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Barbara Zajc

Barbara Zajc

Chemistry

Synthesis & study of fluorinated carcinogen analogs


Email:
Office Phone: (212) 650-8926

Education:

  1. Dipl., M.Sc. and Ph.D. degrees: University of Ljubljana (Slovenia)
  2. Post-doctoral: National Institutes of Health

Biography:

Research

Regioselective fluorination of bioactive molecules. Synthesis and study of fluorinated carcinogen analogs.

Fluorine is a known modulator of biological activity due to a variety of factors; its small van der Waals radius closely resembles that of hydrogen, resulting in little change in steric requirements in the fluoro substituted molecule. On the other hand, its high electronegativity as well as the strength of the carbon-fluorine bond can result in altered biological activity of the fluoro analog. For these reasons fluorine is an attractive substituent for structure-biology studies.

An area we are investigating, through the use of fluorine substitution, is tumor promotion by environmental pollutants at the molecular level. Polycyclic aromatic hydrocarbons (PAHs) constitute an interesting class of compounds for this purpose as these compounds can be fluorinated at various sites, and the net biological result of such substitution can be studied. For example, it has been shown that the 6-fluorobenzo[a]pyrene metabolite 1 shown in Scheme 1 has no tumorigenic activity compared to the protio analog 2 which is a highly active tumorigen. Although diol epoxides such as those shown below are electrophilic and react with DNA, a basic understanding of how the fluorine substitution alters the DNA conformation which in turn influences the biological outcome is presently unknown.



Scheme 1

 

Research is therefore focussed on the following.

 

* Development of methods for regioselective fluorination of polycyclic aromatic hydrocarbons as well as their metabolites. For example, we have shown that 7-hydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene can be selectively fluorinated at position 6 via a unique hydroxyl-directed, regioselective bromination followed by bromine-fluorine exchange. This has been the key to the first synthesis of the 6-fluorobenzo[a]pyrene dihydrodiol, which is a precursor for our studies (Scheme 2).

 



Scheme 2

 

* Synthesis of DNA oligomers containing the protio analogs (Scheme 3). In this respect we have substantial preliminary data on both the chemical and the biological areas.

 



Scheme 3

* Comparisons of DNA oligomers containing the fluoro diol epoxide to those containing the protio analogs. This will involve Circular Dichroic and NMR spectroscopy as well as biological and biochemical experimentation (through collaborations).

* Development of novel strategies for synthesis of other regioselectively fluorinated aromatic hydrocarbons and their putative metabolites.

Chiral recognition studies of PAH derivatives by polysaccharides.

Obtaining enantiopure metabolites of PAH is often critical for our studies. Thus, we have recently investigated the separation of a series of benzo[a]pyrene dihydrodiol analogs by chiral HPLC using polysaccharide-based Daicel columns: Chiralcel CA-I (microcrystalline cellulose triacetate), Chiralcel OG and OF (4-methyl and 4-chlorophenylcarbamate derivatives of cellulose, coated on silica). Our studies show that such polysaccharide based enantioselective columns are extremely efficient for the resolution of chiral metabolites of PAHs. Furthermore, we have evaluated PAH-chiral stationary phase (CSP) interactions on the OG and OF columns by using three close benzo[a]pyrene dihydrodiol analogs shown in Figure 1 that are conformationally different.



Figure 1

We have also carried out comparisons to CSP-1,1'-bi-2-naphthol (Binol) interactions, which have been thoroughly studied by Okamoto et al (J. Am. Chem. Soc. 1996, 118, 4036-4048). Comparison of the enantiomeric resolution of (±)-6-fluoro benzo[a]pyrene dihydrodiol on the OG and OF CSPs to that of (±)-1,1'-bi-2-naphthol revealed that they both show similar trends, i.e. the reversal of elution order of enantiomers. Along with molecular modeling (Figure 2), results indicate possibly similar interactions of (S)-1,1'-bi-2-naphthol and (7S,8S)-isomers of 6-halo-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene with the chiral sorbent.

 


Figure 2

In order to understand better the mechanism(s) in operation for the chiral separation, we plan on investigating a larger number of conformationally restrained PAH dihydrodiols as well as other commercially available polysaccharide based enantioselective columns.

Methodological developments

A significant portion of our research is also directed towards the development of new synthetic methods. One particular area of interest is the fragmentation reactions of ortho esters. For example, we have shown that cis 1,2-amino alcohols can be prepared from cis diols via reaction with 2-acetoxy-2-methylpropionyl chloride (Scheme 4). In this method, an ortho ester is first formed in situ that undergoes fragmentation and trapping by nucleophile. Therefore, such fragmentation chemistry could offer good regio and stereocontrol. We are working towards exploiting this for development of novel structures.



Scheme 4

 

Publications (1995-present)

LeBreton, P. R.; Huang, C-R.; Fernando, H.; Zajc, B.; Lakshman, M. K.; Sayer, J. M.; Jerina, D. M.: Multiple fluorescence lifetimes for oligonucleotides containing single, site-specific modifications at guanine and adenine corresponding to trans addition of exocyclic amino groups to (+)-(7R,8S,9S,10R)- and (-)-(7S,8R,9R,10S)-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a] pyrene, Chem. Res. Toxicol. 1995, 8, 338-348.

Zajc, B.; Lakshman, M. K.: Acyl migration in anions derived from non-enolizable esters of polycyclic aromatic hydrocarbons, J. Org. Chem. 1995, 60, 4936-4939.

Lakshman, M. K.; Zajc, B.: Regio and stereocontrolled synthesis of aryl cis aminoalcohols from cis glycols, Tetrahedron Letters 1996, 37, 2529-2532.

Lakshman, M. K.; Zajc, B.: A rapid, high-yield method for 5'-hydroxyl protection in very reactive and amino group modified nucleosides using dimethoxytrityl tetrafluoroborate, Nucleosides and Nucleotides 1996, 15, 1029-1039.

Page, J. E.; Christner, D. F.; Lakshman, M. K.; Zajc, B.; Oh-hara, T.; Lipinski, L. J.; Ross, H. L.; Agarwal, R.; Szeliga, J.; Yagi, H.; Sayer, J. M.; Jerina, D. M.; Dipple, A.: Effects of polycyclic aromatic hydrocarbon adducts with deoxyguanosine and deoxyadenosine in vivo and in vitro, Polycyclic Aromatic Compounds 1996, 10, 171-178.

Page, J. E.; Zajc, B.; Oh-hara, T.; Lakshman, M. K.; Sayer, J. M.; Jerina, D. M.; Dipple, A.: Sequence context profoundly influences mutagenic potency of trans-opened benzo[a]pyrene 7,8-diol 9,10-epoxide-purine nucleoside adducts in site-specific mutation studies, Biochemistry 1998, 37, 9127-9137.

Lakshman, M. K.; Chaturvedi, S.; Zajc, B.; Gibson, D. T.; Resnick, M.: A general chemoenzymatic synthesis of enantiopure cis ß-aminoalcohols from microbially derived cis glycols, Synthesis 1998, 1352-1356.

Lipinski, L. J.; Ross, H. L.; Zajc, B.; Sayer, J. M.; Jerina, D. M.; Dipple, A.: Effect of single benzo[a]pyrene diol epoxide-deoxyguanosine adducts on the action of DNA polymerases in vitro, Int. J. Oncol. 1998, 13, 269-273.

Custer, L.; Zajc, B.; Sayer, J. M.; Cullinane, C.; Phillips, D. R.; Cheh, A. M.; Jerina, D. M.; Bohr, V. A.; Mazur, S. J.: Stereospecific differences in repair by human cell extracts of synthesized oligonucleotides containing trans-opened 7,8,9,10-tetrahydrobenzo[a]pyrene 7,8-diol 9,10-epoxide N2-dG adduct stereoisomers located within the human K-ras codon 12 sequence, Biochemistry 1999, 38, 569-581.

Zajc, B.: A convenient preparation of N-bromosaccharin, source of electrophilic bromine Synthetic Communications 1999, 29, 1779-1784.

Zajc, B.: Synthesis of (±)-trans-7,8-dihydrodiol of 6-fluoro-benzo[a]pyrene via hydroxyl-directed regioselective functionalization of substituted pyrene, J. Org. Chem. 1999, 64, 1902-1907.

Simhadri, S.; Kramata, P.; Zajc, B.; Sayer, J. M.; Jerina, D. M.; Hinkle, D. C.; Wei, C. S. -J.: Benzo[a]pyrene diol epoxide-deoxyguanosine adducts are accurately bypassed by yeast DNA polymerase zeta in vivo, Mutation Research 2002, 508, 137-145.

Zajc, B.; Grahek, R.; Kocijan, A.; Lakshman, M. K.; Kosmrlj, J.; Lah, J.: Evaluation of the enantiomeric resolution of the proximate carcinogen 7,8-dihydroxy-7,8-dihydro-benzo[a]pyrene, its 6-fluoro and 6-bromo derivatives on polysaccharide columns, J. Org. Chem. 2003, 68, 3291-3294.

Laali, K. K.; Okazaki, T.; Zajc, B.; Lakshman, M. K.; Kumar, S.; Baird, W. M.; Dashwood, W. -M.: Stable ion study of 7,8- and 9,10-dihydrobenzo[a]pyrene (BaP), 6-halo (X = F, Cl, Br)-9,10-dihydro-BaP, 1-methoxy and 3-methoxy-7,8,9,10-tetrahydrobenzo[a]pyren-7-one trans-BaP-dihydrodiol and its O-benzoylated derivative, combined with a comparative DNA binding study of regioisomeric (1,4,2)pyrenyl-CR2OH, Organic and Biomolecular Chemistry 2003, 1, 1509-1516.

Kramata, P.; Zajc, B.; Sayer, J. M.; Jerina, D. M.; Wei, C. S.-J.: A single site-specific trans-opened 7,8,9,10-tetrahydrobenzo[a]pyrene 7,8-diol 9,10-epoxide N2-deoxyguanosine adduct induces mutations at multiple sites in DNA, J. Biol. Chem. 2003, 278, 14940-14948.

 

Courses taught at CCNY

Chem 10301 General Chemistry

Chem 108 (Sophie Davies Medical School) Organic Chemistry

Chem B5200 Spectroscopy and Structural Proof in Organic Chemistry

Chem 261 Organic Chemistry (first semester)

Chem 262 Organic Laboratory

Chem 374 Organic Laboratory