Synthesis and Reactions of a Symmetric Paramagnetic Pyrrolidine Diene

Abstract

Symmetric paramagnetic diene 6 was prepared and proved to be a useful intermediate in 1,4-addition, cheletropic, and Diels-Alder reactions. These methods were used for the synthesis of both aromatic 10, 12 heteroaromatic 15, 23 and heterocyclic ring fused nitroxide rings 7, 14, 16, 25a, 25b. The symmetric dibromo compound 20 led to homobifunctional spin label reagents 21, 22b, 24a, 24b, 27a, 27b heterobifunctional reagents 26a, 28a and to double sensor reagents 26b, 28b.The Diels-Alder reaction is an important tool in organic chemistry for construction of six-membered rings. A great variety of carbo-and heterocyclic ring systems have been synthesized in this way. 1 The synthesis of paramagnetic asymmetric six-membered 2 and five-membered 3 (17) dienes is well documented and their Diels-Alder reactions were also reported. The challenge was the synthesis of paramagnetic symmetric pyrrolidine diene 6 which was expected to be a versatile synthetic intermediate. It is interesting to note that the synthesis of the sterically hindered amine precursor of target molecule 6 was reported 30 years ago, 4 but as far as we know the amine was not oxidized to nitroxide. Stable paramagnetic nitroxides are widely used as spin labels, 5 MRI reagents 6 or co-oxidants 7 and their pharmacological properties are also of interest. 8In this paper we report on the synthesis and reactions of a paramagnetic symmetric pyrrolidine diene 6 in the presence of a paramagnetic free radical centre. Oxidation of γ-nitro ester 1, which was synthesized by Michael addition of nitromethane to an α,β-unsaturated ester, 9 with Nmethylmorpholin-N-oxide (NMO) in the presence of tetrapropylammonium perruthenate (TPAP) resulted not only in the conversion of the nitro group to a formyl group 10 but also in the oxidation of the pyrrolidine ring to pyrroline by forming the double bond yielding the α,β-unsaturated aldehyde ester 2. We found a simpler and cheaper oxidation method with KMnO 4 in basic medium 11 which gave saturated aldehyde ester 3 and also biradical 4 as a byproduct. This can be attributed to the in situ Henry nitro condensation 12 between 1 and 3 followed by oxidation. Treatment of compound 3 with NMO/TPAP yielded 2, demonstrating that a Ley-Griffith oxidation reagent 13 was capable of introducing the double bond. Compound 3 was easily reduced with NaBH 4 in EtOH to saturated paramagnetic 1,4-diol 5a which could be converted to the diiodide 5b through mesylate. The diiodide 5b was converted to the thiol specific reversible bisthiosulfonate reagent 5c by cautious heating of compound 5b with NaSSO 2 Me in DMSO. Elimination of HI from the diiodide 5b was carried out in aqueous NaOH/EtOH solution with heating resulting in the desired diene 6 (Scheme 1).Reagents and conditions: a) NMO (1.5 equiv)/AgOAc (2 equiv)/ K 2 CO 3 (5 equiv)/TPAP (0.1 equiv)/MeCN, 40°C, 12 h (62-75%); b) KOH (1.2 equiv)/MeOH, 0°C, 30 min, then KMnO 4 (0.7 equiv)/ MgSO 4 (1 equiv)/H 2 O, 0°C, 2 h (3: 37%, 4: 59%); c) NaBH 4 (3 equiv)/EtOH, 30 min, r.t. (80%); d) i: MsCl (2.2 equ...

High-resolution helix orientation in actin-bound myosin determined with a bifunctional spin label

“…The spin-labeled amino acid TOAC provides stereospecific attachment to the peptide backbone, but this probe is currently only practical for peptides on the order of 50 amino acids or less (16). For larger proteins, spin labels have been synthesized with bulky substituents to reduce mobility (17), or substitution with additional reactive moieties to confer bifunctionality (18)(19)(20). The smallest and simplest of these derivatives shares its basic structure with the widely used methanethiosulfonate spin label [1-oxyl-2,2,5,5-tetramethyl-Δ3-pyrroline-3-methyl methanethiosulfonate spin label (MTSSL)], with a second MTS group that allows bifunctional targeting of two Cys residues (20) (Fig.…”

New Ethylenedithio-TTF Containing a 2,2,5,5-Tetramethylpyrrolin-1-yloxyl Radical through a Vinylene Spacer and Its FeCl4− Salt—Synthesis, Physical Properties and Crystal Structure Analyses

“…Cyclic voltammograms were measured in benzonitrile at 25 • C using tetra-n-butylammonium hexafluorophosphate as a supporting electrolyte. The obtained redox potentials of donor 1 are summarized in Table 1 together with those of EDT-TTF and 2,2,5,5-tetramethylpyrrolin-1-yloxyl radical derivative 5 [20] measured under the identical conditions. Molecule 1 showed three pairs of one-electron reversible redox waves (E 1 , E 2 and E 3 ).…”

A combined EPR and MD simulation study of a nitroxyl spin label with restricted internal mobility sensitive to protein dynamics

“…Although the inherent flexibility of R1, arising from five dihedral angles (Figure 1), allows the introduction of this spin label (SL) to many accessible sites of a protein it does not allow the unambiguous separation of side-chain motional dynamics from that of the protein. (9)(10)(11)(12)(13)(14) In fact, the rich conformational dynamics of the MTSL label and its distribution can significantly complicate both the analysis and interpretation of CW EPR spectra and long-range distance measurements in proteins using pulsed EPR techniques (11).…”

“…Examples previously employed include introduction of a bulky substituent in the nitroxide ring (12), generation of a second covalent bond between the nitroxide ring and the protein backbone (6,13) and genetically encoded artificial amino acids for labelling specially synthesised probes (14). pyridine ring attached to the nitroxide group (15).…”

“…More recently, Hubbell and co-workers have reported the introduction of nitroxide spin probe (RX) linking to the protein via two cysteine residues (16) with highly constrained motion. However, such spin probes are not straightforward in sample preparation and may significantly perturb the local structure of the protein (13,14).…”