HETEROCYCLES

Paper | Special issue | Vol. 82, No. 1, 2010, pp. 713-728
Received, 9th June, 2010, Accepted, 15th July, 2010, Published online, 16th July, 2010.
DOI: 10.3987/COM-10-S(E)51

■ Synthesis of 8,1’-etheno and 8,2’-ethano Bridged Guanosine Derivatives Using Radical Cyclization

Julian Strohmeier, André Nadler, Daniel Heinrich, Ansgar Fitzner, and Ulf Diederichsen*

Institute of Organic Chemistry, Georg-August-University, Tammannstrasse 2, D-37077 Göttingen, Germany

Abstract

Conformationally constrained nucleosides can be readily generated by radical cyclization reactions. The radical cyclization of two guanosine derivatives containing a 2,2’-dibromovinyl group or a iodovinyl group tethered at the C8 position is described, respectively. The cyclization of the guanosine derivative with the 2,2’-dibromovinyl group initiated by tributyltin hydride formed an anomeric spiro nucleoside with an 8,1’-etheno bridge as the major cyclization product. In contrast, the conversion of guanosine and 2’-deoxyguanosine derivatives carrying the iodovinyl group provided 8,2’-ethano bridged nucleosides as the major products.

INTRODUCTION
The scaffold and conformational preferences of a radical precursor often are decisive for the cyclization Electrochemical experiments: All electrochemical measurements were performed with a three electrodes/two compartments cell. Specpure Pt or glassy carbon (GC) disks (∅ = 5 mm) mounted in teflon supports, were used as working electrodes (WE). A platinum grid and a saturated calomel electrode (SCE) were the counter and reference electrodes, respectively. The WEs were polished to a mirror finishing with wet alumina powder (Buehler Alpha Micropolish II) down to 0.3 µm grade, rinsed with water and with distilled acetonitrile. Prior to each experiment, the electrolyte solutions were thoroughly de-oxygenated directly in the cell with nitrogen (purity > 99.9997%) The electrocatalytic essays were performed at room temperature. The electrochemical measurements were controlled by a potentiostat/galvanostat EG&G PAR model 273amechanism. In order to obtain conformationally restrained nucleosides, Kittaka et al. studied the anomeric radical generation of nucleosides by means of 1,5-radical translocation.1 The subsequent radical cyclization yields uridine spiro nucleosides that are locked in syn-conformation. The respective guanosine spiro nucleosides, not available so far, may serve as valuable tools for inducing a left-handed Z-DNA double helix as Z-DNA requires guanosine nucleotides in syn-conformation.2 Therefore, our interest in 8,1’-etheno bridged guanosine spiro nucleosides is derived from conformationally constrained Z-DNA based on nucleotides that are fixed in the syn-conformation. The nucleotides in syn-conformation are required in every other position of Z-DNA. Since they are less favored due to sterical strain, syn-nucleotides and the left-handed Z-DNA double strand tend to rearrange to the relaxed anti-conformation and the right-handed DNA double helices.2,3 The synthesis of two different guanosine and 2’-deoxyguanosine radical precursors with a 2,2’-dibromovinyl group 1 and a iodovinyl group 2a and 2b tethered at the C8 position is described (Figure 1). The subsequent radical cyclization turned out to be sensitive to the precursor: providing spiro cyclization at the anomeric center is provided as well as annelated six-membered ring formation.

The radical cyclization of the guanosine derivative with the 2,2’-dibromovinyl group formed an anomeric spiro nucleoside 3a with the 8,1’-etheno bridge as the major cyclized product. In contrast, under identical conditions the radical cyclization of the 2’-deoxyguanosine 2a and guanosine 2b with the iodovinyl group generated the 8,2’-ethano bridged nucleosides 4a/4a’ and 4b/4b’ as the major cyclization products.

RESULTS AND DISCUSSION
Synthesis and subsequent radical cyclization of two guanosine radical precursors are described employing two different protecting group and radical precursor strategies. The potential of the two radical precursors 1 and 2a/2b regarding cyclization reaction differs with respect to the influence of the protecting groups on the ribosyl conformation. In addition, the C8-vinyl group contains one or two halogens leading to differences in the radical addition reactivity and the formation of the final product keeping the double bond or leading to an saturated cyclization product.
Nucleoside 9 was synthesized by means of a nucleosidation protocol (Scheme 1). The C8 substituted guanine derivative 5 was prepared using a method described by Pfleiderer et al. and later by Vasella et al.4,5 Preparation of the protected and C8 alkylated guanine was achieved in a four step synthesis starting from commercially available 6-chloro-2,2-diaminopyrimidine. Partial benzylation, nitrosation at position 5, acylation of the remaining free exocyclic amino group and subsequent PPh3 mediated ring closure yielded compound 5. A Silyl-Hilbert-Johnsen nucleosidation in the variant of Vorbrüggen was employed to connect the modified guanine 5 with fully acylated ribofuranose 6.6,7 As a result of the neighboring group effect only the β-isomer 7 was formed. The TBDMS group of compound 7 was removed using aqueous acetic acid; the acetyl groups proved to be not stable when employing the standard TBAF deprotection protocol. The oxidation of the resulting primary alcohol 8 under Dess-Martin conditions yielded compound 9.8,9

The guanosine derivative with the 2,2’-dibromovinyl group 1 was synthesized by means of Corey-Fuchs method.10 The following radical cyclization was provided with Bu3SnH and AIBN. Nevertheless, the best yields for the cyclization were obtained with triethylborane and tributyltin hydride in toluene. The reagents were added over five hours by using a syringe pump and the two anomers 3a and 3b were isolated in an α:β ratio of 1:2.25 (Scheme 2).

The radical cyclization reaction is initiated by bromine atom abstraction from the 2,2’-dibromovinyl group by a stannyl radical likely to be followed by formation of the stabilized anomeric radical by 1,5-radical translocation. The less favorable 5-endo-trig cyclization followed by β-bromine elimination are reasonable steps to yield the two diastereomeric spiro nucleosides 3a and 3b.

Synthesis of C8-iodovinyl-2’-deoxyguanosine and C8-iodovinyl-guanosine derivatives 2a and 2b started from the commercially available nucleotides 10a and 10b employing standard protocols for the first three steps.11,12,13 NBS bromination and subsequent protection of the exocyclic amino group with N,N’-dimethylformamide dimethylacetal gave compounds 11a and 11b in good yields. The completely protected guanosine derivatives 12a and 12b were obtained by TBDMS protection of the hydroxy groups. A modified Stille protocol was employed to generate the C8-vinyl substituted compounds 13a and 13b.14 The introduced vinyl groups were subsequently cleaved under Lemieux-Johnson conditions to yield aldehydes 14a and 14b (Scheme 3).15
The vinyliodide at C8 was introduced by Wittig reaction using (iodomethyl)-tri(m-toloyl)phosphonium iodide 15 as Wittig reagent rather than the standard triphenylphosphonium salt in order to avoid the laborious removal of triphenylphosphine oxide. The cyclization precursors 2a and 2b were obtained in a 1:1 E/Z ratio in the case of 2a and as pure Z compound in the case of 2b, respectively (Scheme 4).

Radical cyclization of vinyliodides 2a and 2b initiated by tributyltin hydride and AIBN in toluene provided two diastereomeric guanosine derivatives (Scheme 5). Separation of the mixture of diastereomers 4a and 4a’ or 4b and 4b’ by flash chromatography was not successful. For further characterization all protecting groups were removed from the ribo nucleoside 4b/4b’ enabling purification by RP-HPLC. Next to the two 8,2’-ethano bridged guanosine nucleosides provided in a 3:1 mixture of diastereomers two side products were identified as an anomeric 1.5:1 mixture of 8-vinyl guanosines. The ratio of the two 8-vinyl guanosine diastereomers was determined by comparing the respective 1H-NMR signals of the vinyl protons at 6.73 and 6.95 ppm. The isolated products allow concluding on the mechanism of radical transfer. It is quite likely that the anomeric radical is generated by vinyl halogen abstraction followed by 1,5-radical translocation as observed for the 2,2’-dibromovinyl guanosine precursor 1. In contrast to the anomeric radical generated from nucleotide 1, the respective radical generated from vinyl iodide 2 does not undergo a 5-endo-trig cyclization. It either is reduced by tributyltin hydride to the vinylguanosine side products or the 8,2’-ethano bridged guanosine derivatives 4 are generated involving a C1’-C2’ radical shift followed by 6-endo-trig radical cyclization as a favored process according to the Baldwin rules.16 The loss of stereogenic information at the anomeric center is a clear indication for an intermediate anomeric radical.

CONCLUSION
Radical cyclization was investigated with respect to ring formation between the anomeric center of guanosine and the C8 position of the nucleobase. Starting from vinyl bromides or vinyl iodides the vinyl radical was translocated to the anomeric center. The succeeding radical stabilization differed with respect to the nucleotide protection scheme and most likely to the respective nucleotide conformation. Whereas the guanosine derivative with 2,2’-dibromovinyl group tethered at the C8 position provided the anomeric spiro nucleoside with the 8,1’-etheno bridge as the major cyclized product, in case of the iodovinyl functionalized guanosine 6-endo-trig radical cyclization yielded the 8,2’-ethano bridged derivative as the major cyclization product.

EXPERIMENTAL
General remarks: All reagents were of analytical grade and used without further purification. Solvents were of the highest grade available. Dry solvents were stored over molecular sieves (4Å). Glass equipment utilized for reactions under inert atmosphere was flame dried before use. 1H- and 13C-NMR spectra were recorded with a Varian Unity 300 spectrometer, a Varian Inova 500 spectrometer or a Varian Inova 600 spectrometer. Chemical shifts are quoted in parts per million (ppm) downfield of TMS. Abbreviations for multiplicities are: s, singulet; d, doublet; t, triplet; q, quartet; m, multiplet; mc centered multiplet; br, broad. Coupling constants are given in Hz. Mass spectrometry was carried out using Finnigan LQC or TSQ 7000 instruments. HRMS spectra were measured with a Bruker APEX-Q IV 7T instrument. Flash chromatography was performed using Merck silica gel 60. Thin layer chromatography (TLC) was carried out using Merck aluminium sheets of silica gel 60 F254. Visualization was accomplished with UV light (254 nm). HPLC analysis was performed using a Pharmacia Äkta basic instrument (pump type P-900, variable wavelength detector) with a linear gradient of A (MilliQ-H2O) to B (acetonitrile/MilliQ-H2O 8:2). Compounds were analyzed using a YMC J’sphere column ODS-H80, RP-C18, 250 x 4.6 mm, 4 µm, 80 Å, with a flow rate of 1 mL per min. Preparative purification was performed using a YMC J’sphere column ODS-H80, RP-C18, 250 × 20 mm, 4 μm, 80 Å, with a flow rate of 10 mL per min.

2’,3’,5’-Tri-O-acetyl-N2,O6-dibenzyl-8-(tert-butyldimethylsiloxymethyl)guanosine (7).
A solution of 2-(N-benzyl)amino-6-benzyloxy-8-(tert-butyldimethylsiloxymethyl)purine (5) (8.50 g, 17.9 mmol) and sodium bis(trimethylsilyl)amide (10.9 g, 13.1 mL, 53.7 mmol) in dry MeCN (300 mL) was stirred at 100 °C for 1 h. Afterwards, the solvent was removed under reduced pressure and a solution of 1,2,3,5-tetra-O-acetyl-β-d-ribofuranose (6) (6.83 g, 21.4 mmol) and TfOSiMe3 (5.17 g, 4.21 mL, 23.3 mmol) in dry MeCN (40.0 mL) was added to the residue and the reaction mixture was stirred at 100 °C for 22 h. The solution was poured into a saturated aqueous NaHCO3 solution (100 mL), the layers were separated and the organic layer was washed with aqueous NaHCO3 solution (2 x 100 mL) and aqueous NaCl solution (2 x 100 mL). The organic layer was dried over MgSO4, the solvent was removed under reduced pressure and the residue was purified by flash chromatography (eluent: EtOAc/pentane 1:3) to furnish 7 as a light-yellow oil. Yield: 5.96 g, 8.12 mmol, 45%. 1H-NMR (300 MHz, C2D2Cl4, 100 °C): δ 0.15 (s, 3 H, SiMe), 0.18 (s, 3 H, SiMe), 0.97 (s, 9 H, tBu), 1.96 (s, 3 H, Me), 2.09 (s, 3 H, Me), 2.11 (s, 3 H, Me), 4.19 (mc, 1 H, H5’), 4.31 (mc, 1 H, H4’), 4.39 (mc, 1 H, H5’), 4.71 (d, 3JH,H = 5.9 Hz, 2 H, NCH2), 4.88 (d, 2JH,H = 12.8 Hz, 1 H, CH2OSi), 4.95 (d, 2JH,H = 13.3 Hz, 1 H, CH2OSi), 5.40 (t, 3JH,H = 6.2 Hz, 1 H, NH), 5.55 (s, 2 H, OCH2), 5.98 (dd, 3JH,H = 6.3 Hz, 5.5 Hz, 1 H, H3’), 6.35 (d, 3JH,H = 4.2 Hz, 1 H, β-H1’), 6.50 (dd, 3JH,H = 5.6 Hz, 4.1 Hz, 1 H, H2’), 7.29-7.51 (m, 10 H, Ph) ppm. 13C-NMR (75 MHz, C2D2Cl4, 100 °C): δ -5.7 (SiMe), 17.8 (C(CH3)3), 20.0, 20.1 (CH3), 25.5 (C(CH3)3), 45.9 (NCH2), 59.3 (COSiMe), 63.1 (C5’), 67.9 (OCH2Ph), 70.7 (C3’), 72.1 (C2’), 78.8 (C4’), 86.7 (C1’), 114.2 (C5), 126.8, 127.2, 127.9, 128.2, 128.3 (Ph), 136.4, 139.2 (Phipso), 148.4, 154.9, 158.6, 160.6 (C2, C4, C6, C8), 168.9, 169.9 (CO) ppm. ESI-MS m/z (rel. %): 734.4 (10) [M + H]+, 756.4 (55) [M + Na]+, 1489.1 (100) [2M + Na]+. HRMS (ESI): 734.32157 (calcd. for C37H48N5O9Si: 734.32157).

2’,3’,5’-Tri-O-acetyl-N2,O6-dibenzyl-8-hydroxymethylguanosine (8).
A solution of 7 (4.29 g, 5.85 mmol) in acetic acid (80%, 150 mL) was stirred at rt for 3 d. The solvent was removed under reduced pressure and the residue was co-evaporated with toluene (3 x 10 mL). The crude product was purified by flash chromatography (eluent: EtOAc/pentane 3:1) to furnish 8 as a colorless oil. Yield: 2.53 g, 4.09 mmol, 70%. 1H-NMR (300 MHz, C2D2Cl4, 100 °C): δ 1.97 (s, 3 H, Me), 2.09 (s, 3 H, Me), 2.12 (s, 3 H, Me), 2.58 (sbr, 1 H, OH), 4.17-4.25 (m, 1 H, H5’), 4.33-4.42 (m, 2 H, H4’, H5’), 4.71 (d, 3JH,H = 5.9 Hz, 2 H, NCH2), 4.85 (s, 2 H, CH2OH), 5.43 (t, 3JH,H = 6.0 Hz, 1 H, NH), 5.56 (s, 2 H, OCH2), 5.90 (dd, 3JH,H = 6.3 Hz, 3JH,H = 6.4 Hz, 1 H, H3’), 6.07 (d, 3JH,H = 4.0 Hz, 1 H, β-H1’), 6.33 (dd, 3JH,H = 5.8 Hz, 3.7 Hz, 1 H, H2’), 7.29-7.50 (m, 10 H, Ph) ppm. 13C-NMR (75 MHz, C2D2Cl4, 100 °C): δ 19.9, 20.1 (CH3), 45.9 (NCH2), 57.9 (CH2OH), 62.9 (C5’), 67.9 (OCH2Ph), 70.4 (C3’), 72.4 (C2’), 79.2 (C4’), 86.9 (C1’), 114.0 (C5), 126.9, 127.2, 127.4, 128.3 (Ph), 136.3, 139.2 (Phipso), 148.7, 154.9, 158.7, 160.6 (C2, C4, C6, C8), 169.0, 169.1, 169.9 (CO) ppm. ESI-MS m/z (rel. %): 620.0 (100) [M + H]+, 642.2 (11) [M + Na]+, 1238.8 (65) [2M + H]+, 1261.0 (18) [2M + Na]+. HRMS (ESI): 620.23569 (calcd. for C31H34N5O9: 620.23510).

2’,3’,5’-Tri-O-acetyl-N2,O6-dibenzyl-8-formylguanosine (9).
A solution of 8 (2.15 g, 5.08 mmol), Dess-Martin periodinane (377 mg, 483 µL, 5.08 mmol) and tert-butyl alcohol in dry DCM (100 mL) was stirred under reflux for 18 h. The reaction mixture was poured into a mixture of saturated aqueous Na2S2O4 solution (30.0 mL) and saturated aqueous NaHCO3 solution (30.0 mL). The layers were separated and the organic layer was washed with aqueous Na2S2O4 solution (2 x 50.0 mL) and aqueous NaHCO3 solution (2 x 50.0 mL). The organic layer was dried over MgSO4, the solvent was removed under reduced pressure and the residue was purified by flash chromatography (eluent: EtOAc/pentane 1:2) to furnish 9 as a colorless oil. Yield: 2.42 g, 3.92 mmol, 85%. 1H-NMR (300 MHz, C2D2Cl4, 100 °C): δ 1.97 (s, 3 H, Me), 2.09 (s, 3 H, Me), 2.12 (s, 3 H, Me), 4.20 (mc, 1 H, H5’), 4.33-4.44 (m, 2 H, H4’, H5’), 4.72 (dd, 2JH,H = 15.6 Hz, 3JH,H = 6.1 Hz, 1 H, NCH2), 4.78 (dd, 2JH,H = 15.3 Hz, 3JH,H = 6.0 Hz, 1 H, NCH2), 5.60 (s, 2 H, OCH2), 5.76 (dd, 3JH,H = 5.9 Hz, 5.9 Hz, 1 H, NH), 5.91 (dd, 3JH,H = 6.3 Hz, 6.3 Hz, 1 H, H3’), 6.26 (dd, 3JH,H = 6.3 Hz, 3.6 Hz, 1 H, H2’), 6.94 (d, 3JH,H = 3.6 Hz, 1 H, β-H1’), 7.29-7.51 (m, 10 H, Ph), 9.84 (s, 1 H, CHO) ppm. 13C-NMR (75 MHz, C2D2Cl4, 100 °C): δ 20.0, 20.1 (CH3), 46.0 (NCH2), 63.0 (C5’), 68.5 (OCH2Ph), 70.4 (C3’), 72.4 (C2’), 79.0 (C4’), 87.0 (C1’), 117.1 (C5), 127.2, 128.2, 128.3, 128.5 (Ph), 135.6, 138.3 (Phipso), 142.6, 154.9, 160.4, 162.7 (C2, C4, C6, C8), 169.1, 170.0 (CO), 182.6 (CHO) ppm. ESI-MS m/z (rel. %): 640.4 (100) [M + Na]+. HRMS (ESI): 618.21919 (calcd. for C31H32N5O9: 618.21945).

2’,3’,5’-Tri-O-acetyl-N2,O6-dibenzyl-8-(2,2’-dibromovinyl)guanosine (1).
A suspension of CBr4 (4.74 g, 14.3 mmol) and zinc powder (933 mg, 14.3 mmol) in dry DCM (43.0 mL) was treated with PPh3 (3.74 g, 14.3 mmol) in dry DCM (12.6 mL) over 1 h. The resulting reaction mixture was stirred at rt for 3 h. Afterwards, a solution of 9 (2.20 g, 3.56 mmol) in a mixture of dry DCM and dry DMF (1:1, 38.0 mL) was added over 1 h and the solution was stirred under reflux over night. The reaction mixture was poured into a mixture of saturated aqueous Na2S2O4 solution (45.0 mL) and DCM (150 mL). The layers were separated and the organic layer was washed with aqueous Na2S2O4 solution (1 x 90.0 mL) and aqueous NaCl solution (1 x 90.0 mL). The organic layer was filtrated over silica gel, the solvent was removed under reduced pressure and the residue was purified by flash chromatography (eluent: EtOAc/pentane 1:2) to furnish 1 as a light yellow-oil. Yield: 1.37 g, 1.77 mmol, 50%. 1H-NMR (300 MHz, C2D2Cl4, 100 °C): δ 1.96 (s, 3 H, Me), 2.10 (s, 3 H, Me), 2.12 (s, 3 H, Me), 4.07-4.23 (m, 1 H, H5’), 4.32-4.40 (m, 2 H, H4’, H5’), 4.71 (d, 3JH,H = 4.9 Hz, 2 H, NCH2), 5.49 (t, 3JH,H = 5.5 Hz, 1 H, NH), 5.59 (s, 2 H, OCH2), 5.86 (d, 3JH,H = 4.1 Hz, 1 H, β-H1’), 5.92 (dd, 3JH,H = 5.9 Hz, 5.9 Hz, 1 H, H3’), 6.36 (dd, 3JH,H = 5.9 Hz, 3.8 Hz, 1 H, H2’), 7.29-7.51 (m, 11 H, Ph, CBr2CH) ppm. 13C-NMR (75 MHz, C2D2Cl4, 100 °C): δ 20.0, 20.1 (CH3), 45.9 (NCH2), 62.8 (C5’), 68.2 (OCH2Ph), 70.4 (C3’), 72.3 (C2’), 79.3 (C4’), 86.9 (C1’), 99.4 (CBr2), 115.4 (C5), 124.6 (CBr2CH), 126.9, 127.1, 127.8, 128.0, 128.2, 128.3, 128.4 (Ph), 136.3, 139.1 (Phipso), 143.2, 153.8, 158.9, 161.0 (C2, C4, C6, C8), 168.8, 168.9, 169.8 (CO) ppm. ESI-MS m/z (rel. %): 796 (94) [M + Na]+, 1568.5 (100) [2M + Na]+. HRMS (ESI): 772.06130 (calcd. for C32H32Br2N5O8: 772.06121).

2’,3’,5’-Tri-O-acetyl-N2,O6-dibenzyl-8,1’-etheno-β-guanosine (3a) and 2’,3’,5’-Tri-O-acetyl-N2,O6-dibenzyl-8,1’-etheno-α-guanosine (3b).
A solution of Bu3SnH (119 mg, 109 µL, 0.41 mmol) and Et3B (137 µL, 0.14 mmol, 1m in hexane) in dry toluene (5.00 mL) was added to a stirred solution of 1 (211 mg, 0.27 mmol) in dry toluene (15.0 mL) at 60 °C over 17 h by using a syringe pump. The solvent was removed under reduced pressure and the residue was solved in aqueous KF solution (10%, 15.0 mL) and stirred at rt for 0.5 h. Et2O (30.0 mL) was added, the layers were separated and the aqueous layer was washed with Et2O (2 x 20.0 mL). The combined organic layers were dried over Na2SO4 and the solvent was removed under reduced pressure. The residue was purified by flash chromatography (eluent: Et2O/pentane 9:1) and afterwards by preparative thin layer chromatography (eluent: DCM/MeOH 97:3) to furnish 3a (30.0 mg, 0.04 mmol, 18%) and 3b (12.0 mg, 0.02 mmol, 8%) as colorless oil. β-anomer 3a: 1H-NMR (300 MHz, CDCl3, rt): δ 1.99 (s, 3 H, Me), 2.06 (s, 3 H, Me), 2.10 (s, 3 H, Me), 4.28-4.45 (m, 2 H, H5’), 4.46-4.53 (m, 1 H, H4’), 4.65 (m, 2 H, NCH2), 5.44 (m, 1 H, NH), 5.48 (s, 2 H, OCH2), 5.67 (d, 3JH,H = 4.5 Hz, 1 H, β-H2’), 6.50 (mc, 1 H, H3’), 6.61 (d, 3JH,H = 6.0 Hz, 1 H, H10), 6.67 (d, 3JH,H = 6.1 Hz, 1 H, H9), 7.25-7.46 (m, 10 H, Ph) ppm. 13C-NMR (75 MHz, CDCl3, rt): δ 20.4, 20.5, 20.7 (CH3), 45.9 (NCH2), 64.0 (C5’), 67.7 (OCH2Ph), 71.3 (C3’), 75.2 (C2’), 79.6 (C4’), 96.8 (C1’), 119.4 (C5), 124.4 (C9), 127.0, 127.1, 127.3, 127.8, 128.1, 128.3, 128.5 (Ph), 136.4 (CCH2O), 139.1 (CCH2N), 140.2 (C10), 152.5 (C4), 154.1 (C8), 158.7 (C2), 160.7 (C6), 168.8, 169.2, 117.5 (CO) ppm. ESI-MS m/z (rel. %): 614.1 (4) [M + H]+, 636.2 (100) [M + Na]+, 1248.9 (96) [2M + Na]+. HRMS (ESI): 614.22470 (calcd. for C32H32N5O8: 614.22454). α-anomer 3b: 1H-NMR (300 MHz, CDCl3, rt): δ 1.69 (s, 3 H, Me), 2.03 (s, 3 H, Me), 2.06 (s, 3 H, Me), 4.14 (dd, 2JH,H = 13.2 Hz, 3JH,H = 3.7 Hz, 1 H, H5’), 4.36 (dd, 2JH,H = 12.7 Hz, 3JH,H = 3.2 Hz, 1 H, H5’), 4.66 (m, 1 H, NH), 4.67 (m, 2 H, NCH2), 5.18 (dd, 3JH,H = 7.8 Hz, 7.8 Hz, 1 H, H3’), 5.23-5.27 (m, 1 H, H4’), 5.50 (s, 2 H, OCH2), 5.58 (d, 3JH,H = 7.5 Hz, 1 H, α-H2’), 6.56 (d, 3JH,H = 6.1 Hz, 1 H, H10), 6.64 (d, 3JH,H = 6.1 Hz, 1 H, H9), 7.25-7.46 (m, 10 H, Ph) ppm. 13C-NMR (75 MHz, CDCl3, rt): δ 20.0, 20.3, 20.7 (CH3), 45.7 (NCH2), 62.2 (C5’), 68.0 (OCH2Ph), 69.4 (C3’), 70.5 (C2’), 79.7 (C4’), 97.5 (C1’), 119.3 (C5), 123.6 (C9), 126.9, 127.1, 127.2, 127.5, 127.9, 128.1, 128.3, 128.5 (Ph), 136.4 (CCH2O), 139.5 (CCH2N), 141.9 (C10), 153.1 (C4), 155.1 (C8), 158.4 (C2), 160.9 (C6), 168.7, 169.8, 1170.5 (CO) ppm. ESI-MS m/z (rel. %): 614.1 (4) [M + H]+, 636.2 (100) [M + Na]+, 1248.9 (96) [2M + Na]+. HRMS (ESI): 614.22470 (calcd. for C32H32N5O8: 614.22454).

N2-[(Dimethylamino)methylen]-3’,5’-bis-O-(tert-butyldimethylsilyl)-8-vinyl-2’-deoxyguanosine (13a).
A solution of 12a (5.50 g, 8.74 mmol), Pd[P(Ph)3]4 (505 mg, 0.44 mmol) and tributyl(vinyl)tin (4.16 g, 13.1 mmol) in dry toluene (100 mL) was degassed and stirred at 95 °C under an argon atmosphere for 12 h. The solvent was removed under reduced pressure and the residue was purified by flash chromatography (eluent: DCM/acetone 4:1) to furnish 13a as a yellow foam. Yield: 4.03 g, 6.78 mmol, 80%. 1H-NMR (600 MHz, CDCl3, rt): δ 0.01-0.10 (m, 12 H, SiCH3), 0.85-0.91 (m, 18 H, Si(CH3)3), 2.13 (ddd, 2JH,H = 13.2 Hz, 3JH,H = 6.3, 3JH,H = 2.7 Hz, 1 H, H2’), 2.65 (ddd, 2JH,H = 13.2 Hz, 3JH,H = 8.6 Hz, 3JH,H = 7.1 Hz, 1 H, H2’), 3.07 (s, 3 H, NCH3), 3.14 (s, 3 H, NCH3), 3.76-3.88 (m, 3 H, H4’, H5’), 4.54-4.59 (m, 1 H, H3’), 5.43 (dd, 3JH,H = 11.0, 2JH,H = 1.5 Hz, 1 H, vinyl-H), 6.41 (dd, 3JH,H = 8.6, 3JH,H = 6.3 Hz, 1 H, H1’), 6.45 (dd, 3JH,H = 17.2 Hz, 2JH,H = 1.5 Hz, 1 H, vinyl-H), 6.92 (dd, 3JH,H = 17.2 Hz, 3JH,H = 11.0 Hz, 1 H, vinyl-H), 8.54 (s, 1 H, N=CH), 9.34 (sbr, 1 H, NH) ppm. 13C-NMR (75 MHz, DMSO-D6, 35 °C): δ -5.6 (SiCH3), -5.6 (SiCH3), -5.0 (SiCH3), -4.7 (SiCH3), 17.5 (C(CH3)3), 17.9 (C(CH3)3), 25.6 (C(CH3)3), 25.6 (C(CH3)3), 34.6 (NCH3), 38.8 (C2’), 40.6 (NCH3), 62.1 (C5’), 71.2 (C3’), 82.1 (C1’), 86.2 (C4’), 119.1 (C=C or C8), 119.5 (C=C or C8), 124.5 (C=C), 144.8 (Ar-C), 150.0 (Ar-C), 156.6 (Ar-C), 157.0 (Ar-C or N=CH), 157.5 (Ar-C or N=CH) ppm. ESI-MS m/z (rel. %): 677.4 (100) [M + H]+, 599.3 (95) [M + Na]+. HRMS (ESI): 577.3347 (calcd. for C27H49N6O5Si2: 577.3348).

N2-[(Dimethylamino)methylen]-2’,3’,5’-tris-O-(tert-butyldimethylsilyl)-8-vinylguanosine (13b).
A solution of 12b (8.00 g, 10.5 mmol), Pd[P(Ph)3]4 (607 mg, 0.53 mmol) and tributyl(vinyl)tin (5.00 g, 15.7 mmol) in dry toluene (120 mL) was degassed and stirred at 95 °C under an argon atmosphere for 12 h. The solvent was removed under reduced pressure and the residue was purified by flash chromatography (eluent: DCM/acetone 4:1) to furnish 13b as a yellow foam. Yield: 6.26 g, 8.82 mmol, 84%. 1H-NMR (300 MHz, CDCl3, rt): δ -0.47 (s, 3 H, SiCH3), -0.14 (s, 3 H, SiCH3), 0.08-0.11 (m, 12 H, SiCH3), 0.67 (s, 9 H, C(CH3)3), 0.92 (s, 9 H, C(CH3)3), 0.94 (s, 9 H, C(CH3)3), 3.08 (s, 3 H, NCH3), 3.12 (s, 3 H, NCH3), 3.79 (dd, 3JH,H = 11.3 Hz, 3JH,H = 3.1 Hz, 1 H, H5’), 3.87 (dd, 3JH,H = 11.3 Hz, 3JH,H = 3.5 Hz, 1 H, H5’), 4.00-4.03 (m, 1 H, H4’), 4.18 (dd, 3JH,H = 4.8 Hz, 3JH,H = 1.2 Hz, 1 H, H3’), 4.46 (dd, 3JH,H = 8.1 Hz, 3JH,H = 4.8 Hz, 1 H, H2’), 5.44 (dd, 3JH,H = 11.0 Hz, 3JH,H = 1.8 Hz, 1 H, vinyl-H), 6.16 (d, 3JH,H = 8.1 Hz, 1 H, H1’), 6.52 (dd, 3JH,H = 17.1 Hz, 3JH,H = 1.7 Hz, 1 H, vinyl-H), 7.02 (dd, 3JH,H = 17.2 Hz, 3JH,H = 11.0 Hz, 1 H, vinyl-H), 8.55 (s, 1 H, N=CH), 8.61 (sbr, 1 H, NH) ppm. 13C-NMR (75 MHz, CDCl3, rt): δ -5.4 (SiCH3), -4.5 (SiCH3), -4.3 (SiCH3), 17.7 (C(CH3)3), 18.1 (C(CH3)3), 18.5 (C(CH3)3), 25.6 (C(CH3)3), 25.9 (C(CH3)3), 26.0 (C(CH3)3), 35.2 (NCH3), 41.3 (NCH3), 63.1 (C5’), 72.0 (C3’), 74.1 (C2’), 85.6 (C1’), 86.2 (C4’), 119.7 (C5), 121.3 (C=C), 124.6 (C=C), 146.6 (Ar-C), 151.5 (Ar-C), 155.9 (Ar-C or N=CH), 157.7 (Ar-C or N=CH), 157.9 (Ar-C or N=CH) ppm. ESI-MS m/z (rel. %): 729.8 (47) [M + Na]+, 1435.8 (100) [2M + Na]+. HRMS (ESI): 707.41599 (calcd. for C33H63N6O5Si3: 707.41623).

N2-[(Dimethylamino)methylen]-8-formyl-3’,5’-bis-O-(tert-butyldimethylsilyl)-2’-deoxyguanosine (14a).
A solution of 13a (3.90 g, 6.77 mmol) in a mixture of dioxane and H2O (2:1, 100 mL) was treated with NaIO4 (2.90 g, 13.5 mmol) and OsO4 (500 µL, 2.0 wt% in tBuOH). The resulting solution was stirred at rt for 3 h. The reaction mixture was poured into a mixture of EtOAc (150 mL) and saturated aqueous Na2SO3 solution (150 mL) and the organic layer was washed with saturated aqueous NaCl solution (3 x 100 mL). The organic layer was dried over Na2SO4, the solvent was removed under reduced pressure and the residue was purified by flash chromatography (eluent: DCM/MeOH 95:5) to furnish 14a as a light-yellow foam. Yield: 2.84 g, 4.90 mmol, 72%. 1H-NMR (300 MHz, DMSO-D6, 35 °C): δ -0.05 (s, 3 H, SiCH3), -0.03 (s, 3 H, SiCH3), 0.10 (s, 6 H, SiCH3), 0.81 (s, 9 H, C(CH3)3), 0.90 (s, 9 H, C(CH3)3), 2.09-2.19 (m, 1 H, H2’), 3.07 (s, 3 H, NCH3), 3.18 (s, 3 H, NCH3), 3.25-3.35 (m, 1 H, H2’), 3.62-3.82 (m, 3 H, H4’, H5’), 4.62 (ddd, 3JH,H = 6.2 Hz, 3JH,H = 3.1 Hz, 3JH,H = 3.1 Hz, 1 H, H3’), 6.96 (dd, 3JH,H = 7.1 Hz, 3JH,H = 7.1 Hz, 1 H, H1’), 8.54 (s, 1 H, N=CH), 9.69 (s, 1 H, COH), 11.68 (sbr, 1 H, NH) ppm. 13C-NMR (75 MHz, DMSO-D6, 35 °C): δ -5.7 (SiCH3), -5.6 (SiCH3), -5.0 (SiCH3), -4.8 (SiCH3), 17.5 (C(CH3)3), 17.8 (C(CH3)3), 25.5 (C(CH3)3), 25.6 (C(CH3)3), 34.8 (NCH3), 37.2 (C2’), 40.9 (NCH3), 62.9 (C5’), 72.5 (C3’), 83.4 (C1’), 87.1 (C4’), 122.2 (C8), 142.2 (Ar-C), 152.0 (Ar-C), 157.6 (Ar-C), 158.3 (Ar-C or N=CH), 158.9 (Ar-C or N=CH), 182.5 (CO) ppm. ESI-MS m/z (rel. %): 601.3 (30) [M + Na]+, 633.4 (100) [M + Na + MeOH]+. HRMS (ESI): 601.2962 (calcd. for C26H46N6O5Si2Na: 601.2960).

N2-[(Dimethylamino)methylen]-8-formyl-2’,3’,5’-tris-O-(tert-butyldimethylsilyl)guanosine (14b).
A solution of 13b (5.20 g, 7.36 mmol) in a mixture of dioxane and H2O (2:1, 120 mL) was treated with NaIO4 (3.15 g, 14.7 mmol) and OsO4 (500 µL, 2.0 wt% in tBuOH). The resulting solution was stirred at rt for 3 h. The reaction mixture was poured into a mixture of EtOAc (150 mL) and saturated aqueous Na2SO3 solution (150 mL) and the organic layer was washed with saturated aqueous NaCl solution (3 x 100 mL). The organic layer was dried over Na2SO4, the solvent was removed under reduced pressure and the residue was purified by flash chromatography (eluent: DCM/MeOH 9:1) to furnish 14b as a yellow foam. Yield: 4.58 g, 6.48 mmol, 88%. 1H-NMR (300 MHz, CDCl3, rt): δ -0.05 (s, 3 H, SiCH3), -0.41 (s, 3 H, SiCH3), -0.12 (s, 3 H, SiCH3), 0.04 (s, 3 H, SiCH3), 0.05 (s, 3 H, SiCH3), 0.11 (s, 3 H, SiCH3), 0.13 (s, 3 H, SiCH3), 0.70 (s, 9 H, C(CH3)3), 0.88 (s, 9 H, C(CH3)3), 0.95 (s, 9 H, C(CH3)3), 3.14 (s, 3 H, NCH3), 3.18 (s, 3 H, NCH3), 3.70 (dd, 2JH,H = 9.1 Hz, 3JH,H = 3.1 Hz, 1 H, H5’), 3.89-4.02 (m, 2 H, H4’, H5’), 4.28 (dd, 3JH,H = 4.8 Hz, 1.1 Hz, 1 H, H3’), 5.03 (dd, 3JH,H = 7.4 Hz, 4.8 Hz, 1 H, H2’), 6.53 (d, 3JH,H = 7.4 Hz, 1 H, H1’), 8.61 (s, 1 H, N=CH), 8.96 (sbr, 1 H, NH), 9.80 (s, 1 H, CHO) ppm. 13C-NMR (75 MHz, CDCl3, rt): δ -5.3 (SiCH3), -5.1 (SiCH3), -4.5 (SiCH3), -4.3 (SiCH3), -4.2 (SiCH3), 17.9 (C(CH3)3), 18.2 (C(CH3)3), 18.5 (C(CH3)3), 25.7 (C(CH3)3), 26.0 (C(CH3)3), 26.0 (C(CH3)3), 35.6 (NCH3), 51.7 (NCH3), 63.1 (C5’), 72.6 (C2’), 72.7 (C3’), 85.8 (C4’), 86.7 (C1’), 122.3 (C5), 144.0 (Ar-C), 153.2 (Ar-C), 158.2 (Ar-C or N=CH), 158.3 (Ar-C or N=CH), 158.5 (Ar-C or N=CH), 181.9 (COH) ppm. ESI-MS m/z (rel. %): 707.4 (100) [M - H]ˉ. HRMS (ESI): 731.3794 (calcd. for C32H60N6O6Si3Na: 731.3800).

N2-[(Dimethylamino)methylen]-8-(2-iodo)vinyl-3’,5’-bis-(tert-butyldimethylsilyl)-2’-deoxyguanosine (2a).
A solution of 16 (1.98 g, 3.46 mmol) and sodium bis(trimethylsilyl)amide (3.50 mL, 3.50 mmol, 1m in THF) in dry THF (50.0 mL) was stirred at rt under an argon atmosphere for 0.5 h. The solution was cooled to -78 °C and a solution of 14a (1.00 g, 1.73 mmol) in dry THF (10.0 mL) was added over 0.5 h. After additional 0.5 h the reaction was quenched by adding saturated aqueous NH4Cl solution (10.0 mL) and the reaction mixture was poured into Et2O (250 mL) and H2O (250 mL). The layers were separated and the organic layer was washed with saturated aqueous NaCl solution (3 x 150 mL). The organic layer was dried over Na2SO4, the solvent was removed under reduced pressure and the residue was purified by flash chromatography (eluent: DCM/acetone 6:1) to furnish 2a as a light-yellow foam. Yield: 790 mg, 1.13 mmol, 65%. 1H-NMR (300 MHz, CDCl3, rt): δ 0.04-0.13 (m, 12 H, SiCH3), 0.88-0.92 (m, 18 H, C(CH3)3), 2.12-2.23 (m, 1 H, H2’), 2.55-2.73 (m, 1 H, H2’), 3.09 (s, 3 H, NCH3), 3.14-3.17 (m, 3 H, NCH3), 3.68-3.98 (m, 3 H, H4’, H5’), 4.50-4.58 (m, 1 H, H3’), 6.33-6.45 (m, 1 H, H1’), 7.00 (d, 3JH,H = 9.1 Hz, 0.5 H, Z-vinyl-H), 7.49 (d, 3JH,H = 14.7 Hz, 0.5 H, E-vinyl-H), 7.59-7.67 (m, 1 H, E-vinyl-H, Z-vinyl-H), 8.55 (s, 0.5 H, N=CH), 8.58 (s, 0.5 H, N=CH), 8.60 (sbr, 0.5 H, NH), 8.67 (sbr, 0.5H, NH) ppm. 13C-NMR (75 MHz, CDCl3, rt): δ -5.3 (SiCH3), -5.2 (SiCH3), -5.1 (SiCH3), -4.6 (SiCH3), -4.5 (SiCH3), 18.0 (C(CH3)3), 18.0 (C(CH3)3), 18.5 (C(CH3)3), 18.5 (C(CH3)3), 25.8 (C(CH3)3), 25.8 (C(CH3)3), 26.0 (C(CH3)3), 26.1 (C(CH3)3), 35.1 (NCH3), 39.7 (C2’), 40.2 (C2’), 41.4 (NCH3), 62.2 (C5’), 62.8 (C5’), 71.0 (C3’), 71.9 (C3’), 85.6 (C1’), 85.9 (C1’), 86.8 (C4’), 87.1 (C4’), 119.3 (C8), 119.7 (C8), 127.6 (vinyl-C), 127.6 (vinyl-C), 132.0 (vinyl-C), 144.4 (Ar-C), 145.6 (Ar-C), 145.8 (Ar-C), 150.6 (Ar-C), 150.9 (Ar-C), 156.2 (Ar-C), 156.4 (Ar-C), 157.8 (Ar-C or N=CH), 157.8 (Ar-C or N=CH), 158.3 (Ar-C or N=CH), 158.4 (Ar-C or N=CH) ppm. ESI-MS m/z (rel. %): 725.2 (100) [M + Na]+. HRMS (ESI): 701.2168 (calcd. for C27H46IN6O4Si2: 701.2169).

N2-[(Dimethylamino)methylen]-8-(Z-2-iodo)vinyl-2’,3’,5’-tris-O-(tert-butyldimethylsilyl)guanosine (2b).
A solution of 16 (1.62 g, 2.83 mmol) and sodium bis(trimethylsilyl)amide (3.00 mL, 3.00 mmol, 1m in THF) in dry THF (40.0 mL) was stirred at rt under an argon atmosphere for 0.5 h. The solution was cooled to -78 °C and a solution of 14b (1.00 g, 1.42 mmol) in dry THF (10.0 mL) was added over 0.5 h. After additional 0.5 h the reaction was quenched by adding saturated aqueous NH4Cl solution (10.0 mL) and the reaction mixture was poured into Et2O (250 mL) and H2O (250 mL). The layers were separated and the organic layer was washed with saturated aqueous NaCl solution (3 x 150 mL). The organic layer was dried over Na2SO4, the solvent was removed under reduced pressure and the residue was purified by flash chromatography (eluent: DCM/acetone 6:1) to furnish 2b as a light-yellow foam. Yield: 764 mg, 0.92 mmol, 65%. 1H-NMR (300 MHz, CDCl3, rt): δ -0.43 (s, 3 H, SiCH3), -0.12 (s, 3 H, SiCH3), 0.10 (s, 3 H, SiCH3), 0.12 (s, 6 H, SiCH3), 0.13 (s, 3 H, SiCH3), 0.69 (s, 9 H, C(CH3)3), 0.91 (s, 9 H, C(CH3)3), 0.95 (s, 9 H, C(CH3)3), 3.08 (s, 3 H, NCH3), 3.12 (s, 3 H, NCH3), 3.78 (dd, 2JH,H = 10.8 Hz, 3JH,H = 6.8 Hz, 1 H, H5’), 3.86 (dd, 2JH,H = 10.8 Hz, 3JH,H = 4.2 Hz, 1 H, H5’), 4.00-4.05 (m, 1 H, H4’), 4.21 (dd, 3JH,H = 4.5 Hz, 1.6 Hz, 1 H, H3’), 4.44 (dd, 3JH,H = 7.5 Hz, 4.5 Hz, 1 H, H2’), 6.11 (d, 3JH,H = 7.5 Hz, 1 H, H1’), 7.51 (d, 3JH,H = 14.6 Hz, 1 H, vinyl-H), 7.65 (d, 3JH,H = 14.6 Hz, 1 H, vinyl-H), 8.54 (s, 1 H, N=CH), 8.57 (sbr, 1 H, NH) ppm. 13C-NMR (75 MHz, CDCl3, rt): δ -5.5 (SiCH3), -5.2 (SiCH3), -4.6 (SiCH3), -4.2 (SiCH3), 17.7 (C(CH3)3), 18.0 (C(CH3)3), 18.4 (C(CH3)3), 25.5 (C(CH3)3), 25.8 (C(CH3)3), 26.1 (C(CH3)3), 35.3 (NCH3), 41.4 (NCH3), 63.1 (C5’), 72.1 (C3’), 74.1 (C2’), 85.7 (C1’ or C4’), 86.0 (C1’ or C4’), 119.9 (C8), 128.4 (vinyl-C), 133.0 (vinyl-C), 145.8 (Ar-C), 151.3 (Ar-C), 156.3 (Ar-C), 157.9 (Ar-C or N=CH), 158.1 (Ar-C or N=CH) ppm. ESI-MS m/z (rel. %): 856.6 (100) [M + Na]+. HRMS (ESI): 833.31272 (calcd. for C33H61IN6O5Si3: 833.31287).

N2-[(Dimethylamino)methylen]-8,2’-ethano-3’,5’-bis-O-(tert-butyldimethylsilyl)-2’-deoxyguanosine (4a/4a’).
A solution of Bu3SnH (41.3 mg, 38.2 µL, 142 µmol) in dry toluene (5.00 mL) was added to a stirred, degassed solution of 2a (50.0 mg, 71.0 µmol) and AIBN (catalytic) in dry toluene (10.0 mL) at 95 °C over 4 h by using a syringe pump. After additional 2 h the solvent was removed under reduced pressure and the residue was purified twice by flash chromatography (eluent: 1. DCM/acetone 3:1, 2. DCM/MeOH 94:6) to furnish 4a/4a’ as a light-yellow solid. Yield: 22.0 mg, 38.2 µmol, 54% (mixture of two diastereomers and traces of α,β-vinylguanosine). ESI-MS m/z (rel. %): 599.4 (100) [M + Na]+, 1175.8 (25) [2M + Na]+. HRMS (ESI): 599.3158 (calcd. for C27H48N6O2Si2Na: 599.3168).

N2-[(Dimethylamino)methylen]-8,2’-ethano-2’,3’,5’-tris-O-(tert-butyldimethylsilyl)guanosine (4b/4b’).
A solution of Bu3SnH (262 mg, 901 µmol) in dry toluene (25.00 mL) was added to a stirred, degassed solution of 2b (750 mg, 901 µmol) and AIBN (catalytic) in dry toluene (100 mL) at 95 °C over 4 h by using a syringe pump. After additional 2 h the solvent was removed under reduced pressure and the residue was purified twice by flash chromatography (eluent: 1. DCM/acetone 3:1, 2. DCM/MeOH 94:6) to furnish 4b/4b’ as a light-yellow solid. Yield: 221 mg, 313 µmol, 35% (mixture of two diastereomers and traces of α,β-vinylguanosine). ESI-MS m/z (rel. %): 729.5 (100) [M + Na]+, 1426.0 (36) [2M + Na]+. HRMS (ESI): 729.3995 (calcd. for C33H62N6O5Si3Na: 729.3982).

8,2’-Ethano-guanosine (16/16’).
A solution of Bu3SnH (262 mg, 901 µmol) in dry toluene (25.00 mL) was added to a stirred solution of 2b (750 mg, 901 µmol) and AIBN (catalytic) in dry toluene (100 mL) at 95 °C over 4 h by using a syringe pump. After additional 2 h the solvent was removed under reduced pressure and the residue was purified two times by flash chromatography (eluent: 1. DCM/acetone 3:1, 2. DCM/MeOH 94:6). A solution of the resulting light-yellow solid and TBAF (852 mg) in dry THF (30.0 mL) was stirred at rt under argon atmosphere for 2 h. The solvent was removed under reduced pressure and the residue was purified by flash chromatography (eluent: acetone/H2O 9:1). The residue was dissolved in TFA solution (0.1%, 10.0 mL) and stirred over night to remove the dimethylaminomethylene group. The solvent was removed under reduced pressure and the crude product was purified by preparative HPLC. Yield: 25.0 mg, 80.0 µmol, 9% (over 3 steps, 3:1 mixture of the α- and β-compound). 1H-NMR (300 MHz, D2O, rt): δ 2.07-2.41 (m, 2 H, CH2), 3.11-3.35 (m, 2 H, CH2), 3.65-3.96 (m, 2 H, H5’), 4.05-4.13 (m, 1 H, H3’), 4.15-4.27 (m, 1 H, H4’), 5.74 (s, 0.75 H, α-H1’ or β-H1’), 5.81 (s, 0.75 H, α-H1’ or β-H1’) ppm. 13C-NMR (75 MHz, D2O, rt): δ 21.9 (CH2), 28.6 (CH2), 63.3 (C5’), 73.1 (C3’), 78.2 (C2’), 86.4 (C4’), 88.5 (α-C1’ or β-C1’), 88.6 (α-C1’ or β-C1’), 113.6 (Ar-C), 149.1 (Ar-C), 157.0 (Ar-C), 158.7 (Ar-C), 165.7 (Ar-C) ppm. ESI-MS m/z (rel. %): 308.1 (25) [M - H]ˉ. HRMS (ESI): 332.0960 (calcd. for C12H15N5O5Na: 332.0965). analytical HPLC (RP-C18, 0 – 15% B [B = MeCN:H2O 8:2] in 30 min): 15.71 min.

ACKNOWLEDGEMENTS
Financial support from the Deutsche Forschungsgemeinschaft (Di 542/5) is gratefully acknowledged.

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