Ring strain strategy for the control of regioselectivity. Gold-catalyzed anti-Markovnikov cycloisomerization initiated tandem reactions of alkynes

Science Bulletin 62 (2017) 352–357

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Ring strain strategy for the control of regioselectivity. Gold-catalyzed anti-Markovnikov cycloisomerization initiated tandem reactions of alkynes Chao Shu a, Long Li a, Tong-De Tan a, Ding-Qiang Yuan a, Long-Wu Ye a,b,⇑ a State Key Laboratory of Physical Chemistry of Solid Surfaces and The Key Laboratory for Chemical Biology of Fujian Province, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China

a r t i c l e

i n f o

Article history: Received 13 December 2016 Received in revised form 4 January 2017 Accepted 6 January 2017 Available online 18 January 2017 Keywords: Gold Heterocycles Ring strain Cyclization Tandem reaction

a b s t r a c t Gold-catalyzed nucleophilic addition to terminal alkynes has received considerable interest in the past decade, as this chemistry offers a highly efficient and regioselective way for CAC, CAH and CAX bond formation. However, such a nucleophilic addition mainly involves a Markovnikov addition. In this short review, the recent progress of the gold-catalyzed 5-endo-dig cycloisomerization-initiated tandem reactions by utilizing the steric strain in ring formation to achieve an anti-Markovnikov regioselectivity was reviewed, including the scope of reactions, mechanism and synthetic applications. Ó 2017 Science China Press. Published by Elsevier B.V. and Science China Press. All rights reserved.

1. Introduction In the past decade, homogeneous gold catalysis has emerged as an extremely powerful tool in organic synthesis due to its high catalytic activities, benign reaction conditions and good tolerance of various functional groups. Recently, gold-catalyzed addition of a heteroatom nucleophile to a CAC multiple bond, in most cases an alkyne, has received considerable interest as this chemistry provides easy access to an incredible variety of different valuable cyclic compounds [1–12]. Especially, the terminal alkynes can undergo various highly regioselective transformations catalyzed by gold, thus providing an efficient way for CAC, CAH and CAX bond formation. However, a Markovnikov regioselectivity was normally observed in this kind of nucleophilic addition, except those involving the formation of indoles driven by aromatization (Scheme 1) [13–15]. A likely reason is that the regioselectivity in such a gold-catalyzed nucleophilic addition is dominated by the electronic nature of alkyne. In other words, b-position of terminal alkyne is more positive, hence this position is easier to be attacked by nucleophiles. With these in mind, we envisioned that the utilization of ring strain strategy might achieve the anti-Markovnikov regioselectivity by reasonable substrate design (Scheme 2). That is, when the

⇑ Corresponding author. E-mail address: [email protected] (L.-W. Ye).

tethered nucleophile attack the electronically favorable bposition of terminal alkyne, the reaction involves the formation of thermodynamically unstable four-membered ring as transition state intermediate. Instead, the reaction involves the formation of thermodynamically stable five-membered ring as transition state intermediate when the corresponding a attack occurs. As a result, the ring strain may outcompete the electronical nature of alkyne in this case, thus leading to the achievement of anti-Markovnikov regioselectivity. Very recently since 2012, our group and others have successfully realized a variety of gold-catalyzed 5-endo-dig cycloisomerization initiated tandem reactions by utilizing the steric strain in ring formation to achieve the anti-Markovnikov regioselectivity (Scheme 2). Thus, this strategy has been applied to the synthesis of various valuable five-membered heterocycles from readily available homopropargyl alcohols or amides. This short review presents an overview of these recent advances by highlighting their specificity and applicability, and the mechanistic rationale is presented where possible.

2. O-Nucleophiles—tandem cycloisomerization/oxidation of homopropargyl alcohols for the synthesis of c-lactones The skeleton of c-Lactone is found in a variety of bioactive natural products as a core structural element [16–23]. In addition, c-Lactones have also proven to be the vital precursors for the construction of many biologically active compounds and natural

http://dx.doi.org/10.1016/j.scib.2017.01.016 2095-9273/Ó 2017 Science China Press. Published by Elsevier B.V. and Science China Press. All rights reserved.

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Markovnikov addition (typical pathway) R

Nu

Nu β

R

α [Au]

β

H

OH

anti-Markovnikov addition? α [Au]

R

1

[Au]

O

tautomerization

O

5-endo-dig

R

R 2-A [Au]

[Au]

[Au]

2-B [Au]

O

Scheme 1. Gold-catalyzed nucleophilic addition to terminal alkynes.

R 2-C

H

H X R

β

α

[M]

XH R

C 6H 4 (3-Cl)

H X

α β [M]

X = O, NPG

R

H

O

α β

[M]

Scheme 2. The anti-Markovnikov regioselectivity driven by the steric strain in ring formation.

products [24–30]. Moreover, they have forged ahead in flavour chemistry [31]. Since their identifications in fruits and vegetables, they have been the subject of intense research for many flavourists. In 2012, our group [32] discovered a gold-catalyzed tandem 5endo-dig anti-Markovnikov cycloisomerization and in situ m-CPBA oxidation sequence for the facile synthesis of various c-lactones in good to excellent yields from readily available homopropargyl alcohols (Scheme 3). That is, the hydroxyl group can selectively attack the electronically unfavorable a-position of alkyne due to steric strain in the formation of four-membered ring if b attack occurs. Of note, a range of functional groups were well tolerated in this transformation, including bromo, azido, protected amino and hydroxy, leading to the corresponding valuable c-lactones. The utility of this methodology was also highlighted by the efficient synthesis of inhibitor of 17b-HSD and formal synthesis of harzialactone A. A plausible mechanism for the above cascade cyclization was proposed based on a variety of control experiments, as shown in Scheme 4. Initially, nucleophilic attack of the hydroxyl group to the Au(I)-coordinated triple bond of homopropargy alcohol 1 forms vinyl gold intermediate 2-A via an anti-Markovnikov 5-endo-dig cyclization, which would be further transformed into the oxocarbenium intermediate 2-D in the presence of acid going through

Scheme 3. Synthesis of c-lactones 2 through gold-catalyzed tandem cycloisomerization/oxidation of homopropargyl alcohols 1.

R [Au]

m-CPBA H

2-D H 2O

H

O

O R

H

H

O

O O

2-E

O

O

R 2

m-CPBA OH

R 2-F

Scheme 4. Mechanistic proposal for the gold-catalyzed cascade cyclization of homopropargyl alcohols 1.

either intermediate 2-B or 2-C. Intermediate 2-D then undergoes subsequent oxidation by m-CPBA to afford the final c-lactone 2. 3. N-Nucleophiles 3.1. Tandem cycloisomerization/oxidation of homopropargyl sulfonamides for the synthesis of enantioenriched c-lactams A year later, our group [33] reported the relevant gold-catalyzed tandem cycloisomerization/oxidation of readily available chiral homopropargyl amides by using similar strategy, thus allowing the efficient synthesis of enantioenriched c lactams by combining the chiral tert-butylsulfinimine chemistry and gold catalysis [34– 36]. Once again, the amide group can selectively attack the electronically unfavorable a-position of alkyne in this case, but not the b-position, as the b attack involves the formation of thermodynamically unstable four-membered transition state intermediate. Different substituted homopropargyl amides were found to work smoothly to afford the desired c lactams 4 in moderate to good yields with excellent ees (Scheme 5). The utility of this methodology was also demonstrated in the synthesis of biologically active compound S-MPP and natural product ( )-bgugaine.

Scheme 5. Synthesis of c-lactams 4 through gold-catalyzed tandem cycloisomerization/oxidation of homopropargyl sulfonamides 3.

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3.2. Tandem cycloisomerization/dimerization of homopropargyl amines for the synthesis of functionalized pyrrolidines In 2013, del Pozo and co-workers [37] described a novel goldcatalyzed cycloisomerization initiated cascade cyclization of homopropargyl amines for the construction of nitrogencontaining tetracycles. In the presence of 5 mol% Ph3PAuCl and 5 mol% AgOTf, a variety of substrates 5 reacted smoothly, furnishing the desired tetracycles 6 in up to 90% yield (Scheme 6). Notably, the cyclized products were obtained as single diastereoisomers in most cases. The proposed mechanism of the above cascade cyclization was depicted in Scheme 7. Firstly, the intramolecular addition of amine 5 to the gold activated triple bond renders intermediate 6-A, which is in equilibrium with its tautomer 6-B. Subsequently, protodeauration would produce the hydroamination product 6-C. The intermediate 6-B would then be trapped by pyrroline 6-C via an enamine attack to afford intermediate 6-D, which undergoes further nucleophilic attack by the ortho position of the N-PMB group, delivering the tetracyclic intermediate 6-E. Finally, aromatization and protodeauration would offer the desired product 6 and regenerate the gold catalyst. Thus, it constitutes a tandem intramolecular hydroamination/formal aza-Diels–Alder reaction of propargylic amino esters.

Scheme 7. Proposed mechanism for the gold-catalyzed cascade cyclization of homopropargyl amines 5.

3.3. Tandem cycloisomerization/dimerization of homopropargyl sulfonamides for the synthesis of chiral pyrrolidines In 2014, our group [38] reported a gold-catalyzed tandem cycloi somerization/dimerization of chiral homopropargyl amides. In the presence of 5 mol% IPrAuNTf2 and 0.5 equiv MsOH, it was found that the cascade cyclization of homopropargyl sulfonamides 3 went smoothly under mild reaction conditions, providing various enantioenriched pyrrolidines 7 in generally good to excellent yields with excellent diastereoselectivities (Scheme 8). Once again, the ee value could be well maintained in this transformation. A plausible reaction mechanism based on a variety of control experiments was proposed, as depicted in Scheme 9. The initial step is a nucleophilic attack of a gold-activated CAC trip bond by the tethered nitrogen via a 5-endo-dig cyclization, leading to the

Scheme 8. Gold-catalyzed cascade cyclization of homopropargyl sulfonamides 3 for the synthesis of pyrrolidines 7.

Scheme 9. Proposed mechanism for the gold-catalyzed cascade cyclization of homopropargyl sulfonamides 3.

vinyl gold intermediate 7-A, which was converted into the dihydropyrrole intermediate 7-B and the 2-hydroxypyrrolidine intermediate 7-C. Finally, both intermediates 7-B and 7-C could be further transformed into the final product 8 co-catalyzed by gold and acid. 3.4. Direct anti-Markovnikov cycloisomerization of homopropargyl amides for the synthesis of chiral 2,3-dihydropyrroles Scheme 6. Synthesis of N-containing tetracycles 6 through gold-catalyzed cascade cyclization of homopropargyl amines 5.

In 2015, our group [39] described a facile synthesis of various enantioenriched 2,3-dihydropyrroles through a gold-catalyzed

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direct 5-endo-dig cycloisomerization of chiral homopropargyl sulfonamides (Scheme 10). Treatment of homopropargyl sulfonamides 3 in the presence of 0.5 equiv of 2,6-dibromopyridine, 2 mol% Et3N and 5 mol% BrettPhosAuNTf2 produced the corresponding 2,3-dihydropyrroles 8, which constitute an important category of heterocycle systems and exist in a large number of bioactive natural products as well as pharmaceuticals [40–45]. In all cases, excellent yields and enantioselectivities were achieved. Of note, the use of a catalytic base as additive could completely inhibit the previous dimer formation and did not affect the Lewis acidity of gold catalyst. This transformation is proposed to proceed through a gold-catalyzed direct 5-endo-dig cyclization of homopropargyl amides but not the gold vinylidene intermediate pathway. 3.5. Gold-catalyzed tandem cycloisomerization/halogenation of chiral homopropargyl sulfonamides Organic halides are not only frequently observed in biologically active natural products and pharmaceutical agents, but also can serve as highly useful and valuable substrates for various synthetic transformations [46–50]. In 2016, our group [51] reported a gold-catalyzed tandem cycloi somerization/fluorination of chiral homopropargyl amides using Selectfluor as fluorinating agent to form functionalized 3fluoropyrrolidin-2-ols in the presence of 5 mol% of BrettPhosAuNTf2 and 2 mol% of Et3N (Scheme 11). Various aromatic and aliphatic substituted homopropargyl sulfoamides 3 were successfully converted to 3-fluoropyrrolidin-2-ols 9 in moderate to good yields with excellent diastereoselectivities. Interestingly, by employing NIS as halogenation agent, this tandem also worked well, leading to the efficient synthesis of highly functionalized 3,3-diiodopyrrolidin-2-ols instead [51]. Thus, this chemistry makes it possible to realize an alkyne tetrafunctionalization with high diastereoselectivity [52]. As shown in Scheme 12, a series of chiral homopropargyl amides 3 were suitable substrates for this transformation, and the corresponding diiodination products 10 were obtained in moderate to good yields. The mechanism shown in Scheme 13 was proposed to explain this novel gold-catalyzed tandem cycloisomerization/halogenation of homopropargyl sulfonamides. At the outset, the intramolecular addition of amide to the triple bond, activated by the gold catalyst,

Scheme 11. Gold-catalyzed tandem cycloisomerization/fluorination of homopropargyl sulfonamides 3.

Scheme 12. Gold-catalyzed tandem cycloisomerization/diiodination of homopropargyl sulfoamides 3.

to render the vinyl gold intermediate via 5-endo-dig cyclization. In the presence of Selectfluor, this intermediate would be converted into 2,3-dihydropyrrole 9-A, which can further react with Selectfluor to offer iminium intermediate 9-B. Finally, intermediate 9-B is trapped by trace of water in the reaction system to produce the corresponding product 9. However, when NIS is employed as halogenation agent, the vinyl gold intermediate would instead be trapped by NIS to provide vinyl iodide 10-A, which is further

Ts N

OH

6 9 HN

Ts N

6 9-A Ts NH

[Au]

[Au]

6 3

Ts N

13A OH

6

Scheme 10. Synthesis of 2,3-dihydropyrroles 8 through gold-catalyzed hydroamination of chiral homopropargyl sulfonamides 3.

F

9-B

6

I 10

Selectfluor

6

F Ts

I

Ts N

H2 O

Ts N

H 2O

6 10-B

[Au]

I I

NIS Ts N

NIS 6

10-A

I

Scheme 13. Proposed mechanism for the gold-catalyzed tandem cycloisomerization/halogenation of homopropargyl sulfonamides 3.

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A plausible mechanism to rationalize the formation of 11 is shown in Scheme 15. The reaction starts with formation of the vinyl gold intermediate 11-A via a direct 5-endo-dig cyclization, followed by the in situ generation of 2,3-dihydropyrrole intermediate 11-B, which is quickly converted into iminium species 11-C catalyzed by gold and acid. Subsequently, the iminium species undergoes facile reduction by hydride to afford the target product 11. 4. Conclusions and outlook

Scheme 14. Synthesis of pyrrolidines 11 through gold-catalyzed tandem cycloisomerization/hydrogenation of homopropargyl sulfonamides 3.

HN

PG

R

[Au]

PG NH R

R 3

11-A [Au] R

[Au]

11-B

Conflict of interest

PG N

11-C

[Au]

PG N

In summary, by utilizing the steric strain in ring formation to achieve the control of regioselectivity, a variety of gold-catalyzed anti-Markovnikov cycloisomerization-initiated tandem reactions have been developed, providing a facile and efficient way for the construction of various synthetically useful heterocycles, especially the optically active N-heterocycles by combining the chiral tert-butylsulfinimine chemistry and gold catalysis [65–75]. In our opinion, this study will continue to be a very exciting area for the expedient synthesis of valuable heterocycles [76]. Besides the control of regioselectivity in catalytic alkyne transformation, this strategy may be applied into other kinds of substrates. For example, the control of regioselectivity in catalytic alkene transformation by steric strain in ring formation, although the relevant 5-endo-trig cyclization is Baldwin Rule forbidden, may result in the development of various novel transition metalcatalyzed anti-Markovnikov cyclization-initiated tandem reactions, thus giving rise to inspiration for new chemistry on the control of regioselectivity driven by ring strain and leading to the divergent synthesis of complex molecules.

hydride [Au]

[Au]

R

PG N

11

Scheme 15. Proposed mechanism for the gold-catalyzed tandem cycloisomerization/hydrogenation of homopropargyl sulfonamides 3.

converted into the iminium intermediate 10-B in the presence of another molecule of NIS. Finally, trace water in the reaction system attacks intermediate 10-B to afford the target product 10. 3.6. Gold-catalyzed tandem cycloisomerization/hydrogenation of chiral homopropargyl sulfonamides for the synthesis of enantioenriched pyrrolidines Chiral pyrrolidines are highly useful and valuable frameworks that can be found in a large number of biologically active natural products and pharmaceutical agents [53–56]. In addition, they can also serve as important organocatalysts [57–60] and ligands in organic synthesis [61–63]. Very recently, our group [64] have realized an efficient and general method for the enantioselective synthesis of various pyrrolidines through gold-catalyzed tandem cycloisomerization/hydrogenation of chiral homopropargyl sulphonamides, which represents the first example of a pyrrolidine synthesis from simple homopropargyl sulfonamide. As described in Scheme 14, it was found that chiral homopropargyl sulfonamides 3 underwent smooth cycloisomerization-initiated hydrogenation, leading to the desired pyrrolidines 11 in excellent yields. Once again, excellent enantioselectivities were achieved in all cases and essentially no epimerization was observed. In addition, the reaction proceeded well with different protecting groups on the nitrogen such as Bs (p-bromobenzenesulfonyl) and benzenesulfonyl.

The authors declare that they have no conflict of interest. Acknowledgments This work was supported by the National Natural Science Foundation of China (21272191, 21572186 and 21622204), the Natural Science Foundation of Fujian Province for Distinguished Young Scholars (2015J06003), the President Research Funds from Xiamen University (20720150045), and NFFTBS (J1310024). References [1] Dorel R, Echavarren AM. Gold(I)-catalyzed activation of alkynes for the construction of molecular complexity. Chem Rev 2015;115:9028–72. [2] Qian D, Zhang J. Gold-catalyzed cyclopropanation reactions using a carbenoid precursor toolbox. Chem Soc Rev 2015;44:677–98. [3] Zheng Z, Wang Z, Wang Y, et al. Au-catalysed oxidative cyclisation. Chem Soc Rev 2016;45:4448–58. [4] Pflästerer D, Hashmi ASK. Gold catalysis in total synthesis – recent achievements. Chem Soc Rev 2016;45:1331–67. [5] Wei Y, Shi M. Divergent synthesis of carbo- and heterocycles via goldcatalyzed reactions. ACS Catal 2016;6:2515–24. [6] Huple DB, Ghorpade S, Liu RS. Recent advances in gold-catalyzed N- and Ofunctionalizations of alkynes with nitrones, nitroso, nitro and nitroxy species. Adv Synth Catal 2016;358:1348–67. [7] Wei F, Song C, Ma Y, et al. Gold carbene chemistry from diazo compounds. Sci Bull 2015;60:1479–92. [8] Yeom HS, Shin S. Catalytic access to a-oxo gold carbenes by N-O bond oxidants. Acc Chem Res 2014;47:966–77. [9] Zhang LM. A non-diazo approach to a-oxo gold carbenes via gold-catalyzed alkyne oxidation. Acc Chem Res 2014;47:877–88. [10] Rudolph M, Hashmi ASK. Gold catalysis in total synthesis – an update. Chem Soc Rev 2012;41:2448–62. [11] Hashmi ASK, Rudolph M. Gold catalysis in total synthesis. Chem Soc Rev 2008;37:1766–75. [12] Jiménez-Núñez E, Echavarren AM. Molecular diversity through gold catalysis with alkynes. Chem Commun 2007;4:333–46. [13] Gimeno A, Cuenca AB, et al. Competitive gold-activation modes in terminal alkynes: an experimental and mechanistic study. Chem Eur J 2014;20:683–8.

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