Development of a Photolabile Amine Protecting Group Suitable for Multistep Flow Synthesis

Full Paper

Han Yueh, Anastasia Voevodin and Aaron B. Beeler* Department of Chemistry and Center for Chemical Methodology and Library Development (CMLD-BU), Boston University, 590 Commonwealth Avenue, Boston, Massachusetts 02215

Received: 3 May 2014; accepted: 10 June 2014

9-Hydroxymethylxanthene derivatives were optimized as a photolabile protecting group for amines in flow chemistry. 9-Methylxanthene and 2-methoxy-9-methylxanthene showed excellent deprotection yields in protic and aprotic solvents, respectively. The protecting group has good stability in acidic, basic, and thermal conditions and was successfully utilized for protection and deprotection of a variety of amines. A multistep continuous-flow synthesis of a piperazinylcarbonyl-piperidine derivative utilized the 2-methoxy-9-methylxanthene as the key protecting group utilized in an orthogonal manner. Keywords: photochemistry, flow chemistry, photolabile protecting group, multistep flow synthesis

1. Introduction deprotection ensuring that conditions would be orthogonal to a Boc protecting group (Figure 1) [19]. Photolabile protecting groups have drawn substantial atten- We first evaluated deprotection utilizing batch and flow tion in synthetic chemistry, due to the mild, neutral deprotection reactors with acetonitrile as solvent (Table 1). The reaction was conditions [1]. Several studies have described carbamoyl deriv- very inefficient with only 33% conversion in batch and 19% in atives as photolabile protecting groups for amines such as flow. Based on the report by Boyd, we evaluated the reaction in an benzoins [2], o-nitrobenzyl [3], 9-fluorenone [4], or 2,5-dime- acetonitrile–water (1:1) mixture. The reaction was significantly thylphenacyl [5] groups. Likewise, 3′-nitrophenylpropyloxy- improved in both batch and flow. We observed 88% conversion carbonyl [6], dimethoxybenzoin carbonate [7], and nitro-sub- after 30 min residence time in flow. stituted benzenesulfenyl groups [8] have been developed as We carried out a broad screen of solvents for the photochemical photolabile groups for protecting hydroxyls and 2-hydroxy- deprotection. Each reaction was carried out in flow with 10 min 1,2,2-triphenylethanone [9], 7-methoxycoumarin [10], anthra- residence time (Figure 2). In a number of organic solvents (dime- cene-9-methanol [11], 2-(1′-hydroxyethyl)-anthraquinone [12], thylformamide [DMF], dichloroethylene [DCE], dioxane, pyri- and o-nitrophenyl [13] for acids, aldehydes, and ketones. These dine, THF, dimethylamphetamine [DMA]), no conversion was photoremovable groups are, by nature, orthogonal to traditional observed. There was low conversion in acetonitrile (19%), etha- protecting groups and have even been demonstrated to have nol (29%), and methanol (34%). However, an excellent conver- orthogonality based on excitation wavelength [14]. Overall, they sion was observed (95%) when the reaction was carried out in have led to advances in many areas including techniques for 2,2,2-trifluoroethanol (TFE). organic synthesis and biochemical applications [15]. Because synthetic utility and application in multistep flow Development of platforms combining photochemistry and chemistry would be limited in aqueous solution or TFE, a more flow chemistry has enabled rapid, scalable, and efficient photo- organic solvent-friendly photolabile protecting group was chemical transformations [16]. Multistep flow synthesis has required. We hypothesized that the dramatic increase in reaction become a powerful tool in the synthesis of small molecules, and efficiency with TFE may be due to an extended triplet lifetime integration of photochemistry in the multistep processes has sig- facilitated by the solvent. To support this premise and poten- nificant potential [17]. tially develop an analog with much better solvent compatibility, In an effort to devise an efficient photolabile protecting group we synthesized xanthene derivatives with varying electronic for amines that would be suitable for multistep flow synthesis, properties. we turned our attention to the known 9-methylxanthene [18]. Synthesis of the substituted xanthenes (Scheme 1) began with Boyd demonstrated amine protections utilizing ultraviolet (UV) condensation of salicylaldehydes 4a–4c and 2-cyclohexenone irradiation in acetonitrile–water. The deprotection reactions (5) catalyzed by Sc(OTf) [20]. The resulting xanthenes 6a–6c afforded good yields for a number of primary amines. Herein, 3 were functionalized with paraformaldehyde in the presence of we report the use of 9-methylxanthene derivatives as effective n-BuLi to afford primary alcohols 7a–7c. The alcohols were photolabile protecting groups for a variety of amines and appli- converted to the carbonates (8a–8c) and used for protection of cation to multistep flow synthesis. N-boc-piperazine to afford carbamates 9a–9c. We utilized our flow reactor for evaluation of the deprotection of xanthene analogs 9a–9c in CH3CN with irradiation (>305 nm) 2. Results and Discussion for 10 min. 1,3-Dimethoxy analog 9b showed no reaction at all. Electron poor 9c had a slight improvement over xanthene 3 with a Xanthene 1 was synthesized from xanthene-9-carboxylic 30% conversion. We were pleased to see a significant improve- acid as reported in the literature [18]. Addition of compound 1 ment with compound 9a which had a 73% conversion. to 1-boc-piperazine (2) in tetrahydrofuran (THF) at room tem- We evaluated xanthene 9a in a broad range of solvents perature afforded carbamate after 1 h. Xanthene 3 would serve (Figure 3). Based on a slight shift in absorbance we carried as our primary substrate for optimization of the photochemical out the reactions with 280 nm cutoff. Interestingly, we observed a nearly inverse solvent dependence compared to compound 3. * Author for correspondence: [email protected] Conversion was 95% in 1,2-dichloroethane and moderate

Figure 1. A) Addition of xanthene protecting group to N-boc-piperazine. B) Photochemical flow reactor

Table 1. Deprotection of N-boc-piperazine in batch and flow reactors optimization, with the exception of phenylalanine methyl ester, which required 1 day to afford a reasonable yield. Overall, the deprotections were successful with no individual optimization. Photodeprotections were performed in dichloroethane and irradiated for 10 and 20 min (>280 nm). Piperazine (entry 2, compound 10) was deprotected in good yield after 20 min (80%). Interestingly, benzyl amine (entry 3, compound 11) showed no conversion after 10 min but good yield after 20 min. This experiment was repeated multiple times with similar results, Reactor Solvent Time (min) Yield (%) and we currently have no explanation for the lack of conversion after 10 min. Boc-protected amino piperidine (entry 4, compound Batch CH3CN 60 33 Flow CH3CN 10 19 12) was deprotected in good yield with 10 min residence time – Batch CH3CN H2O60 77(84%) but showed some decomposition after longer irradiation. – Flow CH3CN H2O10 63Boc-protected diamine (entry 5, compound 13) was depro- Flow CH3CN–H2O20 84 Flow CH CN/H O30 88tected in moderate yield when irradiated for 10 min (65%) with 3 2 a minor increase at 20 min (77%). Primary amine (entry 6, compound 14) was the least successful substrate with only conversion was observed in a number of other non-protic sol- 41% observed. A lower yield (63%) was also observed with a vents. Conversion in aqueous solution and alcohols was signifi- primary amine (entry 7, compound 15). Amino alcohol (entry 8, cantly lower compared to that of 3. We propose that the electron compound 16) was also successfully deprotected in good yield donation of the 2-methoxy group stabilizes the excited triplet state (83%) after 10 min residence time and phenylalanine methyl and hydrogen bonding in protic solvents may diminish such ester (entry 9, compound 17) afforded a similar yield (78%) an effect. after 20 min irradiation. Based on an option of two xanthene protecting groups (3 and A crucial aspect of a synthetically useful protecting group is 9a) to afford protic and aprotic solvent compatibility, we the overall stability under a variety of conditions. Therefore, we believed that this protecting group would serve as practical performed a number of experiments to assess the overall stabil- option in flow synthesis. However, to fully determine their ity of compound xanthene protected piperazine 10 (Table 3). It potential, we evaluated a number of amine substrates for pro- was very stable under basic and acidic conditions with gentle tection and deprotection with 2-methoxy-hydroxymethylxan- heating (entries 1 and 2). It was also stable to NaH at room thene 9a (Table 2). Protection of the amines was carried out in temperature (entry 3) but decomposed in the presence of 1 M THF with 1 equivalent of triethylamine at room temperature. potassium t-butoxide. Most impressively, this protecting group The reactions were allowed to stir for 30 min to an hour. In all was stable under extreme heating conditions of 180 °C in the cases, these conditions afforded excellent yields without microwave for 10 min.

Figure 2. Photodeprotection on compound 3 with various solvents on flow reactor. Deprotection condition: 10 mM compound 3 in solution, 305 nm (long pass), and 10 min residence time

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Scheme 1. Synthesis of substituted-9-methylxanthene protecting groups

Scheme 2. Photochemical deprotection of xanthene analogs

The first application of the 2-methoxy-hydroxymethylxan- time. The reaction mixture was subsequently flowed into the thene protecting group in multistep continuous flow was in the photochemical reactor and irradiated for 15 min. The solu- synthesis of a piperazinylcarbonyl-piperidine derivative uti- tion flowed into a T-junction which introduced a 200-mM lized in medicinal chemistry projects in our laboratory [21]. solution of benzoyl chloride in DCE. After a 10-minute re- The design of our sequential continuous-flow system is out- sidence time, the final product 19 was collected. The com- lined in Figure 4. The flow rate and concentrations are based bined residence time for the three steps was 55 min, and the on 25 mM concentration and 15 min residence time for the overall isolated yield was 42% (average yield per step, 75%). photodeprotection. Protected piperazine 10 (50 mM) was This multistep flow synthesis compared favorably with un- combined with activated ester 18 (60 mM) and diisopropyle- optimized batch reactions which include coupling of 10 and thylamine (DIPEA) (60 mM, 1.2 eq.) at a T-junction. The 18 affording 56% yield and the final acylation affording 59% reaction was incubated at 70 °C with a 30-minute residence yield (33% combined yield).

Figure 3. Solvent evaluation of photodeprotection for xanthene 9a, reactions were carried out at 10 mM with 10 minutes residence time and irradiated at >280 nm

157 Photolabile Amine Protecting Group

Table 2. Protection and photodeprotection utilizing xanthene 8a with a variety of amines Entry Amine Protected compound Protection (%) Deprotection, 10 min (%) Deprotection, 20 min (%) 1 9a 95 92 55

2 10 87 66 80

3 11 94 – 88

4 12 89 84 74

5 13 92 65 77

6 14 85 41 35

7 15 90 60 63

8 16 84 83 84

9 17 69 61 78

Table 3. Stability tests for compound 10, 10 mM in acetonitrile Entry Condition r.t. 1 h 50 °C, 1 h 80 °C, 1 h uW 180 °C, 10 min 1 1 M HCl No decomp. No decomp. No decomp. – 2 1 M TFA No decomp. No decomp. No decomp. – 3 1 M NaH No decomp. –– – 41MtBuOK Decomposed –– – 5 Heating – No decomp. No decomp. No decomp.

3. Conclusion group in a multistep flow synthesis to afford a critical building block for projects in our research group. We have developed a photolabile protecting group for amines based on 9-hydroxymethylxanthene as the core chromophore. The previously reported 9-hydroxymethylxanthene can be uti- 4. Experimental lized in polar, protic solvent environments, and the novel 2-methoxy-9-hydroxymethylxanthene was more compatible with 4.1. General Procedure for Photodeprotection. The reac- organic solvents. The 2-methoxy-9-hydroxymethylxanthene car- tions were performed utilizing flow reactors assembled with bamate showed excellent protection yields and good photodepro- fluorinated ethylene propylene (FEP) tubing (1/32″ inner diam- tection yields with various amines. We have utilized the protecting eter [ID], 1/16″ outer diameter [OD], Cole-Parmer, Vernon

Figure 4. Continuous-flow multistep synthesis of 19

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Hills, IL) and quartz rods (Chemglass Life Sciences, Vineland, 2. (a) Cameron, J. F.; Willson, C. G.; Frechet, J. M. J. J. Chem. Soc., Chem. Commun. 1995, 923–924; (b) Cameron, J. F.; Willson, C. G.; Frechet, J. M. J. NJ). The UV light source was generated by a Xe(Hg) arc lamp J. Am. Chem. Soc. 1996, 118, 12925–12937; (c) Papageorgiou, G.; Corrie, J. E. T. (Newport, Irvine, CA) with Edmund Optics (Barrington, NJ) Tetrahedron 1997, 53,3917–3932; (d) Pirrung, M. C.; Huang, C. Y. Tetrahedron Lett. 1995, 36,5883–5884. long pass filters (280 nm or 305 nm). Solutions of protected 3. (a) Patchornik, A.; Amit, B.; Woodward, R. B. J. Am. Chem. Soc. 1970, amines were flowed through the FEP photoreactor with Harvard 92, 6333–6335; (b) Cameron, J. F.; Frechet, J. M. J.; J. Am. Chem. Soc. 1991, Apparatus (Holliston, MA) PHD Ultra syringe pump, and the 113, 4303–4313. 4. Bucher, G.; Scaiano, J. C.; Sinta, R.; Barclay, G.; Cameron, J. J. Am. flow rates were dependent on the volume of the reactor. Chem. Soc. 1995, 117, 3848–3855. 4.2. Multistep Sequential Synthesis on Flow. The multistep 5. Kammari, L.; Plistil, L.; Wirz, J.; Klan, P. Photochem. Photobiol. Sci., sequential flow synthesis platform was also made by FEP tub- 2007, 6,50–56. 6. (a) Beier, M.; Hoheisel, J. D. Nucleic Acids Res. 2000, 28, e11; (b) ing (1/32″ ID, 1/16″ OD) and quartz rods (Figure 4) equipped Pirrung, M. C.; Dore, T. M.; Zhu, Y.; Rana, V. S. Chem. Commun. 2010, 46, with three reaction zones. The amide coupling with 2-methoxy- 5313–5315. 7. Pirrung, M. C.; Bradley, J. C. J. Org. Chem., 1995, 60,1116–1117. xanthene protected piperazine (10) and activated ester 18 was 8. (a) Patchornik, A.; Amit, B.; Woodward, R. B. J. Org. Chem. 1970, 92, carried out in an FEP coil immersed in a hot water bath. A 40-psi 6333–6335; (b) Wakamatsu, K.; Kouda, M.; Shimaoka, K.; Yamada, H. Tetrahe- back pressure valve was located at the end of this first reaction dron Lett. 2004, 45,6395–6398. 9. Ashraf, M. A.; Russell, A. G.; Whartonb, C. W.; Snaith, J. S. Tetrahedron, zone. The subsequent reactor was irradiated as described for the 2007, 63, 586–593. general photodeprotection procedure to afford a piperazine 10. Schade, B.; Hagen, V.; Schmidt, R.; Herbrich, R.; Krause, E.; Eckardt, T.; adduct which was then benzoylated in the third reactor to afford Bendg, J. J. Org. Chem. 1999, 64, 9109–9117. 11. Singh, A. K.; Khade, P. K. Tetrahedron Lett. 2005, 46, 5563–5566. the final product 19. Purification by column chromatography 12. Ren, M. G.; Bi, N. M.; Mao, M.; Song, Q. H. J. Photochem. Photobiol., A afforded 19 in 42% isolated yield. The flow rate was calculated 2009, 204,13–18. 13. (a) Lage Robles, J.; Bochet, C. G. Org. Lett. 2005, 7, 3545–3547; (b) based on the residence time for the photodeprotection, and the A. Blanc, C. G. Bochet J. Org. Chem 2003, 68, 1138–1141. residence time of the first and third reactors was controlled by 14. (a) Bochet, C. G. Angew. Chem., Int. Ed. Engl. 2001, 40, 2071–2073; (b) reactor volume. For example, the capacity of the photoreactor Bochet, C. G. Tetrahedron Lett. 2000, 41, 6341–6346. μ μ 15. (a) Klan, P.; Solomek, T.; Bochet, C. G.; Blanc, A.; Givens, R.; Rubina, in our design was 350 L, and the flow was 23.33 L/min (1×) M.; Popik, V.; Kostikov, A.; Wirz, J. Chem. Rev. 2013, 113,119–191; (b) to give 15 min residence time for the photodeprotection. The Bochet, C. G. J. Chem. Soc., Perkin Trans. 1 2002, 125–142; (c) Wilcox, M.; flow rate for both of the starting reagents (10 and 18) was set to Viola, R. W.; Johnson, K. W.; Billington, A. P.; Carpenter, B. K.; McCray, J. A.; Guzidowski, A. P.; Hess, G. P. J. Org. Chem. 1990, 55, 1585 –1589; (d) Adams, S. 11.67 μL/min (0.5×), and the volume of first reactor was 700 μL R.; Kao, J. P. Y.; Tsien, R. Y. J. Am. Chem. Soc. 1989, 111, 7957–7968. to give 30 min residence time. The final reagent, benzoyl chlor- 16. (a) Martin, V. I.; Goodell, J. R.; Ingham, O. J.; Porco, J. A. Jr.; Beeler, A. B. J. Org. Chem. 2014, 79, 3838–3846; (b) Maskill, K. G.; Knowles, J. P.; ide in DCE, was injected into the third reactor with a flow rate of Elliott, L. D.; Alder, R. W.; Booker-Milburn, K. I. Angew. Chem., Int. Ed. 2013, 5.83 μL/min (0.25×) which gave the flow rate in the reactor as 52,1499–1502; (c) Hang, Y.;Blackman, M. L.; Leduc, A. B.; Jamison, T. F. Angew. 29.16 μL/min (1.25×), and therefore, the volume for the third Chem., Int. Ed. 2013, 52, 4251; (d) Knowles, J. P.;Elliott, L. D.; Booker-Milburn, K. μ I. Beilstein J. Org. Chem. 2012, 8,2025–2052; (e) Pimparkar, K.; Yen, B.; Goodell, reactor was 292 L to have 10 min residence time. J. R.; Martin, V. I.; Lee, W.-H.; Porco, J. A. Jr.; Beeler, A. B.; Jensen, K. J. J. Flow. Chem. 2011, 2,53. Acknowledgements. We are grateful for funding provided 17. (a) Battilocchio, C.; Hawkins, J. M.; Ley, S. V. Org. Lett. 2013, 15, 2278– 2281; (b) Brodmann, T.; Koos, P.; Metzger, A.; Knochel, P.; Ley, S. V. Org. by Cyclofluidic Ltd. We also thank Drs. Chris Selway, Gary Process Res. Dev. 2012, 16, 1102–1113; (c) Webb, D.; Jamison, T. F. Chem. Sci. Tarver, and Trevor Morgan (Cyclofluidic) for helpful discussion 2010, 1, 675–680. throughout the project. 18. Du, H.; Boyd, M. K. Tetrahedron Lett. 2001, 42, 6645–6647. 19. (a) Chen, H. T.; Neerman, M. F.; Parrish, A. R.; Simanek, E. E. J. Am. Chem. Soc. 2004, 126, 10044–10048; (b) Saleh, M. A.; Compernolle, F.; Van den Brandon, S.; Buysser, W. D.; Hoornaert, G. J. Org. Chem. 1993, 58,690–695; Supporting Information (c) Saleh, M. A.; Compernolle, F.; Van den Brandon, S.; Buysser, W. D.; Hoornaert, G. J. Org. Chem. 1993, 58,690–695. Electronic Supplementary Material (ESM) is available in the 20. Böβ, E.; Hillringhaus, T.; Nitsch, J.; Klussmann, M. Org. Biomol. Chem. 2011, 9, 1744–1748. online version at doi: 10.1556/JFC-D-14-00016. 21. (a) Steinmetzer, T.; Schweinitz, A.; Stürzebecher, A.; Dönnecke, D.; Uhland, K.; Schuster, O.; Steinmetzer, P.; Müller, F.; Friedrich, R.; Than, M. E.; Bode, W.; Stürzebecher, J. J. Med. Chem. 2006, 49,4116–4126; (b) Sriniva- References san, S.; Shafreen, R. M. B.; Nithyanand, P.; Manisankar, P.; Pandian, S. K. Eur. J. Med. Chem. 2012, 45, 6101–6105; (c) Debnath, A. K. J. Med. Chem. 2003, 46, 1. Pillai, V. N. R. Organic Photochemistry; Padwa, A. Ed.; Marcel Dekker: 4501–4515; (d) Efange, S. M. N.; Khare, A. B.; von Hohenberg, K.; Mach, R. H.; New York, 1987, vol. 9, Ch. 3, 225–323. Parsons, S. M.; Tu, Z. J. Med. Chem. 2010, 53, 2825–2835.

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