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Seath-Etal-Synthesis2016-Synthesis-Of-Oxindoles-And-Benzofuranones Synthesis Paper / PSP / Special Topic Synthesis of Oxindoles and Benzofuranones via Oxidation of 2- Heterocyclic BMIDAs Ciaran P. Seath James W. B. Fyfe John J. Molloy Allan J. B. Watson* Department of Pure and Applied Chemistry, WestCHEM, University of Strathclyde, 295 Cathedral St., Glasgow, G1 1XL, 12 examples UK. R BMIDA BS Ox R O up to 99% yield X X [email protected] X = O, NTs • Controlled oxidation via interconversion of boron species • Rapid access to bioactive heterocyclic templates Received: 1b).5 Hydrolysis of 2-oxy-indoles gives the corresponding Accepted: Published online: oxindoles; however, this requires a pre-oxygenated indole as a DOI: starting material (Scheme 1c).6 We recently disclosed a one-pot Abstract: The synthesis of functionalized oxindoles and benzofuranones via synthesis of 2-heterocyclic BMIDA’s,7 which are bench stable, oxidation of 2-BMIDA indoles and benzofurans, respectively, is described. free flowing solids that can be stored indefinitely without Interconversion of boron species (BMIDAàBF3K) was necessary to enable 8 oxidation and overcome boronic acid stability issues associated with a degradation. Here we present the utility of 2-BMIDA indoles difficult BMIDA hydrolysis. Overall, a robust process was developed that and benzofurans as readily accessible precursors to oxindoles allowed access to a small library of oxindole and benzofuranone products and and benzofuranones, respectively, and the scope and limitations facilitated the step-efficient synthesis of biologically-active compounds of this process (Scheme 1d). containing the oxindole pharmacophore. Key words: Boron, oxidation, heterocycles, lactams, lactones a) Buidling the pyrrole nucleus b Y X CO H a b The oxindole motif is present in the core of numerous 2 O N NH2 a N O biologically active natural products as well as pharmaceuticals H H such as tenidap, coreulescine, and semaxanib (Figure 1).1 b) C2-oxidation of indole O Consequently, numerous methods have been and continue to be [Ox] O developed to allow access to this privileged chemotype. N N O H H N H c) Hydrolysis of 2-oxy-indoles Me hydrolysis S Me OR O OH N N N Cl N Me H H O O H d) This work: Brown-type oxidation via boron species interconversion N N O H N 1) KHF2 NH2 H 2) Oxone® O BMIDA O tenidap coerulescine semaxanib N N R R Figure 1 Pharmaceutically relevant oxindoles. Scheme 1 Methods for the synthesis of oxindoles. The most common synthetic approaches towards the oxindole Oxidation of 2-borylated indoles is limited to two single framework forge the pyrrole nucleus either via disconnection at examples using two specific 2-BPin indole substrates.9 This class the amide C-N bond to provide a phenylacetic acid precursor,2 of compounds is generally difficult to access/handle due to or at the C-3 position to afford an anilide precursor (Scheme issues with protodeboronation.10 Consequently, a BMIDA-based 1a).3 Despite significant research, preparation of oxindoles process would offer significant synthetic advantages due to the beginning from the corresponding indole starting materials are accessibility and stability of these organoboron derivatives.8 comparatively limited,4 with oxidative methods often suffering Our initial studies commenced with N-Ts-indole-2-BMIDA 1, from the problem of over-oxidation to deliver isatins (Scheme using well-established Brown-type reaction conditions: H2O2 in Template for SYNTHESIS © Thieme Stuttgart · New York 2016-09-21 page 1 of 7 Synthesis Paper / PSP / Special Topic a range of solvent/water mixtures and in the presence of base A short survey of reaction conditions found that 1 could be (Scheme 2 – see Supporting Information for full details).11 quantitatively converted to 5 upon treatment with aq. KHF2 in MeOH at 70 °C (Scheme 3a). In addition, 5 could be quantitatively converted to the desired oxindole product 3a H O or Oxone® 2 2 ® BMIDA Base O H using Oxone . Combining these two events proved to be N N N Ts Ts Ts straightforward (Table 1). Oxidation of intermediate 5 was 1 3a 4 found to be sluggish in MeOH; however, a solvent switch to [O] PDB acetone before addition of the oxidant was more effective (entry B(OH)2 N 1 vs. entry 2). A short optimization of reaction time and KHF2 Ts 2 stoichiometry (entries 2-5) revealed that the overall two-step process could be completed in 6 hours using 5 equiv. of KHF2 to Scheme 2 Attempted direct hydrolysis/oxidation of 2-BMIDA indole 1. PDB facilitate conversion to 5, ultimately delivering 99% conversion = protodeboronation. to 3a. Interestingly, while oxidation of the intermediate BF3K 5 was facile with Oxone®, NaBO3 and H2O2 were ineffective for Based on the base lability of BMIDA and the well studied slow or oxidation of this intermediate (entries 6 and 7, respectively). fast release of the parent boronic acid,10 we planned an in situ hydrolysis of 1 to deliver 2 that would then be oxidized to deliver 3a. To our surprise, when using a base/oxidant system Table 1 Oxidation of Indole BMIDA via speciation of NaOH/H2O2 at room temperature (fast hydrolysis conditions), i) aq. KHF2 (4.5 M), MeOH BMIDA 70 °C, x h O the BMIDA group remained intact, i.e. no hydrolysis was N N ® observed, even in the presence of excess aq. NaOH. In an Ts ii) Oxone (1.1 equiv), acetone Ts 1 rt, x h 3a attempt to drive the hydrolysis step, the reaction was heated to a 50 C with increasing quantities of NaOH. However, while Entry Conditions 3a (%) ° ® b 1 aq. KHF2 (3 eq), 2 h/ Oxone , 18 h 27 hydrolysis could be induced, these reactions returned the ® c 2 aq. KHF2 (3 eq), 2 h/ Oxone , 18 h 44 protodeboronated product 4 in quantitative yields in all cases, ® c 3 aq. KHF2 (5 eq), 2 h/ Oxone , 18 h 92 presumably due to the sensitivity of 2 under these reaction ® c 4 aq. KHF2 (5 eq), 4 h/ Oxone , 18 h 99 conditions. Attempting to temper the reaction conditions using a ® c 5 aq. KHF2 (5 eq), 4 h/ Oxone , 2 h 99 c slow hydrolysis protocol with either K3PO4 or K2CO3 in the 6 aq. KHF2 (5 eq), 4 h/ NaBO3, 4 h 0 c presence of H2O2 returned starting material (1) only. Changing 7 aq. KHF2 (5 eq), 4 h/ H2O2, 4 h 0 ® the oxidant to Oxone had a small positive effect – hydrolysis aDetermined by HPLC analysis against an internal standard. bOxidant added remained sluggish, requiring extended reaction times or directly. cSolvent switch to acetone before addition of Oxidant. elevated temperatures and although trace quantities of 3a were With optimum conditions in hand, the scope of the reaction was observed under these conditions, protodeboronation product 4 investigated by application to a range of substituted 2-BMIDA dominated along with degradation of 3a. indole architectures (Scheme 4). These initial investigations highlighted a compatibility issue between the conditions required to hydrolyze the BMIDA unit and the stability of the intermediate boronic acid (as well as the i) aq. KHF2 (4.5 M), MeOH BMIDA 70 °C, 4 h O oxindole product). To overcome these issues, we postulated that X X ii) Oxone® (1.1 equiv), acetone a simple boron species interconversion process might be 1 rt, 2 h 3a-l achieved under mild reaction conditions to deliver an organoboron derivative that could then be oxidized under O O O N N Cl N conditions sufficiently mild to inhibit degradation of the Ts Me Ts organoboron intermediate or product. In particular, BMIDA 3a: 92% 3b: 54% 3c: 90% species can be converted to the potassium F Cl Br organotrifluoroborate (BF3K) derivative under relatively mild O O O N N N reaction conditions12 and BF3K species can also be oxidized Ts Ts Ts under mild conditions.13 However, BMIDA-BF3K interconversion 3d: 92% 3e: 90% 3f: 66% on this template was unknown. Accordingly, we evaluated MeO F3CO MeO2C O O O conversion of BMIDA 1 to the BF3K derivative 5. N N N Ts Ts Ts 3g: 88% 3h: 80% 3i: 62% O N BMIDA aq. KHF2 (4.5 M), MeOH BF3K 2 F (a) N N O O O Ts 70 °C, 4 h Ts N N O O 1 5: quant. Ts 3j: 0% (42%)a 3k: 100% 3l: 98% Oxone® (1.1 equiv), acetone O BF3K Scheme 4 Substrate scope of the oxidation process. Isolated yields. (b) N N Ts rt, 18 h Ts aDetermined by 1H NMR analysis. 5 3a: quant. The reaction was found to be tolerant of both electron-donating Scheme 3 Conversion of BMIDA 1 to BF3K 5. and electron-withdrawing groups (3g, 3h, 3i). Halogenated substrates were also effective, providing a synthetic handle for Template for SYNTHESIS © Thieme Stuttgart · New York 2016-09-21 page 2 of 7 Synthesis Paper / PSP / Special Topic further functionalization (3c, 3d, 3e, 3f). In addition, 8 was found to be highly unstable on silica, thereby limiting the azaindoline 3j was delivered in moderate yield by NMR (42%) yield of this process. with the mass balance consisting of the product of protodeboronation. However, this product was found to be Me Me labile to hydrolysis on silica. The N-tosyl group could be Me replaced with a methyl unit but resulted in a decrease in yield of O H N N H N Me the corresponding oxindole (3b). Other N-protecting groups R O 7 H O were not assessed. Lastly, in addition to oxindoles, the oxidation 3a: R = Ts Na/naph piperidine N process was applicable to 2-BMIDA benzofurans to allow access 6: R = H, 90% EtOH, 80 °C H to benzofuranones in excellent yields (3k, 3l).
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