A Metathesis Based Approach to the Synthesis of Aromatic Heterocycles Lisa P. Fishlock, Timothy J. Donohoe and Panayiotis A. Procopiou ‡ Chemistry Research Laboratory, University of Oxford, Mansfield Road, Oxford, OX1 3TA, UK. ‡ GlaxoSmithKline Research & Development Ltd., Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2NY, UK. A Metathesis Based Approach to the Synthesis of Aromatic Heterocycles Lisa P. Fishlock, Timothy J. Donohoe and Panayiotis A. Procopiou ‡ Chemistry Research Laboratory, University of Oxford, Mansfield Road, Oxford, OX1 3TA, UK. ‡ GlaxoSmithKline Research & Development Ltd., Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2NY, UK. Ruthenium-catalysed ring-closing metathesis (RCM) has recently emerged as one of the most powerful tools for the formation of alkenes. 1 Research developed within the group has been focused on altering the premise of the RCM reaction, from simple alkene formation, to one which provides intermediates that are in the correct oxidation state to prepare fully aromatised compounds. 2 This novel protocol could potentially revolutionise the way we approach the disconnection of complex molecules containing aromatic rings. Figure 1. Proposed route to the core of aromatic heterocycles The strategy is therefore to construct the backbone of the molecule, and access the cyclic substrate using RCM. Elimination of the leaving group (LG) will hopefully reveal the desired aromatic heterocycle. Synthesis of disubstituted furans Introduction It was envisaged that the dihydrofuran unit illustrated below could undergo an acid catalysed elimination to reveal the desired furan. The dihydrofuran substrate would be accessed using an enol ether- ene RCM. Scheme 1. Synthesis of disubstituted furans R1R1 R4R4 (iv) Yield (%) C6H5C6H5 i-Pr72 C6H5C6H5 Me56 Cyclopropyli-Pr60 C6F5C6F5 i-Pr75 4-Br-C 6 H 4 C6H5C6H5 58 C6H5C6H5 C6H5C6H FurylC6H5C6H5 50 Table 1. Yield of RCM/aromatisation procedure It is noteworthy that the R 1 substituent is derived from an aldehyde, and R 4 is derived from a carboxylic acid equivalent. Thus, these substituents can be easily introduced from readily available precursors providing great flexibility. Funding was generously provided by GlaxoSmithKline. When a methyl group was introduced as R 3 the RCM reaction was unsuccessful under a variety of conditions, presumably because the catalyst is unable to initiate on the 1,1- disubstituted alkene. The R 2 substituent was investigated using the methyl derivative, and the desired furan was produced in 38% yield with 56% recovered starting alcohol. The backbone was constructed via the sequential ester formation and Takai-Utimoto olefination of a variety of vic-diol mono ethers (Scheme 1). The RCM was executed using Hoveyda-Grubbs second generation catalyst, and the dihydrofuran unit was treated in situ with TFA to yield a range of disubstituted furans in good yields (Table 1). References Synthesis of 2-pyridones and pyridines This mild and novel approach allows installation of groups which can be difficult to incorporate by other methods (e.g. i-Pr, cyclopropyl). Synthesis of trisubstituted furans The synthetic strategy was successfully applied to the synthesis of 2-pyridones via the RCM of the α,β- acrylamide with a leaving group on nitrogen. The aromatisation was carried out with DBU and proceeded in excellent yield to provide a variety of functionalised 2-pyridones. These can be easily transformed into the corresponding pyridines using Comins triflating reagent. Scheme 4. Synthesis of 2-pyridones and pyridines Scheme 3. Synthesis of trisubstituted furan 1) (a) Nicolaou, K. C.; Bulger, P. G.; Sarlah, D. Angew. Chem. Int. Ed. 2005, 44, (b) Fürstner, A. Angew. Chem. Int. Ed. 2000, 39, (c) Armstrong, S. K. J. Chem. Soc., Perkin Trans. 1, 1998, ) (a) Donohoe, T. J.; Orr, A. J.; Bingham, M. Angew. Chem. Int. Ed. 2006, 45, (b) Donohoe, T. J.; Orr, A. J; Gosby, K.; Bingham, M. Eur. J. Org. Chem., 2005, ) (a) Donohoe, T. J.; Fishlock, L. P.; Lacy, A. R.; Procopiou, P. A. Org. Lett. 2007, 9, 953. (c) Donohoe, T. J.; Kershaw, N. M.; Orr, A. J.; Wheelhouse, K. M. P.; Fishlock, L. P.; Lacy, A. R.; Bingham. M.; Procopiou, P. A. Tetrahedron, 2007, in press. Scheme 2. Attempted synthesis of trisubstituted furan Figure 2. Proposed disconnection of the furan core R1R1 R2R2 R3R3 R4R4 (iii) Yield (%) (iv) Yield (%) CO 2 MeHHH9894 CO 2 MeHHMe8889 CO 2 MeHHCF CO 2 MeMeHH9793 CO 2 MePhHH5992 CO 2 MePhHMe7193 CO 2 MeHPhH0- 2-PyridylHHH Methyl-2-pyridylHHH QuinolylHHH QuinoxalylHHH95 Table 2. Yields of RCM and aromatisation steps Additional functionalisation This approach has also been applied to dipyridones by using a double RCM and aromatisation strategy. The dihydropyridone intermediate can also be subjected to an alternative aromatisation procedure using bromine to form 3-benzyloxy substituted pyridones in good yield. Further functionalisation can be achieved by bromination at the 3- and 5- positions of the 2-pyridone. These compounds can allow access to an extensive number of tetrasubstituted pyridines. Scheme 5. Additional functionalisation of 2-pyridones