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X-ray Structures of Succinimidyl Halobenzoates

Constantin Mamat
Daniel Holger Weiß
2 and
Martin Köckerling
Helmholtz-Zentrum Dresden-Rossendorf, Institut für Radiopharmazeutische Krebsforschung, Bautzner Landstraße 400, D-01328 Dresden, Germany
Anorganische Festkörperchemie, Institut für Chemie, Universität Rostock, Albert-Einstein-Straße 3a, D-18059 Rostock, Germany
Author to whom correspondence should be addressed.
Crystals 2017, 7(3), 90;
Submission received: 10 February 2017 / Revised: 14 March 2017 / Accepted: 15 March 2017 / Published: 20 March 2017


The crystal and molecular structures of five succinimidyl halobenzoates are reported. Corresponding derivatives with the respective halo-radionuclide (18F, 76Br, 123I/124I/125I/131I) were prepared and used for the radiolabeling of biologically active (macro-)molecules (peptides, proteins, antibodies) under mild labeling conditions. All compounds were crystalized from petroleum ether/ethyl acetate mixtures.

1. Introduction

The radiolabeling of large biologically active molecules such as peptides, proteins or antibodies is an ongoing issue in radiopharmacy [1,2,3,4,5]. Harsh reaction conditions (high temperature, organic solvents, oxidizing conditions) for the introduction of the radionuclide and the selective radiolabeling, together with the sensitivity of the applied biomolecules are often the main problems. Thus, novel radiolabeling building blocks were elaborated for a selective and mild insertion of the radionuclide under physiological friendly conditions. Succinimidyl esters which belong to activated esters [6] play a major role for this purpose.
mSIB, oSIB, and pSIB with radioiodine were first developed [7,8,9,10] and are the basis of all other (radio)halogenated succinimidyl esters described in this paper. They are important for labeling purposes with radioiodine (123I/124I/125I/131I) [11,12,13,14,15,16,17,18,19,20]. [18F]SFB is the most applied building block in fluorine-18 chemistry. Additionally, a carbon-11-containing SFB derivative was synthesized in the past [21]. In the meantime, several commercial suppliers deliver the non-radioactive SFB compound. [76Br]SBrB is rarely applied [22,23].
In this paper, we synthesized the non-radioactive esters SFB 3a, SClB 3b, SBrB 3c, o-SIB 3d, and p-SIB 3e which are commonly in use as non-radioactive standards to analyze radiolabeling via TLC and HPLC analyses and determined the molecular structure of these compounds via single crystal XRD.

2. Results and Discussion

2.1. Synthesis and Chemistry

In general, the radiolabeling of biomacromolecules (peptides, proteins, and antibodies) follows a two-step procedure. Normally, these compounds were not directly radiolabeled. For this purpose, the radionuclide-containing building block was prepared first. In the case of radiofluorine 18F, electron-demanding ethyl benzoates were applied [24,25] and the fluorine was introduced via a nucleophilic substitution SNAr of the trimethylammonium group in most of the cases [2]. Newer developments are based on the use of iodonium salts [26,27] or nickel complexes [28] as precursors. Afterwards, the ethyl group was cleaved followed by the introduction of the succinimidyl group.
In the case of radiobromine and radioiodine, both radionuclides were classically inserted by an electrophilic substitution SEAr (radiohalodestannylation) using stannyl precursors.
The second step involves the actual labeling of the (sensitive) biomacromolecule. Mostly, free amine groups were used for this labeling reaction under mild conditions (room temperature, aqueous solvents, non-oxidizing conditions, short reaction times). The overview is outlined in Scheme 1.
Various ways to prepare the halogenated succinimidyl esters are known from the literature. Several are based on the Steglich esterification of N-hydroxy succinimide with the halobenzoic acid and DCC, EDC or TSTU as coupling reagent [11,29,30,31,32,33]. Others used halobenzyl alcohols under radical conditions [34,35] or halobenzoic acid and N,N′-succinimidyl carbonate [36]. Transition metal catalyzed reactions are also applied such as palladium catalyzed coupling reactions with CO [37] or with formyl derivatives [38] as well as Ru-catalyzed reactions using the respective benzaldehyde [39].
Application of halobenzoyl chlorides or the use of the Steglich esterification are the most convenient synthesis methods with the highest yields and the shortest reaction times. In our case, the synthesis of all succinimidyl esters was accomplished using N-hydroxy succinimide (2) which was treated with the respective halobenzoyl chlorides (1ae) in anhydrous THF and triethylamine as base [16]. All succinimidyl esters 3ae were obtained in high yields of 77% to 92%. The purification of 3ae was accomplished via a short column chromatography. The reaction path is outlined in Scheme 2.

2.2. X-ray Structure Determination

Single crystals of 3a to 3e were obtained by the slow evaporation method. The crystal and experimental as well as the structure refinement parameters for the single crystal X-ray structure determinations are summarized in Table 1. The crystals of all five compounds consist of neutral succinimidyl halobenzoate molecules. The chloro-, bromo- and iodo-derivatives 3b, 3c and 3d, which have the halogen atom attached on the para-position of the benzoate ring, are isotypic. Figure 1, Figure 2 and Figure 3 show the molecular structures of the five compounds.
The interatomic distances for all five compounds are found within the expected ranges. Selected atom distances and mean plane angles are listed in Table 2. A different packing of the molecules is observed only in crystals of the fluoro compound 3a and the ortho-iodobenzoate 3e, resulting in different space groups. The two carbon–oxygen bonds of C5 in all five structures differ significantly in length. Generally, the much shorter C5–O4 lengths compared to C5–O3 indicate a strong double bond character and a single bond character for C5–O3. The mean planes through the halo benzoate moieties are tilted towards the mean planes through the succinimidyl moieties by angles ranging from 70.7° (3c) to 80.5° (3d), such that the two ring systems are arranged almost perpendicular to each other. Because of the lack of acidic protons, no classical hydrogen bonds are observed in the structures (see below).
Furthermore, the surrounding of the nitrogen atoms of the succinimidyl residue in compounds 3ae can be described as follows. These atoms show nearly planar bonding geometry, with a maximum deviation of 0.08 Å out of the C1–C4–O3 plane. The presence of an adjacent single bound oxygen atom can act to pyramidalize the N bonding geometries, but in these cases it is minimal due to the strong conjugation between the N atom and two carbonyl groups. This nearly planar behavior can be explained by the partial double bond character of the N1–C1–O1 and the N1–C4–O2 amide function.
Figure 4 demonstrates exemplarily the packing of the molecules of 3b in a view along the b axis of the unit cell. The dotted lines included in the figure show the shortest center distances of the phenyl rings (brown dotted lines) and the shortest intermolecular O....H distances (green dotted lines). Weak π–π interactions with distances between the planes of the aromatic phenyl rings of 4.181 Å and 4.586 Å as well as weak “non-classical” hydrogen bonds with the shortest acceptor–donor distance of 3.264(1) Å (in 3b) are responsible for the final arrangement of molecules.

3. Conclusions

In this paper, we have synthesized four succinimidyl halobenzoate derivatives which are used in radiopharmacy as prostetic groups with the respective halo radionuclides. The structures of all derivatives were elucidated.

4. Experimental Section

4.1. General

NMR spectra were recorded on an Agilent DD2 (400 or 600 MHz) with ProbeOne probe. Chemical shifts of the 1H, 13C and 19F spectra were reported in parts per million (ppm) using TMS for 1H and 13C spectra and CFCl3 for 19F spectra as internal standard. Chromatographic separations and TLC detections were carried out with Merck Silica Gel 60 (63–200 μm) and Merck Silica Gel 60 F254 sheets, respectively. TLCs were developed by visualization under UV light (λ = 254 nm). Anhydrous THF was purchased from Acros (Geel, Belgium) or SigmaAldrich (Schnelldorf, Germany). N-Hydroxysuccinimide (2), all benzoyl chlorides 1ae and Et3N were used as received without further purification. Crystallographic data were collected with a Bruker–Nonius Apex-X8 CCD-diffractometer (Bruker, Madison, WI, USA) with Mo-Kα radiation (λ = 0.71073 Å) at 123 K. The structures were solved by direct methods using SHELXS-97 and refined against F2 on all data by full matrix least-squares refinements using the program suites from G. M. Sheldrick [40,41,42]. Data corrections including multi-scan absorption corrections were applied to the data sets using the Bruker AXS software [43]. All non-hydrogen atoms were refined anisotropically; all hydrogen atoms bonded to C atoms were placed on geometrically calculated positions and refined using riding models. CCDC 1524925 (3a), CCDC 1504220 (3b), CCDC 1505323 (3c), CCDC 1505325 (3d), and CCDC 1505324 (3e) contain the supplementary crystallographic data of the compounds. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via

4.2. General Synthesis Procedure

N-Hydroxysuccinimide (2, 150 mg, 1.33 mmol) was dissolved in anhydrous THF (10 mL), Et3N (197 mg, 1.95 mmol) was added and the mixture was cooled to 0 °C. Next, the respective halobenzoyl chloride 1ae (1.56 mmol) was added dropwise, the solution was stirred at 0 °C for 60 min and at rt for 2 h. Afterwards, the reaction was quenched with water (15 mL) and extracted with ethyl acetate (3 × 15 mL). The combined organic layers were separated and dried over Na2SO4. The solvent was removed and the crude product was purified via flash chromatography (petroleum ether/ethyl acetate 2:1) to yield compounds 3ae (77%–92%) as colorless solids.

4.2.1. Succinimidyl 4-Fluorobenzoate (SFB, 3a)

Yield: 283 mg, 92%. M.p. 112 °C. 1H NMR (400 MHz, CDCl3): δ = 2.90 (s, 4H, CH2), 7.19 (t, 3J = Hz, Ar–H), 8.16 (dd, 3J = Hz, 3JH,F = Hz, 2H, Ar–H); 13C NMR (101 MHz, CDCl3): δ = 25.8 (CH2), 116.4 (d, 2JC,F = 22.3 Hz, C–Hmeta), 121.5 (d, 4JC,F = 3.2 Hz, Cipso) 133.5 (d, 3JC,F = 9.9 Hz, C–Hortho), 161.0 (C=O), 167.0 (d, 1JC,F = 257.6 Hz, Cpara), 169.3 (C=O); 19F NMR (376 MHz, CDCl3): δ = −101.3 ppm.

4.2.2. Succinimidyl 4-Chlorobenzoate (SClB, 3b)

Yield: 290 mg, 88%. M.p. 206 °C. 1H NMR (600 MHz, CDCl3): δ = 2.91 (s, 4H, CH2), 7.50 (d, 3J = 8.6 Hz, Hmeta), 8.07 (d, 3J = 8.6 Hz, Hortho); 13C NMR (151 MHz, CDCl3): δ = 25.8 (CH2), 123.7 (Cipso), 129.5 (Cmeta), 132.0 (Cortho), 141.8 (Cpara), 161.3 (C=O), 169.2 (C=Osucc).

4.2.3. Succinimidyl 4-Bromobenzoate (SBrB, 3c)

Yield: 300 mg, 77%. M.p. 224 °C. 1H NMR (600 MHz, CDCl3): δ = 2.91 (s, 4H, CH2), 7.67 (d, 3J = 8.6 Hz, Hmeta), 8.56 (d, 3J = 8.6 Hz, Hortho); 13C NMR (151 MHz, CDCl3): δ = 25.8 (CH2), 124.2 (Cpara), 130.6 (Cipso), 132.1, 132.5 (Cmeta + Cortho), 161.4 (C=O), 169.2 (C=Osucc).

4.2.4. Succinimidyl 2-Iodobenzoate (o-SIB, 3d)

Yield: 410 mg, 91%. M.p. 134 °C. 1H NMR (600 MHz, CDCl3): δ = 2.91 (s, 4H, CH2), 7.28 (dt, 4J = 1.4 Hz, 3J = 7.7 Hz, 1H, HAr), 7.48 (t, 3J = 7.7 Hz, 1H, HAr), 8.08 (d, 3J = 8.0 Hz, HAr), 8.11 (dd, 4J = 1.5 Hz, 3J = 7.7 Hz, 1H, HAr); 13C NMR (151 MHz, CDCl3): δ = 25.9 (CH2), 95.9 (CAr), 128.3 (CHAr), 129.5 (CAr), 132.4 (CHAr), 134.7 (CHAr), 142.3 (CHAr), 161.4 (C=O), 169.1 (C=Osucc).

4.2.5. Succinimidyl 4-Iodobenzoate (p-SIB, 3e)

Yield: 402 mg, 90%. M.p. 162 °C. 1H NMR (600 MHz, CDCl3): δ = 2.91 (s, 4H, CH2), 7.83 (d, 3J = 8.5 Hz, Hortho), 7.89 (d, 3J = 8.5 Hz, Hmeta); 13C NMR (151 MHz, CDCl3): δ = 25.8 (CH2), 103.5 (Cpara), 124.7 (Cipso), 131.8 (Cortho), 138.5 (Cmeta), 161.7 (C=O), 169.2 (C=Osucc).


Patrick Wieder (HZDR) is gratefully acknowledged for the support during the syntheses.

Author Contributions

Constantin Mamat performed the syntheses and the NMR analyses; Martin Köckerling and Daniel Holger Weiß performed the XRD experiments and analyzed the data; Constantin Mamat and Martin Köckerling contributed equally by writing the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.


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Scheme 1. General labeling procedure using radiolabeling building blocks based on radiohalogenated (18F, 76Br, 123/124/125/131I) succinimidyl benzoates.
Scheme 1. General labeling procedure using radiolabeling building blocks based on radiohalogenated (18F, 76Br, 123/124/125/131I) succinimidyl benzoates.
Crystals 07 00090 sch001
Scheme 2. Synthesis path to the succinimidyl halobenzoates 3ae.
Scheme 2. Synthesis path to the succinimidyl halobenzoates 3ae.
Crystals 07 00090 sch002
Figure 1. A view of the molecular structures of 3a (left) and 3b (right), showing the atom labeling scheme. Displacement ellipsoids are drawn at the 50% probability level.
Figure 1. A view of the molecular structures of 3a (left) and 3b (right), showing the atom labeling scheme. Displacement ellipsoids are drawn at the 50% probability level.
Crystals 07 00090 g001
Figure 2. A view of the molecular structures of 3c (left) and 3d (right), showing the atom labeling scheme. Displacement ellipsoids are drawn at the 50% probability level.
Figure 2. A view of the molecular structures of 3c (left) and 3d (right), showing the atom labeling scheme. Displacement ellipsoids are drawn at the 50% probability level.
Crystals 07 00090 g002
Figure 3. The molecular structure of 3e with the atom labeling scheme. Displacement ellipsoids are drawn at the 50% probability level.
Figure 3. The molecular structure of 3e with the atom labeling scheme. Displacement ellipsoids are drawn at the 50% probability level.
Crystals 07 00090 g003
Figure 4. The packing of the molecules of 3b in an expanded view of the unit cell along the b axis. The shortest contacts between the phenyl rings are shown as brown dotted lines and the shortest intermolecular O....H distances as green dotted lines.
Figure 4. The packing of the molecules of 3b in an expanded view of the unit cell along the b axis. The shortest contacts between the phenyl rings are shown as brown dotted lines and the shortest intermolecular O....H distances as green dotted lines.
Crystals 07 00090 g004
Table 1. Crystal data and structure refinement for compounds 3ae.
Table 1. Crystal data and structure refinement for compounds 3ae.
Formula weight (g·mol−1)237.18253.63298.09345.08345.08
Temperature (K)123
Wavelength (Å)0.71073
Crystal systemmonoclinicmonoclinicmonoclinicmonoclinicorthorhombic
Space groupP21/cP21/nP21/nP21/nPbca
Unit cell dimensions
a (Å)11.6331(6)8.7157(7)8.554(2)8.566(3)12.1900(3)
b (Å)5.4971(3)5.7238(5)5.800(1)5.817(2)8.5246(2)
c (Å)17.041(1)22.598(2)22.844(6)23.374(8)22.0618(6)
β (°)103.992(2)90.470(4)92.20(1)93.27(2)90.00
Volume (Å3)1057.4(1)1127.3(2)1132.5(5)1162.8(7)2292.6(1)
Density (calcd.) (g·cm−3)1.4901.4941.7481.9712.000
Absorpt. coeff. (mm−1)0.120.343.642.762.76
Crystal size (mm3)0.05 × 0.05 × 0.010.22 × 0.11 × 0.060.62 × 0.40 × 0.210.15 × 0.15 × 0.10
Refinement methodFull matrix—least-squares
Measured reflections19468257966751510067238241
2 θmax (°)27.336.645.447.933.1
GoF on F21.
R1 [I > 2σ(I)]0.0540.0440.0500.0300.025
wR2 (all data)0.1330.1280.1440.0670.064
Larg. diff. peak/hole (e·Å3)0.28/−0.220.64/−0.661.88/−1.652.82/−2.340.76/−1.49
Table 2. Selected atom distances [Å] and mean plane angles [°].
Table 2. Selected atom distances [Å] and mean plane angles [°].
Distance or Angle3a3b3c3d3e
C=O carbonyl [Å]1.189(3)1.187(1)1.191(1)1.196(2)1.195(1)
C–O carbonyl [Å]1.389(2)1.392(1)1.398(1)1.395(2)1.400(1)
C=O succin. (av.) [Å]1.2001.2061.2091.2021.208
C–Hal [Å]1.356(3)1.737(1)1.893(1)2.094(1)2.095(1)
∢ mean plane [°] (halobenzoyl/succinimidyl residues)76.272.970.780.571.6

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Mamat, C.; Weiß, D.H.; Köckerling, M. X-ray Structures of Succinimidyl Halobenzoates. Crystals 2017, 7, 90.

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Mamat C, Weiß DH, Köckerling M. X-ray Structures of Succinimidyl Halobenzoates. Crystals. 2017; 7(3):90.

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Mamat, Constantin, Daniel Holger Weiß, and Martin Köckerling. 2017. "X-ray Structures of Succinimidyl Halobenzoates" Crystals 7, no. 3: 90.

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