Functionalization of photochromic dithienylmaleimides †

Photochromic dithienylmaleimides are well known molecular switches, but for applications the suitable functionalization of the photochromic sca ﬀ old is required. We report here synthetic routes to dithienylmaleimides, which are functionalized at three di ﬀ erent positions: at each of the thiophene moieties and the maleimide nitrogen. A Perkin-type condensation of two thiophene precursors is used as the key step to assemble the maleimide core, which allows the synthesis of non-symmetrically substituted dithienylmaleimides, such as photochromic amino acids. A di ﬀ erent approach to the maleimide core is provided by the reaction of a dithienylmaleic anhydride with amines or hydrazides leading to maleimide protected dithienylmaleimides and photochromic labeled natural amino acids. The photochromic properties of the new photoswitches were investigated showing reversible photochromism in polar organic solvents.


Introduction
5][6] In the eld of life sciences, molecular switches have been used to control enzyme activity, [7][8][9][10] Watson-Crick base pairing, [11][12][13] the regulation of neuronal activity by photochromic ligands for ion channels and receptors, [14][15][16][17][18][19][20] antibiotic effects 21,22 and even the agility of a living organism 23 by light.This broad applicability is one of the reasons why photopharmacology has evolved into a vibrant eld of research. 24Various photochromic molecules, like azobenzenes, 25 spiropyrans, 26 spirooxazines, 26 fulgides 27 and diarylethenes 28,29 have been developed.All these photoswitches can be reversible toggled between two isomers using light.The well investigated dithienylethenes (DTEs), including dithienylmaleimides, are characterized by a nearly quantitative photochemical conversion between the photoisomers, which are oen thermally stable.Irradiation with light of a specic wavelength switches the DTEs between their open and closed photoisomers, which differ in conformational exibility and electronic conjugation (Fig. 1).
Many DTEs show high fatigue resistance. 28Despite their outstanding photophysical properties the synthesis of DTEs, in particular of non-symmetric derivatives, is laborious. 13,30ifferent synthetic routes for the preparation of dithienylmaleimides were established.2][33] However, only nitrogen protected maleimides can be used and the synthesis of non-symmetric compounds is challenging.5][36] The synthesis of diarylmaleimides by intramolecular Perkin condensation of two independently prepared precursors gives selective access to non-symmetric diarylmaleimides. 10,37,38Compared to diarylperuorocyclopentenes and diarylcyclopentenes, diarylmaleimides are more hydrophilic and better water soluble, which is valuable for applications in biology and pharmacy.The absorption maxima of diarylmaleimides are shied to higher wavelengths and thus the photoisomerization can be induced by light with lower energy reducing potential cell damage. 280][41][42][43] However, a better synthetic access to functionalized photochromic dithienylmaleimides is desirable in order to extend their applications.Herein we discuss the synthesis of functionalized dithienylmaleimides substituted on each thiophene moiety and the maleimide nitrogen atom.

Synthesis
Functionalization of the maleimide nitrogen atom.The transformation of diarylmaleic anhydrides into their corresponding diarylmaleimides provides an easy access to compounds with a functionalized maleimide nitrogen atom. 28omplex functionalities or protecting groups can be introduced at the maleimide nitrogen by reaction with amines or hydrazides.We used the adapted synthetic approach of Scandola et al. 36 for the synthesis of anhydride 4 as precursor (Scheme 1).
Methyl ester 2 was converted to its potassium salt 3 and condensed in a Perkin reaction with carboxylic acid 1 yielding the photochromic maleic anhydride 4. The anhydride moiety allows the subsequent functionalization with hydrazides or amines (Scheme 2).Therefore maleic anhydride 4 was treated with a-Cbz protected L-glutamic acid g-hydrazide 44 (5) and a-Cbz protected L-lysine to give amino acids 6 and 7 with a photochromic dithienylmaleimide on each sidechain.Photochromic tripeptides forming hydrogels with different aggregation modes mainly depending on the switch moiety were recently reported. 45The reaction of hydrazine hydrate in acetic acid as solvent and 1,2-dimethylhydrazine dihydrochloride, respectively, with maleic anhydride 4 afforded the maleimide nitrogen protected dithienylmaleimides 8 and 9 in good yields (Scheme 2).Remarkably, the formation of any maleic hydrazide or other tautomers was not observed.The protected maleimides 8 and 9 could be used for further functionalizations on the thiophene moieties by palladium-catalyzed cross coupling reactions or other reactions using the reactivity of the heteroaryl chlorides.
Functionalization as photochromic amino acid.Recently, DTE-based non-natural amino acids were synthesized and successfully introduced into small peptides. 46However, their water-solubility is limited due to the diaryl per-uorocyclopentene core and therefore we developed a more polar dithienylmaleimide amino acid.Compounds 13a and 13b were prepared by a Perkin condensation 10,37,38 of the thiophene precursors 10 and 11 bearing a protected primary amino or carboxyl group, respectively (Scheme 3).The Alloc group was chosen as a suitable protection for the amine as it is stable during the synthesis of compound 12. Diester thiophene 11 provides in 4-position the carboxylic ester giving the maleimide core in the Perkin condensation.The ester in 2-position will serve as carboxylate of the amino acid.Both carboxylic acids were protected as methyl ester.Alloc group and methyl ester of 12 were cleaved simultaneously with boron tribromide giving amino acid 13a in 47% yield, accompanied by 20% of the Alloc amino acid 13b as byproduct.A selective non-hydrolytic deprotection of the methyl ester of 12 is possible in low yield using lithium iodide in a polar aprotic solvent. 47,48A large excess of lithium iodide and reux were necessary to achieve conversion; several solvents were tested with best yields in acetone (see ESI, Table S1 †).Standard basic hydrolytic conditions for the deprotection of the methyl ester afforded the deprotected maleic anhydride (see ESI, Scheme S2 †).The synthesis of thiophene 10 is depicted in Scheme 4. Bromination 49 of 2-methylthiophene ( 14) and subsequent Rosenmund-von Braun reaction 50 giving nitrile 16 were performed according to literature procedures.
The reduction of nitrile 16 with lithium aluminum hydride followed by immediate protection with allyl chloroformate afforded carbamate 17 in good yield.Using Fmoc chloride instead led to the respective Fmoc derivative in lower yields and caused the formation of side products in the subsequent Friedel-Cras acylation.The yield of glyoxylester 18 in the Friedel-Cras acylation depends critically on the sequence of the reagent addition.Best results were obtained by mixing 17 and methyl chlorooxoacetate before adding aluminum chloride in small portions.Quenching the reaction with saturated sodium hydrogen carbonate solution avoids the addition of hydrochloric acid to the allyl double bond.Aminolysis with aqueous ammonia converted the glyoxylester 18 in high yield into compound 10.The overall yield for 10 aer six steps is 22%.
Thiophene 11 was prepared by esterication 51 of methylthiophene acid 19 in the presence of thionyl chloride followed by Friedel-Cras acylation and nally a thallium trinitrate (TTN) mediated oxidative rearrangement 52 (Scheme 5).All intermediates were isolated in good to excellent yields with an overall yield of 68% for three steps.Initial moderate yields for the Friedel-Cras acylation of around 40% signicantly increased to 77% aer rigorous removal of stabilizers from the solvent chloroform.
Functionalization by Suzuki coupling.Dithienylmaleimides are conveniently synthesized by the Perkin-type condensation.The reaction of two precursors yields the maleimide core without the need for protection of the maleimide nitrogen.Scheme 6 shows the intramolecular Perkin condensation of the two chlorosubstituted precursors 22 and 23.Both precursors can be differently functionalized by Suzuki coupling before used in the Perkin condensation yielding non-symmetric dithienylmaleimides.
Recently, we described the synthesis of symmetric diarylmaleimides, with thiophene moieties functionalized by palladium-catalysis prior to the condensation reaction. 10Based on this strategy we prepared a small series of non-symmetric diarylmaleimides (Scheme 7).
The Perkin condensation to the maleimide core was performed under basic conditions combining the different thiophenes.Scheme 8 summarizes the synthesis of the nonsymmetric photoswitches 35-37.
Upon irradiating a methanol solution of the ring-open form of compound 12 with UV light (312 nm), the absorption band at  250 nm immediately decreases.Simultaneously, new absorption maxima at 232 nm, 378 nm and 550 nm arise (Fig. 2) causing the color change of the sample from slightly yellow to purple.The isosbestic points indicate a clean conversion between two components.Compared to typical DTEcyclopentenes the absorption maxima are red shied.The photostationary state was reached aer 42 s of irradiation (Herolab, 312 nm, 6 W) and the open form can be regained by irradiation with visible light (>420 nm) for 5 min.The photoswitchable amino acid 12 is stable over at least seven closing/ opening cycles (Fig. 3).
The absorption maxima and their corresponding extinction coefficients for the open and closed form of all synthesized photochromic compounds are summarized in Table 1.Interestingly, the long wavelength absorption maximum of compound 13a is blue shied to 537 nm compared to photoswitches 12 and 13b, which may indicate an interaction of the Alloc group with the dithienylmaleimide core.In contrast the selective removal of the methyl ester has almost no inuence on the photochromic properties.In comparison to bischloro dithienylmaleimide 24 the functionalized maleimides 35-37 show a bathochromic shi in their absorption maxima of the closed photoisomer.The enlarged p-system of the substituted thiophenes can explain this shi to higher wavelengths.

Conclusions
In summary, we have prepared several photochromic dithienylmaleimides.Maleimide nitrogen atom functionalized derivatives were obtained by the reaction of dithienylmaleic anhydride with different hydrazides and amines.Using a Perkin-type condensation non-symmetric dithienylmaleimides were synthesized including a photochromic amino acid and dithienylmaleimides with different aromatic substituents on each thiophene moiety.Reversible photoisomerization in dimethylsulfoxide and methanol was observed for all synthesized photochromic compounds.

4-(4-(5-(Aminomethyl
A solution of BBr 3 (1 M in CH 2 Cl 2 , 2.0 mL, 2.00 mmol) was added to a solution of compound 12 (92 mg, 0.20 mmol) in anhydrous CH 2 Cl 2 (6 mL) in a crimp top vial.The mixture was heated to 40 C for 5 h.Then water (4 mL) was added via syringe and the suspension was stirred at 40 C for additional 30 min.Aer cooling to room temperature the solvent was removed at the rotary evaporator.Purication by automated reversed phase ash column chromatography (MeCN/H 2 O with 0.05% TFA, 3-100% MeCN) yielded compound 13a (34 mg, 47%) as yellow solid and compound 13b (18 mg, 20%) as yellow solid.

Alternative procedure to obtain 13b
Compound 12 (40 mg, 0.09 mmol) was dissolved in acetone (10 mL) and LiI (350 mg, 2.60 mmol) was added.The mixture was heated to 100 C overnight.Aer cooling to room temperature it was quenched with 1 M aqueous HCl solution (5 mL) and diluted with CH 2 Cl 2 (5 mL).The phases were separated and the aqueous phase was extracted with CH 2 Cl 2 (3 Â 5 mL).The combined organic phases were dried over Na 2 SO 4 and the solvent was removed at the rotary evaporator.Automated reversed phase ash column chromatography (MeCN/H 2 O with 0.05% TFA, 3-100% MeCN) yielded compound 13b (14 mg, 35%) as yellow solid.

General procedure A: Suzuki coupling
To a suspension of Pd 2 (dba) 3 (5 mol%), XPhos (10 mol%), the appropriate boronic acid (1.5 eq.) and K 3 PO 4 (1.5 eq.) in 1,4dioxane (0.5 M) the appropriate ester (1.0 eq.) was added.The resulting mixture was heated to 100 C and stirred overnight.Aer cooling to room temperature the reaction mixture was diluted with EtOAc and the organic phase was washed two times with water.The organic phase was dried over MgSO 4 , ltered and the solvent was removed under reduced pressure.

General procedure B: aminolysis
An NH 4 OH solution (25% in H 2 O) (10.0 eq.) was added to a solution of the appropriate oxoacetate (1.0 eq.) in THF (0.3 M) at 0 C. The reaction was stirred for 1 h at room temperature and then quenched with water.The aqueous phase was extracted with EtOAc.The combined organic phases were dried over MgSO 4 , ltered and the solvent was removed under reduced pressure.

General procedure C: Perkin condensation
KOtBu (1 M in THF) (1.2 eq.) was added to a solution of the appropriate amide (1.0 eq.) in THF (0.2 M) at 0 C. Aer 90 min stirring at 0 C the appropriate ester (1.0 eq.) was added at 0 C and stirred overnight at room temperature.The reaction was quenched with 1 M HCl and diluted with EtOAc.The organic phase was washed three times with water and one time with brine.The organic phase was dried over MgSO 4 , ltered and the solvent was removed under reduced pressure.

Fig. 1
Fig. 1 Reversible photochemical isomerization of a dithienylmaleimide between the open and closed photoisomer by irradiation with light of different wavelength.

Fig. 2
Fig. 2 Changes of the UV-Vis absorption spectra of dithienylmaleimide amino acid 12 (50 mM in MeOH) upon light irradiation with 312 nm; arrows indicate the changes of the absorption maxima over 42 s irradiation in periods of 6 s (Herolab, 6 W); the cuvettes show the color of the solution before and after irradiation.

Fig. 3
Fig. 3 Cycle performance of the dithienylmaleimide amino acid 12 (50 mM in MeOH).Changes in absorption at 554 nm were measured during alternate irradiation with light of 312 nm for 60 s (Herolab, 6 W) and 530 nm (CREE-XP green, 700 mA) for 5 min.