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PubMed2004

Inhibition of DNA alkylation damage with inorganic salts.

Human exposure to alkylating agents metabolized from tobacco- and food-borne carcinogens occurs regularly. Dietary inorganic compounds such as selenium and vanadium have been shown previously to provide chemoprotective benefits in rat and human trials. Here, we present biochemical data on the ability of inorganic compounds to protect DNA from alkylation damage. An enzyme cleavage assay is used to observe alkylated DNA. Simple salts (e.g., NaCl or NiCl(2)) did not prevent DNA alkylation, whereas anionic oxo species (e.g., Na(2)SeO(4) or Na(3)VO(4)) did inhibit alkylation. We propose that these oxo species behave as nucleophilic targets for the electrophilic alkylating agents, thereby preventing DNA damage.

Alkylation↗
PubMed1995

Alkyl dihydroxyacetone phosphate synthase in glycosomes of Trypanosoma brucei.

Alkyl-dihydroxyacetone phosphate synthase (E.C. 2.5.1.26), the key enzyme in ether phospholipid biosynthesis, was demonstrated to be present in Trypanosoma brucei. The distribution of alkyl-dihydroxyacetone phosphate synthase was found to be identical to that of dihydroxyacetone phosphate acyltransferase (E.C. 2.3.1.42), which has previously been shown to be exclusively associated with the glycosome fraction (Opperdoes, F.R. (1984) FEBS Lett. 169, 35-39). Studies with gradient purified glycosomes indicated that the formation of alkyl-dihydroxyacetone phosphate was completely dependent on the presence of acyl-dihydroxyacetone phosphate. The glycosomal alkyl-dihydroxyacetone phosphate synthase activity was characterized with respect to its pH optimum, Triton X-100 sensitivity and the dependency on the concentration of the substrates palmitoyl-dihydroxyacetone phosphate and hexadecanol. Using thin-layer chromatographic and alkaline hydrolysis procedures the reaction product was identified as alkyl-dihydroxyacetone phosphate. Alkyl-dihydroxyacetone phosphate synthase was resistant to proteolytic inactivation by trypsin in intact glycosomes but not in Triton X-100 disrupted glycosomes. It is concluded that T. brucei glycosomes contain the enzymes responsible for glycero-ether bond formation analogous to mammalian peroxisomes.

Alkyl and Aryl Transferases↗
PubMed1983

Enzymatic methylation of chemically alkylated DNA and poly(dG-dC) X poly(dG-dC) in B and Z forms.

The enzymatic methylation of chemically alkylated DNA and of poly(dG-dC) X poly(dG-dC) by beef brain DNA(cytosine-5-)-methyltransferase have been tested. The alkylation by dimethylsulfate, which yields mostly 7 methylguanine (m7G) and 3 methyladenine (m3A) do not affect the enzymatic methylation. The dimethylsulfate alkylated poly(dG-dC) X poly(dG-dC) converted into the Z-form in the presence of MgCl2, is just as well methylated as the native or the alkylated polynucleotide in the B-form. The alkylation of DNA or of poly(dG-dC) X poly(dG-dC) by methylnitrosourea yields, in addition to the above base modifications described for dimethylsulfate, methylphosphotriesters and O6-methylguanine. The enzymatic methylation of these substrates modified by methylnitrosourea is decreased. This decrease is proportional to the extent of the chemical alkylation of the substrate.

Alkylation↗
PubMed1983

Study of two alkylating derivatives: the p-isothiocyanato- and the p-methylisothiocyanato-clonidine.

Pharmacological experiments with isolated rat aorta and radioligand binding studies in rat cerebral membranes were performed with the p-isothiocyanato (p-NCS) and p-methylisothiocyanato (p-CH2-NCS) derivatives of clonidine in order to assess their selectivity for alpha 1- and alpha 2-adrenoceptors, and to characterize their ability to alkylate alpha-adrenoceptors. Preincubation of rat aortic strips with both derivatives produced non-parallel rightward shifts in the dose-response curves of noradrenaline and significantly depressed the maximum response in a manner characteristic of irreversible receptor antagonists. The p-CH2-NCS derivative was slightly more potent than the p-NCS derivative. Further analysis of the data indicated that treatment of rat aorta with a 30 microM concentration of the p-CH2-NCS derivative alkylated all but 2.4 percent of the alpha-adrenoceptors, whereas a 100 microM concentration of the p-NCS derivative was required to produce a similar degree of alpha-adrenoceptor alkylation. Radioligand binding studies indicate an apparent 2 fold alpha2-adrenoceptor selectivity for the p-NCS derivative. In contrast, the p-CH2-NCS derivative displayed 7 fold selectivity for alpha 1-adrenoceptors. Interestingly, both alkylating derivatives of clonidine produced dose-dependent contractile responses in rat aorta with pD2 values of 6.30 and 5.56 for the p-NCS and p-CH2-NCS derivatives, respectively, relative to a pD2 of 7.67 for clonidine. The order of potency of the two alkylating derivatives of clonidine for producing contraction of rat aorta is the opposite of that for antagonizing the contractile effects of noradrenaline. The results suggest that the p-NCS and p-CH2-NCS derivatives of clonidine non-competitively antagonize noradrenaline by irreversibly alkylating alpha-adrenoceptors.

Alkylating Agents↗
PubMed1992

Alkyl phosphonic acids and sulfonic acids in the Murchison meteorite.

Homologous series of alkyl phosphonic acids and alkyl sulfonic acids, along with inorganic orthophosphate and sulfate, have been identified in water extracts of the Murchison meteorite after conversion to their t-butyl dimethylsilyl (tBDMS) derivatives. The methyl, ethyl, propyl, and butyl compounds have been observed in both series. Five of the eight possible alkyl phosphonic acids and seven of the eight possible alkyl sulfonic acids through C4 have been identified. Abundances decrease with increasing carbon number as observed of other homologous series indigenous to Murchison. Concentrations range downward from approximately 380 nmol/gram in alkyl sulfonic acid series, and from 9 nmol/gram in the alkyl phosphonic acid series. Inorganic phosphate is present at about 25 micromoles/gram.

Alkanesulfonic Acids↗
PubMed1992

Base alterations in yeast induced by alkylating agents with differing Swain-Scott substrate constants.

The base alterations induced by four alkylating agents, methyl methanesulfonate (MMS), ethyl methanesulfonate (EMS), N-nitroso-N-methylurea (MNU), and N-nitroso-N-ethylurea (ENU), have been determined at the URA3 locus in the yeast Saccharomyces cerevisiae. The mutagen treatment was carried out on yeast cells in the logarithmic phase of growth. The mutants were selected by their resistance to 7.3 mM-5-fluoroorotic acid at pH 3.8. DNA sequence analysis was carried out by the dideoxy chain termination method. The alkylating agents were selected for their widely differing Swain-Scott substrate constants (s values), which are as follows: MMS, s = 0.83; EMS, s = 0.67; MNU, s = 0.42; ENU, s = 0.26. A higher s value is correlated with a higher ratio of 7-alkylguanine to O6-alkylguanine in native DNA in vitro. 125 forward mutations from URA3----ura3 were sequenced with marked differences in the mutational spectra being observed as the s value changed. Five hotspots were recorded for the four alkylating agents. They were all G.C----A.T transition mutations. There was one common hotspot for all of them; there were two additional ones for the two ethylating agents (ENU and EMS) and two different ones for MNU. Four of the five hotspots have the 5'-GG-3' sequence with the 3'-guanine mutated. It was seen that MMS, which has the highest Swain-Scott substrate constant, yielded the widest array of mutational types. As the substrate constants decreased, the types of mutations became more and more restricted to the G.C----A.T transitions and the A.T----T.A transversions. The transitions are consistent with the concept that mutations arise from O6-alkylation of guanine and alkylation of thymine. The transversions are consistent with the notion of N1-alkylation of adenosine or adenylic acid.

Alkylating Agents↗
PubMed1990

Affinity alkylation of human placental 3 beta-hydroxy-5-ene-steroid dehydrogenase and steroid 5----4-ene-isomerase by 2 alpha-bromoacetoxyprogesterone: evidence for separate dehydrogenase and isomerase sites on one protein.

We have copurified human placental 3 beta-hydroxy-5-ene-steroid dehydrogenase and steroid 5----4-ene-isomerase, which synthesize progesterone from pregnenolone and androstenedione from fetal dehydroepiandrosterone sulfate, from microsomes as a homogeneous protein based on electrophoretic and NH2-terminal sequencing data. The affinity alkylator, 2 alpha-bromoacetoxyprogesterone, simultaneously inactivates the pregnene and androstene dehydrogenase activities as well as the C21 and C19 isomerase activities in a time-dependent, irreversible manner following first order kinetics. At four concentrations (50/1-20/1 steroid/enzyme M ratios), the alkylator inactivates the dehydrogenase activity (t1/2 = 1.5-3.7 min) 2-fold faster than the isomerase activity. Pregnenolone and dehydroepiandrosterone protect the dehydrogenase activity, while 5-pregnene-3,20-dione, progesterone, and androstenedione protect isomerase activity from inactivation. The protection studies and competitive kinetics of inhibition demonstrate that the affinity alkylator is active site-directed. Kitz and Wilson analyses show that 2 alpha-bromoacetoxyprogesterone inactivates the dehydrogenase activity by a bimolecular mechanism (k3' = 160.9 l/mol.s), while the alkylator inactivates isomerase by a unimolecular mechanism (Ki = 0.14 mM, k3 = 0.013 s-1). Pregnenolone completely protects the dehydrogenase activity but does not slow the rate of isomerase inactivation by 2 alpha-bromoacetoxyprogesterone at all. NADH completely protects both activities from inactivation by the alkylator, while NAD+ protects neither. From Dixon analysis, NADH competitively inhibits NAD+ reduction by dehydrogenase activity. Mixed cofactor studies show that isomerase binds NAD+ and NADH at a common site. Therefore, NADH must not protect either activity by simply binding at the cofactor site. We postulate that NADH binding as an allosteric activator of isomerase protects both the dehydrogenase and isomerase activities from affinity alkylation by inducing a conformational change in the enzyme protein. The human placental enzyme appears to express the pregnene and androstene dehydrogenase activities at one site and the C21 and C19 isomerase activities at a second site on the same protein.

3-Hydroxysteroid Dehydrogenases↗
PubMed2005

Assessment of the combined approach of N-alkylation and salt formation to enhance aqueous solubility of tertiary amines using bupivacaine as a model drug.

Quaternary prodrug types of poorly water-soluble tertiary amines have been shown to exhibit significantly enhanced solubilities as compared to the parent amine. In the present study the combined effect of N-alkylation and salt formation to enhance aqueous solubility of tertiary amines have been investigated using bupivacaine as a model compound. X-ray structure analyses of selected salts were included to investigate the potential existence of correlations between salt solubility and crystal packing modes. Alkyl groups were methyl, ethyl, propyl, and butyl and the derivatives were isolated as their iodide salts. Chloride, mesylate, formate, acetate, glycolate, and tosylate salts were obtained by anion exchange of the N-methyl-bupivacaine derivative. N-Alkylation and salt formation afforded quaternary ammonium salts possessing pH-independent aqueous solubilities far exceeding that of the parent tertiary amine (up to a factor of 3200 at pH 8). A moderate reduction in solubility with increasing length of the alkyl chain was observed for the iodide salts of the N-alkylated bupivacaine derivatives. In case of the N-methyl-bupivacaine derivative variation of the counterion had a significant impact on the solubility with the iodide salt being 200 times less soluble than the chloride salt. X-ray analysis revealed that both the alkyl substituent and the anionic counterion influenced salt packing modes, however, in an unpredictable manner making establishment of quantitative correlations between crystal packing and solubility difficult even for a series of closely related derivatives.

Alkylation↗
PubMed2004

Transglycosylation reaction of xylanase B from the hyperthermophilic Thermotoga maritima with the ability of synthesis of tertiary alkyl beta-D-xylobiosides and xylosides.

The recombinant xylanase B (XynB) of Thermotoga maritima MSB8 was characterized and was found to cleave p-nitrophenyl beta-D-xyloside via the transglycosylation reaction in the previous study. XynB was activated in the presence of alcohols, and XynB activity was increased by iso-propanol (2M) to 2.1-fold. This type of activation was investigated and was shown to be due to the transglycosylation activity with p-nitrophenyl beta-D-xylobioside being converted to alkyl beta-D-xylobiosides in the presence of XynB and alcohols. Through the transglycosylation reaction, alkyl beta-xylosides and xylobiosides were simultaneously produced in the presence of xylan and alcohols. Primary alcohols were found to be the best acceptors. The highest yields of alkyl beta-xylosides and xylobiosides were 33% and 50% of the total sugar, respectively. XynB showed a great ability to transfer xylose and xylobiose to secondary alcohol acceptors, and was unique for being able to synthesize the tertiary alkyl beta-xylosides and xylobiosides with high yields of 18.2% and 11.6% of the total sugar, respectively. This is the first report of a xylanase with the ability to synthesize tertiary alkyl beta-xylosides and xylobiosides. The specificity of the beta-linkage was confirmed by the proton nuclear magnetic resonance ((1)H NMR). Thus, XynB of T. maritima appears to be an ideal enzyme for the synthesis of useful alkyl beta-xylosides and xylobiosides.

Alkylation↗
PubMed2006

Residues of genotoxic alkyl mesylates in mesylate salt drug substances: real or imaginary problems?

Mesylate esters of short-chain (n = 1-3) alcohols are reactive, direct-acting, genotoxic and possibly carcinogenic alkylating agents. Their chemical and biological properties appear to correlate well with Swain-Scott s constants; for example, high S(N)1 character (low s value) is associated with enhanced carcinogenic potential, but also a rapid hydrolysis rate. Concerns over the possible formation of such esters during the preparation of mesylate salt drug substances, by addition of methane sulfonic acid (MSA) to the free base dissolved in an alcoholic solvent, have led regulatory agencies to require applicants to demonstrate that the synthetic method employed does not lead to the presence of detectable levels of alkyl mesylates. Mechanistic considerations, relating mainly to the extremely low nucleophilicity of the mesylate anion, and experimental data, both indicate that alkyl mesylates should not be formed (except from MSA impurities) during mesylate salt synthesis. Mechanistic arguments also predict that residues of alkyl halides (possibly formed in the preparation of amine hydrochlorides or hydrobromides) could represent a similar or greater potential hazard than alkyl mesylates. The perceived risk of alkyl mesylate formation seems to rely on mistaken assumptions and so the concerns appear unjustified. Further reassurance could be achieved however by applying a variety of strategies during synthesis, including pH control, and use of high-purity MSA or of a non-hydroxylic reaction solvent.

Alkylating Agents↗
PubMed1999

Effects of alkylating agents on dopamine D(3) receptors in rat brain: selective protection by dopamine.

Dopamine D(3) receptors are structurally highly homologous to other D(2)-like dopamine receptors, but differ from them pharmacologically. D(3) receptors are notably resistant to alkylation by 1-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), which readily alkylates D(2) receptors. We compared EEDQ with N-(p-isothiocyanatophenethyl)spiperone (NIPS), a selective D(2)-like receptor alkylating agent, for effects on D(3) and D(2) receptors in rat brain using autoradiographic analysis. Neither agent occluded D(3) receptors in vivo at doses that produced substantial blockade of D(2) receptors, even after catecholamine-depleting pretreatments. In vitro, however, D(3) receptors were readily alkylated by both NIPS (IC(50)=40 nM) and EEDQ (IC(50)=12 microM). These effects on D(3) sites were blocked by nM concentrations of dopamine, whereas microM concentrations were required to protect D(2) receptors from the alkylating agents. The findings are consistent with the view that alkylation of D(3) receptors in vivo is prevented by its high affinity for even minor concentrations of endogenous dopamine.

Alkylating Agents↗
PubMed1997

Influence of quinone methide reactivity on the alkylation of thiol and amino groups in proteins: studies utilizing amino acid and peptide models.

Quinone methides (QMs) are electrophiles formed in several biological processes including direct oxidations of 4-alkylphenols by cytochromes P450. These species may be responsible for the adverse effects of certain phenolic compounds through protein alkylation, but little information is available concerning specific targets or the resulting mechanisms of cell injury. The present goal was to determine the most likely sites of adduct formation among competing protein nucleophiles utilizing QMs of varying electrophilicity. Reactions of poorly reactive, moderately reactive, and highly reactive QMs, 2,6-di-tert-butyl-4-methylene-2,5-cyclohexadienone (BHT-QM), 6-tert-butyl-2-(2'-hydroxyl-1',1'-dimethylethyl)-4-methylene- 2,5-cyclohexadienone (BHTOH-QM), and 2-tert-butyl-6-methyl-4-methylene-2,5-cyclohexadienone (BDMP-QM), respectively, were investigated in aqueous solutions with nucleophilic amino acids. Each QM rapidly formed a thioether derivative of cysteine with little or no competition from the addition of water (hydration). The alpha-amino groups were the primary sites of alkylation for all other amino acids examined including lysine, histidine, tyrosine, and serine, and the pseudo-first order rates were 5 to 8-fold greater than the rates of hydration. Alkylation of the side chain nitrogens of lysine and histidine occurred at about one-fourth the rate of hydration for BDMP-QM, but no reaction was detectable for BHT-QM and no reactions occurred between QMs and amino acid hydroxyl groups. The results indicate that, based on chemical reactivity, peptide alkylation should occur in the order cysteine thiol > N-terminal amino > N epsilon-lysine = NIm-histidine, with side chain modifications occurring only with the more electrophilic QMs. Reactions of QMs with the tripeptide Gly-His-Lys confirmed the results with amino acids as N alpha-glycine alkylation predominated, but side chain adducts also formed with BHTOH-QM and BDMP-QM. Human hemoglobin was treated with QMs, hydrolyzed, and assayed by HPLC-thermospray mass spectrometry. This work revealed that N epsilon-lysine was the main alkylation site, emphasizing the importance of factors, in addition to chemical reactivity, which influence protein modification by electrophiles.

Alkylation↗
PubMed2002

Preparation of oxidized glucose-crosslinked N-alkylated chitosan membrane and in vitro studies of pH-sensitive drug delivery behaviour.

Glucose was oxidized to generate a glucose dialdehyde and chitosan (CS) was hydrophobicallly modified with butyl bromide, octyl bromide and dexyl bromide. The analysis of IR and X-ray diffraction results of CS derivatives confirms that the hydrogen bonds and crystallinity were weakened by the incorporation of pendant alkyl. The permeability coefficient P and diffusion coefficient D for model drug vitamin B2 through oxidized glucose-crosslinked alkylated CS membrane were determined under different pH media. The results show that for the same alkylated CS in different pH media, P and D decrease with an increase in pH; for different alkxylated CSs in acidic media, P and D diminish with the increase in the length of alkyl side, which is supposedly originated from the enhancement of hydrophobicity. In alkali medium, P and D show a rising trend with the increase in the length of alkyl chain, which might be related to the loose stacking of network as it occurs to shrink in alkali medium. The preliminary cytotoxicity assay indicates that oxidized glucose-crosslinked alkylate CS membrane is non-toxic in vitro.

Alkylation↗
PubMed1987

Active-site-directed reductive alkylation of xanthine oxidase by imidazo[4,5-g]quinazoline-4,9-diones functionalized with a leaving group.

A new class of purine antimetabolites, directed toward xanthine oxidase, was designed by employing some of the features found in the bioreductive alkylator mitomycin C. The design involved functionalizing the purine-like imidazo[4,5-g]quinazoline ring system as a quinone (4,9-dione) bearing a 2 alpha leaving group. Due to the presence of the electron-deficient quinone ring, the leaving group cannot participate in alkylation reactions. Reduction to the hydroquinone (4,9-dihydroxy) derivative, however, permits elimination of the leaving group to afford an alkylating quinone methide. In spite of the electronic differences, both quinone and hydroquinone derivatives of the imidazo[4,5-g]quinazoline system are able to enter the purine-utilizing active site of the enzyme. Thus, the hypoxanthine-like quinone derivative [2-(bromomethyl)-3-methylimidazo[4,5-g]quinazoline-4,8, 9(3H, 7H)-trione] and its hydroquinone derivative can act as reducing substrates for the enzyme, resulting in conversion to the xanthane-like 6-oxo derivatives. Hydrolysis studies described herein indicate that the hypoxanthine-like hydroquinone derivative eliminates HBr to afford an extended quinone methide species. The observed alkylation of the enzyme by this derivative may thus pertain to quinone methide generation and nucleophile trapping during enzymatic oxidation at the 6-position. Enzymatic studies indicate that the hypoxanthine-like quinone is an oxidizing suicide substrate for the enzyme. Thus, the reduced enzyme transfers electrons to this quinone, and the resulting hydroquinone inactivates the enzyme. As with mitomycin C, reduction and quinone methide formation are necessary for alkylation by the title quinone. This system is therefore an example of a purine active-site-directed reductive alkylator.(ABSTRACT TRUNCATED AT 250 WORDS)

Alkylation↗
PubMed1997

17 alpha (haloacetamidoalkyl) estradiols alkylate the human estrogen receptor at cysteine residues 417 and 530.

Results obtained in a previous study suggested that cysteine residues in the estrogen receptor were covalent attachment sites for four 17 alpha-(haloacetamidoalkyl) estradiols (halo, bromo or iodo; alkyl, methyl, ethyl, or propyl). To identify the putative concerned cysteines, we expressed wild-type and various cysteine --> alanine mutants of the human estrogen receptor in COS cells and determined their ability to be alkylated by the four electrophiles. The quadruple mutant, in which all the cysteines (residues 381, 417, 447, and 530) of the hormone-binding site were changed to alanines, showed very little electrophile labeling, whereas the four single mutants (C381A, C417A, C447A, and C530A) were alkylated as efficiently as the wild-type receptor. These results (i) demonstrate that cysteine residues were covalent attachment sites of electrophiles and (ii) indicate that more than one cysteine residue could be alkylated. Analysis of three double mutants (C381A/C530A, C417A/C530A, and C447A/C530A) provided strong evidence that only C417 and C530 were sites for electrophile covalent attachment. Since C530 was also alkylated by tamoxifen aziridine, a nonsteroidal affinity-labeling agent, we propose a selective mode of superimposition of tamoxifen-class antiestrogens with estradiol, which could account for the relative positioning of the two types of ligands in the receptor hormone-binding pocket. According to the structure of the hormone-binding pocket of nuclear receptors, as inferred from crystallographic studies and general sequence alignment of hormone-binding domains, C417 and C530 appear to be (1) located at the extreme border or in structural elements involved in delineation of the hormone-binding pocket, (2) spatially in close proximity to each other, and (3) in positions highly homologous to those of glucocorticoid receptor sites alkylated by affinity- and photoaffinity-labeling agents, respectively.

Affinity Labels↗
PubMed2003

Small molecules that mimic the thiol-triggered alkylating properties seen in the natural product leinamycin.

Reaction of the antitumor agent leinamycin with cellular thiols results in conversion of the natural product to a DNA-alkylating episulfonium alkylating agent via an intriguing sequence of chemical reactions. To establish whether the chemistry first seen in leinamycin represents a general motif that can function in various molecular frameworks, construction of greatly simplified analogues containing only the "core" funcional groups anticipated to be necessary for thiol-triggered generation of an alkylating agent was undertaken. For this purpose, the "stripped-down" leinamycin analogue 7-(3-methyl-but-2-enyl)-1-oxo-1H-lambda4-benzo[1,2]dithiol-3-one (4) was synthesized. Treatment of 4 with thiol under several different conditions results in efficient conversion of the compound to cyclized 2,3-dihydro-benzo[b]thiophene-7-carboxylic acid products (13) that are envisioned to arise from Markovnikov addition of solvent to an intermediate episulfonium ion (14). Thus, the relatively simple molecule 4 is able to mimic the thiol-triggered alkylating properties displayed by the natural product leinamycin. This work helps define why the core functional groups required thiol-accelerated generation of an alkylating intermediate from leinamycin and indicates that substantially altered analogues of the natural product may retain alkylating properties. In a broader context, the results provide evidence that the unique cascade of chemical reactions first seen in the context of leinamycin represents a general motif that can operate in a variety of molecular frameworks.

Alkylating Agents↗
PubMed2000

Engineering acyclic stereocontrol in the alkylation of vinylglycine-derived dianions: asymmetric synthesis of higher alpha-vinyl amino acids.

A generalizable synthesis of higher L-alpha-vinyl amino acids is presented. The strategy pursued here involves the introduction of the amino acid side chain via the alkylation of a chiral, vinylglycine-derived dianionic dienolate, bearing the (-)-8-(beta-naphthyl)menthyl (d'Angelo) auxiliary. A model is presented that postulates a favored "exo-entended" conformation for this dienolate, leading to C(alpha)-alkylation at the si face. The model invokes internal amidate chelation to control ester enolate geometry and soft-soft interactions between the polarizable beta-naphthyl ring of the auxiliary and the extended pi-system of the dienolate to shield the re face. Heats of formation for four conformers of this dianion were calculated for their semiempirical optimized geometries (PM3). The results support the notion that in these vinylglycine-derived dianionic dienolates, "exo" conformations are considerable lower in energy than their "endo" counterparts, with the "exo-entended" conformation being most favorable. In fact, the d'Angelo auxiliary gives a greater degree of acyclic stereocontrol in this system when compared with the (-)-8-phenylmenthyl (Corey) and trans-2-(beta-naphthyl)cyclohexyl auxiliaries, using isobutyl iodide and benzyl bromide as model electrophiles. These dianions are generated from the corresponding dehydrobutyrine esters via sequential deprotonation with LDA and n-BuLi (2 equiv). When alkylations are carried out at -78 degrees C in THF-HMPA, they proceed in 65-81% yields, with both regiocontrol (deconjugative alpha-alkylation is preferred over gamma-alkylation) and a great degree of acyclic stereocontrol [91:9 to >/=98:2 diastereomeric ratios (10 examples)]. The auxiliary may be recovered in high yield (generally 90%) using a modification of Gassman's "anhydrous hydroxide" conditions, in which considerably higher temperatures are employed. Among the side chains introduced directly are those of butyrine, leucine, ornithine, phenylalanine, aspartate, valine, and norvaline. The lysine side chain is elaborated via a 4-step sequence from the alkylation product obtained with 1-chloro-4-iodobutane as electrophile. Importantly, to our knowledge, this work represents the first asymmetric synthesis of L-alpha-vinyl analogues of m-tyrosine, ornithine, and lysine, known time-dependent inhibitors for amino acid decarboxylases.

Alkylation↗
PubMedDate not supplied

Alkylation of DNA by 1,3-dialkyl-3-acyltriazenes: correlation of biological activity with chemical behavior.

The reactions of calf thymus DNA with four 1,3-dialkyl-3-acyltriazenes were studied alone or in the presence of pig liver esterase in pH 7.4 phosphate buffer for varying lengths of time. The best alkylating agent in the absence of esterase was determined to be 1,3-dimethyl-3-carbethoxytriazene (DMC), followed in order by 1-(2-hydroxyethyl)-3-methyl-3-carbethoxytriazene (HMC), 1-(2-hydroxyethyl)-3-methyl-3-acetyltriazene (HMA), and 1-(2- chloroethyl)-3-methyl-3-carbethoxytriazene (CMC). This order is the same as that for the rate of decomposition of the various acyltriazenes in pH 7.5 phosphate buffer. The extent of calf thymus DNA alkylation by CMC was found to be dependent on both the reaction buffer and the ionic strength of the medium. Alkylation by CMC alone in low ionic strength glycine buffer produced large quantities of 7-(2-chloroethyl)guanine and 7-(2-hydroxyethyl)guanine. The products of DNA alkylation observed at neutral pH are consistent with N(2)-N(3) heterolysis of the triazene, resulting in the N(1) alkyldiazonium ion as the sole alkylating species. In the presence of esterase, CMC showed an enhanced rate of product formation. Furthermore, the product distribution shifted dramatically from mainly hydroxyethylation to predominantly methylation. CMC is postulated to undergo initial enzymatic deacylation, leading to two different alkyldiazonium ions which competitively alkylate DNA. HMC, on the other hand, was little affected by the esterase. The enzyme-catalyzed reaction showed a small increase in methylation and a smaller decrease in hydroxyethylation.(ABSTRACT TRUNCATED AT 250 WORDS)

Alkylating Agents↗