Nucleosides. Part LX. Synthesis and Characterization of Monomeric Cordycepin-Vitamin and Cordycepin-Lipid Conjugates Model Substances for Biodegradable Ester and Carbonate Linkages in Conjugates and Potential Inhibitors of HIV-1 Replication

Monomeric 3′-deoxyadenosine (cordycepin) was modified at the 2′-O- ( 13–18 ) and 5′-O-position ( 25–29 ) by the vitamins E, D₂, and A and by the two lipids 1,2-di-O-palmitoylglycerol and 1,2-di-O-hexadecylglycerol via succinate or carbonate linkages. The base-labile conjugates afforded protection groups like the 2-(4-nitro-phenyl)ethoxycarbonyl (npeoc) and monomethoxytrityl group (MeOTr) that are cleavable without harming the ester and carbonate bonds, respectively. Monomeric conjugates of cordycepin and vitamin E, vitamin D₂, 1,2-di-O-palmitoylglycerol, and 1,2-di-O-hexadecylglycerol (see 13, 14, 17, 18, 25, 26, 28 , and 29 ) inhibited HIV-1-induced syncytia formation 1.7 to 6.2 fold compared to 1.5-fold for cordycepin (see Table); IC₅₀ values for 25 and 28 were 257 and 267 m?M , respectively. In addition, the monomeric cordycepin-vitamin and -lipid conjugates inhibited HIV-1 RT activity 28–49% which compares with a 13% inhibition of HIV-1 RT observed for cordycepin. The minimal inhibition of HIV-1-induced syncytia formation and HIV-1 RT activity did not proceed by the activation of RNase L. The monomeric conjugates tested ( 13, 14 ) increased PKR expression.     

Nucleotides. Part XLIV. Synthesis,characterization, and biological activity of monomeric and trimeric cordycepin-cholesterol conjugates and inhibition of HIV-1 replication

The antivirally active 3′-deoxyadenylyl-(2′–5′)-3′-deoxyadenylyl-(2′–5′)-3′-deoxyadenosine (cordycepin trimer core) was modified at the 2′- or 5′-terminus, by attachment of cholesterol via a carbonate bond (→ 15 ) or a succinate linker (→ 16 and 27 ) to improve cell permeability. The corresponding monomeric conjugates 4 , 7 , and 21 of cordycepin were prepared as model substances to study the applicability of the anticipated protecting groups – the monomethoxytrityl (MeOTr), the (tert-butyl)dimethylsilyl (tbds), and the β -eliminating 2-(4-nitrophenyl)ethyl (npe) and 2-(4-nitrophenyl)ethoxycarbonyl (npeoc) groups – for the final deblocking steps without harming the ester bonds of the conjugate trimers. The syntheses were performed in solution using phosphoramidite chemistry. The fully protected trimer conjugates 13 , 14 , and 26 as well as all intermediates were characterized by elemental analyses, UV and ¹H-NMR spectra. The deblocked conjugates 15 , 16 , and 27 were pure according to HPLC and showed the correct compositions by mass spectra. Comparative biological studies indicated that cordycepincholesterol conjugate trimers 16 and 27 were 333- and 1000-fold, respectively, more potent inhibitors of HIV-1-induced syncytia formation than cordycepin trimer core.     

Nucleotides Part LI. Synthesis and biological activities of (2′-5′)adenylate trimer conjugates with 2′-terminal 3′-O(ω-hydroxyalkyl) and 3′-O-(ω-carboxyalkyl) spacers

An efficient strategy for the synthesis of (2′-5′)adenylate trimer conjugates with 2′-terminal 3′-O-(ω-hydroxyalkyl) and 3′-O-(ω-carboxyalkyl) spacers is reported. Npeoc-protected adenosine building blocks 37--40 for phosphoramidite chemistry carrying a 3′-O-[11-(levulinoyloxy)undecyl], 3′-O-{2-[2-(levulinoyloxy)ethoxy]ethyl}, 3′-O-[5-(2-cyanoethoxycarbonyl)pentyl], and 3′-O-{5-[(9H-fluoren-9-ylmethoxy)carbonyl]pentyl} moiety, respectively, were prepared (npeoc = 2-(4-nitrophenyl)ethoxycarbonyl). Condensation with the cordycepin (3′-deoxyadenosine) dimer 1 led to the corresponding trimers 42, 43, 47 , and 48. Whereas the levulinoyl (lev) and 9H-fluoren-9-ylmethyl (fm) blocking groups could be cleaved off selectively from the trimers 42, 43 , and 48 yielding the intermediates 44, 45 , and 49 for the synthesis of the 3′-O-(ω-hydroxyalkyl)trimers 53, 54 and the cholesterol conjugates 59--61 , the 2-cyanoethyl (ce) protecting group of 47 , however, could not be removed in a similar manner from the carboxy function. Trimer 47 served as precursor for the preparation of the trimer 55 with a terminal 3′-O-(5-carboxypentyl)adenosine moiety. The metabolically stable 3′-O-alkyl-(2′--5′)A derivatives were tested regarding inhibition of HIV-1 syncytia formation and HIV-1 RT activity. Only the conjugate 59 showed significant effects, whereas the trimers 53--55 and the conjugates 60 and 61 were less potent inhibitors, even at 100-fold larger concentrations.     

Nucleotides. Part XXXV. Synthesis of 3′-deoxyadenylyl-(2′–5′)-3′-deoxyadenyIyl-(2′–ω)-9-(ω-hydroxyalkyl)adenines

Via the phosphotriester approach, new structural analogs of (2′–5′)oligoadenyiates, namely 3′-deoxyadenylyl-(2′–5′)-3′-dcoxyadenylyl-(2′–ω)-9-(ω-hydroxyalkyl)adenines 18 – 21 , have been synthesized (see Scheme) which should preserve biological activity and show higher stability towards phosphodiesterases. The newly synthesized oligonucleotides 18 – 21 have been characterized by ¹H-NMR spectra, TLC, and HPLC analysis.     

Synthesis and properties of 3′-deoxyadenylate trimer dA2′p5′A2′p5′A

The trimeric 3′-deoxyadenylyl-(2′→5′)-3′-deoxyadenylyl-(2′→5′)-3′-deoxyadenosine ($\text{1}$$\text{2}$) was synthesized via the phosphotriester approach starting from cordycepine ($\text{1}$). Various physical data have been determined and compared with those of the ribo-A2′p5′A2′p5′A analog.     

Nucleotides. Part LV. Synthesis and application of a novel linker for solid-phase synthesis of modified oligonucleotides

Various bifunctional amino-protecting groups such as the phthaloyl, succinyl, and glutaryl group were investigated as potential linker molecules for attachment to solid-support materials. Pentane-1,3,5-tricarboxylic acid 1,3-anhydride ( 16 ) offered the best properties and reacted with the amino groups of differently sugar-protected adenosine (see 20 and 22 ), cytidine (see 29 ), and guanosine derivatives (see 32 ) to the corresponding 2-(2-carboxyethyl)glutaryl derivatives 23 , 24 , 30 , and 33 . The usefulness of the new linker-type molecules was demonstrated by the solid-support synthesis of the potentially antivirally active 3′-deoxyadenylyl-(2′–5′)-2′-adenylic acid 2′-{2-[(adenin-9-yl)methoxy]ethyl} ester ( 38 ) starting from the 2′-end with N⁶,N⁶-[2-(2-carboxyethyl)glutaryl]-9-{{2-[(4,4′-dimethoxytrityl)ethoxy]methyl}adenine ( 12 ).     

Nucleotides. Part XXVIII. Chemical syntheses of the 2′-5′-cordycepin-trimer core

The chemical synthesis of 3′-deoxyadenyly-(2′-5′)-3′-deoxyadenylyl-(2′-5′)-3′-deoxyadenosine ( 30 ; trimeric cordycepin) is described by three different routes using various protecting groups and applying the phosphotriester approach. The intermediates have been isolated and characterized by elemental analyses and spectroscopic means. High yields of 30 have been obtained on deprotection making this biologically very active compound available in preparative scale.     

INTERMEDIATES IN THE ROOM TEMPERATURE FLASH PHOTOLYSIS OF ADENINE AND SOME OF ITS DERIVATIVES

The transient absorption spectra of aqueous solutions of adenine, 2′-deoxyadenosine, 2′-deoxyadenosine-5′-phosphate and 2′-deoxyadenylyl-(3′-5′)-2′-deoxyadenosine have been determinated at different pH values using conventional flash photolysis. Reactives intermediates produced in the flash photolysis of these adenine derivatives present similar absorption regions: two higher intensity bands in the UV and 560–720 nm wavelength region and a third weaker band at 420–560 nm. On the basis of the effects produced by triplet quenchers and/or electron scavengers the bands have been assigned to hydrated electrons, radical cations, radical anions and/or neutral radicals resulting from neutralization reactions of the charged radicals. The results indicate that the bases photoionize via a triplet state under these conditions.     

Nucleotides. Part LXXVIII

Several N(‐hydroxyalkyl)‐2,4‐dinitroanilines were transformed into their phosphoramidites (see 5 and 6 in Scheme 1) in view of their use as fluorescence quenchers, and modified 2‐aminobenzamides (see 9, 10, 18 , and 19 in Scheme 1) were applied in model reactions as fluorophors to determine the relative fluorescence quantum yields of the 3′‐Aba and 5′‐Dnp‐3′‐Aba conjugates (Aba=aminobenzamide, Dnp=dinitroaniline). Thymidine was alkylated with N‐(2‐chloroethyl)‐2,4‐dinitroaniline ( 24 ) to give 25 which was further modified to the building blocks 27 and 28 (Scheme 3). The 2‐amino group in 29 was transformed by diazotation into the 2‐fluoroinosine derivative 30 used as starting material for several reactions at the pyrimidine nucleus (→ 31, 33 , and 35 ; Scheme 4). The 3′,5′‐di‐O‐acetyl‐2′‐deoxy‐N²‐[(dimethylamino)methylene]guanosine ( 37 ) was alkylated with methyl and ethyl iodide preferentially at N(1) to 43 and 44 , and similarly reacted N‐(2‐chloroethyl)‐2,4‐dinitroaniline ( 24 ) to 38 and the N‐(2‐iodoethyl)‐N‐methyl analog 50 to 53 (Scheme 5). The 2′‐deoxyguanosine derivative 53 was transformed into 3′,5′‐di‐O‐acetyl‐2‐fluoro‐1‐{2‐[(2,4‐dinitrophenyl)methylamino]ethyl}inosine ( 54 ; Scheme 5) which reacted with 2,2′‐[ethane‐1,2‐diylbis(oxy)]bis[ethanamine] to modify the 2‐position with an amino spacer resulting in 56 (Scheme 6). Attachment of the fluorescein moiety 55 at 56 via a urea linkage led to the doubly labeled 2′‐deoxyguanosine derivative 57 (Scheme 6). Dimethoxytritylation to 58 and further reaction to the 3′‐succinate 59 and 3′‐phosphoramidite 60 afforded the common building blocks for the oligonucleotide synthesis (Scheme 6). Similarly, 30 reacted with N‐(2‐aminoethyl)‐2,4‐dinitroaniline ( 61 ) thus attaching the quencher at the 2‐position to yield 62 (Scheme 7). The amino spacer was again attached at the same site via a urea bridge to form 64 . The labeling of 64 with the fluorescein derivative 55 was straigthforward giving 65 . and dimethoxytritylation to 66 and further phosphitylation to 67 followed known procedures (Scheme 7). Several oligo‐2′‐deoxynucleotides containing the doubly labeled 2′‐deoxyguanosines at various positions of the chain were formed in a DNA synthesizer, and their fluorescence properties and the T_ms in comparison to their parent duplexes were measured (Tables 1–5).     

Carotenoids of Rhizobia. I. New Carotenoids from Rhizobium lupini

The main pigments of Rhizobium lupini were 2,3,2′,3′-di-trans-tetrahydroxy-β,β-caroten-4-one and 2,3,2′,3′-di-trans-tetrahydroxy-β,β-carotene. As minor components 7,8,7′,8′-tetrahydro-ψ, ψ-carotene (ζ-carotene), β, β-carotene (β-carotene), and tentatively, a 2,3,2′(or 3′)-trihydroxy-β, β-caroten-4-one and a 2,3,2′(or 3′)-trihydroxy-β, β-carotene were identified.     

Synthesis of (5′S)‐5′‐C‐Alkyl‐2′‐deoxynucleosides

We describe the synthesis of (5′S)‐5′‐C‐butylthymidine ( 5a ), of the (5′S)‐5′‐C‐butyl‐ and the (5′S)‐5′‐C‐isopentyl derivatives 16a and 16b of 2′‐deoxy‐5‐methylcytidine, as well as of the corresponding cyanoethyl phosphoramidites 9a , b and 14a , b , respectively. Starting from thymidin‐5′‐al 1 , the alkyl chain at C(5′) is introduced via Wittig chemistry to selectively yield the (Z)‐olefin derivatives 3a and 3b (Scheme 2). The secondary OH function at C(5′) is then introduced by epoxidation followed by regioselective reduction of the epoxy derivatives 4a and 4b with diisobutylaluminium hydride. In the latter step, a kinetic resolution of the diastereoisomer mixture 4a and 4b occurs, yielding the alkylated nucleoside 2a and 2b , respectively, with (5′S)‐configuration in high diastereoisomer purity (de=94%). The corresponding 2′‐deoxy‐5‐methylcytidine derivatives are obtained from the protected 5′‐alkylated thymidine derivatives 7a and 7b via known base interconversion processes in excellent yields (Scheme 3). Application of the same strategy to the purine nucleoside 2′‐deoxyadenine to obtain 5′‐C‐butyl‐2′‐deoxyadenosine 25 proved to be difficult due to the sensitivity of the purine base to hydride‐based reducing agents (Scheme 4).     

Nucleotides. Part XXXVI. Syntheses and biological characterization of phosphorothioate analogues of (3′–5′)adenylate trimer

The four protected diastereoisomcrs 7a / 7b and 8a / 8b P-thioadenylyl-(3′–5′)-P-thioadenylyI-(3′–5′)-adenosine were synthesized, separated, and deblocked to the free oligonucleotides (Scheme). Biochemical characterization of these (3′–5′)phosphorothioate analogues of adenyiate trimer indicate that these compounds, and the corresponding 5′-monophosphates, neither bind to nor activate RNase L, and are considered to be valuable control compounds in screening experiments.     

Synthesis and properties of 5-(1,2-dihaloethyl)-2′-deoxyuridines and related analogues

The regiospecific reaction of 5-vinyl-3′,5′-di-O-acetyl-2′-deoxyuridine ( 2 ) with HOX (X = Cl, Br, I) yielded the corresponding 5-(1-hydroxy-2-haloethyl)-3′,5′-di-O-acetyl-2′-deoxyuridines 3a-c . Alternatively, reaction of 2 with iodine monochloride in aqueous acetonitrile also afforded 5-(1-hydroxy-2-iodoethyl)-3′,5′-di-O-acetyl-2′-deoxyuridine ( 3c ). Treatment of 5-(1-hydroxy-2-chloroethyl)- ( 3a ) and 5-(1-hydroxy-2-bromoethyl)-3′,5′-di-O-acetyl-2′-deoxyuridine ( 3b ) with DAST (Et₂NSF₃) in methylene chloride at -40° gave the respective 5-(1-fluoro-2-chloroethyl)- ( 6a , 74%) and 5-(1-fluoro-2-bromoethyl)-3′,5′-di-O-acetyl-2′-deoxyuridine ( 6b , 65%). In contrast, 5-(1-fluoro-2-iodoethyl)-3′,5′-di-O-acetyl-2′-deoxyuridine ( 6e ) could not be isolated due to its facile reaction with methanol, ethanol or water to yield the corresponding 5-(1-methoxy-2-iodoethyl)- ( 6c ), 5-(1-ethoxy-2-iodoethyl)- ( 6d ) and 5-(1-hydroxy-2-iodoethyl)-3′,5′-di-O-acetyl-2′-deoxyuridine ( 3c ). Treatment of 5-(1-hydroxy-2-chloroethyl)- ( 3a ) and 5-(1-hydroxy-2-bromoethyl)-3′,5′-di-O-acetyl-2′-deoxyuridine ( 3b ) with thionyl chloride yielded the respective 5-(1,2-dichloroethyl)- ( 6f , 85%) and 5-(1-chloro-2-bromoethyl)-3′,5′-di-O-acetyl-2′-deoxyuridine ( 6g , 50%), whereas a similar reaction employing the 5-(1-hydroxy-2-iodoethyl)- compound 3c afforded 5-(1-methoxy-2-iodoethyl)-3′,5′-di-O-acetyl-2′-deoxyuridine ( 6c ), possibly via the unstable 5-(1-chloro-2-iodoethyl)-3′,5′-di-O-acetyl-2′-deoxyuridine intermediate 6h . The 5-(1-bromo-2-chloroethyl)- ( 6i ) and 5-(1,2-dibromoethyl)-3′,5′-di-O-acetyl-2′-deoxyuridine ( 6j ) could not be isolated due to their facile conversion to the corresponding 5-(1-ethoxy-2-chloroethyl)- ( 6k ) and 5-(1-ethoxy-2-bromoethyl)-3′,5′-di-O-acetyl-2′-deoxyuridine ( 61 ). Reaction of 5-(1-hydroxy-2-bromoethyl)-3′,5′-di-O-acetyl-2′-deoxyuridine ( 3b ) with methanolic ammonia, to remove the 3′,5′-di-O-acetyl groups, gave 2,3-dihydro-3-hydroxy-5-(2′-deoxy-β-D-ribofuranosyl)-furano[2,3-d]pyrimidine-6(5H)-one ( 8 ). In contrast, a similar reaction of 5-(1-fluoro-2-chloroethyl)-3′,5′-di-O-acetyl-2′-deoxyuridine ( 6a ) yielded (E)-5-(2-chlorovinyl)-2′-deoxyuridine ( 1b , 23%) and 5-(2′-deoxy-β-D-ribofuranosyl)furano[2,3-d]pyrimidin-6(5H)-one ( 9 , 13%). The mechanisms of the substitution and elimination reactions observed for these 5-(1,2-dihaloethyl)-3′,5′-di-O-acetyl-2′-deoxyuridines are described.     

Asymmetric Synthesis of 3-Hydroxyprolines by Photocyclization of C(1′)-Substituted N-(2-Benzoylethyl)glycine Esters

The chiral N-(2-benzoylethyl)-N-tosylglycine esters 5a–h and the α-amino-γ-keto ester 6 were prepared from γ-(tosylamino) alcohols 7a–h . Irradiation of compounds 5a–c, e gave cis-3-hydroxyproline esters 20–23 (Scheme 6), partly with complete asymmetric induction by the C(1′)-substituent, whereas 6 gave enantiomerically pure 4-hydroxy-4-phenyl-L -proline esters 24 in good yield but low de (Scheme 6). The de of the photocyclization depended on the nature and/or size of the C(1′)-substituents. Irradiation of ketones 5d and 5f , bearing H-atoms at C(γ) with respect to the keto function, gave cyclobutanols (Scheme 9) in low yields besides the preferred Norrish-type-II cleavage product. Cyclopentanol 25 was a by-product of the photocyclization of 5c as a result of H? C(δ) abstraction from the t-Bu group. The structure of products 20, 22 , and 24a, b was established by NMR or X-ray analyses.     

Mechanismus der photochemischen Methanol-Addition an 2-allylierte Aniline

Mechanism of the Photochemical Addition of Methanol to 2-Allylated Anilines We studied in methanol the photoreaction of the 2-allylated anilines, given in Scheme 3 (cf. also [ 1 ]). Irradiation of N-methyl-2-(1′-methylallyl)aniline ( 15 ) with a high pressure mercury lamp yielded trans- and cis-1,2,3-trimethylindoline (trans- and (cis- 34 ) as well as erythro- and threo-2-(2′-methoxy-1′-methylpropyl)-N-methylaniline (erythro- and threo- 35 ; Scheme 7). When the corresponding aniline d₃- 15 , specifically deuterated in the 1′-methyl group, was irradiated in methanol, a mixture of trans- and cis-d₃- 34 , and of erythro- and threo-d₃- 35 was obtained. Successive dehydrogenation of the mixture of cis/trans-d₃- 34 by Pd/C in boiling xylene and by MnO₂ in boiling benzene lead to the corresponding indole d₃- 36 (cf. Scheme 9), the ¹H- and ²H-NMR. spectra of which showed that both cis-d₃- and trans-d₃- 34 had bound the deuterium labeled methyl group exclusively at C(3). The ¹H- and ²H-NMR. analyses of the separated methanol addition products revealed that erythro-d₃- 35 contained the deuterium label to at least 95% in the methyl group at C(1′), and threo-d₃- 35 to 50% in CH₃? C(1′) and to 50% in CH₃? C(2′) (cf. Scheme 9). To confirm these results 2-(1′-ethylallyl)aniline ( 16 ) was irradiated in methanol, whereby a complex mixture of at least 6 products was obtained (cf. Scheme 11). Two products were identified as trans- and cis-3-ethyl-2-methylindoline (trans- and cis- 37 ). The four other products represented erythro- and threo-2-(1′-ethyl-2′-methoxypropyl)aniline (erythro- and threo- 39 ) as major components, and erythro- and threo-2-(2′-methoxy-1′-methylbutyl)aniline (erythro- and threo- 40 ). These results clearly demonstrate that the methanol addition products must arise from spirodienimine intermediates of the type of trans- 9 and cis- 11 (R¹ = CD₃ or C₂H₅, R² = CH₃ or H; Scheme 2) which are opened solvolytically with inversion of configuration by methanol. Thus, cis- 11 (R¹ = CD₃, R² = CH₃) must lead to a 1:1 mixture of threo- 13 and threo- 14 (i.e.) a 1:1 distribution of the deuterium labelled methyl group between C(1′) and C(2′) in threo- 35 ) The formation of erythro-d₃- 35 with at least 95% of the deuterium label in the methyl group at C(1′) indicates that trans- 9 (R¹ = CD₃, R² = CH₃) reacts with methanol regioselectively (> 95%) at the C(2), C(3) bond. Similarly, the formation of the methanol addition products in the photoreaction of 16 (Scheme 11) can be explained. Since the indolines, formed in both photoreactions, show no alteration in the position of the subsituent at C(1′) with respect to the starting material we suppose that the diradical 7 (R¹ = CD₃ or C₂H₅, R² = CH₃ or H; Scheme 2) is a common intermediate which undergoes competetive 1.3 and 1.5 ring closure yielding the spirodienimines and the indolines. This conception is supported by irradiation experiments with N, 3,5-trimethyl-2-(1′-methylally)aniline ( 17 ) and 2-(2′-cyclohexenyl)-N-methylaniline ( 18 ) in methanol. In the former case the formation of spirodienimines is hindered by the methyl group at C(3) for steric reasons, thus leading to a ratio of the indoline to the methoxy compounds of about 6.3 as compared with ca. 1.0 for 15 (cf. Scheme 12). On the other hand, no methoxy compounds could be detected in the reaction mixture of 18 (cf. Scheme 13) which indicates that in this case the 1.3 ring closure cannot compete with the 1.5 cyclization in the corresponding cyclic diradical of the type 7 (R¹–C(1′)–C(2′) is part of a six-membered ring; Scheme 2). We suppose that the diradicals of type 7 are formed by proton transfer in an intramolecular electron-donor-acceptor (EDA) complex arising from the excited single state of the aniline chromophor and the allylic side chain. This idea is supported by the fluorescence specta of 2-allylated N-methylanilines (cf. Fig.1-4) which show pronounced differences with respect to the corresponding 2-alkylated anilines. Furthermore, the anilines 18 and 20 when irradiated in methanol in the presence of an excess of trans-1,3-pentadiene undergo preferentially an intermolecular addition to the diene, thus yielding the N-(1′-methyl-2′-butenyl)anilines 52 and 51 , respectively (Scheme 15), i.e. as one would expect the diene with its low lying LUMO is a better partner for an EDA complex than the double bond of the allylic side chain.