Amino acid synthesis via ring opening of N-sulphonyl aziridine-2-carboxylate esters with organometallic reagents

Stereoselective synthesis of S-norvaline and related amino acids through a common intermediate

A divergent, enantioselective synthetic strategy is reported to produce the non-proteinogenic, biologically active natural amino acids norvaline, 5-hydroxy-4-oxo-L-norvaline, and ɣ-oxonorvaline. These were synthesized in good yields (45-75%) from the common starting material (S)-allylglycine obtained by asymmetric transfer allylation of glycine Schiff base using the Corey catalyst derived from cinchonidine in more than 97% enantiomeric excess.

Chemistry and biology of the aminosulfonate cysteinolic acid: discovery, distribution, synthesis and metabolism

D-Cysteinolic acid is a zwitterionic aminosulfonate found in marine (and occasionally freshwater) environments. It is distributed in a wide range of algae (red, green and brown algae and diatoms), and some bacteria and sea animals. It was discovered in 1957 and in spite of its long history, its biosynthesis and degradation is poorly understood. Cysteinolic acid is found conjugated to steroids, lipids and arsenosugars, and the cysteinolic acid motif is found within the structures of various capnoid and sulfoceramide sulfonolipids. This review provides an historical account of the discovery of D-cysteinolic acid and related molecules, its distribution and occurrence within marine and freshwater organisms, routes for its chemical synthesis, and summarizes knowledge and speculations surrounding its biosynthesis, degradation and bioconversions.

Synthesis of the Alkylsulfonate Metabolites Cysteinolic Acid, 3-Amino-2-hydroxypropanesulfonate, and 2,3-Dihydroxypropanesulfonate

Chiral hydroxy- and aminohydroxysulfonic acids are widespread in the marine and terrestrial environment. Here we report simple methods for the synthesis of d- and l-cysteinolic acid (from (Boc-d-Cys-OH)₂ and (Boc-l-Cys-OH)₂, respectively), R- and S-3-amino-2-hydroxypropanesulfonate (from S- and R-epichlorohydrin, respectively), and R- and S-2,3-dihydroxypropanesulfonate (from S- and R-epichlorohydrin, respectively). d-Cysteinolate bile salts were generated by coupling with cholic and chenodeoxycholic acids. A series of single-crystal 3D X-ray structures confirmed the absolute configurations of the aminosulfonates. By comparison of optical rotation, we assign naturally occurring 3-amino-2-hydroxypropanesulfonate from Gateloupia livida as possessing the R-configuration. This simple synthetic approach will support future studies of the occurrence, chemotaxonomic distribution, and metabolism of these alkylsulfonates.

Palladium-Catalyzed Regioselective and Stereospecific Ring-Opening Cross-Coupling of Aziridines: Experimental and Computational Studies

Aziridines, i.e., the smallest saturated N-heterocycles, serve as useful building blocks in synthetic organic chemistry. Because of the release of the large ring strain energy accommodated in the small ring, (ca. 27 kcal/mol), aziridines undergo ring-opening reactions with a variety of nucleophiles. Therefore, among the synthetic reactions utilizing aziridines, regioselective ring-opening substitutions of aziridines with nucleophiles, such as heteroatomic nucleophiles (e.g., amines, alcohols, and thiols) and carbonaceous nucleophiles (e.g., carbanions, organometallic reagents, and electron-rich arenes), constitute a useful synthetic methodology to synthesize biologically relevant β-functionalized alkylamines. However, the regioselection in such traditional ring-opening substitutions of aziridines is highly dependent on the substrate combination, and stereochemical control is challenging to achieve, especially in the case of Lewis acid-promoted variants. Therefore, the development of robust catalytic ring-opening functionalization methods that enable precise prediction of regioselectivity and stereochemistry is desirable. In this direction, our group focused on the highly regioselective and stereospecific nature of the stoichiometric oxidative addition elementary step of 2-substituted aziridines into Pd(0) complexes in an S_N2 fashion. In conjunction with the recent advancements in transition-metal-catalyzed cross-coupling reactions of alkyl pseudohalides containing a C(sp³)-Q (Q = O, N, S, etc.) bond, aziridines can be used as nonclassical alkyl pseudohalides in regioselective and stereospecific cross-couplings.In this Account, starting from the background of transition-metal-catalyzed ring-opening functionalization of aziridines, our contributions to the palladium-catalyzed regioselective and stereoinvertive cross-couplings of aziridines with organoboron reagents to form C(sp³)-C, C(sp³)-B, and C(sp³)-Si bonds have been compiled. The developed methods allow the syntheses of medicinally important amine compounds, e.g., enantioenriched β-phenethylamines, β-amino acids, and their boron and silyl surrogates, from readily available enantiopure aziridine substrates. Notably, the regioselectivity of the ring opening can be switched by appropriate selection of the catalyst (i.e., Pd/NHC vs Pd/PR₃ systems). Computational studies rationalized the detailed mechanisms of the full catalytic cycle and the regioselectivity and stereospecificity of the reactions. The computational results suggested that the interactions operating between the Pd catalyst and aziridine substrate play important roles in determining the regioselection of the aziridine ring-opening event (i.e., oxidative addition). Also, the computational results rationalized the role of water molecules in promoting the transmetalation step through the formation of a Pd-hydroxide active intermediate. This Account evidences the benefits of synergistic collaborations between experimental and computational methods in developing novel transition-metal-catalyzed cross-coupling reactions.

Genetic encoding of 2-aryl-5-carboxytetrazole-based protein photo-cross-linkers

Three γ-heteroatom-substituted N-methylpyrroletetrazole-lysines (mPyTXKs) were synthesized and subsequently incorporated into proteins site-specifically via genetic code expansion. The γ-seleno-substituted derivative, mPyTSeK, showed excellent incorporation efficiency in Escherichia coli and allowed site-selective photo-cross-linking of the GST dimer. Furthermore, the mPyTSeK-cross-linked GST dimer can be cleaved under mild oxidative conditions. The incorporation of mPyTXKs into proteins in mammalian cells was also demonstrated. Lastly, the recombinantly expressed mPyTSeK-encoded Grb2 was shown to covalently capture its interaction partner, EGFR, in mammalian cell lysate, which was subsequently released after treatment with H2O2.

Solid-phase route to Fmoc-protected cationic amino acid building blocks

Diamino acids are commonly found in bioactive compounds, yet only few are commercially available as building blocks for solid-phase peptide synthesis. In the present work a convenient, inexpensive route to multiple-charged amino acid building blocks with varying degree of hydrophobicity was developed. A versatile solid-phase protocol leading to selectively protected amino alcohol intermediates was followed by oxidation to yield the desired di- or polycationic amino acid building blocks in gram-scale amounts. The synthetic sequence comprises loading of (S)-1-(p-nosyl)aziridine-2-methanol onto a freshly prepared trityl bromide resin, followed by ring opening with an appropriate primary amine, on-resin N(β)-Boc protection of the resulting secondary amine, exchange of the N(α)-protecting group, cleavage from the resin, and finally oxidation in solution to yield the target γ-aza substituted building blocks having an Fmoc/Boc protection scheme. This strategy facilitates incorporation of multiple positive charges into the building blocks provided that the corresponding partially protected di- or polyamines are available. An array of compounds covering a wide variety of γ-aza substituted analogs of simple neutral amino acids as well as analogs displaying high bulkiness or polycationic side chains was prepared. Two building blocks were incorporated into peptide sequences using microwave-assisted solid-phase peptide synthesis confirming their general utility.

Ring opening of pymisyl-protected aziridines with organocuprates

The pyrimidine-2-sulfonyl (pymisyl) group is introduced as a new protecting group that can be used to activate aziridines towards ring opening. It is readily introduced and removed under mild conditions. Regioselective ring opening of pymisyl-protected 2-methyl-aziridine with organocuprates gives the corresponding sulfonamides in high yields, and the pymisyl group can subsequently be removed upon treatment with a thiolate. The versatility of this new nitrogen protecting group is illustrated with a new synthesis of Selegiline, a monoamine oxidase-B inhibitor marketed for the treatment of Parkinson's disease.

First synthesis of N-[(aziridin-2-yl)methyl]benzimidazolequinone and analysis of toxicity towards normal and Fanconi anemia cells

A diazole is N-substituted with 1-trityl-2-methylaziridine and demethylated and oxidised with NBS under acidic conditions to give a benzimidazolequinone; this novel anti-tumour agent is marginally more cytotoxic than mitomycin C (MMC) towards the normal human fibroblast cell line GM00637, while the MMC-hypersensitive human Fanconi anaemia (FA) cell line, PD20i, lacking the FANCD2 protein, is also hypersensitive to the benzimidazolequinone, with expression of FANCD2 protein decreasing sensitivity to both MMC and the benzimidazolequinone.

Investigation of a flexible enantiospecific approach to aziridines

A flexible approach to functionalized and enantioenriched aziridines has been developed from an intermediate aziridinylmethyl tosylate. This protocol features a Cu-catalyzed Grignard substitution reaction that allows a range of functionalized organic fragments to be incorporated. Moreover, a convenient one-pot procedure is outlined that allows the trityl group to be exchanged for a range of common N-protecting groups.

A General, Convergent Strategy for the Construction of Indolizidine Alkaloids: Total Syntheses of (−)-Indolizidine 223AB and Alkaloid (−)-205B

N-Toluenesulfonyl aziridines comprise effective second electrophiles in the solvent controlled three-component linchpin union of silyl dithianes for the stereocontrolled convergent elaboration of protected 1,5-amino alcohols. This tactic, in conjunction with a one-flask sequential cyclization, constitutes an effective general strategy for the construction of indolizidine and related alkaloids, illustrated here with the total syntheses of (-)-indolizidine 223AB (1) and alkaloid (-)-205B (2).

Synthesis of α-amino acids by reaction of aziridine-2-carboxylic acids with carbon nucleophiles

A variety of homochiral alpha-amino acids have been prepared in good yield via regioselective reaction of higher order cuprates with (2S)-N-para-toluenesulfonylaziridine-2-carboxylic acid 4. The reaction was much less regioselective and low yielding when higher order cuprates were reacted with the more hindered aziridine carboxylic acid 30, the principal products being protected beta-amino acids. Reaction of lithium trimethylsilylacetylide with the aziridine acid 30, however, gave a protected alpha-amino acid which was converted to the protected isoleucine ester 37.

Total Synthesis of the Neotropical Poison-Frog Alkaloid (−)-205B

[reaction: see text]. A stereocontrolled total synthesis of the neotropical poison-frog alkaloid (-)-205B (1) has been achieved, employing a dithiane three-component linchpin coupling, a one-pot sequential construction of the embedded indolizidine ring, and ring-closing metathesis (RCM) to arrive at the novel 8b-azaacenaphthylene ring system comprising the alkaloid. The synthesis proceeded with a longest linear sequence of 19 steps, affording (-)-1 in 5.6% overall yield.

Synthesis of the Aziridinomitosene Skeleton by Intramolecular Michael Addition: α-Lithioaziridines and Nonaromatic Substrates

The bicyclic pyrrole ketone 16 has been prepared by using an oxaza-Claisen rearrangement, followed by nitrogen deprotection. Coupling with the stannylaziridine mesylate 15a or nosylate 15b affords 17. Conversion to 40 provides a substrate for generation of an alpha-lithioaziridine 41 by tin lithium exchange. An intramolecular Michael addition pathway for 41 has been demonstrated by the isolation of 46 in 20% yield under conditions where the intermediate enolate 43 is trapped by selenenylation, but competing proton transfer gives 42. The synthetic potential of the process is limited by stability problems at the stage of the enolate 43 or the protonated product 44.

?2-amino acids?syntheses, occurrence in natural products, and components of ?-peptides1,2

Although they are less abundant than their alpha-analogues, beta-amino acids occur in nature both in free form and bound to peptides. Oligomers composed exclusively of beta-amino acids (so-called beta-peptides) might be the most thoroughly investigated peptidomimetics. Beside the facts that they are stable to metabolism, exhibit slow microbial degradation, and are inherently stable to proteases and peptidases, they fold into well-ordered secondary structures consisting of helices, turns, and sheets. In this respect, the most intriguing effects have been observed when beta2-amino acids are present in the beta-peptide backbone. This review gives an overview of the occurrence and importance of beta2-amino acids in nature, placing emphasis on the metabolic pathways of beta-aminoisobutyric acid (beta-Aib) and the appearance of beta2-amino acids as secondary metabolites or as components of more complex natural products, such as peptides, depsipeptides, lactones, and alkaloids. In addition, a compilation of the syntheses of both achiral and chiral beta2-amino acids is presented. While there are numerous routes to achiral beta2-amino acids, their EPC synthesis is currently the subject of many investigations. These include the diastereoselective alkylation and Mannich-type reactions of cyclic- or acyclic beta-homoglycine derivatives containing chiral auxiliaries, the Curtius degradation, the employment of transition-metal catalyzed reactions such as enantioselective hydrogenations, reductions, C-H insertions, and Michael-type additions, and the resolution of rac. beta2-amino acids, as well as several miscellaneous methods. In the last part of the review, the importance of beta2-amino acids in the formation of beta-peptide secondary structures is discussed.

Synthetic Enantiopure Aziridinomitosenes: Preparation, Reactivity, and DNA Alkylation Studies

An enantiocontrolled route to aziridinomitosenes had been developed from l-serine methyl ester hydrochloride. The tetracyclic target ring system was assembled by an internal azomethine ylide cycloaddition reaction based on silver ion-assisted intramolecular oxazole alkylation and cyanide-induced ylide generation via a labile oxazoline intermediate (62 to 66). Other key steps include reductive detritylation of 26, methylation of the N-H aziridine 56, oxidation of the sensitive cyclohexenedione 68 to quinone 70, and carbamoylation using Fmoc-NCO. Although the aziridinomitosene tetracycle is sensitive, a range of protecting group manipulations and redox chemistry can be performed if suitable precautions are taken. A study of DNA alkylation by the first C-6,C-7-unsubstituted aziridinomitosene 11a has been carried out, and evidence for DNA cross-link formation involving nucleophilic addition to the quinone subunit is described.

Biphenyls as potent vitronectin receptor antagonists. Part 2: biphenylalanine ureas

Vitronectin receptor (alpha(V)beta(3)) antagonism has been implicated in a variety of disease states, like restenosis, osteoporosis and cancer. In this work, we present the development of a novel class of biphenyl vitronectin receptor antagonists. Identified from a focused combinatorial library based on para-bromo phenylalanine, these compounds show nanomolar affinity to the vitronectin receptor and display unprecedented SAR. Their binding mode can be rationalized by computational docking studies using the X-ray structure of alpha(V)beta(3).

Aziridines: epoxides' ugly cousins?

Aziridines, the nitrogenous analogues of epoxides, have until recently excited far less interest amongst synthetic organic chemists than their oxygenated counterparts, with some justification. A range of reviews concerned with the physical properties, synthesis (asymmetric and otherwise), reactions and utility of aziridines exists; this review briefly summarizes the similarities and differences between oxiranes and their nitrogenated analogues, concentrating on the underlying properties of aziridines and recent developments in their chemistry. In addition to descriptions of the physical nature of aziridines, especially those features which underpin their utility as synthetic intermediates, the sections beneath describe reactions involving alkylative ring-opening and synthesis of aziridines.