LETTER 749 Synthesis of 3-Nitropyrrolidines via Dipolar Cycloaddition Reactions Using a Modular Flow Reactor SynthesisMarcus of 3-Nitropyrrolidines Baumann, Ian R. Baxendale, Steven V. Ley* Innovative Technology Centre, Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK Fax +44(1223)767798; E-mail: [email protected] Received 20 November 2009 This modular platform consists of a dual pumping unit Abstract: The generation and subsequent use of unstabilised azomethine ylides in dipolar cycloaddition reactions within a flow which delivers the dissolved starting materials into a mix- microreactor is demonstrated. The 3-nitropyrrolidines produced ing device and directs the combined reaction stream were furthermore subjected to chemoselective hydrogenation reac- through a selection of convection flow coils (CFC) and/or tions using the H-Cube® system. To ensure product purities in ex- glass tubes13 filled with solid-supported reagents and cess of 90–95%, immobilised scavengers were successfully scavengers that can be maintained at a desired tempera- employed. ture and pressure. Furthermore, this system can be run Key words: microreactor, flow chemistry, pyrrolidine, dipolar cy- with the Flow Commander software also available from cloaddition, solid-supported reagents Vapourtec12 allowing for the use of a front-end liquid han- dler and a UV-directed fraction collector. The synthesis of highly functionalised heterocyclic com- H O pounds is of central importance to most modern medicinal N N chemistry programmes1 since these building blocks are N N N OH N N H N H readily manipulated to compounds with favourable phar- H -proline -prolinyl tetrazole nicotine L macophoric properties. Accordingly, the synthetic chem- L ist is expected to deliver these entities in a flexible, high- O yielding fashion yet avoiding time-consuming purifica- H O NH Ar tion procedures as well as hazardous or obnoxious chem- HO OTMS NH O 2 3 2 N ical inputs wherever possible. In recent studies we and N N 4 Ar H others have attempted to address some of these issues by NC using modular flow reactors in concert with automation H vildagliptin Jörgensen's catalyst methods and in-line immobilised reagents and scavengers levetiracetam packed into glass columns to effect clean product forma- Figure 1 Biologically and synthetically interesting pyrrolidines tion. We have already demonstrated the success of these meth- For the preparation of 3-nitropyrrolidine derivatives and ods in the preparation of various heterocyclic scaffolds in order to access more structural variety in the pyrroli- 5 6 7 8 such as oxazoles, oxazolines, pyrazoles, triazoles, thi- dine ring, we have chosen to use a dipolar cycloaddition 9 azoles, and imidazoles using a variety of flow microreac- process involving nonstabilised azomethine ylides and ni- tors. Here we show how these devices can be applied tro alkenes.14 This reaction also readily allows for the sub- further to the efficient assembly of 3-nitropyrrolidines and sequent differentiation of both nitrogen functionalities related structures as potentially useful building blocks for and hence increases the flexibility towards any subse- synthesis since pyrrolidine-based compounds have been quent transformation (Scheme 1). shown to display a wide variety of biological activities.10 These structures can be found in alkaloids such as nicotine In order to perform initial cycloaddition studies, toluene or in amino acids typified by proline, as well as many drug was selected as the preferred solvent to dissolve both the substances such as the anticonvulsant Levetiracetam or nitro alkene as well as the commercially available N-(meth- the oral antihyperglycemic agent Vildagliptin (Figure 1). oxymethyl)-N-(trimethylsilyl)benzylamine coupling part- In addition, many chiral pyrrolidines have been described ner at concentrations up to 0.2 M. The further addition of as privileged structures with numerous applications in the 1.0 equivalent of TFA to the nitro alkene solution was field of asymmetric organocatalysis.11 found to be crucial in order to generate the dipole even at the elevated temperatures of the flow reaction. When In our investigations below we make use of the R2+/R4 these stock solutions were mixed in a standard static mix- 12 flow system commercially available from Vapourtec. ing tee and flowed through a 10 mL CFC heated to 120 °C, with a residence time of between 30–90 minutes, the corresponding 3-nitropyrrolidine was formed. Subse- SYNLETT 2010, No. 5, pp 0749–0752 16.3.2010 Advanced online publication: 25.01.2010 quent optimisation led to the observation that acetonitrile, DOI: 10.1055/s-0029-1219344; Art ID: D33709ST in most cases, was a superior solvent to toluene as it was © Georg Thieme Verlag Stuttgart · New York 750 M. Baumann et al. LETTER more easily removed, and the starting materials could be By being able to carefully adjust the reaction parameters prepared at higher concentrations (up to 0.5 M). Further- of the flow reactor more sensitive nitropyrrolidines such more, real time product analysis via LC-MS was more as those with labile halides on heterocyclic ring systems convenient in acetonitrile as opposed to toluene due to the (1, 2, and 5) or the bicyclic octahydroisoindoline deriva- high UV absorption of the toluene signal at key monitor- tive 3 could also be readily obtained in high yield. ing wavelengths. Having prepared this first set of nitropyrrolidine deriva- tives we wished to further simplify our protocol by avoid- ing the use of the strong TFA acid in order to generate the reactive dipole. In relation to some of our earlier studies on monolithic reactor cartridges we were attracted by the concept of a reloadable fluoride monolith which we had previously found very powerful in a number of trifluo- romethylation reactions using Ruppert’s reagent (TMS- 17b CF3) in a flow process. After preparation and charging of this ion-exchange monolith according to previously published literature procedures3e,16,17 we started our inves- Scheme 1 Optimised flow set-up for formation of 3-nitropyrrolidines tigations in the azomethine ylid cycloaddition chemistry. We were pleased to confirm that indeed high conversions We have also shown that by passing the exiting flow of the starting materials could be achieved even at reduced stream through a glass column packed with an immobil- temperatures of between 50–80 °C in comparison to our ised benzylamine scavenger resin (QP-BZA,15 2.5 equiv) original procedure. Furthermore, we were able to reduce followed by a plug of silica gel (1 cm path length), it was the overall reaction time including the QP-BZA-assisted possible to remove any unreacted nitro alkene and simul- removal of excess nitro alkene starting material to less taneously release the desired nitropyrrolidine from its ini- than one hour. Using the fluoride monolith also resulted in tially formed TFA salt. By using the above set of increased yields of 3-nitropyrrolidine products when conditions a small collection of 3-nitropyrrolidines compared to the previous method using TFA (Figure 3, (Figure 2) was quickly generated using temperatures compounds 1, 3, 9). Encouraged by these new results we ranging from 60–120 °C and affording high-purity prod- expanded the initial work by varying the type of dipolaro- ucts in good isolated yields. phile. Again, a small collection of differently substituted pyrrolidine products was prepared (Figure 3) including O N 2 N O interesting substituents such as sulfonates, phosphonates, 2 Br N Bn and esters. Starting from D-menthyl acrylate we also in- MeO vestigated the impact of a chiral auxiliary on the diastere- N S Bn omeric ratio of the product outcome. However, under all Cl N conditions evaluated (time, temperature, stoichiometry, 1, 76% , 77% 2 and solvent modifications) only a 1:1 mixture of diastereo- mers was ever observed by NMR analysis. NO O 2 Bn NO Bn 2 NO 2 Bn N N N Bn N O OBn O N H O F Cl N P Bn Bn O O 3, 74% 5, 82% 4, 79% EtO OEt MeO F CO 3 O N 2 NO NO 2 2 11, 86% 12, 89% 13, 92% O N Bn N Bn N Bn O N Bn O Bn N 6, 87% 7, 81% 8, 91% O S O O N Bn 2 NO 2 N 14, 83% 15, 87% Bn NO 2 Cl N N O N 2 O N Br 2 Bn N Bn N O N 2 H 10, 93% 9, 88% S Bn 1, 83% , 84% 9, 91% Figure 2 Collection of 3-nitropyrrolidines prepared in flow 3 Figure 3 Pyrrolidines prepared using a fluoride monolith in flow Synlett 2010, No. 5, 749–752 © Thieme Stuttgart · New York LETTER Synthesis of 3-Nitropyrrolidines 751 To further elaborate these 3-nitropyrrolidine derivatives References and Notes into other building blocks we exploited their inherent (1) (a) Name Reactions in Heterocyclic Chemistry; Li, J. J., Ed.; chemoselectivity. This was accomplished by selective hy- John Wiley and Sons, Inc.: Hoboken / NJ, 2005. (b) The drogenation of the nitro group over the benzyl group using Organic Chemistry of Drug Design and Drug Action, 2nd the H-Cube® flow hydrogenator.18 Employing an ethanol– ed.; Silverman, R. B., Ed.; Elsevier Academic Press: New ethyl acetate (1:1 mixture) solvent system the clean reduc- York, 2004. (c) Heterocyclic Chemistry, 4th ed.; Joule, J. A.; tion of the nitro group to the corresponding amine was Mills, K., Eds.; Blackwell Publishing: Cambridge MA, achieved on passage through a Raney Nickel filled car- 2008. (d) The Practice of Medicinal Chemistry, 3rd ed.; Wermuth, C. G., Ed.; Elsevier Academic Press: Burlington tridge at 60 °C in the presence of catalytic amounts of ace- MA, 2008. tic acid (Figure 4). (2) (a) Ley, S. V.; Baxendale, I. R. Chimia 2008, 63, 162. (b) Baxendale, I.