Enantiomerically pure alpha-amino acid synthesis via hydroboration-suzuki cross-coupling

A straightforward stereoselective synthesis of meso-, (S,S)- and (R,R)-2,6-diaminopimelic acids from cis-1,4-diacetoxycyclohept-2-ene

A straightforward synthesis of meso-2,6-diaminopimelic acid (DAP) meso-1 was developed from 1,4-diacetoxycyclohept-2-ene (2) via an oxidative ring cleavage. Subsequently, an enantio-divergent synthesis of (S,S)- and (R,R)-1 was performed using a homochiral monoacetate 7 available from 2 by enzymatic desymmetrization.

Microwave-assisted RCM for the synthesis of carbocyclic peptides

Microwave irradiation dramatically improves the efficiency of ring closing metathesis (RCM) reactions of resin-attached peptides and the technology is illustrated by the highly selective synthesis of dicarba analogues of alpha-conotoxin IMI.

Organoboron Reagents in the Preparation of Functionalized ?-Amino Acids

Over the past decade, major advances in the preparation and utilization of organoboron reagents have been applied to virtually all areas of organic synthesis. The present review collates recent examples of the use of organoboron reagents in the synthesis of α-amino acids and their derivatives. Aryl- and alkenylboronic acids have been used in the asymmetric synthesis of α-amino acids through conjugate addition to unsaturated amino acids and the Petasis three-component coupling reaction. Additionally, α-amino acid derivatives with organoboron functionality on the side-chain have been prepared and used in metal-catalyzed cross-coupling reactions to prepare cross-linked amino acids and complex cyclic peptide natural products.

Synthesis of ω-(Hetero)arylalkynylated α-Amino Acid by Sonogashira-Type Reactions in Aqueous Media

Mild conditions are described that allow the palladium-catalyzed cross-coupling of C(alpha)-alkynylated glycine with a wide variety of electron-rich and electron-poor aryl and heteroaryl halides in aqueous media.

Controlled Synthesis of (S,S)-2,7-Diaminosuberic Acid: A Method for Regioselective Construction of Dicarba Analogues of Multicystine-Containing Peptides

A method to facilitate regioselective formation of multiple dicarba isosteres of cystine is described. A sequence of ruthenium-catalyzed cross metathesis and rhodium-catalyzed hydrogenation of nonproteinaceous allylglycine derivatives has been developed to achieve high-yielding and unambiguous formation of diaminosuberic acid derivatives. Allylglycine derivatives readily undergo ruthenium-catalyzed metathesis and hydrogenation to yield diaminosuberic acid derivatives in near quantitative yield. Under the same experimental conditions, prenylglycine was found to be inert to both Grubbs' and Wilkinson's catalyzed metathesis and hydrogenation, respectively, but was readily activated for metathesis via cross metathesis with Z-butene. Subsequent cross metathesis of the metathesis-formed crotylglycine derivative, followed by hydrogenation, yielded the second diaminosuberic acid derivative in excellent yield.

Stereoselective total synthesis of cis- and trans-3-hydroxypipecolic acid

[reaction: see text] 3-Hydroxypipecolic acid, a nonproteinogenic cyclic alpha-amino acid, is a common structural moiety found in a large number of natural and synthetic compounds of medicinal significance. Utilizing D-serine as a chiral template, the present research describes efficient and straightforward routes to cis- and trans-3-hydroxypipecolic acids in enantiopure form. The key steps in the syntheses involve chelation-controlled addition of a homoallyl Grignard reagent to a protected serinal derivative toward stereoselective formation of the corresponding syn-amino alcohol adduct 3. On the other hand, zinc borohydride-mediated chelation-controlled reduction of a serine-derived alpha-aminoketone precursor leads to the formation of the corresponding anti-amino alcohol adduct 4 with high stereoselectivity. Following an efficient sequence of reactions, the above amino alcohol derivatives were subsequently transformed to the corresponding cis- and trans- title compounds, respectively.

Palladium-Catalyzed Cross-Coupling Reactions of 2-Iodo-4-(phenylchalcogenyl)-1-butenes

[reaction: see text] 2-Iodo-4-(phenylchalcogenyl)-1-butenes 2 or 3, which are derived from the ring-opening reaction of methylenecyclopropanes (MCPs) 1 by iodine, can be applied to some palladium-catalyzed cross-coupling reactions such as the Sonogashira, Heck, Kumada, Suzuki, and Negishi reactions under mild conditions to give the corresponding coupling products in good yields. These reactions proceeded smoothly at room temperature (20 degrees C) in most cases without any phosphine ligand and additive. The phenylchalcogenyl group plays an important role in the reactions and a plausible reaction mechanism has been proposed.

L-cysteine, a versatile source of sulfenic acids. Synthesis of enantiopure alliin analogues

[reaction: see text] l-Cysteine is a stimulating starting product for the generation of transient sulfenic acids, such as 4, 6, 9, and 15, which add to suitable acceptors, allowing formation of sulfoxides showing a biologically active residue. These sulfoxides are easily isolated in enantiomerically pure form. For instance, N-(tert-butoxycarbonyl)-l-cysteine methyl ester (1a) furnished in few steps sulfenic acid 9a, which was readily converted into (R,S(S))-(2-tert-butoxycarbonylamino-2-methoxycarbonyl-ethylsulfinyl)ethene (22), the methyl ester of Boc-protected nor-alliin. Moreover, the addition of 9a to 2-methyl-1-buten-3-yne has led to a sulfur epimeric and separable mixture of (R)-2-(2-tert-butoxycarbonylamino-2-methoxycarbonyl-ethylsulfinyl)-3-methyl-buta-1,3-dienes 10a and 11a, still possessing a "masked" sulfenic acid function, producible from their cysteine moieties once the dienes have been converted into the desired derivatives.

Facile synthesis of multisubstituted buta-1,3-dienes via Suzuki-Miyaura and Kumada cross-coupling strategy of 2,4-diiodo-buta-1-enes with arylboronic acids and Grignard reagents

One-pot Suzuki-Miyaura-type and Kumada-type cross-coupling reactions of 2,4-diiodo-buta-1-enes with arylboronic acids and alkyl/aryl magnesium bromides were carried out in the presence of accessibly simple catalysts under mild conditions. As a result, some 1,1,2-trisubstituted buta-1,3-dienes were obtained including the Tamoxifen-type, which have potential adjuvant therapy in women who have suffered from breast cancer and cyclooxygenase-2-type (COX-2-type) inhibitors, some of which have been proved to elicit efficient anti-inflammatory analgesic activities and less adverse gastrointestinal side effects and to be very useful in the prophylactic treatment of a wide variety of cancers and neurodegenerative disorders.

Complex Peptidyl Nucleoside Antibiotics: Efficient Syntheses of the Glycosyl Nucleoside Amino Acid Cores

Employing an amino acid chiral template strategy, the present research describes a general and highly efficient protocol for the rapid construction of enantiopure furanosyl and pyranosyl nucleoside amino acid cores as present in various complex peptidyl nucleoside antibiotics. Starting from easily available d-serine, the strategy and the approach involve rapid and efficient stereoselective synthesis of five- or six-membered lactone amino alcohols, followed by incorporation of the required functionalities of the target molecules on these strategically functionalized chiral templates.

Enantioselective synthesis of non-natural aromatic alpha-amino acids

We present two complementary methods for the stereoselective synthesis of non-natural alpha-amino acids with aromatic or heteroaromatic side chains. One approach is based on the chemical transformation of methionine, whereas the other applies the stereoselective Myers alkylation of glycine. The resulting product types differ in the linker length between glycine and the aromatic substituent. Since methionine and pseudoephedrine are available in both absolute configurations, R- or S-configured enantiopure amino acids with either C(2) or C(3) linkers can be obtained on gram scales. In each case the key step of the synthesis is hydroboration of the unsaturated building blocks 9 and 17, followed by palladium-catalyzed Suzuki cross-coupling with aryl halides. Attention must in certain cases be paid to the stereochemical integrity when basic Suzuki conditions are applied. Our initial difficulties are reported as well as the final "racemization-proof" procedures. The protecting groups chosen for the alpha-amino acids should be compatible with solid-phase peptide synthesis. This was confirmed by the successful synthesis of a series of tripeptides.

A Short Diastereoselective Synthesis of Orthogonally Protected Diaminosuccinic Acid Derivatives

Homogeneous, Rh-catalyzed hydrogenation of heteromeric olefinic glycine dimers presents an efficient route to diastereomerically pure, orthogonally protected diaminosuccinic acid derivatives depending on the double bond geometry of the starting material. The products were obtained as racemates.

Straightforward synthesis of sphinganines via a serine-derived Weinreb amide

Sphinganines can be synthesized in just three steps from easily prepared serine-derived Weinreb amide 4. Pre-deprotonation of the acidic (N-H and O-H) protons of 4 allows for its efficient conversion to amino ketones 5. Such ketones can be selectively reduced to either erythro- or threo-sphinganines. Partially protected sphinganines 11 are also readily accessible in five steps from 4. Thus, Weinreb amide 4 represents one of the most versatile templates described to date for sphinganine synthesis.

Stereoselective Synthesis of α- and β-l-C-Fucosyl Aldehydes and Their Utility in the Assembly of C-Fucosides of Biological Relevance

An efficient synthesis of O-benzylated derivatives of the title sugar aldehydes via thiazole addition to tri-O-benzyl-l-fuconolactone followed by highly stereoselective deoxygenation of the resulting thiazolylketose and thiazole to formyl transformation is described. Wittig olefination of these aldehydes with galactopyranose and glucopyranose 6-phosphoranes and reduction of the resulting alkenes afforded alpha- and beta-linked (1-->6)-L-C-fucosyl disaccharides, namely, beta-L-C-Fuc-(1-->6)-alpha-D-Gal, alpha-L-C-Fuc-(1-->6)-alpha-D-Gal, and alpha-L-C-Fuc-(1-->6)-alpha-d-Glc. The alpha-anomer of the above C-fucosyl aldehydes was transformed into a C-fucosylmethyl triphenylphosphonium iodide from which the corresponding C-fucosylmethylene phosphorane was generated upon treatment with BuLi. This phosphorane reacted with the Garner aldehyde (N-Boc D-serinal acetonide) and its one-carbon higher homologue to give alkenes whose reduction and unveiling of the glycinyl group from the oxazolidine ring afforded C-fucosyl alpha-amino acids, namely alpha-L-linked C-fucosyl serines and C-fucosyl asparagines. As a final test of the synthetic utility of the title aldehydes, the beta-anomer was employed as starting material in the stereoselective synthesis of both R- and S-epimer L-C-fucosyl phenylhydroxy acetates. One epimer was obtained by reaction of the sugar aldehyde with phenylmagnesium bromide, oxidation of the resulting alcohol to ketone, addition of 2-lithiothiazole to the latter, and transformation of the thiazole ring into the carboxyl group through an aldehyde intermediate. The other epimer was obtained by the same procedure and inverting the timing of phenyl and thiazolyl group addition. In both routes, the key step establishing the configuration of the quaternary carbon atom of the aliphatic chain was the highly stereoselective addition of the organometal to the ketone intermediate.

An efficient and highly stereocontrolled route to bulgecinine hydrochloride

(-)-Bulgecinine is a nonproteinogenic amino acid component present in bulgecins A, B, and C, antibiotic glycopeptides derived from Pseudomonas acidophila and Pseudomonas mesoacidophila. In combination with beta-lactam antibiotics, bulgecins exihibit a unique synergistic antibacterial activity against various Gram-negative microorganisms. Utilizing d-serine as a chiral template and employing a highly regio- and stereoselective intramolecular amidomercuration-oxidation protocol in the key pyrrolidine ring forming step, an efficient total synthetic route to enantiopure bulgecinine is reported herein.

Highly Functionalized Organolithium Reagents for Enantiomerically Pure α-Amino Acid Synthesis

[reaction: see text] Highly functionalized l-serine-derived organolithium reagents have been generated and reacted with a variety of electrophiles, delivering novel enantiomerically pure adducts. These adducts were then converted into homochiral amino alcohols and novel nonproteinogenic alpha-amino acids, including an aspartic acid mimic that has been synthesized in an enantiomerically pure form for the first time.