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PubMed2006

Photogeneration and reactivity of 1,n-diphenyl-1,n-azabiradicals.

The 1,5-diphenyl-1,5-azapentanediyl biradical Ia was generated by photolysis of 1,2-diphenylazacyclopentane (pyrrolidine 1a). Among the reaction pathways followed by Ia, C-N bond reformation with ring closure was found to be the predominating process, as determined by separate irradiation of either of the pure enantiomers of 1a. Disproportionation was a minor process and took place only via H abstraction by the C5 benzylic radical. Another minor pathway was C5-aryl coupling, with formation of 5-phenyl-2,3,4,5-tetrahydro-1H-benzo[b]azepine (4a), which is equivalent to photo-Claisen rearrangement of 1a. Likewise, the 1,4-diphenyl-1,4-azabutanediyl biradical Ib was generated by photolysis of 1,2-diphenylazacyclobutane (azetidine 1b). This species underwent predominating C2-C3 cleavage, as indicated by the extensive styrene formation. Although N1-C4 bond reformation also took place, this is not the major pathway occurring from Ib. Besides, C4-aryl coupling to give 4-phenyl-1,2,3,4-tetrahydroquinoline (4b) was also observed. All the possible reaction pathways were theoretically studied at the UB3LYP/6-31G computational level; the results were found to be in good agreement with the experimental observations.

Journal Article↗
PubMed2006

The profound effect of fluorine substitution on the reactivity and regioselectivity of nucleophilic substitution reactions of strained heterocycles. A study of aziridine and its derivatives.

Unlike the synthetically exploited oxiranes and thiiranes, aziridines that lack electron-withdrawing substituents, such as acyl or sulfonyl functionalities at nitrogen, are rather unreactive. As expected, three-membered aziridine 6 was calculated to be significantly more reactive than azetidine 7 in nucleophilic cleavage by ammonia, a typical nucleophile. The reactivity of 7 was about the same as that of an acyclic model compound, 8, when release of ring strain in the transition state was taken into account. Fluorine due to its similar size but vastly different electronegativity has been substituted for hydrogen as a means of modifying chemical properties for varied applications. In the present investigation, the effect of fluorine substitution at aziridine positions other than nitrogen was studied. Computations at the MP2(Full)/6-311++G(d,p)//MP2(Full)/6-31+G(d) level found a vast preference for attack by ammonia at the 3-position of 2-fluoroaziridine in the gas phase at 298 K. When release of ring strain was taken into account, this compound reacted more than 10(11) times faster than 6. The reaction rate with trans-2,3-difluoroaziridine was about twice that of 2-fluoroaziridine, while its diastereomer reacted with ammonia considerably slower. Acyclic fluorinated amine model compounds were employed to assess the generality of the effects produced by fluorine substitution. The results were rationalized by the energy contributions of strain energy releases, stabilization of the leaving group, and the relative electrostatic energies of the heterocycles in the transition states. The more reactive fluoroaziridines underwent nucleophilic attack at rates comparable to those of N-acetylaziridine.

Fluorine↗
PubMed2006

Aqueous phosphoric acid as a mild reagent for deprotection of tert-butyl carbamates, esters, and ethers.

Aqueous phosphoric acid (85 wt %) is an effective, environmentally benign reagent for the deprotection of tert-butyl carbamates, tert-butyl esters, and tert-butyl ethers. The reaction conditions are mild and offer good selectivity in the presence of other acid-sensitive groups, including CBZ carbamates, azetidine, benzyl and methyl esters, TBDMS, and methyl phenyl ethers. The mildness of the reaction is further demonstrated in the synthesis of clarithromycin derivative, in which a tert-butyl ester is removed in the presence of cyclic carbamate, lactone, ketal, acetate ester, and epimerizable methyl ketone functionalities. The reaction preserves the stereochemical integrity of the substrates. The reactions are high yielding, and the workup is convenient.

Carbamates↗
PubMed1996

Synthesis of 4,4-Disubstituted beta-Lactams by Regiospecific Electrophile- and Silver-Induced Ring Expansion of 2,2-Disubstituted 1-Methoxycyclopropylamines.

2,2-Disubstituted 1-methoxycyclopropylamines underwent regiospecific ring expansion to 1,4,4-trisubstituted 2-azetidinones by N-chlorination with tert-butyl hypochlorite and subsequent rearrangement with silver tetrafluoroborate. Upon thermolysis, 4,4-disubstituted beta-lactams suffer a characteristic ring opening to afford beta,gamma-unsaturated carboxylic amides. The reduction of 1,4,4-trisubstituted 2-azetidinones with lithium aluminum hydride afforded 1,2,2-trisubstituted azetidines.

Journal Article↗
PubMed1997

An Approach to Clavams and 1-Oxacephams from Hydroxy Acids.

[2 + 2] Cycloaddition of chlorosulfonyl isocyanate to chiral alkyl vinyl ethers bearing a sterogenic center in the alkyl part of the ether afford the corresponding azetidin-2-ones with relatively good asymmetric induction in certain cases. Reactions were shown to depend on steric requirements of the ligands at the stereogenic center. The model that rationalizes the stereochemical outcome is based on the s-cis conformation of the vinyl ether in which the bulkiest of the ligands is situated in the plane of the double bond, and the next most demanding substituent is placed gauche to the double bond.

Journal Article↗
PubMed2006

The special five-membered ring of proline: An experimental and theoretical investigation of alkali metal cation interactions with proline and its four- and six-membered ring analogues.

The interaction of the alkali metal cations, Li+, Na+, and K+, with the amino acid proline (Pro) and its four- and six-membered ring analogues, azetidine-2-carboxylic acid (Aze) and pipecolic acid (Pip), are examined in detail. Experimentally, threshold collision-induced dissociation of the M+(L) complexes, where M = Li, Na, and K and L = Pro, Aze, and Pip, with Xe are studied using a guided ion beam tandem mass spectrometer. From analysis of the kinetic energy dependent cross sections, M(+)-L bond dissociation energies are measured. These analyses account for unimolecular decay rates, internal energy of reactant ions, and multiple ion-molecule collisions. Ab initio calculations for a number of geometric conformations of the M+(L) complexes were determined at the B3LYP/6-311G(d,p) level with single-point energies calculated at MP2(full), B3LYP, and B3P86 levels using a 6-311+G(2d,2p) basis set. Theoretical bond energies show good agreement with the experimental bond energies, which establishes that the zwitterionic form of the alkali metal cation/amino acid, the lowest energy conformation, is formed in all cases. Despite the increased conformational mobility in the Pip systems, the Li+, Na+, and K+ complexes of Pro show higher binding energies. A meticulous examination of the zwitterionic structures of these complexes provides an explanation for the stability of the five-membered ring complexes.

Cations↗
PubMed2004

(+)- and (-)-syn-2-isobutyl-4-methylazetidine-2,4-dicarboxylic acids from the extract of Monascus pilosus-fermented rice (red-mold rice).

The structures of two enantiomeric azetidine-type amino acids isolated from the n-butanol-soluble fraction of the 70% ethanol extract of red-mold rice fermented with Monascus pilosus were established to be (+)- [1; (+)-monascumic acid] and (-)-syn-2-isobutyl-4-methylazetidine-2,4-dicarboxylic acids [2; (-)-monascumic acid] based on spectroscopic methods.

Dicarboxylic Acids↗
PubMed2002

In search of high stereocontrol for the construction of cis-disubstituted cyclopropane compounds. Total synthesis of a cyclopropane-configured urea-PETT analogue that is a HIV-1 reverse transcriptase inhibitor.

[reaction: see text] A new azetidine-ligated dirhodium(II) catalyst that possesses a l-menthyl ester attachment provides significant diastereocontrol and high enantiocontrol for the formation of cis-cyclopropane products from reactions of substituted styrenes with diazo esters.

Cyclopropanes↗
PubMed2005

Highly diastereoselective allylic azide formation and isomerization. Synthesis of 3(2'-amino)-beta-lactams.

The stereoselective anti SN2' attack of NaN3 to 3-alkenyl-3-bromo-azetidin-2-ones gave a mixture of diastereomeric azides in fast equilibrium. The [3,3]-sigmatropic rearrangement of allylic azides occurred with complete stereocontrol, allowing the equilibrium to be directed preferentially toward the (E)- or (Z)-isomer, useful precursors of 3(2'-amino)-beta-lactams. [reaction: see text]

Allyl Compounds↗
PubMed2006

Decarboxylative ring contractions and olefin insertions of vinyl oxazinanones.

[Structure: see text] 6-Vinyl oxazinanones undergo catalytic, diastereoselective, decarboxylative ring contraction to form vinyl azetidines in good yield. Performing the decarboxylation in the presence of Michael acceptors results in decarboxylative olefin insertion to provide diastereoenriched substituted vinyl piperidines.

Journal Article↗
PubMed2003

Novel function of ascorbic acid as an angiostatic factor.

Endothelial permeability is increased by vascular endothelial cell growth factor and decreased by antioxidants. Whether or not l-ascorbic acid (Asc), which decreases endothelial permeability by stimulating the endothelial barrier function, is anti-angiogenic (angiostatic) remains unknown. We examined the role of Asc on angiogenesis using two assay systems. At first, the potential role of Asc on four steps of angiogenesis was investigated in cultured bovine microvascular endothelial cells. Asc inhibited the formation of vessel-like tubular structures of endothelial cells cultured on Matrigel; however, it did not decrease the activity of plasminogen activator (PA), which creates the space into which vascular vessels extend. Furthermore, even at high concentrations, Asc did not inhibit either the proliferation or migration of endothelial cell cultures. Secondly, whether Asc inhibited in vivo angiogenesis or not was studied on chick chorioallantoic membrane (CAM) during the 4-6 days of embryogenesis when neovascularization is rapid. It also revealed that angiogenesis was dose-dependently inhibited by Asc from 0.5 micro mol/CAM with half-maximal inhibition at 2.5 micro mol/CAM. Because it was previously reported that the endothelial barrier function decreases permeability via the stimulation of collagen synthesis induced by Asc, we treated CAM with the inhibitor of collagen synthesis, l-azetidine 2-carboxylic acid (AzC). This compound partially attenuated the angiostatic function of Asc on CAM. To understand the involvement of an antioxidant activity in the angiostatic function of Asc, we further examined the effect of glutathione (GSH), which is an endogenous antioxidant, on angiogenesis in CAM and endothelial cells. GSH inhibited CAM angiogenesis, as well as the formation of vessel-like tubular structures of endothelial cell cultures on Matrigel. Both Asc and GSH inhibited hydrogen peroxide (H(2)O(2)) induced tubular morphogenesis. These findings suggest that Asc affects angiogenesis through both its antioxidant properties and the stimulation of collagen synthesis. As the angiostatic activity of Asc may be one of the many effects involved in host resistance to the growth or invasiveness of solid cancer, it may be useful as a supplementary therapy in various angiogenic diseases.

Angiogenesis Inhibitors↗
PubMed2002

Differential expression of two stress-inducible hsp70 genes by various stressors.

Inducible HSP70s encoded by two almost identical hsp70 genes, known as hsp70.1 and hsp70.3 are located in a tandem array on mouse chromosome 17. Duplication of this gene has been found in various higher organisms. However, the role of two almost identical hsp70 genes is still unclear. To elucidate the ambiguity of expression pattern between hsp70.1 and hsp70.3, we investigated the induction of each hsp70 isoforms by several HSP70-inducible stressors in wild type (hsp70.1+/+ and hsp70.3+/+) and the hsp70.1-/- (hsp70.1-/- and hsp70.3+/+) murine embryo fibroblast (MEF) cells, and the M-1 mouse cortical collecting duct cell line. Each induced hsp70 isoforms by heat shock were very similar at transcriptional and translational levels in wild type and hsp70.1-/- MEF cells. The mRNA stabilities of both hsp70.1 and hsp70.3, even in two kinds of hsp70.3 transcripts, were also very similar. L-azetidine-2-carboxylic acid, sodium arsenite, CdCl2 and ZnCI2 caused induction of both isoforms of the hsp70 genes, even though their expression levels were variable. NaCl caused induction of just hsp70.1 gene expression. H2O2 failed to induce the expression of either hsp70 genes in MEF cells, caused induction of hsp70.1 gene expression in the M-1 cell line. Hsp70 accumulation by H2O2 and NaCl treatment was mainly due to hsp70.1 expression. Our studies demonstrated that two hsp70 genes respond differentially to types of stress.

Animals↗
PubMed2006

Syntheses and pharmacokinetic studies of prodrug esters for the development of oral carbapenem, L-084.

We discovered an orally active carbapenem, L-084, through pharmacokinetic studies on various prodrug esters of (1R,5S,6S)-6-[(R)-1-hydroxyethyl]-l-methyl-2-[1-(1,3-thiazolin-2-yl)azetidin-3-yl]thio-1-carbapen-2-em-3-carboxylic acid (LJC11,036). L-084 showed a strong antimicrobial activity against Gram-positive and Gram-negative bacteria and exhibited the highest intestinal absorption among synthesized prodrugs of LJC11,036.

Administration, Oral↗
PubMed2005

Substrate specificity and functional characterisation of the H+/amino acid transporter rat PAT2 (Slc36a2).

Functional characteristics and substrate specificity of the rat proton-coupled amino acid transporter 2 (rat PAT2 (rPAT2)) were determined following expression in Xenopus laevis oocytes using radiolabelled uptake measurements, competition experiments and measurements of substrate-evoked current using the two-electrode voltage-clamp technique. The aim of the investigation was to determine the structural requirements and structural limitations of potential substrates for rPAT2. Amino (and imino) acid transport via rPAT2 was pH-dependent, Na(+)-independent and electrogenic. At extracellular pH 5.5 (in Na(+)-free conditions) proline uptake was saturable (Km 172+/-41 muM), demonstrating that rPAT2 is, relative to PAT1, a high-affinity transporter.PAT2 preferred substrates are L-alpha-amino acids with small aliphatic side chains (e.g. the methyl group in alanine) and 4- or 5-membered heterocyclic amino and imino acids such as 2-azetidine-carboxylate, proline and cycloserine, where both D- and L-enantiomers are transported. The major restrictions on transport are side chain size (the ethyl group of alpha-aminobutyric acid is too large) and backbone length, where the separation of the carboxyl and amino groups by only two CH(2) groups, as in beta-alanine, is enough to reduce transport. Methylation of the amino group is tolerated (e.g. sarcosine) but increasing methylation, as in betaine, decreases transport. A free carboxyl group is preferred as O-methyl esters show either reduced transport (alanine-O-methyl ester) or are excluded. The structural characteristics that determine the substrate specificity of rPAT2 have been identified. This information should prove valuable in the design of selective substrates/inhibitors for PAT1 and PAT2.

Amino Acid Transport Systems↗
PubMed2002

Three component coupling reactions of N-acetyl-2-azetine- rapid stereoselective entry to 2,3,4-trisubstituted tetrahydroquinolines.

N-Acetyl-2-azetine undergoes Lewis acid catalysed [4 + 2]-cycloaddition with imines derived from aromatic amines and gave a 1:1 mixture of exo-endo diastereoisomeric azetidine cycloadducts which reacted further with aromatic amine, to give 2,3,4-trisubsitituted tetrahydroquinolines in good to excellent yield, predominantly as one diastereoisomer.

Journal Article↗
PubMed2004

Preparation of beta-and gamma-lactams via ring closures of unsaturated carbamoyl radicals derived from 1-carbamoyl-1-methylcyclohexa-2,5-dienes.

1-Carbamoyl-1-methylcyclohexa-2,5-dienes produced the corresponding delocalised 1-carbamoyl-1-methylcyclohexa-2,5-dienyl radicals on treatment with radical initiators. At temperatures above ca. 300 K dissociation to produce toluene and aminoacyl (carbamoyl) radicals took place. The alternative dissociation of the 1-carbamoyl-1-methylcyclohexa-2,5-dienyl radicals to release methyl radicals and an aromatic amide did not compete. Aminoacyl radicals with allyl, butenyl or similar side chains underwent cyclisations. Moderate yields of N-benzyl-azetidin-2-ones and N-benzyl-pyrrolidin-2-ones were isolated for a range of substituents. The main by-products were N-benzyl-N-alkenylformamides. Ring closure did not take place to a significant extent for precursors with alk-2-ynyl or 2-cyanoalkyl side chains. An improved yield of 1,3-dibenzylazetidin-2-one was obtained by use of lauroyl peroxide as initiator and by inclusion of methyl thioglycolate as polarity reversal catalyst.

Journal Article↗
PubMed2004

Preparation of beta- and gamma-lactams from carbamoyl radicals derived from oxime oxalate amides.

A general synthetic route to oxime oxalate amides was developed and applied to the preparation of molecules incorporating N-benzyl-N-alkenyl amides linked with acetone oxime or benzaldoxime units. In addition, 2-substituted-thiazolidine-4-carboxylic acid methyl ester amides of oxalyl benzaldoxime were also prepared. It was shown by EPR spectroscopy that the oxalyl benzaldoxime amides dissociated to produce benziminyl and carbamoyl (aminoacyl) radicals when photolysed with 4-methoxyacetophenone as a photosensitizer. Carbamoyl radicals derived from N-alk-3-enyl oxime oxalate amides underwent ring closure to afford pyrrolidin-2-ones. The analogous N-alk-2-enyl precursors afforded azetidin-2-ones. Reactions of the cyclohexenyl and cinnamyl oxime oxalate amides afforded a bicyclic beta-lactam and a 3-benzyl-substituted beta-lactam respectively. Interestingly, both products were isolated as hydroxylated compounds. A thiazolidine-derived oxime oxalate amide containing an isobutenyl side chain also dissociated with production of the corresponding thiazolidinyl-carbamoyl radical, as shown by EPR spectroscopy. GC-MS evidence indicated that this radical cyclised to afford some of the corresponding penicillin derivative

Amides↗