Synthesis of New Chiral Peptide Nucleic Acid (PNA) Monomers by a Simplified Reductive Animation Method

Abstract

The aim of this work was the preparation of four new peptide nucleic acid (PNA) monomer backbone by reductive animation of N^α-Boc-protected chiral amino aldehydes, derived from Leu, Phe, Tyr(Bzl), and Thr(Bzl), with methyl glycinate. To the crude 2-substituted methyl N-(2-Boc-aminoethyl)glycinates obtained, thymin-1-ylacetic acid was coupled using TBTU procedure in a one-pot reaction. PNA monomers were isolated and characterized.     

Photochemical synthesis of novel dideoxynucleosides

A series of 2',3'-dideoxynucleosides based on the apiose family was prepared from photochemical ring-expansion of a common cyclobutanone precursor. The starting ketone, (+/-) 3-[2'-(benzoyloxy)ethyl]-2,2-dimethylcyclobutanone (12) was prepared from commercially available (+/-)alpha-pinene. Since the optically pure antipodes of alpha-pinene are also commercially available, these nucleosides can be prepared optically pure using the identical procedure.     

Rapid synthesis of 2-desoxy-2-amino-3-phosphocholine-glycerinic-acid- alkylester, 1-alkyl-1-desoxy- and 1-O-alkyl-2-desoxy-2-amino-sn-glycero-3- phosphocholines, -3-phospho-N,N'-dimethylethanolamine and -3-phospho-Fmoc- serine-methylester.

A convenient sequence for the rapid synthesis of 2-desoxy-2-amino-3-phosphocholine-glycerinic-acid-alkylester , 1-alkyl-1-desoxy- and 1-O-alkyl-2-amino-2-desoxy-3-phospho-derivatives is described. Key steps are the reaction of 1-carbonyloxyalkyl-, 1-alkyl- or 1-O-alkyl-amino-alcohols with phosphorus oxychloride to 1-carbonyloxyalkyl-, 1-alkyl- or 4-substituted 2-chloro-2-oxo-1,3,2-oxazaphospholane followed by nucleophilic displacement with choline tosylate, 1-bromoethane-2-ol or Fmoc-L-serine-methylester and subsequent hydrolysis to 2-amino-lysophospholipids giving the desired compounds in yields ranging between 68% and 81%. Several 2-amino-lysophospholipid analogs can then be prepared by this synthetic scheme utilizing the same oxazaphospholane intermediate. A brief method for the preparation of 2-amino-3-hydroxy-propionic-acid-pentyl- and -octylester from L-serine is described, opening a facile access to chiral precursors of phospholipid analogs.     

Design and synthesis of conformationally frozen peptide nucleic acid backbone: chiral piperidine PNA as a hexitol nucleic acid surrogate

The design and facile synthesis of novel chiral piperidine PNA from naturally occurring 4-hydroxy-L-proline is reported. The stereospecific ring-expansion reaction to get six-membered piperidine derivative from 5-membered pyrrolidine derivative is exploited for this synthesis. The resulting conformationally constrained PNA is utilized for the synthesis of PNA mixmers and the concept is substantiated by UV-Tm studies of the resulting PNA(2):DNA complexes.     

Camptothecin analogs with bulky, hydrophobic substituents at the 7-position via a Grignard reaction

By developing a new synthetic procedure for introduction of side chains onto the camptothecin ring system, we were able to achieve the preparation of a number of analogs bearing bulky, hydrophobic groups directly attached to the 7-position. These include 7-tert-butylcamptothecin, 7-benzylcamptothecin and the corresponding 10,11-methylenedioxycamptothecins. This method involves the reaction of an appropriate orthoaminobenzonitrile with various Grignard reagents to give the corresponding orthoaminoketones. Friedlander condensation of the latter with the key tricyclic ketone leads to 7-substituted camptothecin analogs. We report the activity of these compounds as topoisomerase I poisons and their ability to inhibit growth of selected tumor cell lines.     

Enantioselective synthesis of 2- and 3-substituted 2,3-dihydro[1,4]dioxino[2,3-b]pyridine derivatives and enantiomeric purity control by capillary electrophoresis

A rapid and simple procedure for enantioselective preparation of 2- and 3-substituted 2,3-dihydro[1,4]dioxino[2,3-b]pyridine derivatives (A and B, respectively) is described. The enantiomeric purity of each isomer was determined by capillary electrophoresis using a dual-cyclodextrin system (S-beta-CD/beta-CD) dissolved in formic acid-ammonia buffer (pH 4, ionic strength 50 mM).     

A facile synthesis of 5-(perfluoroalkyl)-pyrimidines.

In the paper a synthetic two stage procedure is described for the preparation of perfluoroalkylated derivatives of uracil and its nucleosides. Using copper bronze a perfluoroalkyl-copper-complex is formed from perfluoralkyl iodides in polar aprotic solvents, such as DMSO, and under inert conditions. The reaction of this complex with uracil, uridine and 2-deoxyuridine leads to the corresponding 5-substituted perfluoralkyl derivatives. It is shown by mass spectra that the substitution always takes place at the 5-position of the pyrimidine. The chemical and physical properties of the formed compounds are described.     

An improved route to the synthetic of diphenyl alpha-(diethoxythiophosphorylamino)methylphosphonates

An improved method for the synthesis of Diphenyl alpha-(diethoxythiophosphorylamino)methylphosphonates under mild conditions is described. It consists of the reaction of diethyl thiophosphoramidate (1) with triphenyl phosphite (3) and a substituted benzylaldehyde or ketone (2) by a one-pot procedure with the aid of acetyl chloride.     

Asymmetric aldol reaction using boron enolates

The protocol for the preparation of boron enolates and their subsequent reaction with aldehydes is described, providing convenient access to beta-hydroxy ketones in good yields and with high stereoselectivities. The reaction consists of three steps: first, the ketone is rapidly converted to the corresponding boron enolate, by exposure to a chlorodialkylborane and tertiary amine base, which is then reacted in situ with the aldehyde. Finally, oxidative workup of the resultant boron aldolate provides aldol adduct. The reaction procedure requires approximately 28 h to complete over a 2-d period, consisting of 5 h to set up the reaction, whereupon the reaction mixture is left at -20 degrees C overnight (16 h), followed by 7 h for workup and purification.     

A concise asymmetric route for the synthesis of a novel class of glucocorticoid mimetics containing a trifluoromethyl-substituted alcohol

An asymmetric route was developed for the synthesis of a class of novel glucocorticoid receptor ligand derivatives 1. The key step of this synthesis involves a diastereoselective addition of chiral sulfoxide anion to a trifluoromethyl ketone precursor. The resulting diastereomers are readily separable and can be converted to the corresponding chiral epoxide and chiral alkyne intermediates (2 and 3). This sequence of reactions is suitable for large-scale preparation of these chiral intermediates and derivatives of 1. The absolute stereochemistry of the biologically active enantiomer of these GR ligands has also been determined.     

Synthesis and absolute configuration assignment of 5-amino-1,3,5-triphenyl-pentane-1,3-diol stereoisomers

Starting from 2,4,6-triphenylpyrylium perchlorate, 5-amino-1,3,5-triphenyl-pentane-1,3-diol stereoisomers 4 were obtained in a simple two-step synthesis: reaction with hydroxylamine, and reduction with LAH of the resulting 2-isoxazoline ketone derivative 2. The eight stereoisomers of 4 were separated in a single shot on a chiral stationary phase cellulose tris(3,5-dimethylphenylcarbamate) (Chiralcel OD-H). The absolute configuration of the title compounds, intermediate 2-isoxazoline ketone 2 and isoxazoline alcohol derivative 3 were determined using a combination of diastereoselective synthesis, affiliation of the sign in chemical interconversion method, and X-ray determination. 2-Isoxazoline ketone 2 enantiomers and isoxazoline alcohol 3 enantiomers were obtained by chiral HPLC on Chiralpak AD column. 2-Isoxazoline ketone 2 enantiomers can be racemized via a retro Michael addition.     

Reduction of 2-Substituted Cyclohexanones by Saccharomyces Cerevisiae

An enzymatic reduction of 2-substituted cyclohexanones mediated by Saccharomyces cerevisiae was studied with respect to the stereochemical course and optical purity of the products. Reduction of ketones 1b-1f resulted in separable diastereoisomeric mixtures of cis- and trans-stereoisomers of 2-substituted cyclohexanols (2b-2f and 3b-3f) having the (S) absolute configuration at the chiral center bearing the hydroxyl functionality with high enantiomeric purity. Reduction of ketone 1a yielded mixture of cis-(1S, 2R)- and trans-(1R, 2R)-stereoisomers (2a and 3a) with lower enantiomeric purity. Changes in the nature of the C(2)-substituent affect the stereochemical course of the biotransformation. However, they significantly influenced the enantiomeric purity of the products. The diastereoselectivity of the process was studied as well; high diastereoselectivity was observed with the substrates 1a, 1e and 1f.     

Microwave-assisted preparation of the quorum-sensing molecule 2-heptyl-3-hydroxy-4(1H)-quinolone and structurally related analogs

An optimized procedure for the efficient preparation of 2-heptyl-3-hydroxy-4(1H)-quinolone (Pseudomonas quinolone signal or PQS) and a diverse range of structurally related 2-alkyl-4-quinolones with biological activity is presented. The two-step synthesis begins with the formation of α-chloro ketones by the coupling of a Weinreb amide (2-chloro-N-methoxy-N-methylacetamide) and an appropriate Grignard reagent. The resulting α-chloro ketones can be reacted with commercially available anthranilic acids under microwave irradiation conditions to furnish the desired 2-alkyl-4-quinolone products. As a typical example, the synthesis of PQS, a molecule involved in quorum sensing in the pathogenic bacterium Pseudomonas aeruginosa, is described in detail. The first step of this process (α-chloro ketone formation) takes ～10 h in total to complete from commercially available bromoheptane and 2-chloro-N-methoxy-N-methylacetamide. The second step (microwave-assisted reaction with anthranilic acid) takes ～14 h in total to complete (the reaction typically proceeds in ～30 min, with work-up and purification requiring ～13 h).     

3-substituted BINOL Schiff bases and their reductive products for catalytic asymmetric addition of diethylzinc to aldehydes

Three new chiral 3-substituted BINOL Schiff bases and their reductive 3-arylaminomethyl BINOL products have been synthesized and used for the asymmetric addition reaction of diethylzinc to aldehydes in the presence of titanium tetraisopropoxide. The reaction provided optically active secondary alcohols in high yield (86-100%) and good enantioselectivity (up to 92% ee).     

Controlling the substrate selectivity of deacetoxycephalosporin/deacetylcephalosporin C synthase

Deacetoxycephalosporin/deacetylcephalosporin C synthase (DAOC/DACS) is an iron(II) and 2-oxoglutarate-dependent oxygenase involved in the biosynthesis of cephalosporin C in Cephalosporium acremonium. It catalyzes two oxidative reactions, oxidative ring-expansion of penicillin N to deacetoxycephalosporin C, and hydroxylation of the latter to give deacetylcephalosporin C. The enzyme is closely related to deacetoxycephalosporin C synthase (DAOCS) and DACS from Streptomyces clavuligerus, which selectively catalyze ring-expansion or hydroxylation reactions, respectively. In this study, structural models based on DAOCS coupled with site-directed mutagenesis were used to identify residues within DAOC/DACS that are responsible for controlling substrate and reaction selectivity. The M306I mutation abolished hydroxylation of deacetylcephalosporin C, whereas the W82A mutant reduced ring-expansion of penicillin G (an "unnatural" substrate). Truncation of the C terminus of DAOC/DACS to residue 310 (Delta310 mutant) enhanced ring-expansion of penicillin G by approximately 2-fold. A double mutant, Delta310/M306I, selectively catalyzed the ring-expansion reaction and had similar kinetic parameters to the wild-type DAOC/DACS. The Delta310/N305L/M306I triple mutant selectively catalyzed ring-expansion of penicillin G and had improved kinetic parameters (K(m) = 2.00 +/- 0.47 compared with 6.02 +/- 0.97 mm for the wild-type enzyme). This work demonstrates that a single amino acid residue side chain within the DAOC/DACS active site can control whether the enzyme catalyzes ring-expansion, hydroxylation, or both reactions. The catalytic efficiency of mutant enzymes can be improved by combining active site mutations with other modifications including C-terminal truncation and modification of Asn-305.     

Synthesis of axially chiral benzamides utilizing tricarbonyl(arene)chromium complexes

Axially chiral N,N-diethyl 2,6-disubstituted benzamides were stereo-selectively prepared utilizing planar chiral (arene)chromium complexes as an enantiomerically active form by following two methods. Ortho-lithiation of the enantiomerically pure planar chiral tricarbonyl(N,N-diethyl 2-methylbenzamide)chromium complex followed by electrophilic quenching gave axially chiral 2-methyl-6-substituted N,N-diethyl benzamide chromium complexes. Photo-oxidative demetalation produced the chromium-free axially chiral benzamides as optically active compounds. An alternative method for the preparation of axial chiral benzamides is an enantioselective lithiation at the benzylic methyl of meso tricarbonyl(N,N-diethyl 2,6-dimethylbenzamide)chromium with appropriate chiral lithium amide base followed by quenching with alkyl halides.     

Reduction of 2-Substituted Cyclohexanones by Saccharomyces Cerevisiae

An enzymatic reduction of 2-substituted cyclohexanones mediated by Saccharomyces cerevisiae was studied with respect to the stereochemical course and optical purity of the products. Reduction of ketones 1b-1f resulted in separable diastereoisomeric mixtures of cis- and trans-stereoisomers of 2-substituted cyclohexanols (2b-2f and 3b-3f) having the (S) absolute configuration at the chiral center bearing the hydroxyl functionality with high enantiomeric purity. Reduction of ketone 1a yielded mixture of cis-(1S, 2R)- and trans-(1R, 2R)-stereoisomers (2a and 3a) with lower enantiomeric purity. Changes in the nature of the C(2)-substituent affect the stereochemical course of the biotransformation. However, they significantly influenced the enantiomeric purity of the products. The diastereoselectivity of the process was studied as well; high diastereoselectivity was observed with the substrates 1a, 1e and 1f.     

Equilibrium position, kinetics, and reactor concepts for the adipyl-7-ADCA-hydrolysis process

One of the building blocks of cephalosporin antibiotics is 7-amino-deacetoxycephalosporanic acid (7-ADCA). It is currently produced from penicillin G using an elaborate chemical ring-expansion step followed by an enzyme-catalyzed hydrolysis. However, 7-ADCA-like components can also be produced by direct fermentation. This is of scientific and economic interest because the elaborate ring-expansion step is performed within the microorganism. In this article, the hydrolysis of the fermentation product adipyl-7-ADCA is studied. Adipyl-7-ADCA can be hydrolyzed in an equilibrium reaction to adipic acid and 7-ADCA using glutaryl-acylase. The equilibrium reaction yield is described as a function of pH, temperature, and initial adipyl-7-ADCA concentration. Reaction rate equations were derived for adipyl-7-ADCA-hydrolysis using three (pH-independent) reaction rate constants and the apparent equilibrium constant. The reaction rate constants were calculated from experimental data. Based on the equilibrium position and reaction rate equations the hydrolysis reaction was optimized and standard reactor configurations were evaluated. It was found that equilibrium yields are high at high pH, high temperature and low-initial adipyl-7-ADCA concentration. The course of the reaction could be described well as a function of pH (7-9), temperature (20-40 degrees C) and concentration using the reaction rate equations. It was shown that a series of CSTR's is the best alternative for the process.     

Asymmetric reduction of prochiral ketones to chiral alcohols catalyzed by plants tissue

As an important organic compound, chiral alcohols are the key chiral building blocks to many single enantiomer pharmaceuticals. Asymmetric reduction of the corresponding prochiral ketones to produce the chiral alcohols by biocatalysis is one of the most promising routes. Asymmetric reduction of different kinds of non-natural prochiral ketones catalyzed by various plants tissue was studied in this work. Acetophenone, 4'-chloroacetophenone and ethyl 4-chloroacetoacetate were chosen as the model substrates for simple ketone, halogen-containing aromatic ketone and beta-ketoesters, respectively. Apple (Malus pumila), carrot (Daucus carota), cucumber (Cucumis sativus), onion (Allium cepa), potato (Soanum tuberosum), radish (Raphanus sativus) and sweet potato (Ipomoea batatas) were chosen as the biocatalysts. It was found that these kinds of prochiral ketoness could be reduced by these plants tissue with high enantioselectivity. Both R- and S-form configuration chiral alcohols could be obtained. The e.e. and chemical yield could reach about 98 and 80% respectively for acetophenone and 4'-chloroacetophenone reduction reaction with favorable plant tissue. And the e.e. and yield for ethyl 4-chloroacetoacetate reduction reaction was about 91 and 45% respectively.     

Structural Revision of the Acetal Intermediates in Brassinolide Synthesis

Acetalization of a steroidal ketone possessing an isopropylidenedioxy moiety in the other part of the molecule by acetal exchange with 2-ethyl-2-methyl-l,3-dioxolane was found to give an acetal with a sec-butylidenedioxy moiety. By this additional exchange reaction, a new chiral center was generated in the sec-butylidenedioxy moiety, and hence an unnecessary stereochemical complexity was added. To circumvent the problem, 2,2-dimethyl-l,3-dioxolane was employed for the acetalization. By adopting this procedure, several intermediates in Mori’s brassinolide synthesis could be obtained in pure and crystalline states. The present improved synthesis furnished 30 g of brassinolide in a 7.0% overall yield from stigmasterol, in contrast to 3.0% by the original procedure.