Catalyzed and Uncatalyzed Modifications of Nucleosides, Synthesis of Hippadine, and Deuterated 1,2,3-Triazoles

The C4 amide carbonyl of O-t-butyldimethylsilyl-protected thymidine, 2’-deoxyuridine, and 3’-azidothymidine (AZT) was activated by reaction with (benzotriazol-1-yloxy)tris(dimethylamino) phosphonium hexafluorophosphate (BOP) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in THF as solvent. This led to the formation of corresponding O4-(benzotriazol-1-yl) derivatives, which are reactive intermediates. Substitution at the C4 position was then carried out by reactions with alkyl and aryl amines, and thiols. Typically, reactions were conducted as a two-step, one-pot transformation, and also as a one-step conversion. After examining the reactions, the formation of 1-(4-pyrimidinyl)-1H-benzotriazole-3-oxide derivatives from the pyrimidine nucleosides was identified. However, these too underwent conversion to the desired products. C4 modified pyrimidine nucleosides were desilylated using standard conditions. Desilylated 3’-azido derivatives obtained from AZT were also converted to the 3’-amino derivatives by catalytic reduction. All products were evaluated for their abilities to inhibit cancer cell proliferation and for antiviral activities. Some compounds displayed moderate inhibitory activity against proliferation of murine leukemia (L1210), human cervix carcinoma (HeLa), and human T-lymphocytic (CEM) cell lines. Many were seen to be active against HIV-1 and HIV-2, and one was active against herpes simplex virus-1 (HSV-1). Evaluations of the structures and activities indicated that the methyl group at the C5 position is important for biological activity. Chemoselective N-arylation of 8-vinyladenine nucleosides can be carried out with the Pd(OAc)2/Xantphos/Cs2CO3 combination. All the other ligands such as DPEPhos, DPPF, and BIPHEP in combination with Pd(OAc)2, and the complex Pd-118 resulted in Heck arylation, exclusively. Both aryl iodides and bromides can be used under these conditions. Generally, all reactions resulted in N-arylated products in good yields, along with small amounts of Heck-like products and C,N-diarylated products. However, the Heck-like products were observed mostly in the reactions of aryl iodides. The results from the Pd-catalyzed N-arylation reactions of deoxy and ribonucleosides were very similar, but a higher catalyst loading and temperature for reactions of the ribonucleoside was required. A modular, one-pot approach was utilized for the synthesis of diaryl products via sequential C–C reaction and C–N arylation. The generality of chemoselective arylation of simple substrates was tested by exposing p-aminostyrene under N-arylation and Heck-arylation conditions. The results indicated that in this case as well, the Pd/Xantphos/Cs2CO3 combination was effective for chemoselective N-arylation and the Pd/DPEPhos/Cs2CO3 combination was effective for chemoselective Heck-arylation. In order to synthesize nucleosides adducts produced by a cis ring-opening of benzo[a]pyrene diol epoxide 1, diastereoselective synthesis of (±)-10β-amino-7β,8α,9β-trihydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene was carried out in nine steps from (±)-7β,8α-dibenzoyloxy-7,8,9,10-tetrahydrobenzo[a]pyrene. The (±)-7β,8α-dibenzoyloxy-7,8-dihydrobenzo[a]pyrene was converted to the diol epoxide and then reacted with lithium chloride and acetic anhydride to give a peracyl trans chloro triol with a chloride at the benzylic position. Displacement of chloride by azide, followed by deacylation, and reduction of the azide afforded the requisite amino triol. B[a]P-deoxyadenosine adducts were synthesized by the reaction of this amino triol with a 6-fluoropurine 2’-deoxyriboside derivative. The two adducts obtained from this reaction were separated by preparative TLC and the chirality in each was assigned by comparing their circular dichroism data with the literature. The 2-fluoro-2’-deoxyinosine derivative required for the synthesis of B[a]P-deoxyguanosine adducts, was synthesized by a modified approach utilizing a C6 modification protocol for guanosine nucleosides via the amide activation by BOP. However, the reaction of 2-fluoro-2’-deoxyinosine derivative with the amino triol was unsuccessful. Hence, the hydrochloride salt of amino triol was prepared and then reacted with 2-fluoro-2’-deoxyinosine derivative. This reaction yielded two B[a]P-deoxyguanosine adducts, which were separated by preparative TLC and the chirality in each was assigned by comparing their circular dichroism data with literature. However, careful NMR analysis of B[a]P-dA and dG adducts indicated that the products were not the anticipated cis ring-opened nucleoside adducts as previously reported, but the data were more consistent with trans ring-opened B[a]P DE1-nucleoside adducts. This information suggested that the amino triol synthesized had the undesired trans stereochemistry at the C9 and C10 positions. This was further confirmed by careful evaluation of chemical shift and coupling constant data of synthesized azido triol with known data of trans and cis ring-opened azido triols. By utilizing PPh3/I2 mediated amidation reaction as a key step, a simple approach was developed for the synthesis of hippadine via anhydrolycorinone. N-(Piperonoyl)indoline was synthesized by reacting piperonylic acid and indoline in the presence of PPh3/I2 andiPr2NEt. The combination of polymer-supported PPh3/I2 in place of PPh3/I2 was also very effective under these conditions, and both combinations gave comparable yields of N-(piperonoyl)indoline. However, in CDCl3, the 1H NMR data of the amide obtained was missing one aromatic resonance. The structure of amide obtained via these amidation reactions was further confirmed by obtaining 1H NMR in C6D6 at 70 °C and COSY data. The amide was then cyclized using PhI(OTFA)2 and BF3 to give anhydrolycorinone that was finally oxidized by DDQ to give hippadine in an overall yield of 13%, over three steps. Deuteration at the C-5 position of the 1,2,3-triazole structure was carried out efficiently during the triazole-forming step by using a copper-catalyzed azide−alkyne cycloaddition (CuAAC) reaction. Reactions of alkynes and azides were conducted in a biphasic medium of CH2Cl2/ D2O, using the CuSO4 and Na ascorbate. The mild reaction conditions allow the applicability of this method to relatively high complex substrates, such as nucleosides. Generally, good yields and high levels of deuterium incorporation were observed in all cases. Using appropriately deuterated precursors, partially to fully deuterated triazoles were also assembled under the same conditions. The competition of deuteration vs protonation in the CuAAC reaction was evaluated by conducting a reaction of phenyl azide with 4-ethynyltoluene with equimolar H2O and D2O. Higher hydrogen atom incorporation in the triazole products was observed as compared to deuterium (protonated vs deuterated triazoles were obtained in a 2.7:1 ratio)

Synthesis of Deuterated 1,2,3-Triazoles

The copper-catalyzed azide–alkyne cycloaddition (CuAAC) is a highly effective method for the selective incorporation of deuterium atom into the C-5 position of the 1,2,3-triazole structure. Reactions of alkynes and azides can be conveniently carried out in a biphasic medium of CH₂Cl₂/D₂O, using the CuSO₄/Na ascorbate system. The mildness of the method renders it applicable to substrates of relatively high complexity, such as nucleosides. Good yields and high levels of deuterium incorporation were observed. A reaction conducted in equimolar H₂O and D₂O showed 2.7 times greater incorporation of hydrogen atom as compared to deuterium. This is consistent with the H⁺ and D⁺ ion concentrations in H₂O and D₂O, respectively. With appropriately deuterated precursors, partially to fully deuterated triazoles were assembled where the final deuterium atom was incorporated in the triazole-forming step

Facile functionalization at the C4 position of pyrimidine nucleosides via amide group activation with (benzotriazol-1-yloxy)tris(dimethylamino)phosphonium hexafluorophosphate (BOP) and biological evaluations of the products (vol 15, pg 1130, 2017)

Correction for 'Facile functionalization at the C4 position of pyrimidine nucleosides via amide group activation with (benzotriazol-1-yloxy)tris(dimethylamino)phosphonium hexafluorophosphate (BOP) and biological evaluations of the products' by Hari K. Akula, et al., Org. Biomol. Chem., 2017, DOI: 10.1039/c6ob02334g.crosscheck: This document is CrossCheck deposited related_article: http://dx.doi.org/10.1039/C6OB02334G copyright_licence: The Royal Society of Chemistry has an exclusive publication licence for this journal copyright_licence: This article is freely available. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence (CC BY 3.0) history: Received 11 January 2017; Accepted 11 January 2017; Advance Article published 19 January 2017; Version of Record published 1 February 2017status: publishe

Facile functionalization at the C4 position of pyrimidine nucleosides via amide group activation with (benzotriazol-1-yloxy)tris(dimethylamino)phosphonium hexafluorophosphate (BOP) and biological evaluations of the products

Reactions of O-t-butyldimethylsilyl-protected thymidine, 2'-deoxyuridine, and 3'-azidothymidine (AZT) with (benzotriazol-1-yloxy)tris(dimethylamino)phosphonium hexafluorophosphate (BOP) leads to activation of the C4 amide carbonyl by formation of putative O(4)-(benzotriazol-1-yl) derivatives. Subsequent substitution with alkyl and aryl amines, thiols, and alcohols leads to facile functionalization at this position. Reactions with amines and thiols were conducted either as a two-step, one-pot transformation, or as a one-step conversion. Reactions with alcohols were conducted as two-step, one-pot transformations. In the course of these investigations, the formation of 1-(4-pyrimidinyl)-1H-benzotriazole-3-oxide derivatives from the pyrimidine nucleosides was identified. However, these too underwent conversion to the desired products. Products obtained from AZT were converted to the 3'-amino derivatives by catalytic reduction. All products were assayed for their abilities to inhibit cancer cell proliferation and for antiviral activities. Many were seen to be active against HIV-1 and HIV-2, and one was active against herpes simplex virus-1 (HSV-1).crosscheck: This document is CrossCheck deposited related_article: http://dx.doi.org/10.1039/C7OB90013A related_data: Supplementary Information identifier: Mahesh K. Lakshman (ORCID) copyright_licence: The Royal Society of Chemistry has an exclusive publication licence for this journal history: Received 26 October 2016; Accepted 16 December 2016; Advance Article published 5 January 2017; Version of Record published 1 February 2017status: publishe

Ruthenium-Catalyzed C–H Bond Activation Approach to Azolyl Aminals and Hemiaminal Ethers, Mechanistic Evaluations, and Isomer Interconversion

C­(sp³)–N bond-forming reactions between benzotriazole and 5,6-dimethylbenzotriazole with <i>N</i>-methylpyrrolidinone, tetrahydrofuran, tetrahydropyran, diethyl ether, 1,4-dioxane, and isochroman have been conducted using RuCl₃·3H₂O/<i>t</i>-BuOOH in 1,2-dichloroethane. In all cases, <i>N</i>1 and <i>N</i>2 alkylation products were obtained, and these are readily separated by chromatography. One of these products, 1-(isochroman-1-yl)-5,6-dimethyl-1<i>H</i>-benzotriazole, was examined by X-ray crystallography. It is the first such compound to be analyzed by this method, and notably, the benzotriazolyl moiety is quasi-axially disposed, consistent with the anomeric effect. This has plausible consequences, not observed previously. In contrast to other hemiaminal ether-forming reactions, which proceed via radicals, this Ru-catalyzed process is not suppressed in the presence of a radical inhibitor. Therefore, an oxoruthenium-species-mediated rapid formation of an oxocarbenium intermediate is believed to occur. In the radical-trapping experiment, previously unknown products containing both the benzotriazole and the TEMPO unit have been identified. In these products, it is likely that the benzotriazole is introduced via a Ru-catalyzed C–N bond formation, whereas C–O bond-formation with TEMPO occurs via a radical reaction. We show that reactions of THF with TEMPO are influenced by ambient light. A competitive reaction of THF and THF-<i>d</i>₈ with benzotriazole indicated that C–H bond cleavage occurs ca. 5 times faster than C–D cleavage. This is comparable to other metal-mediated radical reactions of THF but lower than that observed for a reaction catalyzed by <i>n</i>-Bu₄N⁺I^–. Detailed mechanistic experiments and comparisons are described. The catalytic system was also evaluated for reactions of benzimidazole, imidazole, 1,2,4-triazole, and 1,2,3-triazole with THF, and successful reactions were achieved in each case. In the course of our studies, we discovered an unexpected but significant isomerization of some of the benzotriazolyl hemiaminal ethers. This is plausibly attributable to the pseudoaxial orientation of the heterocycle in the products and the stability of oxocarbenium ions, both of which can contribute to C–N bond cleavage and reformation. Predominantly, the <i>N</i>2-isomers rearrange to the <i>N</i>1-isomers <i>even upon storage at low temperature!</i> This previously unknown phenomenon has also been studied and described