Diversely C8-functionalized adenine nucleosides via their underexplored carboxaldehydes.

The potentially versatile N-unprotected 8-formyl derivatives of adenosine and 2'-deoxyadenosine are highly underexploited for C8 modifications of these nucleosides. Only in situ formation of 8-formyladenosine is known and a single application of an N-benzoyl derivative has been reported. On the other hand, 8-formyl-2'-deoxyadenosine and its applications remain unknown. Herein, we report straightforward, scalable syntheses of both N-unprotected 8-formyladenine nucleoside derivatives, and demonstrate broad diversification at the C8 position by hydroxymethylation, azidation, CuAAC ligation, reductive amination, as well as olefination and fluoroolefination with modified Julia and a Horner-Wadsworth-Emmons reagents.

Reduction of amine N-oxides by diboron reagents.

Facile reduction of alkylamino-, anilino-, and pyridyl-N-oxides can be achieved via the use of diboron reagents, predominantly bis(pinacolato)- and in some cases bis(catecholato)diboron [(pinB)(2) and (catB)(2), respectively]. Reductions occur upon simply mixing the amine N-oxide and the diboron reagent in a suitable solvent, at a suitable temperature. Extremely fast reductions of alkylamino- and anilino-N-oxides occur, whereas pyridyl-N-oxides undergo slower reduction. The reaction is tolerant of a variety of functionalities such as hydroxyl, thiol, and cyano groups, as well as halogens. Notably, a sensitive nucleoside N-oxide has also been reduced efficiently. The different rates with which alkylamino- and pyridyl-N-oxides are reduced has been used to perform stepwise reduction of the N,N'-dioxide of (S)-(-)-nicotine. Because it was observed that (pinB)(2) was unaffected by the water of hydration in amine oxides, the feasibility of using water as solvent was evaluated. These reactions also proceeded exceptionally well, giving high product yields. In constrast to the reactions with (pinB)(2), triethylborane reduced alkylamino-N-oxides, but pyridine N-oxide did not undergo efficient reduction even at elevated temperature. Finally, the mechanism of the reductive process by (pinB)(2) has been probed by (1)H and (11)B NMR.

Discovery of a potent tubulin polymerization inhibitor: synthesis and evaluation of water-soluble prodrugs of benzophenone analog.

Prodrugs have proven to be very useful in enhancing aqueous solubility of sparingly water-soluble drugs, thereby increasing in vivo efficacy without a need of special excipients. In vitro and in vivo evaluations of a number of amino acid prodrugs of 1, a previously identified potent tubulin polymerization inhibitor and cytotoxic against various cancer cell lines led to the discovery of 3·HCl (l-valine attached) which is highly efficacious in mouse xenografts bearing human cancer. Pharmacokinetic analysis in rats revealed that compound 1 was released immediately upon administration of 3·HCl intravenously, with rapid clearance of 3·HCl indicating the effective cleavage of prodrug. Compound 3·HCl (CKD-516) has now been progressed to phase 1 clinical trial.

O6-(benzotriazol-1-yl)inosine derivatives for C6 modification of purine nucleosides.

A new class of reactive nucleosides, O(6)-(benzotriazol-1-yl) derivatives of inosine and 2'-deoxyinosine, have been developed via reaction of silyl-protected or unprotected inosine and 2'-deoxyinosine with 1H-benzotriazol-1-yloxy-tris(dimethylamino)phosphonium hexafluorophosphate (BOP). Alternatively, the silyl-protected O(6)-(benzotriazol-1-yl) derivatives can be synthesized via reaction of protected inosine and 2'-deoxyinosine with triphenylphosphine/iodine/1-hydroxybenzotriazole. These new O(6)-(benzotriazol-1-yl) inosine derivatives are excellent reagents for the synthesis of other nucleoside analogues via S(N)Ar reaction with a range of nucleophiles.

Synthesis of N6 ,N6-Dialkyl Adenine Nucleosides With In Situ Formed Hexaalkylphosphorus Triamides.

Reactions between secondary amines and phosphorus trichloride (PCl3) leads to the formation of the corresponding tris(dialkylamino)phosphines or hexaalkylphosphorus triamides [HAPT: (R2N)3P]. Reaction of silyl-protected 2'-deoxyinosine and acetyl-protected inosine with the in situ formed HAPT and iodine (I2) leads to the formation of N6,N6-dialkyl adenosine and 2'-deoxyadenosine. In some cases the stoichiometry of the amine is important as also the use of a tertiary amine base. The effect of amine stoichiometry on the reaction of HAPT with I2 has been studied by 31P{1H} NMR.

Synthetic utility of an isolable nucleoside phosphonium salt.

The reaction of O6-benzyl-3',5'-bis- O-( tert-butyldimethylsilyl)-2'-deoxyxanthosine with 1H-benzotriazol-1-yloxy-tris(dimethylamino)phosphonium hexafluorophosphate (BOP) yielded the nucleoside C-2 tris(dimethylamino)phosphonium hexafluorophosphate salt as a stable, isolable species. This is in contrast to reactions of inosine nucleosides with BOP, where the in situ formed phosphonium salts undergo subsequent reaction to yield O6-(benzotriazol-1-yl)inosine derivatives. The phosphonium salt obtained from the 2'-deoxyxanthosine derivative can be effectively used to synthesize N2-modified 2'-deoxyguanosine analogues. Using this salt, a new synthesis of an acrolein-2'-deoxyguanosine adduct has also been accomplished.

A novel polymer supported approach to nucleoside modification.

Polymer-supported O(6)-(benzotriazol-1-yl)inosine derivatives (Pol-I and Pol-dI) have been synthesized reasonably effectively via reaction of nucleoside phosphonium salts with polymer-linked HOBt (Pol-HOBt). In constast to solution chemistry, use of polymer-supported BOP (Pol-BOP) did not lead to efficient nucleoside loading. Presence of the nucleosides on the support could be readily detected by MAS NMR. Exposure of the polymer-supported nucleosides, Pol-I and Pol-dI, to alcohol, phenol, thiol and amine nucleophiles caused cleavage from the support leading directly to the C-6 modified nucleoside analogues. To our knowledge, these are the first examples of the application of such technology for nucleoside modification. Where possible, results of reactions with the polymer-supported nucleosides are compared to those from solution chemistry, providing insight into the differences between the two techniques. These new polymer-supported nucleosides can be conveniently utilized for diversity-oriented synthesis.

Unusual deoxygenation and reactivity studies related to O6-(benzotriazol-1-yl)inosine derivatives.

New and unusual developments related to the chemistry of O6-(benzotriazol-1-yl)inosine derivatives are reported. First, a simple, scalable method for their syntheses via the use of PPh3/I2/HOBt has been developed and has been mechanistically investigated by 31P(1H) NMR. Studies were then conducted into a unique oxygen transfer reaction between O6-(benzotriazol-1-yl)inosine nucleosides and bis(pinacolato)diboron (pinB-Bpin) leading to the formation of C-6 (benzotriazol-1-yl)purine nucleoside derivatives and pinB-O-Bpin. This reaction has been investigated by 11B(1H) NMR and compared to pinB-O-Bpin obtained by oxidation of pinB-Bpin. The structures of the C-6 (benzotriazol-1-yl)purine nucleosides have been unequivocally established via Pd-mediated C-N bond formation between bromo purine nucleosides and 1H-benzotriazole. Finally, short and extremely simple synthesis of 1,N6-ethano- and 1,N6-propano-2'-deoxyadenosine are reported in order to demonstrate the synthetic versatility of the O6-(benzotriazol-1-yl)inosine nucleoside derivatives for the assembly of relatively complex compounds.

Synthetic, crystallographic, computational, and biological studies of 1,4-difluorobenzo[c]phenanthrene and its metabolites.

1,4-Difluorobenzo[c]phenanthrene (1,4-DFBcPh) and its putative metabolites, the dihydrodiol and diol epoxides, have been synthesized and structurally characterized, and the extent of DNA binding by the metabolites has been assessed. 1,4-DFBcPh and 1,4-difluoro-10-methoxybenzo[c]phenanthrene were prepared by photochemical cyclization of appropriate naphthylphenylethylenes. The dihydrodiol was synthesized from 1,4-difluoro-10-methoxybenzo[c]phenanthrene, and the diol epoxides were diastereoselectively synthesized from the dihydrodiol. Interesting differences were noted in 1H NMR spectra of the series 1 (syn) diol epoxides of benzo[c]phenanthrene (BcPh) and 1,4-DFBcPh; the BcPh diol epoxide displays a quasi-diequatorial orientation of the hydroxyl groups, but in the 1,4-DFBcPh case these are diaxially disposed. This difference probably stems from the presence of the fjord-region fluorine atom in 1,4-DFBcPh. A through-space, fjord-region H-F coupling has also been observed for 1,4-DFBcPh and its derivatives. Comparative X-ray crystallographic analyses of BcPh and 1,4-DFBcPh and their dihydrodiols show that introduction of fluorine increases the molecular distortion by about 6-7 degrees . As a guide to estimating the molecular distortion and its effects, and for comparison with the X-ray structures in known cases, optimized structures of BcPh, 1,4-DFBcPh, and 1,4-DMBcPh (the dimethyl analogue) as well as their dihydrodiols and diol epoxides were computed. Relative aromaticities of these compounds were assessed by nucleus-independent chemical shift calculations, and 13C NMR chemical shifts were computed by gauge-inducing atomic orbital calculations. 1,4-DFBcPh and its dihydrodiol were subjected to metabolism, and the amount of DNA binding in human breast cancer MCF-7 cells was assessed. The extent of DNA binding was then compared with that for BcPh and its dihydrodiol and the potent carcinogen benzo[a]pyrene. The 1,4-DFBcPh series 2 (anti) diol epoxide-derived DNA adducts were also compared with those arising from intracellular oxidation of the dihydrodiol with subsequent DNA binding. These experiments showed that increased molecular distortion decreased metabolic activation to the terminal metabolites but that diol epoxide metabolites that are formed are the DNA-damaging species.

O6-(benzotriazol-1-yl)inosine derivatives: easily synthesized, reactive nucleosides.

A novel class of O6-(benzotriazol-1-yl)inosine as well as the corresponding 2'-deoxy derivatives can be conveniently prepared by a reaction between sugar-protected or -unprotected inosine or 2'-deoxyinosine nucleosides and 1H-benzotriazol-1-yloxy-tris(dimethylamino)phosphonium hexafluorophosphate (BOP). The reaction appears to proceed via a nucleoside phosphonium salt, and in the absence of any additional nucleophile, the released 1-hydroxybenzotriazole undergoes reaction with the formed phosphonium salt leading to the requisite O6-(benzotriazol-1-yl)inosine or 2'-deoxyinosine derivatives. Isolation and characterization of the phosphonium salt as well as analysis by 31P{1H} NMR appear to be consistent with this reaction pathway. The resulting O6-(benzotriazol-1-yl)inosine derivatives are effective as electrophilic nucleosides, undergoing facile reactions with a variety of nucleophiles such as alcohols, phenols, amines, and a thiol. Unusual and challenging nucleoside derivatives such as an aryl-bridged dimer, a nucleoside-amino acid conjugate, and a nucleoside-nucleoside dimer have also been synthesized from the O6-(benzotriazol-1-yl)-2'-deoxyinosine derivative. Finally, a fully protected DNA building block, the O6-(benzotriazol-1-yl)-2'-deoxyinosine 5'-O-DMT 3'-O-phosphoramidite, has been prepared and a preliminary evaluation of its use for DNA modification has been performed. Results from these studies indicate several important facts: A single, simple methodological approach provides a class of stable, isolable ribo and 2'-deoxyribonucleoside derivatives that possess excellent reactivity for SNAr chemistry with a wide range of nucleophiles. Also, a benzotriazolyl nucleoside phosphoramidite appears to be a suitable reagent for incorporation into DNA for purposes of site-specific DNA modification.

Syntheses of 1,3-Imidazoline-2-thione and 2-Phenylimino-1,3-thiazoline combinatorial libraries through different sequences of the same components.

We have developed combinatorial libraries of new 1,3-imidazoline-2-thiones 5 and 2-phenylimino-1,3-thiazolines 7 by way of different reaction sequences of the same three components, gamma-chloroacetoacetanilides 1, amines 2, and isothiocyanates 3 in a parallel synthetic fashion. One of the building blocks, the gamma-chloroacetoacetanilides 1, was prepared by the sequential reaction of 4-methylene-oxetan-2-one (ketene dimer) with chlorine and various anilines. The condensation of 1 with amines gave dihydrofuran 4 intermediates that when reacted with 3 afforded the 1,3-imidazoline-2-thiones 5. On the other hand, reaction of 3 with 2 provided substituted thioureas 6 that were reacted with 1 to yield 2-phenylimino-1,3-thiazolines 7.

Synthesis and reactions of 2-chloro- and 2-tosyloxy-2'-deoxyinosine derivatives.

Convenient syntheses of 2-chloro- and 2-tosyloxy-2'-deoxyinosine as their tert-butyldimethylsilyl ethers are described. Both compounds can be synthesized via a common route and rely on commercially available 2'-deoxyguanosine. The present method leading to the chloro nucleoside is operationally simpler compared to previously reported glycosylation techniques where isomeric products were obtained. Both electrophilic nucleosides can be used for the preparation of N-substituted 2'-deoxyguanosine analogues via displacement of the leaving groups, and a comparison of their reactivities shows the chloro analogue to be superior. Interestingly, a Pd catalyst-mediated, two-step, one-pot conversion of an allyl-protected chloro nucleoside intermediate to the final modified 2'-deoxyguanosine derivatives is also feasible. On the basis of these observations, initial assessments of Pd-catalyzed aryl amination as well as a C-C cross-coupling have also been performed with the chloro and tosyloxy nucleoside substrates. Results indicate a potentially high synthetic utility of 2-chloro-2'-deoxyinosine and in many instances this derivative can supplant the bromo and fluoro analogues that are more cumbersome to prepare or are not readily available.

Synthesis of pyrene and benzo[a]pyrene adducts at the exocyclic amino groups of 2'-deoxyadenosine and 2'-deoxyguanosine by a palladium-mediated C-N bond-formation strategy.

Single-electron oxidation of the carcinogenic hydrocarbon benzo[a]pyrene (BaP) is thought to result in a radical cation intermediate and this species has been proposed to cause alkylation at the nitrogens of the purine nucleobases. Although several different nucleoside adducts have been isolated as arising from this mode of metabolic activation, there are no selective, total syntheses of the stable exocyclic amino group adducts formed by the single-electron oxidation of any hydrocarbon with the purine 2'-deoxynucleosides to date. In this paper we disclose the synthesis of the model adducts N(6)-(1-pyrenyl)-2'-deoxyadenosine and N(2)-(1-pyrenyl)-2'-deoxyguanosine as well as the first synthesis of the carcinogen-linked nucleoside derivatives N(6)-(6-benzo[a]pyrenyl)-2'-deoxyadenosine and N(2)-(6-benzo[a]pyrenyl)-2'-deoxyguanosine via a palladium-mediated C-N bond formation. Two different coupling strategies were attempted: coupling of an aryl bromide with a suitably protected nucleoside and the coupling of an arylamine with a suitable halonucleoside. The former had somewhat limited applicability in that only N(6)-(1-pyrenyl)-2'-deoxyadenosine was prepared by this method; on the other hand, the latter was more general. However, there are noteworthy differences in the amination reactions at the C-6 and C-2 positions. Reactions at the C-6 resulted in the competing formation of a 1:2 amine-nucleoside adduct in addition to the desired monoaryl nucleoside. Such a dimer formation was not observed at the C-2. The C-2 adducts, however, displayed an interesting conformational behavior.