N-Alkyl cysteine-assisted thioesterification of peptides

A new method for the preparation of peptide thioester by the post-solid phase peptide synthesis (SPPS) approach was developed. A series of N-alkyl cysteine derivatives were prepared and used as the C-terminus residue of the peptides prepared by the Fmoc SPPS. The synthetic peptides released from resin by TFA were readily converted to the peptide thioester in aqueous 3-mercaptopropionic acid (MPA) without significant side reactions.     

Facile solid-phase synthesis of head-side chain cyclothiodepsipeptides through a cyclative cleavage from MeDbz-resin

Head to side-chain cyclothiodepsipeptides were conveniently prepared through a cyclative cleavage using the MeDbz linker. Briefly, the peptide sequence was elongated on a MeDbz-Gly-ChemMatrix resin and reacted with 4-nitrophenyl chloroformate, followed by treatment with DIEA to render an activated cyclic N-acyl-N′-methylurea-resin. Removal of the Cys protecting group and further treatment with DIEA allowed the formation of the thiolactone with the concomitant release of the cyclic peptide from the resin.     

A convenient synthesis of new α-aminoalkylphosphonates, aromatic analogues of arginine as inhibitors of trypsin-like enzymes

A simple and efficient protocol for the synthesis of new N-protected α-aminoalkylphosphonic diphenyl esters--aromatic analogues of arginine--is presented. The crucial, guanylating step was achieved using S-ethyl-N,N′-di(Boc)isothiourea in chloroform and in the presence of Et₃N and HgCl₂. Deprotection of the derivatives obtained was performed using trifluoroacetic acid in CH₂Cl₂ or hydrogenolysis over Pd/C. The products are potent inhibitors of trypsin.     

Chemical Synthesis of Proteins with Base-Labile Posttranslational Modifications Enabled by a Boc-SPPS Based General Strategy Towards Peptide C-Terminal Salicylaldehyde Esters

Chemical synthesis of proteins bearing base-labile post-translational modifications (PTMs) is a challenging task. For instance, O-acetylation and S-palmitoylation PTMs cannot survive Fmoc removal conditions during Fmoc-solid phase peptide synthesis (SPPS). In this work, we developed a new Boc-SPPS-based strategy for the synthesis of peptide C-terminal salicylaldehyde (SAL) esters, which are the key reaction partner in Ser/Thr ligation and Cys/Pen ligation. The strategy utilized the semicarbazone-modified aminomethyl (AM) resin, which could support the Boc-SPPS and release the peptide SAL ester upon treatment with TFA/H₂O and pyruvic acid. The non-oxidative aldehyde regeneration was fully compatible with all the canonical amino acids. Armed with this strategy, we finished the syntheses of the O-acetylated protein histone H3(S10ac, T22ac) and the hydrophobic S-palmitoylated peptide derived from caveolin-1.     

Diastereoselective synthesis of unsaturated 1,2-amino alcohols from α-hydroxy allyl ethers using chlorosulfonyl isocyanate

Diastereoselective synthesis of 1,2-amino alcohols was achieved from a highly diastereoselective allylic amination reaction of α-hydroxy allyl ethers using chlorosulfonyl isocyanate. Diastereoselectivities varied depending on the stereochemistry of the ethers used and the stability of the carbocation intermediate obtained during the reaction. We propose that this CSI reaction is the results of either a S_Ni or S_N1 mechanism, according to the stability of the carbocation intermediate.     

Reaction of indium ate complexes with allylic compounds. Controlling SN2/SN2′ selectivity by solvents

Vinyl and methylindium ate complexes (indates) were prepared and both the tendency of immigration and regioselectivity toward cinnamyl bromide were investigated. The vinyl group was more preferably transferred than the Me group, giving a regioisomeric mixture of S_N2 and S_N2′ products. The ratio of S_N2/S_N2′ selectivity can be controlled by solvents; in the presence of polar solvents, such as N-butylpyrrolidone (NBP) and THF, the S_N2′ product was mainly obtained, whereas the S_N2 product was selectively prepared in solutions containing hexane. The vinylindium compound, generated by the reaction of allylic-type diindium reagents with imine, was also converted to the corresponding vinyl indate, which was allowed to react with allyl chloride to give a three-component coupling product.     

Solid-phase peptide synthesis in the reverse (N --> C) direction

[reaction: see text] A new strategy for SPPS in the reverse direction based on the use of 2-Cl-trityl resin, an allyl ester as the temporary protecting group, and Cu(OBt)(2)/DIPCDI or HATU/DIEA as the coupling method is described. These conditions ensure good yields with minimal racemization of the C-terminal residue.     

新型双烯丙基化手性二胺的合成

通过烯丙基溴化锌对手性亚胺的两次不对称加成, 合成了光活性的双烯丙基化的新型手性二胺. 首先经N-Boc保护和Dess-Martin氧化制得了光活性的(S)-N-Boc-3-苯基-2-氨基丙醛. (S)-N-Boc-3-苯基-2-氨基丙醛与(S)-缬氨基酸甲酯缩合生成手性亚胺, 在七水合三氯化铈(CeC1₃•7H₂O)催化以及底物的手性诱导作用下, 该手性亚胺与烯丙基溴化锌经不对称加成反应合成了高光活性的单烯丙基化手性胺. 该手性胺与2-噻吩甲醛缩合生成新的手性亚胺, 再次通过不对称烯丙基加成反应合成了含有四个手性中心的高光学纯的双烯丙基化手性二胺. 目标化合物和关键中间体的结构均被¹H NMR, ¹³C NMR及MS确证.     

Synthesis of the C6–C18 bis-tetrahydrofuran fragment of the proposed structure of iriomoteolide-2a via stepwise double SN2 cyclization reactions

The C6–C18 bis-tetrahydrofuran (bis-THF) fragment of the proposed structure of iriomoteolide-2a has been synthesized via stepwise double intramolecular S_N2-type etherifications. The C11 and C16 stereogenic centers could be secured in the forms of propargyl alcohols by asymmetric transfer hydrogenation of the corresponding propargyl ketones. The C9–C12 THF ring was first constructed via a tandem asymmetric dihydroxylation (AD)–S_N2 sequence while the C13–C16 THF ring was later installed via an intramolecular S_N2 reaction of a chiral propargyl mesylate. During the latter THF ring formation, epimerization at the propargylic carbon was not observed. Since the initially proposed (9R,11S,12R) configuration of iriomoteolide-2a has recently been revised to (9S,11R,12S), the established synthesis of the C6–C18 bis-THF fragment could be easily amended by using the opposite enantiomers of the chiral ligands for AD and asymmetric transfer hydrogenation.     

The use of a cysteinyl prolyl ester (CPE) autoactivating unit in peptide ligation reactions

A peptide containing a cysteinyl prolyl ester (CPE) moiety at the C-terminus (CPE peptide) is spontaneously transformed into a diketopiperazine thioester via an intramolecular N-S acyl shift reaction, followed by diketopiperazine formation. The CPE peptide can be ligated with a Cys-peptide in a one-pot procedure. The peptide diketopiperazine thioester can also be transformed into a peptide thioester by intermolecular thiol-thioester exchange with external thiol compounds such as sodium mercaptoethanesulfonate. Since CPE peptides can be prepared by standard Fmoc solid-phase synthesis, it is a versatile alternative to the peptide thioester, providing a flexible ligation strategy that promises to be useful in polypeptide synthesis.     

Rapid and high-yield solution-phase synthesis of DOTA-Tyr-octreotide and DOTA-Tyr-octreotate using unprotected DOTA

An improved method for the solution-phase derivatization of Tyr³-Lys⁵(Dde)-octreotide (TOC(Dde)) and Tyr³-Lys⁵(Dde)-octreotate (TATE(Dde)) with the macrocyclic chelator DOTA (1,4,7,10-tetraazacyclododecane-N′,N″,N?,N?-tetraacetic acid) has been developed. The fully protected parent peptides were assembled via solid-phase peptide synthesis (SPPS) using Fmoc-strategy. After cleavage from the solid support, disulfide bond formation was carried out using H₂O₂. Both TOC(Dde) and TATE(Dde) were successfully coupled with DOTA in the presence of NHS, EDCI and DIPEA in a water/DMF solvent system. Yields of the coupling reaction were >98% within only 2 h with no detectable formation of sideproducts. This method for the preparation of DOTATOC, DOTATATE and other DOTA-peptide conjugates is therefore a rapid and economic alternative to the currently used methods.     

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.     

Convenient Preparation and Spectroscopic Characterization of 7R-Hydroxymatairesinol

The preparation of 7R-HMR (allo-hydroxymatairesinol) is reported by: (a) NaBH₄ kinetic reduction of 7R/7S diastereomeric mixture; and (b) epimerization of the C7 hydroxyl group by Mitsunobu reaction and subsequent ester hydrolysis. The availability of highly pure target compound (7R-HMR) made it possible to confirm the structure of the target compound and to complete the full spectroscopic characterization.     

Total synthesis of (+)-cylindricine C

A stereoselective total synthesis of (+)-cylindricine C has been achieved starting with (S)-N-Boc-2-pyrrolidinone. The key elements of this synthesis involve the sequence of reactions including BF₃-mediated addition of the allyl Grignard reagent to the cyclic imine, spirocyclization via enamine formation, and intramolecular Michael addition to form the tricyclic core.     

An improved synthesis of aziridine-2,3-dicarboxylates via azido alcohols—epimerization studies

The reasons for epimerization of 3-azido-2-hydroxysuccinates observed during the ring opening of epoxides or cyclic sulfites with sodium azide is now clarified. It is caused by the high acidity of the proton at the 3-position. This is proven by a proton deuterium exchange in assays with either D₂O or DCl containing solvents. The anti-3-azido-2-hydroxysuccinates serve as intermediates for enantiomerically pure trans-aziridine-2,3-dicarboxylates for which an optimized synthetic pathway is presented. The first example of an enantiomerically pure mixed diester of the aziridine-2,3-dicarboxylic acid the synthesis of the allyl ethyl ester is reported herein.     

An efficient synthesis of a novel analog of octreotide with an unnatural l-lysine-like tetrazolyl amino acid

A new cyclic octreotide-like octapeptide was prepared by incorporation of an unnatural tetrazolyl amino acid, an analog of Fmoc-l-lysine, into the peptide chain. The new tetrazolyl amino acid, (S)-2-{[2-(9H-fluoren-9-yl)acetoxy]amino}-6-(1H-tetrazol-1-yl)hexanoic acid, was obtained by azidation of Fmoc-l-lysine trifluoroacetate with sodium azide in the presence of triethyl orthoformate in glacial acetic acid. The linear peptide sequence was prepared using efficient Fmoc/t-Bu solid-phase peptide synthesis (SPPS). Cyclization of the octapeptide was carried out via oxidation with iodine. The structure and purity of the cyclic octapeptide were confirmed by LC–MS, MALDI-TOF MS/MS analysis as well as 1D/2D ¹H and ¹³C NMR spectroscopy.     

A convenient synthesis of (1S,2R)-1,2-indene oxide and trans-(1S,2S)-2-bromo-1-indanol via oxazaborolidine-catalyzed borane reduction

A facile synthesis of (1S,2R)-indene oxide with >99% e.e. and (1S,2S)-trans-2-bromo-1-indanol with 87% e.e. has been established by employing CBS-oxazaborolidine-catalyzed asymmetric borane reduction of 2-p-toluenesulfonyloxy-1-indanone using N-ethyl-N-isopropylaniline–borane complex as the borane carrier.     

Synthesis of trisubstituted alkenes by reductive dehydroxylation of Baylis-Hillman adducts using polymethylhydrosiloxane (PMHS) and catalytic B(C6F5)3

B(C₆F₅)₃ as a catalyst and polymethylhydrosiloxane as a hydride source have been employed for the reductive dehydroxylation of Baylis-Hillman adducts wherein the hydride adds in an S_N2′ manner onto the unactivated allyl alcohol moiety with concomitant elimination of the hydroxy group along with double bond migration. The products formed were found to be E in the case of ester adducts and Z in the case of nitrile adducts.     

Improved synthesis of a [4.4]-spirolactam β-turn mimetic as surrogate of the didemnin side chain dipeptide Pro-N-Me-d-Leu

An efficient synthesis of [4.4]-spirolactam restricted derivatives of the didemnin side chain dipeptide l-Pro-N-Me-d-Leu is described. This methodology involves: (a) peptide coupling of N-Boc-2-allylproline with d-Leu-OBn; (b) OsO₄/NaIO₄ mediated allyl oxidation and intramolecular cyclization to the corresponding cyclic hemiaminals; and (c) NaBH₄ mediated reduction of an intermediate N-acyliminium ion. This synthetic strategy gave significant better results than the previously reported strategies for the synthesis of [4.4]-spirolactam β-turn mimetics.     

Catalytic asymmetric synthesis of ethyl (1R,2S)-dehydrocoronamate

A synthesis of (1R,2S)-dehydrocoronamic acid ethyl ester was developed employing a regio- and enantioselective palladium-catalysed nucleophilic ring-opening of 3,4-epoxy-1-butene with a glycine anion equivalent as the key enantiodifferentiating step. The desired selectivity was achieved using Trost’s naphthyl ligand. The subsequent activation of the free hydroxyl group and ring-closure by intramolecular S_N2 reaction gave the desired amino acid ethyl ester.