New approaches towards the synthesis of 1,2,3,4-tetrahydro isoquinoline-3-phosphonic acid (TicP).

Two new strategies for the efficient synthesis of racemic 1,2,3,4-tetrahydroisoquinoline-3-phosphonic acid (TicP) (±)-2 have been developed. The first strategy involves the electron-transfer reduction of the easily obtained α,ß-dehydro phosphonophenylalanine followed by a Pictet-Spengler cyclization. The second strategy involves a radical decarboxylation-phosphorylation reaction on 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (Tic). In both strategies, the highly electrophilic N-acyliminium ion is formed as a key intermediate, and the target compound is obtained in good yield using mild reaction conditions and readily available starting materials, complementing existing methodologies and contributing to the easy accessibility of (±)-2 for further research.

Synthesis and biological activity of dehydrophos derivatives.

The synthesis of dehydrophos derivatives featuring modified peptide chains, characterized by the presence of substituents in the vinyl moiety, or possessing a phosphonic acid monoalkyl ester other than the monomethyl ester one, has been accomplished by a versatile procedure based on Horner-Wadsworth-Emmons olefination with suitable aldehydes and on the selective hydrolysis of the dialkyl phosphonate group. Such derivatives have been tested against a series of bacterial strains, using the naturally occurring peptide, dehydrophos, for comparison. Thus, the effects of the aforementioned structural variations on antimicrobial activity have been studied.

Stereoselective Synthesis of α-Amino-C-phosphinic Acids and Derivatives.

α-Amino-C-phosphinic acids and derivatives are an important group of compounds of synthetic and medicinal interest and particular attention has been dedicated to their stereoselective synthesis in recent years. Among these, phosphinic pseudopeptides have acquired pharmacological importance in influencing physiologic and pathologic processes, primarily acting as inhibitors for proteolytic enzymes where molecular stereochemistry has proven to be critical. This review summarizes the latest developments in the asymmetric synthesis of acyclic and phosphacyclic α-amino-C-phosphinic acids and derivatives, following in the first case an order according to the strategy used, whereas for cyclic compounds the nitrogen embedding in the heterocyclic core is considered. In addition selected examples of pharmacological implications of title compounds are also disclosed.

Cyclopalladation and reactivity of amino esters through C-H bond activation: experimental, kinetic, and density functional theory mechanistic studies.

The orthopalladation, through C-H bond activation, of a large number of amino esters and amino phosphonates derived from phenylglycine, and having different substituents at the aryl ring and the C-α atom, as well as on the N-amine atom, has been studied. The experimental observations indicated an improvement in the yields of the orthopalladated compounds when the N-amine and/or the C-α atom are substituted, when compared with the unsubstituted methyl phenylglycinate derivatives. In contrast, substitutions at the aryl ring do not promote significant changes in the orthometalation results. Furthermore, the use of hydrochloride salts of the amino esters has also been shown to have a remarkably favorable effect on the process. All these observations have been fully quantified at different temperatures and pressures by a detailed kinetic study in solution in different solvents and in the presence and absence of added Brønsted acids and chloride anions. The data collected indicate relevant changes in the process depending on these conditions, as expected from the general background known for cyclopalladation reactions. An electronic mechanism of the orthopalladation has been proposed based on DFT calculations at the B3LYP level, and a very good agreement between the trends kinetically measured and the theoretically calculated activation barriers has been obtained. The reactivity of the new orthopalladated amino phosphonate derivatives has been tested and it was found that their halogenation, alkoxylation and carbonylation resulted in formation of the corresponding functionalized ortho-haloaminophosphonates, ortho-alkoxyaminophosphonates and oxoisoindolinylphosphonates.

Engineering strategy to improve peptide analogs: from structure-based computational design to tumor homing.

We present a chemical strategy to engineer analogs of the tumor-homing peptide CREKA (Cys-Arg-Glu-Lys-Ala), which binds to fibrin and fibrin-associated clotted plasma proteins in tumor vessels (Simberg et al. in Proc Natl Acad Sci USA 104:932-936, 2007) with improved ability to inhibit tumor growth. Computer modeling using a combination of simulated annealing and molecular dynamics were carried out to design targeted replacements aimed at enhancing the stability of the bioactive conformation of CREKA. Because this conformation presents a pocket-like shape with the charged groups of Arg, Glu and Lys pointing outward, non-proteinogenic amino acids α-methyl and N-methyl derivatives of Arg, Glu and Lys were selected, rationally designed and incorporated into CREKA analogs. The stabilization of the bioactive conformation predicted by the modeling for the different CREKA analogs matched the tumor fluorescence results, with tumor accumulation increasing with stabilization. Here we report the modeling, synthetic procedures, and new biological assays used to test the efficacy and utility of the analogs. Combined, our results show how studies based on multi-disciplinary collaboration can converge and lead to useful biomedical advances.

Nanoparticle-induced vascular blockade in human prostate cancer.

The tumor-homing pentapeptide CREKA (Cys-Arg-Glu-Lys-Ala) specifically homes to tumors by binding to fibrin and fibrin-associated clotted plasma proteins in tumor vessels. Previous results show that CREKA-coated superparamagnetic iron oxide particles can cause additional clotting in tumor vessels, which creates more binding sites for the peptide. We have used this self-amplifying homing system to develop theranostic nanoparticles that simultaneously serve as an imaging agent and inhibit tumor growth by obstructing tumor circulation through blood clotting. The CREKA nanoparticles were combined with nanoparticles coated with another tumor-homing peptide, CRKDKC, and nanoparticles with an elongated shape (nanoworms) were used for improved binding efficacy. The efficacy of the CREKA peptide was then increased by replacing some residues with nonproteinogenic counterparts, which increased the stability of the peptide in the circulation. Treatment of mice bearing orthotopic human prostate cancer tumors with the targeted nanoworms caused extensive clotting in tumor vessels, whereas no clotting was observed in the vessels of normal tissues. Optical and magnetic resonance imaging confirmed tumor-specific targeting of the nanoworms, and ultrasound imaging showed reduced blood flow in tumor vessels. Treatment of mice with prostate cancer with multiple doses of the nanoworms induced tumor necrosis and a highly significant reduction in tumor growth.

Quantum-chemical predictions of redox potentials of carbamates in methanol.

Redox potentials for two stepwise anodic oxidations of a series of carbamates in methanolic solution have been calculated using ab initio and DFT quantum mechanical methods. Hartree-Fock method and Density Functional Theory at the B3LYP level, together with 6-31G(d), 6-31G(d,p) and 6-311++G(2df,2p) basis sets have been used for the calculation. The Polarizable Continuum Model (PCM) is used to describe the solute-solvent interactions of carbamates, and those of their radical-cations and cations. Linear relationships were observed between the theoretically predicted redox potential values and the corresponding anodic peak potentials obtained by cyclic voltammetry or the corresponding calculated energies of the Highest Occupied Molecular Orbital (HOMO) of these carbamates.

Practical access to the proline analogs (S,S,S)- and (R,R,R)-2-methyloctahydroindole-2-carboxylic acids by HPLC enantioseparation.

An efficient methodology for the preparation of the α-tetrasubstituted proline analog (S,S,S)-2-methyloctahydroindole-2-carboxylic acid, (S,S,S)-(αMe)Oic, and its enantiomer, (R,R,R)-(αMe)Oic, has been developed. Starting from easily available substrates and through simple transformations, a racemic precursor has been synthesized in excellent yield and further subjected to HPLC resolution using a cellulose-derived chiral stationary phase. Specifically, a semipreparative (250 mm × 20 mm ID) Chiralpak® IC column has allowed the efficient resolution of more than 4 g of racemate using a mixture of n-hexane/tert-butyl methyl ether/2-propanol as the eluent. Multigram quantities of the target amino acids have been isolated in enantiomerically pure form and suitably protected for incorporation into peptides.

A straightforward route to enantiopure alpha-substituted derivatives of (2S,3aS,7aS)-octahydroindole-2-carboxylic acid.

High yielding and remarkably selective alkylations of a suitably protected derivative of (2S,3aS,7aS)-octahydroindole-2-carboxylic acid are described. The fused bicyclic structure of this proline analogue greatly influences the stereochemical outcome of direct alkylation reactions taking place at the alpha-carbon and provides access to alpha-substituted analogues with retention of the configuration. The overall procedure allows the preparation of enantiopure alpha-substituted derivatives of this Oic isomer, suitably protected for their incorporation into peptides, in a straightforward manner.

Versatile methodology for the synthesis and α-functionalization of (2R,3aS,7aS)-octahydroindole-2-carboxylic acid.

An improved strategy for the effective synthesis of enantiomerically pure (2R,3aS,7aS)-octahydroindole-2-carboxylic acid (Oic), based on the formation of a trichloromethyloxazolidinone derivative, has been developed. Additionally, the completely diastereoselective α-alkylation of such oxazolidinone provides a very convenient and concise route to enantiopure α-tetrasubstituted derivatives of this Oic stereoisomer.

Amide-triazole isosteric substitution for tuning self-assembly and incorporating new functions into soft supramolecular materials.

The proof-of-concept for the modular synthesis of new functional soft gel materials based on amide-triazole isosteric replacement has been demonstrated. A coassembly approach of isosteric amino acid-based hydrogelators was fruitfully applied for fine-tuning the release of entrapped drugs.

Aldolase-catalyzed synthesis of conformationally constrained iminocyclitols: preparation of polyhydroxylated benzopyrrolizidines and cyclohexapyrrolizidines.

A straightforward chemo-enzymatic synthesis of new polyhydroxylated benzopyrrolizidines and cyclohexapyrrolizidines is developed. The two-step strategy consists of l-fuculose-1-phosphate aldolase variant F131A-catalyzed aldol addition of dihydroxyacetone phosphate to rac-N-benzyloxycarbonylindoline-2-carbaldehyde as well as (2S*,3aS*,7aS*)- and (2S*,3aR*,7aR*)-N-benzyloxycarbonyloctahydroindole-2-carbaldehydes and a subsequent one-step catalytic deprotection-reductive amination.

Dynamic kinetic resolution of 1,3-dihydro-2H-isoindole-1-carboxylic acid methyl ester: asymmetric transformations toward isoindoline carbamates.

Asymmetric syntheses of isoindoline carbamates have been successfully achieved through enzyme-mediated dynamic kinetic resolution processes and without requirement of metal or acid-base catalyst for the substrate racemization. Optically active carbamates were obtained in good yields and an excellent degree of stereoselectivity when Pseudomonas cepacia lipase (PSL) was used as biocatalyst, with diallyl or dibenzyl carbonates being both adequate reagents in alkoxycarbonylation reactions.

Efficient access to enantiomerically pure cyclic alpha-amino esters through a lipase-catalyzed kinetic resolution.

A series of alpha-amino acid derivatives containing the 2,3-dihydroindole or octahydroindole core have been chemoenzymatically synthesized in good overall yields and high enantiomeric purity under mild reaction conditions using lipases for the introduction of chirality. Candida antarctica lipase type A has shown excellent activity and high enantiodiscrimination ability towards the two cyclic amino esters used as substrates. The selectivity of the process proved to be greatly dependent on the alkoxycarbonylating agent. Thus, the enzymatic kinetic resolution of methyl indoline-2-carboxylate has been successfully achieved using 3-methoxyphenyl allyl carbonate, whereas (2R,3aR,7aR)-benzyl octahydroindole-2-carboxylate required the less reactive diallyl carbonate.

Towards the stereoselective synthesis of alpha-methylated (2S,3aS,7aS)-octahydroindole-2-carboxylic acid.

A high yielding and remarkably stereoselective alpha-methylation reaction of the (2S,3aS,7aS) stereoisomer of octahydroindole-2-carboxylic acid, (S,S,S)-Oic, suitably protected is described. The severe steric hindrance imposed by the fused cyclohexane ring, which prevents the application of Seebach's self-reproduction of chirality methodology, accounts for the formation of (S,S,S)-(alphaMe)Oic with high selectivity and retention of configuration.