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Received 19 Aug 2014 | Accepted 24 Nov 2014 | Published 12 Jan 2015

Alkaloids constitute a large family of natural products possessing diverse biological properties. Their unique and complex structures have inspired numerous innovations in synthetic chemistry. In the realm of late-stage CH functionalization, alkaloids remain a signicant challenge due to the presence of the basic amine and a variety of other functional groups. Herein we report the rst examples of dirhodium(II)-catalysed intermolecular CH insertion into complex natural products containing nucleophilic tertiary amines to generate a CC bond. The application to a diverse range of alkaloids and drug molecules demonstrates remarkable chemoselectivity and predictable regioselectivity. The capacity for late-stage diversication is highlighted in the catalyst-controlled selective functionalizations of the alkaloid brucine. The remarkable selectivity observed, particularly for site-specic CH insertion at N-methyl functionalities, offers utility in a range of applications where efcient installation of synthetic handles on complex alkaloids is desired.

DOI: 10.1038/ncomms6943

Late-stage CH functionalization of complex alkaloids and drug molecules via intermolecular rhodium-carbenoid insertion

Jing He1, Lawrence G. Hamann1, Huw M.L. Davies2 & Rohan E.J. Beckwith1

1 Department of Global Discovery Chemistry, Novartis Institutes for BioMedical Research, 250 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA. 2 Department of Chemistry, Emory University, 1515 Dickey Drive, Atlanta, Georgia 30322, USA. Correspondence and requests for materials should be addressed to H.M.L.D. (email: mailto:[email protected]

Web End [email protected] ) or to R.E.J.B. (email: mailto:[email protected]

Web End [email protected] ).

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ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6943

The decline in the number of new drug approvals in recent years despite high levels of R&D investment has spurred a number of alternative approaches towards drug discovery

within the pharmaceutical industry1. This includes a resurgence in phenotypic screens, and along with that a renewed interest in exploiting the unique structural diversity of natural products to modulate disease-relevant processes25. Drug-discovery efforts around natural product optimization have been underexplored, owing in part to the synthetic challenge of effecting useful modications in the face of structural complexity and diverse functionality. Advances in synthetic methodology are therefore essential to enable progression of natural product hits of compelling phenotype into suitable probes for elucidating mechanism of action and towards the development of new therapeutics6,7.

CH functionalization is one such approach that has the potential to revolutionize how complex organic molecules, such as natural products, are made through the ability to selectively and efciently transform CH bonds in a predictable manner under mild conditions. Over the last two decades, the eld of CH functionalization has experienced explosive growth, and a number of highly effective transformations have been developed. Many compelling examples have been reported on the utilization of CH functionalization as key strategic reactions in total synthesis, illustrating novel retrosynthetic analysis and step-economical synthesis813. In recent years, considerable efforts have been expended on the late-stage CH functionalization of biologically compelling natural products and drug-like molecules1422. This has been challenging, because although the pace of advances in CH functionalization has been impressive, in many instances the new methodology is limited to a narrow range of substrates and functional groups. Many of the most effective CH functionalization processes rely on the use of directing groups, which need to be introduced and then removed23. Consequently, the late-stage CH functionalization examples demonstrated to date often use rather specialized substrates and are of limited generality.

Our aim is to select structurally complex molecules of compelling biological activity and then attempt to devise approaches to execute site-selective modication of CH bonds in such compounds bearing multiple functional groups. The overarching goal is to establish a toolkit of reactions and reagents amenable to effecting functionalizations on a variety of complex molecules in a relatively predictive and selective manner for broad application. Alkaloids represent an extremely challenging class of natural products for direct CH functionalization, because they typically contain a basic amine and a variety of other reactive functionality. In particular, a basic amine can impede many CH-activation methodologies, as the nitrogen functionality may coordinate to and poison the catalyst2426 or generate undesirable side reactions27,28. Only a few examples of selective oxidation29,30 or CH amination of alkaloid derivatives have been reported, although in the case of the latter aza-ylide products are also formed and in many cases are the exclusive product27. One of the most effective methods for site-selective CH functionalization, without resorting to the use of directing groups, has been the CH insertion chemistry of rhodium-bound donor/acceptor carbenes31. Herein we demonstrate for the rst time that such highly reactive intermediates are indeed capable of undergoing site-selective CH functionalization of a range of complex alkaloids and drug molecules possessing a tertiary amine. We show that our effective strategy enables CH functionalization at methyl, methylene or methine sites proximal to the basic amine, with little to no aza-ylide formation. This study constitutes a proof of concept that we envision can be readily extended to a wide range of applications,

where efcient installation of synthetic handles on complex alkaloids may nd utility. For instance, such a paradigm would be expected to greatly facilitate the ability to derivatize payloads for attachment to monoclonal antibodies for antibodydrug conjugate systems32, and in probe design for target identication or mechanism of action studies in a chemical genetics context27,33.

ResultsRhodium-carbenoid-mediated CH functionalization of brucine. Brucine (1) is a readily available complex natural product, which contains 22 CH bonds in different chemical environments. The presence of two doubly activated allylic sites, a complex multi-ring architecture, a lactam functionality, an electron-rich phenyl ring, methyl ethers, as well as a basic tertiary amine offered a challenging target on which to commence these studies. A metal-free carbene approach to derivatize brucine was recently reported, which yielded a ring-expanded product through formation of an aza-ylide species followed by a [1,2]-Stevens rearrangement34. Indeed, the use of metallocarbenoids to access an aza-ylide intermediate followed by ring expansion has been widely described3537. Interested in effecting CH insertions via a carbenoid approach, we were intrigued as to whether the presence of a suitable rhodium catalyst and appropriate temperature might inuence the nature of the reaction with brucine, allowing for CH insertion over aza-ylide formation or catalyst poisoning, despite the presence of the nucleophilic amine. To the best of our knowledge, the ability to effect such chemistry on substrates possessing a tertiary amine, especially in structurally complex alkaloids, has not been previously reported31,38.

As illustrated in Fig. 1a, we rst compared the inuence of a dirhodium catalyst in contrast to a metal-free system. Brucine was treated with excess methyl-p-bromophenyl diazoacetate (2) in the presence or absence of the well-established carbenoid CH insertion catalyst Rh2(S-DOSP)4 (ref. 31) in triuorotoluene at 83 C (oil bath temperature). We observed that the Rh2(S

DOSP)4-catalysed reaction provided CH insertion product 3 in 20% yield as a single diastereomer, in addition to the expected Stevens rearrangement product 4 in 58% yield with 1.2:1 diastereomeric ratio (dr). When the same reaction was carried out in the absence of Rh2(S-DOSP)4, formation of 3 and 4 was not observed, strongly suggesting that effective functionalization of brucine was mediated via the dirhodium catalyst. Attempting the Rh2(S-DOSP)4-catalysed reaction at ambient temperature in either dichloromethane or triuorotoluene failed to afford compound 3 or 4. The differential outcome observed when conducting the reaction at room temperature as opposed to 83 C led us to rationalize that a higher temperature enables a more rapid kinetic dissociation of the amine from the dirhodium catalyst. No longer incapacitated, the catalyst is free to effect diazo decomposition and carbenoid formation.

Control experiment. Having isolated aza-ylide 5, we postulated whether formation of CH insertion product 3 actually proceeded through a non-rhodium-bound ylide intermediate (as proposed in the formation of 4)34 aided by thermal activation. Accordingly, a control experiment was conducted as presented in Fig. 1b. Aza-ylide 5 was alternatively prepared in situ through deprotonation of the corresponding triuoroacetate salt 6 at 0 C. The reaction was heated to 83 C in a pressure vessel for 30 min, yielding exclusively [1,2]-Stevens rearrangement product 4 in 1:1 dr. Since CH insertion product 3 was not observed, it suggests that ylide formation and carbenoid CH insertion proceed through different reaction pathways. Indeed, we speculate that product 3 is formed via direct CH insertion

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NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6943 ARTICLE

MeO2C

2 mol % Rh2(S-DOSP)4

CO2Me

CF3Ph, 83 C, 3h

MeO

3 4

58% Yield; 1.2:1 dr

No catalyst

CF3Ph, 83 C, 3h

20% Yield

Brucine (1)

MeO2C

+ Br

CO2Me

2 mol % Rh2(S-DOSP)4

DCM, rt, 3 h 9% conv.

0% Formation of 3, 4 and 5; only 35% decomposition of 1

(4 equiv.)

MeO

TFA

CO2Me

MeO

CO2Me

KHMDS THF/CF3Ph 0 C, 30 min

80% Yield 1:1dr

83 C, 30 min

MeO

Figure 1 | Donor/acceptor rhodium-carbenoid-mediated functionalization of brucine (1). (a) Investigations into the role of the catalyst and temperature. (b) Control experiment provided mechanistic insights for the formation of CH insertion product 3. Conversion, yield and dr were determined by

1H NMR with addition of 4-dimethylaminopyridine (DMAP) after work-up as an internal standard. rt, room temperature.

induced by the metal carbene39,40. This being the case, it is remarkable that 3 is the sole CH insertion product observed from a molecule that has numerous potential sites for CH functionalization. The subtle balance between electronic, steric and stereoelectronic effects can often result in exquisite regiocontrol in the CH functionalization reactions of donor/ acceptor rhodium carbenes31, but such control has not been previously demonstrated with such a complex substrate.

Inuence of dirhodium catalysts. The electrophilicity and steric environment of the metallocarbenoids can be affected by the nature of the catalyst31. Hence, we were eager to explore what inuence various dirhodium catalysts would have on the site selectivity of the reaction (Table 1). The bulky dirhodium catalyst Rh2(TPA)4 promoted formation of the CH insertion product, giving 3 in 50% yield as a single diastereomer. Subsequent investigations into other achiral rhodium catalysts (Table 1, entries 25) revealed that Rh2(TPA)4 is the most effective catalyst for carbenoid-mediated CH insertion. In addition, the formation of Stevens rearrangement product 4 is a competing pathway for which the chemoselectivity between the formation of compounds 3 and 4 is catalyst dependent. Furthermore, the two enantiomers of chiral catalysts afforded different results.

For example, only the use of the S-enantiomer of the DOSP ligand allowed for CH insertion (Table 1, entries 7 and 8). A most unexpected result was obtained with the very bulky chiral catalyst Rh2(BTPCP)4 refs 41,42. Under the standard reaction conditions, the Rh2(BTPCP)4-catalysed reactions suffered very poor conversion, irrespective of whether the R- or S-enantiomer of the catalyst was used (Table 1, entries 11 and 12). Careful 1H nuclear magnetic resonance (NMR) analysis of the crude reaction mixture revealed an alternative CH insertion product (7) had been generated in low yield with Rh2(S-BTPCP)4. Increasing the catalyst loading of

Rh2(S-BTPCP)4 to 20 mol% afforded compound 7 in 39% yield as a single diastereomer, albeit with moderate conversion (Fig. 2a). Extensive one- and two-dimensional NMR experiments conrmed that this new compound 7 was generated through CH insertion into the tertiary CH bond adjacent to the amine. The ability for Rh2(S-BTPCP)4 to enable selective CH insertion at the methine position is unexpected, because a bulky catalyst should generally favour functionalization of a less-sterically encumbered CH bond42. The catalyst screen reveals that one can manipulate the favoured site for functionalization in a catalyst-controlled manner, allowing one to rapidly probe three alternative sites on a complex molecule (Fig. 2a).

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Table 1 | Inuence of dirhodium catalyst on CH carbene insertion of brucine (1).

Conversion, yield and dr were determined by 1H NMR analysis with addition of DMAP during work-up as an internal standard, unless otherwise specied. *1,2-Dichloroethane was used as solvent.

wIsolated yield.

Diazo scope. Studies with a small set of aryldiazoacetates of distinct electronic properties indicate that the nature of the carbenoid also plays a role in the chemoselectivity of the rhodiumcarbenoid insertion (Table 2, entries 13). Donor/acceptor carbenoids bearing electron-decient aryl substituents favoured CH insertion product 9 over aza-ylide formation (Table 2, entries 1 and 3), whereas an electron-donating substituent provides a less-electrophilic rhodium-carbenoid for which exclusive formation of ring-expanded product 10 was observed (Table 2, entry 2). Reaction with methyl diazomalonate (8c) or ethyl diazoacetate (8d) failed to generate any CH insertion or Stevens rearrangement product (Table 2, entries 4 and 5), suggesting the importance of the donor group in the efciency of intermolecular CH insertion reactions under these conditions.

CH functionalization of securinine. To explore reaction generality, we applied the Rh2(TPA)4-mediated approach to other structurally complex alkaloids and tertiary amine-containing drug molecules (Figs 3 and 4). The GABAA antagonist securinine (12)

is a tricyclic alkaloid possessing two olens in conjugation with a lactone functionality, in addition to the tertiary amine (Fig. 3a). Treatment of 12 with methyl-p-bromophenyl diazoacetate (2) and Rh2(TPA)4 in triuorotoluene at 83 C selectively gave CH insertion product 13 (44% yield, 2.2:1 dr), with no competing cyclopropanation at either olen43. Furthermore, although securinine contains four CH bonds adjacent to the amine (two methine and two diastereotopic methylene in nature), only a single methylene CH bond undergoes carbene insertion. Unlike the case of brucine in which substrate control afforded a highly diastereoselective CH insertion product, with securinine the

diastereoselectivity was rather moderate, and was inuenced by the dirhodium catalyst itself (Fig. 3a).

CH functionalization of apovincamine. Further application of this method to apovincamine (14a) led to bis-cyclopropanation of the electron-rich indole ring 16, with only minimal formation of the CH insertion product (Fig. 3b)44. Attempts with other dirhodium catalysts provided similar reaction outcomes. It is well established that aromatic rings are sterically protected from reaction with rhodium donor/acceptor carbenoids when they are at least 1,4-disubstituted45. Accordingly, the iodinated apovincamine analogue 14b was prepared and evaluated in the reaction, and in this instance the CH insertion product 15b was obtained in improved yield (47% yield) as a single diastereomer. Although 14b contains one methine and four methylene CH bonds adjacent to the amine, the CH insertion occurred selectively at the benzylic methylene site rather than alpha to the amine. Previously, it has been reported that benzylic CH bonds are also favourable sites for CH insertion, due to the capacity of p-systems to stabilize the neighbouring buildup of positive charge in the transition state46.

Selective N-methyl CH insertion. CH functionalization induced by dirhodium-bound donor/acceptor carbenoids has been shown to be initiated by a hydride transfer event, and sites that are capable of stabilizing positive charge buildup at carbon are electronically favoured40. However, the general order of reactivity of CH bonds in competition reactions is typically methineBmethylene4methyl, because although tertiary sites are

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NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6943 ARTICLE

MeO

CO2Me

2 mol% Rh2(Oct)4

(4 equiv.)

20 mol%

Rh2(S-BTPCP)4

Brucine (1)

Rh2(TPA)4

2 mol%

MeO2C

CO2Me

MeO2C

MeO

4 7

39% Yield (48% conv.); 0% Yield of 3 & 4

(1.2:1 dr) 74% Yielda

50% Yield;20% Yield of 4 (1:1 dr)

O O

C12H25

Rh2(S-DOSP)4

Rh2(S-PTAD)4

Rh2(TPA)4

Rh2(S-BTPCP)4

Figure 2 | Optimization of rhodium-catalysed CH carbene insertion of brucine (1). (a) Catalyst inuence on site-selective CH functionalization of brucine (1). (b) Structures of dirhodium catalysts. Conversion, yield and dr were determined by 1H NMR analysis with addition of 4-dimethylaminopyridine after work-up as an internal standard, unless otherwise specied. aIsolated yield.

electronically most activated, donor/acceptor carbenoids are sterically very demanding31. In the alkaloids studied so far the reactions have been remarkably site selective, suggesting that the majority of electronically activated methine and methylene sites in these alkaloids are sterically inaccessible. Therefore, we chose to explore the reactions of alkaloids containing N-methyl groups. Functionalization of electronically activated methyl CH bonds has been observed, but only a few examples of CH insertion into methyl CH bonds in the presence of activated methylene and/or methine CH bonds have been reported42,4749, and none of those feature a basic amine. We found for alkaloids possessing an N-Me functionality, the most favoured product in each case arose from CH insertion into the said N-methyl group (Fig. 4). With dextromethorphan (17), for example, carbene insertion at the N-methyl CH bond was the most favoured product despite methylene sites adjacent to the nitrogen atom and the phenyl ring. It is worth noting that no insertion was observed at the accessible methyl ether site. Indeed, CH insertion at the N-methyl site was such a favoured pathway that we were able to effect the reaction at room temperature affording 87% yield of C H insertion product (1:1 dr), with complete conversion of 17. The structurally related yet more elaborate thebaine (18) smoothly underwent CH insertion at the N-methyl site (52%, 1.4:1 dr), without any competing reactivity despite the electron-rich aryl group and the 1,3-diene functionality. It appears that regardless of conformation or neighbouring functionalities, in all systems

explored the donor/acceptor carbenoid derived from 2 selectively inserts into the primary CH bond adjacent to nitrogen. In addition, the apparent ease of insertion into the N-methyl group enables some reactions to be conducted at room temperature, with diazo 2 used as the limiting reagent, as was the case with noscapine (19), although the structurally related bicuculline (20) required more forcing conditions. Generally, the diastereo-selectivity of the Rh2(TPA)4-mediated N-methyl insertion reactions afforded diastereomeric ratios in the 1:1-2:1 range, and the application of chiral dirhodium catalysts failed to improve on this (Table 3). Sercloremine (21) represents an interesting substrate in that its relatively simple structure suggests little potential for steric differentiation between the methylene sites and the terminal methyl site adjacent to the amine, however, the N-methyl CH insertion product is again exclusively formed in 62% yield. It should be noted that Stevens-type rearrangement products were not observed in any of the N-methyl-containing systems explored in this study and presumably is no longer a competing pathway. It is likely that the ease of accessibility the primary CH bond offers has a signicant impact on the site-selectivity of this reaction.

We have successfully devised and implemented an effective strategy for non-directed CH functionalization of nucleophilic tertiary amine-containing complex natural products and drug molecules. We demonstrate that these traditionally challenging substrates are capable of undergoing rhodium-

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ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6943

Table 2 | Inuence of diazo reagents on rhodium-catalysed carbene insertion of brucine (1).

DMAP, 4-dimethylaminopyridine; NMR, nuclear magnetic resonance.*Conversion, yield and dr were determined by 1H NMR analysis with addition of DMAP during work-up as an internal standard, and where formed, the corresponding product is given in bold font in parentheses next to the yield.

wNot determined, but cannot be excluded owing to the presence of a small amount of material isolated as an intractable mixture of products.

zThe corresponding aza-ylide 11 was obtained as triuoroacetate salt in 4% isolated yield.

H O O

MeO2C

2 mol% Rh2(TPA)4

4 equiv. 2 CF3Ph, 83 C, 3 h

2 mol% Rh2(TPA)4

4 equiv. 2 CF3Ph, 83 C, 3 h

H 56% Yield; 1:2 dr

62% Yield; 2.6:1 dr

52% Yield; 1:1.2 dr 49% Yield; 4.9:1 dr

Securinine (12)

GABAA antagonist

Apovincamine

vasodilator

44% Yield; 2.2:1 dr

Rh2(S-DOSP)4 Rh2(R-DOSP)4

Rh2(S-PTAD)4 Rh2(R-PTAD)4

MeO2C

MeO2C N

CO2Me

MeO2C

CO2Me

14a: R = H 15a, R = H, 4% Yield 14b: R = I 15b, R = I, 47% Yield

16, 20% Yield,1.9:1 dr

Figure 3 | Rhodium-catalysed CH insertion of alkaloids securinine and apovincamine. (a) Diastereoselective CH carbene insertion is achieved by using chiral rhodium catalysts. (b) Introduction of steric hindrance could protect electron-rich aromatic rings from cyclopropanation and enable desired CH insertion. Yield and dr were determined by 1H NMR analysis with addition of 4-dimethylaminopyridine after work-up as an internal standard, unless otherwise specied.

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NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6943 ARTICLE

2 mol% Rh2(TPA)4

CO2Me

N CO2Me

Secondary CH bonds adjacent to the nitrogen atom

Secondary CH bonds adjacent to aryl groups

(4 equiv.) 2

CF3Ph 83 C

Double activated CH bonds in the 1,3-dioxolane moiety

H H

22, 87% Yielda;

1:1 dra

24, 51% Yielda1.2:1 dra

25 ,63% Yielda2.1:1 dra

23, 52% Yielda1.4:1 dra

N O

Dextromethorphan (17)b Cough suppressant

26, 62% Yielde

O O

Cl Noscapine (19)c Cough suppressant

Sercloremine (21)

Antidepressant

Thebaine (18)

Cytotoxic alkaloid

Bicuculline (20)d GABAA antagonist

Figure 4 | CH insertion in N-methyl-containing natural products and drug molecules. aYield and dr were determined by 1H NMR with 4-dimethylaminopyridine as an internal standard. bReaction was carried out in dichloromethane under room temperature. cReaction was carried out with 0.5 equiv. of 2 in dichloromethane at room temperature (54% recovered SM). dReaction was carried out with 0.5 equiv. of 2 (52% recovered SM).

eIsolated yield.

Table 3 | Chiral dirhodium catalysts screen using bicuculline(20) as a substrate.

DMAP, 4-dimethylaminopyridine; NMR, nuclear magnetic resonance.

Yield and dr were determined by 1H NMR analysis with addition of DMAP during work-up as an internal standard.

catalysed CH insertion reactions to selectively install a new CC bond. The methods we describe are effectively implemented on a diverse set of alkaloids possessing a wide variety of architectures, functionalities and potentially reactive sites. The use of donor/acceptor carbenoids is the key to our ability to achieve such remarkable chemoselectivity and predictable regioselectivity. In addition, our approach obviates the challenging issue of undesired aza-ylide formation commonly observed with carbenoids in the presence of basic nitrogen-containing substrates, and in doing so the work signicantly expands and enables the scope of this eld27. The highly efcient generation of three distinct brucine derivatives in a catalyst-controlled fashion

(Fig. 2a), showcases the potential of this approach to offer late-stage diversication of complex molecules. It is worth noting that the site of CH insertion is routinely in proximity to the amine moiety. We look upon this consistency, in particular the remarkable selectivity for CH insertion into N-methyl-containing alkaloids, as a robust and highly predictable approach for site-specic functionalization of complex molecules, which could be applied late on in a synthetic route for direct derivatization adjacent to an amine, or for the introduction of a synthetic handle for use in bioconjugation strategies for chemical biology studies.

Methods

Materials. Materials were obtained from commercial suppliers and used as received, or prepared according to standard procedures, unless otherwise noted. Sercloremine (21) and apovincamine (14a) were obtained from the Novartis compound archive. Methyl-p-bromophenyl diazoacetate (2) as well as its analogues 8a and 8b were synthesized according to a previously reported procedure43. All reactions were conducted under an inert atmosphere of dry nitrogen. Analytical thin-layer chromatography was performed on Kieselgel 60 F254 (250 mm silica gel)

glass plates and compounds were visualized with ultraviolet light 254 nm). Flash column chromatography was performed using Kieselgel 60 (230400 mesh) silica gel with ethyl acetate/hexanes as eluent, unless indicated otherwise.

General spectroscopic methods. 1H NMR spectra were measured at 400 MHz on a Bruker Avance instrument and reported in parts per million (d, p.p.m.). Coupling constants (J-values) were reported in Hertz (Hz), with multiplicity reported following usual convention: s singlet, d doublet, t triplet, q quartet,

dd doublet of doublets, m multiplet and br broad. The proton signal of the

residual, non-deuterated solvent (d 7.26 for CHCl3) was used as an internal reference for 1H NMR spectra. 13C NMR spectra were completely hetero-decoupled and measured at 100 MHz. Residual chloroform (d 77.23) was used as an internal reference. Preparative high-performance liquid chromatography was performed using Waters Autopurication system with a photodiode array detector. Preparative supercritical uid chromatography was performed using a Thar (Waters) SFC 80 preparative system with a Waters 2489 UV/visible detector. All tested compounds were found to be 495% pure (unless stated otherwise) as determined by liquid chromatography-UV (LC-UV)/electrospray ionization (ESI)-mass spectrometry (MS), recorded using an Acquity G2 Xevo OTof mass spectrometer (accuracy o5 p.p.m.) with an electrospray ionization source and Acquity ultra performance liquid chromatograph (conditions: Acquity UPLC BEH C181.7 mm 2.1 50 mm column, solvent A: water 0.1% formic acid, solvent B:

acetonitrile 0.1% formic acid, gradient: from 2 to 98% B in 4.4 min, 1.0 ml min 1

ow rate and 50 C). Ammonium salts, which were generated in preparative high-performance liquid chromatography with triuoroacetic acid or formic acid as a modier, were converted to the corresponding free amine with 4 equiv. MP-carbonate (Biotage) in 0.1 M dichloromethane for 1 h at room temperature. 1H NMR,

13C NMR and high-resolution mass spectra (HRMS) are provided for all compounds. For NMR spectra and detailed analysis of NMR assignments, see

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ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6943

Supplementary Figs 128; Supplementary Notes 15, respectively. For supercritical uid chromatography analysis and ion chromatography analysis of 6 see Supplementary Notes 6,7. See Supplementary Methods for the characterization data of compounds not listed in this section.

Synthesis of 3. An oven-dried 10 ml two-necked round-bottom ask tted with a condenser was charged with brucine (1) (133.2 mg, 0.338 mmol) and Rh2(TPA)4(9.7 mg, 6.8 mmol) in degassed triuorotoluene (3.0 ml) under a nitrogen atmosphere. A solution of methyl-p-bromophenyl diazoacetate (2) (345 mg, 1.35 mmol) in degassed triuorotoluene (3.8 ml) was slowly added via a syringe pump over 1 h under nitrogen atmosphere at 83 C (oil bath temperature). The mixture was stirred at this temperature for 2 h and then concentrated in vacuo. 1H NMR analysis with addition of 5.0 mg 4-dimethylaminopyridine as an internal standard indicated 3 was obtained in 50% yield as a single diastereomer and 4 in 20% yield (1:1 dr). The resulting crude was then puried through ash column chromatography (050% EtOAc/heptane) twice to afford 3 as a white solid (92.9 mg, 44% isolated yield, Rf 0.04 in 50% EtOAc/heptane). To aid analysis, 3 was isolated as

the corresponding TFA salt. 1H NMR (400 MHz, chloroform-d) d 7.76 (s, 1H), 7.59 (d, J 8.0 Hz, 2H), 7.44 (d, J 8.0 Hz, 2H), 6.91 (s, 1H), 6.01 (t, J 4.0 Hz, 1H),

4.77 (br, 1H), 4.36 (d, J 8.0 Hz, 1H), 4.35 (m, 1H), 4.24 (dd, J 16.0, 8.0 Hz, 1H),

4.06 (d, J 12.0 Hz, 1H), 3.964.06 (m, 2H), 3.91 (s, 3H), 3.90 (s, 3H), 3.86 (d,

J 16.0 Hz, 1H), 3.63 (s, 3H), 3.26 (br, 1H), 3.20 (dd, J 16.0, 8.0 Hz, 1H), 2.73

(dd, J 16.0, 4.0 Hz, 1H), 2.422.51 (m, 2H), 2.29 (t, J 16.0 Hz, 1H), 2.17 (d,

J 16.0 Hz, 1H), 1.67 (d, J 16.0 Hz, 1H), 1.39 (dt, J 8.0, 4.0 Hz, 1H). 13C NMR

(100 MHz, chloroform-d) d 170.9, 169.2, 161.7 (CF3), 150.7, 147.5, 135.9, 135.5, 133.2, 132.9, 132.7, 130.3, 124.1, 118.8, 105.1, 101.3, 76.9, 66.7, 64.2, 63.5, 59.7, 56.5,53.2, 52.5, 52.0, 51.4, 47.1, 46.1, 42.1, 30.5, 25.6. HRMS (ESI-time of ight (TOF)) [M H] calculated for C32H34BrN2O6: 621.1600; found: 621.1575. LC-UV/ESI

MS retention time: 1.70 min. Purication of the crude reaction mixture also afforded 4 contaminated with small amount of impurity. A subsequent purication through ash column chromatography (050% EtOAc/heptane) provided the two diastereomers of 4 as separate compounds (24.2 mg, 11% isolated yield, Rf 0.30

in 50% EtOAc/heptane; 17.2 mg, 8% isolated yield, Rf 0.25 in 50% EtOAc/hep

tane). 4a, diastereomer 1: 1H NMR (400 MHz, chloroform-d) d 7.82 (s, 1H), 7.54 (d, J 8.6 Hz, 2H), 7.40 (d, J 8.6 Hz, 2H), 6.72 (s, 1H), 5.86 (q, J 3.8 Hz, 1H),

4.38 (dd, J 16.4, 5.0 Hz, 1H), 4.27 (m 1H), 4.10 (dm, J 16.4 Hz, 1H), 3.98 (d,

J 11.1 Hz, 1H), 3.94 (s, 1H), 3.90 (s, 3H), 3.893.85 (m, 4H), 3.63 (s, 3H), 3.33

3.22 (m, 1H), 3.03 (d, J 16.0 Hz, 1H), 2.97 (t, J 7.5 Hz, 1H), 2.93 (m, 2H), 2.83

(s, 1H), 2.00 (dt, J 16.0, 4.0 Hz, 1H), 1.961.81 (m, 2H), 1.60 (dt, J 8.0, 4.0 Hz,

1H), 1.41 (d, J 15.5 Hz, 1H). 13C NMR (100 MHz, chloroform-d) d 173.3, 170.0,

149.4, 146.3, 142.8, 138.3, 135.9, 131.7, 128.8, 128.4, 123.8, 121.5, 105.9, 101.1, 75.8,68.7, 65.2, 62.9, 59.6, 56.8, 56.4, 53.2, 53.1, 49.4, 48.3, 47.0, 42.4, 39.5, 35.3, 31.7. HRMS (ESI-TOF) [M H] calculated for C32H34BrN2O6: 621.1600; found:

621.1594. LC-UV/ESI-MS retention time: 2.88 min. 4b, diastereomer 2: 1H NMR (400 MHz, chloroform-d) d 7.81 (s, 1H), 7.48 (d, J 8.2 Hz, 2H), 7.26 (br, 2H), 6.67

(s, 1H), 5.52 (s, 1H), 4.284.13 (m, 2H), 4.09 (d, J 10.4 Hz, 1H), 4.00 (dm,

J 14.8 Hz, 1H), 3.94 (s, 3H), 3.90 (s, 3H), 3.89 (s, 3H), 3.68 (d, J 15.2 Hz, 1H),

3.46 (d, J 4.2 Hz, 1H), 3.10 (dd, J 16.2, 8.3 Hz, 1H), 2.96 (s, 1H), 2.882.59 (m,

3H), 2.502.29 (m, 2H), 2.071.95 (m, 1H), 1.751.61 (m, 2H), 1.43 (dt, J 10.5,

4.5 Hz, 1H). 13C NMR (100 MHz, chloroform-d) d 173.9, 170.5, 149.1, 146.5, 142.9, 142.1, 136.6, 131.5, 128.3, 128.1 (br), 124.1, 121.6, 105.7, 101.2, 79.3, 68.5, 67.0,64.5, 60.4, 56.8, 56.4, 53.0, 52.1, 49.1, 47.9, 45.8, 42.2, 40.8, 35.2, 29.3. HRMS (ESITOF) [M H] calculated for C32H34BrN2O6: 621.1600; found: 621.1600. LC

UV/ESI-MS retention time: 3.03 min.

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Acknowledgements

We acknowledge support from Novartis Institutes for BioMedical Research and the NSF under the CCI Center for Selective CH Functionalization, CHE-1205646. We thank Professor Djamaladdin G. Musaev (Emory University), Dr Andrew Patterson and Dr Hasnain Malik for insightful discussion. J.H. gratefully acknowledges the Education Ofce of the Novartis Institutes for BioMedical Research Inc. for receipt of a Presidential Postdoctoral Fellowship. We thank Changming Qin (Emory University) for providing Rh2(BTPCP)4 catalyst. We thank Jinhai Gao and Melissa Grondine for providing NMR and preparative SFC separation support, respectively.

Author contributions

R.E.J.B. and H.M.L.D. conceived and supervised this study. J.H., L.G.H., H.M.L.D. and R.E.J.B. were involved with experimental design and results discussion. J.H. conducted the experiments and analysed the data. J.H., H.M.L.D. and R.E.J.B. wrote the manuscript. All authors edited the manuscript.

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How to cite this article: He, J. et al. Late-stage CH functionalization of complex alkaloids and drug molecules via intermolecular rhodium-carbenoid insertion. Nat. Commun. 6:5943 doi: 10.1038/ncomms6943 (2015).

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Alkaloids constitute a large family of natural products possessing diverse biological properties. Their unique and complex structures have inspired numerous innovations in synthetic chemistry. In the realm of late-stage C-H functionalization, alkaloids remain a significant challenge due to the presence of the basic amine and a variety of other functional groups. Herein we report the first examples of dirhodium(II)-catalysed intermolecular C-H insertion into complex natural products containing nucleophilic tertiary amines to generate a C-C bond. The application to a diverse range of alkaloids and drug molecules demonstrates remarkable chemoselectivity and predictable regioselectivity. The capacity for late-stage diversification is highlighted in the catalyst-controlled selective functionalizations of the alkaloid brucine. The remarkable selectivity observed, particularly for site-specific C-H insertion at N-methyl functionalities, offers utility in a range of applications where efficient installation of synthetic handles on complex alkaloids is desired.

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Title

Late-stage C-H functionalization of complex alkaloids and drug molecules via intermolecular rhodium-carbenoid insertion

Author

He, Jing; Hamann, Lawrence G; Davies, Huw M L; Beckwith, Rohan E J

Pages

5943

Publication year

2015

Publication date

Jan 2015

Publisher

Nature Publishing Group

e-ISSN

20411723

Source type

Scholarly Journal

Language of publication

English

DOI

https://doi.org/10.1038/ncomms6943

ProQuest document ID

1644477251

Late-stage C-H functionalization of complex alkaloids and drug molecules via intermolecular rhodium-carbenoid insertion

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