stationary phase with a trimethylacetyl (Tma) group proved much more effective than the one with the Fmoc group. For the 53 analytes studied, the stationary phase with the Tma group resolved 39, while the one with the Fmoc group resolved 19. Separation factors achieved for the stationary phase with the Tma group are also significantly higher than those for the stationary phase with the Fmoc group. The stationary phase with the (-)-2-(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)propionyl (Tapa) group provides very different selectivity profile when compared to the one with the Tma group. In most of the proline stationary phases studied, pi-pi interaction is not the dominant interaction for the enantioselective recognition.     

Self‐Assembly of Tetraphenylalanine Peptides

Three different tetraphenylalanine (FFFF) based peptides that differ at the N‐ and C‐termini have been synthesized by using standard procedures to study their ability to form different nanoassemblies under a variety of conditions. The FFFF peptide assembles into nanotubes that show more structural imperfections at the surface than those formed by the diphenylalanine (FF) peptide under the same conditions. Periodic DFT calculations (M06L functional) were used to propose a model that consists of three FFFF molecules defining a ring through head‐to‐tail NH₃⁺???^?OOC interactions, which in turn stack to produce deformed channels with internal diameters between 12 and 16 Å. Depending on the experimental conditions used for the peptide incubation, N‐fluorenylmethoxycarbonyl (Fmoc) protected FFFF self‐assembles into a variety of polymorphs: ultra‐thin nanoplates, fibrils, and star‐like submicrometric aggregates. DFT calculations indicate that Fmoc‐FFFF prefers a parallel rather than an antiparallel β‐sheet assembly. Finally, coexisting multiple assemblies (up to three) were observed for Fmoc‐FFFF‐OBzl (OBzl = benzyl ester), which incorporates aromatic protecting groups at the two peptide terminals. This unusual and noticeable feature is attributed to the fact that the assemblies obtained by combining the Fmoc and OBzl groups contained in the peptide are isoenergetic.     

Synthesis and Gelation Capability of Fmoc and Boc Mono-substituted Cyclo(L-Lys-L-Lys)s

Fmoc or Boc mono-substituted cyclo(L-Lys-L-Lys)s were synthesized via the reaction of lysine cyclic dipeptide with Fmoc N-hydroxysuccinimide este(Fmoc-OSu) and di-tert-butyl dicarbonate[(Boc)₂O], respectively. The resulted mono-substituted cyclo(L-Lys-L-Lys)s(2-4) by means of test tube inversion method served as organogelators enabled to form stable thermo-reversible organogels in alcoholic, substituted benzene and chlorinated solvents, with the minimum gelation concentration(MGC) in a range of 1%-4%(mass fraction). The transmission electron microscopy(TEM) and scanning electron microscopy(SEM) observations reveal that these gelators self-assembled into 3D nanofiber, nanoribbon or nanotube network structures. The rheological measurement exhibited that the storage modulus of gels is higher than the loss one, and the complex viscosity is reduced linearly with the increasing of scanning frequency. The fluorescence spectrum of compound 2 in 1,2-dichloroethane and benzene demonstrates that the emission peak of Fmoc at 320 nm has red-shifted and the intensity decreases gradually, while the intensity of the emission peak at 460 nm substantially enhances as a function of concentration, indicating the existence of π-π sta- cking interactions and the formation of J-type aggregates. Meanwhile, compound 4 self-assembled into nanotubes via the stacking of multiple bilayer membranes. Fmoc and Boc disubstituted cyclo(L-Lys-L-Lys)(3) holds the relatively lower MGC values, showing the stronger gelation ability in most selected organic solvents due to the presence of both Fmoc and Boc groups.     

Fmoc-based solid-phase synthesis of adenylylated peptides using diester-type adenylylated amino acid derivatives

Phosphodiester-type adenylylated (AMPylated) Ser, Thr, and Tyr derivatives were developed for Fmoc solid phase peptide synthesis of AMPylated peptides. One-pot/sequential reaction consisting of condensation of an N-nonprotected adenosine derivative and Fmoc-Ser/Thr/Tyr-OAllyl using allyl-N,N-diisopropylchlorophosphoramidite and subsequent oxidation with m-chloroperbenzoic acid gave phosphotriester-type AMPylated Ser/Thr/Tyr derivatives. After Pd(0)-mediated deprotection of allyl groups, the resulting phosphodiester-type AMPylated Ser/Thr/Tyr derivatives were successfully incorporated into peptides by standard Fmoc solid phase peptide synthesis without significant side reactions including dehydroalanine formation.     

Helicity Control of Polymeric Backbones with Alternating cis-trans Double Bonds in Cyclopolymerized Dipropargyl Amides

We report the synthesis of new helical polymeric structures having alternating cis and trans double bonds and chiral amino acid side chains by metathesis cyclopolymerization. The polymer helicity, which is generated by the interaction between fluorenylmethyloxycarbonyl (Fmoc) groups in the side chains, is dramatically affected by solvents. A thorough experimental and theoretical analysis including nuclear magnetic resonance, atomic force microscopy, and density functional theory and molecular mechanics calculations suggests that the helicity of both backbone and side chains are determined by anti-syn rotation of the carbamate groups and by the different interactions of the Fmoc groups with solvents.     

Fmoc-OPhth,the reagent of Fmoc protection

Fmoc-OSu has been widely used for Fmoc protection of amino groups, especially amino acids, in solid phase peptide synthesis. However, it has been recognized that Fmoc-βAla-OH is formed as a by-product via the Lossen rearrangement during the reaction. Since we reconfirmed the formation of Fmoc-βAla-OH during the preparation of Fmoc-AA-OH by Fmoc-OSu, Fmoc-OPhth was designed and synthesized as a new Fmoc reagent to avoid the formation of Fmoc-βAla-OH. Furthermore, Fmoc protection by Fmoc-OPhth and Fmoc-SPPS were evaluated. The various Fmoc-amino acids prepared by Fmoc-OPhth were carried out in good yields and these are applicable in Fmoc-SPPS.     

Synthesis of Sulfotyrosine‐Containing Peptides by Incorporating Fluorosulfated Tyrosine Using an Fmoc‐Based Solid‐Phase Strategy

Tyrosine O‐sulfation is a common protein post‐translational modification that regulates many biological processes, including leukocyte adhesion and chemotaxis. Many peptides with therapeutic potential contain one or more sulfotyrosine residues. We report a one‐step synthesis for Fmoc‐fluorosulfated tyrosine. An efficient Fmoc‐based solid‐phase peptide synthetic strategy is then introduced for incorporating the fluorosulfated tyrosine residue into peptides of interest. Standard simultaneous peptide‐resin cleavage and removal of the acid‐labile side‐chain protecting groups affords the crude peptides containing fluorosulfated tyrosine. Basic ethylene glycol, serving both as solvent and reactant, transforms the fluorosulfated tyrosine peptides into sulfotyrosine peptides in high yield.     

A convenient route to N-[2-(Fmoc)aminoethyl]glycine esters and PNA oligomerization using a Bis-N-Boc nucleobase protecting group strategy

A simple and practical synthesis of the benzyl, allyl, and 4-nitrobenzyl esters of N-[2-(Fmoc)aminoethyl]glycine is described starting from the known N-(2-aminoethyl)glycine. These esters are stored as stable hydrochloride salts and were used in the synthesis of peptide nucleic acid monomers possessing bis-N-Boc-protected nucleobase moieties on the exocyclic amino groups of ethyl cytosin-1-ylacetate, ethyl adenin-9-ylacetate and ethyl (O(6)-benzylguanin-9-yl)acetate. Upon ester hydrolysis, the corresponding nucleobase acetic acids were coupled to N-[2-(Fmoc)aminoethyl]glycine benzyl ester or to N-[2-(Fmoc)aminoethyl]glycine allyl ester in order to retain the O(6) benzyl ether protecting group of guanine. The Fmoc/bis-N-Boc-protected monomers were successfully used in the Fmoc-mediated solid-phase peptide synthesis of mixed sequence 10-mer PNA oligomers and are shown to be a viable alternative to the currently most widely used Fmoc/Bhoc-protected peptide nucleic acid monomers.     

A new set of orthogonal-protecting groups for oligosaccharide synthesis on a polymeric support

The hydroxyl-protecting groups, levulinoyl (Lev) and 9-fluorenylmethoxycarbonyl (Fmoc), and the amino-protecting group 2,2,2-trichloroethoxycarbonyl (Troc), offer an ideal set of orthogonal-protecting groups which are compatible with oligosaccharide synthesis on a methylpolyethyleneglycol (MPEG) support using a p-alkyloxybenzyl-type linker.     

Towards a biomimetic poly-aminoketone foldamer: synthesis of a triply protected monomer and its coupling to a dimer, trimer and tetramer

The design of a new biomimetic foldamer, relying on the weak amine-carbonyl interaction for secondary structure formation, is presented. The efficient synthesis of a triply protected monomer starting from glycidol was developed. This monomer contains a dioxolane-protected keto group that will allow liberation of the ketone functionality in the backbone once construction of the oligomeric backbone is complete. This monomer contains two additional orthogonal protecting groups at its two termini, the Fmoc and the TBDMS groups. The Fmoc group in particular permits oligomerisation towards the N terminus as seen in Fmoc solid phase peptide synthesis. Construction and full characterisation of a ketone-protected dimer, trimer and tetramer are reported.     

One-pot synthesis of Weinreb amides employing 3,3-dichloro-1,2-diphenylcyclopropene (CPI-Cl) as a chlorinating agent

The synthesis of N^α-protected amino alkyl Weinreb amides starting from the corresponding α-amino acids as well as carboxylic acids has been delineated through the in situ generation of acid chlorides using CPI-Cl as a chlorinating agent. The protocol is simple; the reaction conditions employed were mild, and compatible with all the three commonly used urethane protecting groups namely, Boc, Cbz and Fmoc groups. The resulting Weinreb amides are obtained in good yields as optically pure products.     

Solid-supported synthesis of cryptand-like macrobicyclic peptides

A straightforward method for the solid-supported synthesis of cryptand-like bicyclic peptides (1-5) on a backbone amide linker has been described. For the branching, two novel easily available building blocks, viz. N-(4-methoxytrityl)-N-(2-nitrobenzenesulfonyl)-protected N,N-bis(2-aminoethyl)-beta-alanine (6) and N-(9-fluorenylmethoxycarbonyl) protected iminodiacetic acid monoallyl ester (7), have been employed. The key steps of the synthesis are as follows: (i) stepwise coupling of one amino acid and 6 to the secondary amino group of the linker; (ii) removal of the 2-nitrobenzenesulfonyl group and SPPS by the Fmoc chemistry, using 7 as the penultimate and tert-butoxycarbonyl (Boc) protected glycine as the last amino acid; (iii) removal of the 4-methoxytrityl protection and subsequent SPPS by the Fmoc chemistry; (iv) removal of the allyl and Fmoc groups, followed by cyclization; and (v) removal of the Boc and tert-butyl groups, followed by cyclization. Final cleavage from the support and removal of benzyl-derived protecting groups gives the desired bicyclic products.     

赖氨酸环二肽有机凝胶因子的合成及其选择性凝胶化与染料吸附

在利用半胱氨酸修饰赖氨酸环二肽制备对称性四肽的过程中, 通过两种脱除Trt(三苯甲基)的方法分别得到含有Fmoc(芴甲氧羰基)的非环与大环四肽产物, 其结构得到了核磁、质谱、红外、元素分析等证实。 它们能使多种有机溶剂凝胶化, 且具有热可逆性, 由扫描电子显微镜(SEM)可观察到凝胶内部均为三维网络结构。 在体积分数低至0.1%的含氯有机溶剂/水两相体系中, 它们依然可以进行选择性凝胶化。 此外, 该有机凝胶干胶由于内部微纳米网络结构以及Fmoc基团的存在, 可以直接从水溶液中吸收多种染料分子, 且吸附能力随温度的升高而提高。     

A convenient, general synthesis of 1,1-dimethylallyl esters as protecting groups for carboxylic acids

[reaction: see text] Carboxylic acids were converted in high yield to their 1,1-dimethylallyl (DMA) esters in two steps. Palladium-catalyzed deprotection of DMA esters was shown to be compatible with tert-butyl, benzyl, and Fmoc protecting groups, and Fmoc deprotection could be carried out selectively in the presence of DMA esters. DMA esters were also shown to be resistant to nucleophilic attack, suggesting that they will serve as alternatives to tert-butyl esters when acidic deprotection conditions need to be avoided.     

Facile amide bond formation from carboxylic acids and isocyanates

A wide variety of carboxylic acids in the form of their salts condense with aryl isocyanates at room temperature with loss of carbon dioxide to give the corresponding amides in high yield. Application of the reaction to acyl isocyanates gives unsymmetric imides. The reaction is compatible with hydroxyl groups and both Fmoc and Boc protecting groups for amines and is applicable to aliphatic, aromatic, and heteroaromatic acids.     

Orthogonality and compatibility between Tsc and Fmoc amino-protecting groups

New deprotection conditions that provide a complete orthogonality between Tsc and Fmoc amino-protecting groups are described. The potential of these orthogonal deprotection conditions was then demonstrated by the efficient solid-phase synthesis of branched peptides 20 and 21 using doubly protected amino acids such as Tsc-Lys(Fmoc)-OH 4c and Fmoc-Lys(Tsc)-OH 4d.     

Solid-Phase Total Synthesis of Bacitracin A

An efficient solid-phase method for the total synthesis of bacitracin A is reported. This work was undertaken in order to provide a general means of probing the intriguing mode of action of the bacitracins and exploring their potential for use against emerging drug-resistant pathogens. The synthetic approach to bacitracin A involves three key features: (1) linkage to the solid support through the side chain of the L-asparaginyl residue at position 12 (L-Asn(12)), (2) cyclization through amide bond formation between the alpha-carboxyl of L-Asn(12) and the side chain amino group of L-Lys(8), and (3) postcyclization addition of the N-terminal thiazoline dipeptide as a single unit. To initiate the synthesis, Fmoc L-Asp(OH)-OAllyl was attached to a PAL resin. The chain of bacitracin A was elaborated in the C-to-N direction by sequential piperidine deprotection/HBTU-mediated coupling cycles with Fmoc D-Asp(OtBu)-OH, Fmoc L-His(Trt)-OH, Fmoc D-Phe-OH, Fmoc L-Ile-OH, Fmoc D-Orn(Boc)-OH, Fmoc L-Lys(Aloc)-OH, Fmoc L-Ile-OH, Fmoc D-Glu(OtBu)-OH, and Fmoc L-Leu-OH. The allyl ester and allyl carbamate protecting groups of L-Asn(12) and L-Lys(8), respectively, were simultaneously and selectively removed by treating the peptide-resin with palladium tetrakis(triphenylphosphine), acetic acid, and triethylamine. Cyclization was effected by PyBOP/HOBT under the pseudo high-dilution conditions afforded by attachment to the solid support. After removal of the N-terminal Fmoc group, the cyclized peptide was coupled with 2-[1'(S)-(tert-butyloxycarbonylamino)-2'(R)-methylbutyl]-4(R)-carboxy-Delta(2)-thiazoline (1). The synthetic peptide was deprotected and cleaved from the solid support under acidic conditions and then purified by reverse-phase HPLC. The synthetic material exhibited an ion in the FAB-MS at m/z 1422.7, consistent with the molecular weight calculated for the parent ion of bacitracin A (MH(+) = C(73)H(84)N(10)O(23)Cl(2), 1422.7 g/mol). It was also indistinguishable from authentic bacitracin A by high-field (1)H NMR and displayed antibacterial activity equal to that of the natural product, thus confirming its identity as bacitracin A. The overall yield for the solid-phase synthesis was 24%.