and Enantioselective Reductive Cyclization of Alkyne- Tethered

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Rhodium-Catalyzed Highly Regio- and Enantioselective Reductive Cyclization of Alkyne-Tethered Cyclohexadienones Krishna Kumar Gollapelli, Sangeetha Donikela, Nemali Manjula, and Rambabu Chegondi ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04054 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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Rhodium-Catalyzed Highly Regio- and Enantioselective Reductive Cyclization of AlkyneTethered Cyclohexadienones Krishna Kumar Gollapelli,a, ‡ Sangeetha Donikela,b, ‡ Nemali Manjula,b Rambabu Chegondi*,a aOrganic

& Biomolecular Chemistry Division, bNatural Products Chemistry Division,

CSIR-Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad 500007, India. a,bAcademy

‡

of Scientific and Innovative Research (AcSIR), New Delhi 110025, India.

Authors contributed equally to this work.

ABSTRACT

Rhodium-catalyzed

asymmetric

hydrogenation

of

alkyne-tethered

cyclohexadienones enables highly regio- and enantioselective reductive cyclization to afford cishydrobenzofurans and cis-hydroindoles in high yields. Desymmetrization of 1,3-diyne-tethered cyclohexadienones was also explored, wherein the intramolecular coordination of a Rh-complex with the cyclohexadienone ring induces exclusive regioselectivity. Mechanistic studies including hydrogen-deuterium crossover experiments suggested that hydrogen activation is the ratedetermining step for tandem reductive cyclization. Moreover, this highly practical and atomeconomical transformation has tolerance to many functional-groups with a broad range of substrate

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scope, allowing further transformations to expand the structural complexity of the bicyclic scaffolds.

KEYWORDS: Desymmetrization, Asymmetric hydrogenation; Rhodium, Cyclohexadienones, cis-Hydroindoles, cis-Hydrobenzofurans

INTRODUCTION Methods for rapid construction of chiral structural motifs from readily available substrates have always fascinated organic and medicinal chemists due to their application in both natural product synthesis and small molecule therapeutics for drug discovery. Chiral cis-hydrobenzofurans and cis-hydroindoles are such class of privileged structural motifs and are widely present in many natural products (Figure 1).1 The oxidative dearomatization of phenols followed by catalytic enantioselective desymmetrization is the most convenient method to build this bicyclic framework.2 However, few asymmetric methods have been reported, and so novel, highly enantioselective and atom efficient approaches are needed to access these desirable scaffolds.

Figure 1. cis-Hydrobenzofurans and cis-hydroindoles as structural motifs in natural products.

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Over the last decade, Hayashi, Rovis, Gaunt and Sasai research groups have successfully demonstrated the application of organocatalysis in the tether-mediated asymmetric desymmetrization of prochiral cyclohexadienones.3 Although, very few examples of transitionmetal

catalyzed

desymmetrization

of

cyclohexadienones

to

prepare

chiral

cis-

hydrobenzofuranones have been reported.4-7 In 2013, Tian and Lin co-workers have demonstrated a copper-catalyzed asymmetric borylative enyne cyclization of the prochiral cyclohexadienone with excellent enantioselectivity (Scheme 1a).5a The high regioselectivity was achieved through coordination of catalyst with the propargylic oxygen atom. Recently, Lautens6a and Tian/Lin6b research groups independently reported an efficient Rh-catalyzed enantioselective arylative cyclization of alkyne-tethered cyclohexadienones. However, substrate scope of these methods is limited to O-tethered terminal or alkyl substituted alkynes, which gives only aryl substituted cishydrobenzofurans.

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Scheme 1. Catalytic asymmetric desymmetrization of alkyne-tethered cyclohexadienones

In continuation of our efforts aimed at desymmetrization of C2-symmetric molecules,8 we envisioned that Rh-catalyzed asymmetric hydrogenation of alkyne-tethered cyclohexadienones would be more general and atom efficient for the synthesis of enantioselective cishydrobenzofurans and cis-hydroindoles (Scheme 1b). Despite of the fact that the elemental hydrogen is commercially inexpensive, cleanest chemical reductant, catalytic hydrogenation mediated C-C bond formation reaction has been limited to hydroformylation9 and related Fischertropsch-type reactions10 for many years. Recently, Krische and co-workers extensively studied the capturing of organometallic intermediates in catalytic reductive coupling reactions.11,12 Despite such exciting developments, desymmetrization of prochiral cyclohexadienones via asymmetric 4


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hydrogenation was still unexplored prior to this study. We hypothesized that the high regio- and enantioselectivity could be achieved through the coordination of a cationic Rh-complex with diene A followed by key symmetry breaking Michael addition of intermediate B (Scheme 1b). RESULTS AND DISCUSSION Studies began with evaluation of various Rh(I)- and Ir(I)-catalysts (2.5 mol%) with common chiral bisphosphine ligands (L1-L7, 5 mol%) in the tandem reductive cyclization of 1,6-dienyne 1a13 in presence of H2 atmosphere in dichloroethane (0.1 M) at room temperature (Table 1). Pleasingly, both [Rh(COD)Cl]2 and [Ir(COD)Cl]2 with various ligands evaluated led to required product 2a in good yields and moderate enantioselectivity along with appreciable quantities of over hydrogenated product 3a (entries 1-6). The use of more hindered ligands DTBM-SEGPHOS, L5 and Trost ligand L6 provided modest yields and poor enantioselectivity (entries 7-9). Interestingly, Rh(COD)2X catalyst, where counter ion X is OTf and SbF6 in presence of BINAP (L2) or SEGPHOS (L3) ligands furnished 2a in high yield with excellent enantioselectivity (>99% ee). This clearly indicates that the precatalyst with less strongly coordinated counter ion is more effective in catalytic hydrogenation. Subsequently, the more bulky DM-SEGPHOS ligand, L7 was examined to further improve the yield of enone 2a (entries 14 & 15). Unfortunately, the desired product was obtained with low yields, similar to the previous observations (entries 7-9). Notably, Rh(COD)2OTf catalyst ligated by simple (S)-BINAP (L2) provided the best results, furnishing 2a as the major product (2a/3a = 91:9; 80% yield) in 99% ee (Table 1, entry 10).

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Table 1. Optimization of reaction conditionsa,b

[a] The reaction was carried out with 1a (0.1 mmol), catalyst (2.5 mol%), ligand (5 mol%) in dichloroethane (2 mL) under H2 (g) atmosphere at room temperature. [b] Yield of the isolated products 2a and 3a. [c] Determined by HPLC analysis using a chiral stationary phase. [d] Ratio determined by 1H NMR analysis of crude reaction mixture. [e] Observed exclusive cis-selectivity.

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With the optimal reaction conditions identified, the generality of the present enantioselective reductive cyclization was investigated with various alkyne-tethered cyclohexadienones 1 (Table 2). Despite the presence of bulky R1 substituents (Me, Et, iPr, C2H4OTBS and Ph) at the substrate’s prochiral quaternary carbon center, the reaction proceeded smoothly with high yields (75–85%) and excellent enantioselectivities (96–99% ee, entries 2a–2e). Diverse functional groups were well tolerated including electron-donating and electron-withdrawing groups at the para, meta and ortho positions of the ethynylphenyl ring, providing the corresponding products with good yields and excellent enantioselectivities (2f–m). However, electron-donating substituents at para-position as well as ortho-substituents provided their corresponding products (2f and 2m, respectively) in lower enantioselectivities in comparison to the other examples. Naphthyl and thiophene substituted alkynes yielded the corresponding products 2n and 2o respectively, with higher yields and enantioselectivities. Internal alkyne-tethered cyclohexadienones substituted with aliphatic groups also afforded corresponding reductive cyclization products 2u–2x in good yields (56–83%) with excellent enantiomeric excess (>98%). Interestingly, terminal alkyne-tethered 1,6-enynes were well tolerated in the asymmetric hydrogenation reaction and exhibited higher reactivity than internal alkynes. Thus, the terminal alkyne substrates gave the corresponding products 2p–2t in excellent yields (75–86%) and enantioselectivities (>98% ee). In addition, N-tethered cyclohexadienones were also suitable substrates furnishing the corresponding hydroindoles 2y, 2z and 2za in high yields (72-86%). It is also noted that the Boc-protected hydroindoles 2y, 2z have lower enantioselectivity (83 & 87% ee, respectively) compared to Ts-protected hydroindole 2z (98% ee). The practicality of this reaction is also proved by the preparation of product 2y on a large scale. The C-tethered 1,6-enyne under the standard reaction conditions failed to give the desired cyclization product 2zb, presumably due to the absence of the Thorpe−Ingold effect14. The

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relative and absolute configurations of cis-hydrobenzofuran 2g were unambiguously assigned as S and S by X-ray crystallography (Figure 2).15 Table 2. Alkyne-Tethered-Cyclohexadienone Scopea,b,c,d

aReaction

conditions: [a] 1 (0.5 mmol), Rh(COD)2OTf (2.5 mol %), (S)-BINAP (5 mol%) in dichloroethane (5 mL) under H2 (g) atmosphere at room temperature. [b] Yield of the isolated products. [c] Determined by HPLC analysis using a chiral stationary phase. [d] Observed exclusive cis-selectivity. 8


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Figure 2. ORTEP diagram of compound 2g at the 30% probability level.

Next, we sought to investigate the scope of 1,3-diyne-tethered cyclohexadienones 4 in the asymmetric hydrogenation reaction (Table 3). To the best of our knowledge, this is the first time in which the desymmetrization of 1,3-diyne-tethered cyclohexadienones 4 has been explored. In this case, regioselective hydro-metallation followed by carbocyclization proceeds from the alkyne proximal to the cyclohexadienone ring. The exclusive reverse regioselectivity of this reaction as compared to the previous report16 is probably due to the intramolecular coordination of dienone with Rh-complex. As shown in Table 3, with the R1 substituent as methyl, ethyl and phenyl group, the reactions proceeded smoothly with high yields (75–86%) and excellent enantioselectivities (97% ee, 5a–5c). Similarly, desymmetrization of other aromatic and aliphatic substituted 1,3diynes were successful to give the corresponding cis-hydrobenzofuranones 5d–5h in good yields (52–74%) with high enantiomeric excess (85–99%). In addition, aromatic and aliphatic substituted N-tethered 1,3-diyne substrates also gave the corresponding hydroindoles 5i–5m in good yields (63–75%) and enantioselectivities (88–96% ee).

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Table 3. 1,3-Diyne-Tethered Cyclohexadienone Scopea,b,c,d

aReaction

conditions: [a] 4 (0.5 mmol), Rh(COD)2OTf (2.5 mol %), (S)-BINAP (5 mol%) in dichloroethane (5 mL) under H2 (g) atmosphere at room temperature. [b] Yield of the isolated products. [c] Determined by HPLC analysis using a chiral stationary phase. [d] Observed exclusive cis-selectivity. To get further insight into the mechanism of this tandem reaction, a set of experiments were conducted under an atmosphere of D2 and with an isotopically labelled substrate (Scheme 2). Reductive cyclization of 1t in the presence of D2 under standard reaction conditions furnished monodeuterated product d1-2t with 71% yield. Similarly, catalytic reductive cyclization of 1a and 10


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1t under a mixed atmosphere of H2 and D2 afforded respective products 2a/d1-2a and 2t/d1-2t with ~30% deuterium incorporation (kH/kD = >2). In addition, a kinetic isotope effect (KIE) of kH/kD = ~1 was observed using 1:1 ratio of 1t/d1-1t in the intermolecular isotopic study. KIE measurement from two parallel reactions also gave similar result (see Supporting Information). These experiments suggest that the oxidative addition of hydrogen is probably rate-determining for rhodium-catalyzed enantioselective reductive cyclization reaction.12

Scheme 2. Deuterium-labelling experiments and study of the kinetic isotope effect. In light of these experiments and previous reports,12a,b a plausible mechanism for rhodiumcatalyzed reductive cyclization is depicted in Scheme 3. Initially, monohydride catalyst LnRh(I)H is generated through the heterolytic activation of elemental hydrogen with cationic rhodium catalyst LnRhX, followed by syn-hydro metalation of alkyne 1a into LnRhH to give vinyl rhodium intermediate I which then undergoes subsequent carobocyclization. The key enantioselective desymmetrization proceeds through the differentiation of two enantiotopic functional groups in 11


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cyclohexadienone via favourable six-membered chair transition state I to provide oxa-πallylrhodium intermediate II. The oxidative addition of enolate-intermediate II with H2 provides Rh(III) intermediate III followed by reductive elimination releases the product 2a along with the regeneration of active catalyst LnRh(I)H to complete the catalytic cycle.

Scheme 3. Plausible mechanism for tandem reductive cyclization reaction. To demonstrate the synthetic utility of this tandem reaction, the bicyclic products were further functionalised into more complex tricyclic products with excellent diastereoselectivity (Scheme 4). Initially, cis-hydrobenzofuran 2w was converted to known tricyclic product 76b through tandem desilylation oxa-Michael addition followed by ozonolysis via alkene 6 which determined that the absolute configurations of the products were consistent with the X-ray crystallographic analysis of compound 2g. It is worth to mention that the tricyclic diketone 7 was an architecture unit of bioactive natural product incarviditone.1 The hydrogenation of compound 6 in the presence of PdC furnished endo-8a and exo-8b with 3:1 ratio of diastereoselectivity in 71% yield. The removal of the Boc group from 2y using trifluoroacetic acid followed by the treatment of the free amine with phenyl isocyanate afforded corresponding aza-Michael product 9 exclusively with 76% 12


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overall yield. Subsequent hydrogenation of compound 9 gave endo-product 10 in 68% yield with good diastereoselectivity (12:1). In addition, deprotection of 2y and subsequent allylation with 2,3dibromopropene in the presence of K2CO3 gave vinyl bromide 11 in 74% yield. The radical cyclization of 11 in the presence of Bu3SnH/AIBN furnished 5-exo-trig product 12 in 61% yield.

Scheme 4. Synthetic utility

CONCLUSION In summary, we have developed an atom-economical, highly regio- and enantioselective reductive cyclization of cyclohexadienone-containing 1,6-dienynes using a Rh-catalyzed asymmetric hydrogenation reaction. This tandem desymmetrization process proceeded through 13


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selective hydrometalation of the alkyne and subsequent enantioselective 1,4-addition to cyclohexadienone to afford cis-hydrobenzofurans or cis-hydroindoles in high yields with excellent enantioselectivities.

Desymmetrization of 1,3-diyne-tethered cyclohexadienones was also

explored for the first time, wherein the intramolecular coordination of a Rh-complex with the cyclohexadienone ring induces exclusive regioselectivity. The reductive cyclization has high functional group compatibility and a broad range of substrate scope. The practicality of this method has been further demonstrated with a gram-scale reaction. Finally, the bicyclic products could be subjected to various transformations in a diastereoselective manner for elaborating synthetic utility. Further desymmetrization studies on alkyne-tethered cyclohexadienones are underway in our laboratory. EXPERIMENTAL SECTION General Procedure: To a stirred solution of akyne-tethered meso-cyclohexadienone 1 or 4 (0.5 mmol, 100 mol %) in DCE (5 mL, 0.1 M) at room temperature was added Rh(COD)2OTf (5.8 mg, 0.0125 mmol, 2.5 mol %) and (S)-BINAP (16 mg, 0.025 mmol, 5 mol %). The reaction mixture was purged with hydrogen gas, and the reaction was allowed to stir at room temperature under hydrogen atmosphere until complete consumption of starting material (monitored by TLC). Afterwards, solvent was evaporated under reduced pressure and residue was purified by flash column chromatography on silica gel (EtOAc in hexane) to give the desired product 2 or 5. [Note: For racemic products, rec-BINAP ligand was used in place of (S)-BINAP and followed the same procedure]

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ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] ORCID Rambabu Chegondi: 0000-0003-2072-7429 Author Contributions ‡KKG

and SD contributed equally to this work.

Funding Sources The authors declare no competing financial interest. Supporting Information The supporting information is available free of charge via the Internet at http://pubs.acs.org. Experimental details, characterization data, and spectra, X-ray crystallographic data for compound 2g. ACKNOWLEDGMENT The Department of Science and Technology (DST), New Delhi, India, is acknowledged for the award of a Start-up Research Grant (SB/FT/CS-109/2013). KKG thanks DST and SD thanks UGC for a research fellowships. We thank Laboratory of X-ray Crystallography, CSIR-IICT for X-ray analysis. We gratefully acknowledge to Dr. S. Chandrasekhar (Director, CSIR-IICT) for his 15


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support and valuable discussions. This work is dedicated to Professor D. Basavaiah (University of Hyderabad) for his outstanding contributions to organic chemistry. REFERENCES (1) For cis-hydrobenzofuran containing natural products, see: (a) Castro, J. M.; Salido, S.; Altarejos, J.; Nogueras, M.; Sánchez, A. Tetrahedron 2002, 58, 5941–5949. (b) Bringmann, G.; Lang, G.; Gulder, T. A. M.; Tsuruta, H.;

Mühlbacher, J.; Maksimenka, K.; Steffens, S.;

Schaumann, K.; Stöhr, R.; Wiese, J.; Imhoff, J. F.; Perovic-Ottstadt, S.; Boreiko, O.; Müller, W. E. G. Tetrahedron 2005, 61, 7252–7265. (c) Chen, Y.-Q.; Shen, Y.-H.; Su, Y.-Q.; Kong, L.-Y.; Zhang, W.-D. Chem. Biodiversity 2009, 6, 779–783. (d) Wegner, J.; Ley, S. V.; Kirschning, A.; Hansen, A.-L.; Garcia, J. M.; Baxendale, I. R. Org. Lett. 2012, 14, 696–699. For cis-hydroindoles containing natural products, see (e) Dyke, S. F.; Quessy, S. N.; In The Alkaloids; R. G. A. Rodrigo, R. G. A.; Ed.; Academic Press: New York, 1981; Vol. 18, pp 1–98. (f) Jin, Z.; Nat. Prod. Rep. 2011, 28, 1126−1142. (2) (a) Sun, W.; Li, G.; Hong, L.; Wang, R. Org. Biomol. Chem. 2016, 14, 2164–2176. (b) Kalstabakken, K. A.; Harned, A. M. Tetrahedron 2014, 70, 9571−9585. For recent review on enantioselective desymmetrization reactions, see: (c) Zeng, X.-P.; Cao, Z.-Y.; Wang, Y.-H.; Zhou, F.; Zhou, J. Chem. Rev. 2016, 116, 7330−7396. (3) For organocatalyzed desymmetrization of cyclohexadienones, see: (a) Hayashi, Y.; Gotoh, H.; Tamura, T.; Yamaguchi, H.; Masui, R.; Shoji, M. J. Am. Chem. Soc. 2005, 127, 16028−16029. (b) Vo, N. T.; Pace, R.D.M.; O’Hara, F.; Gaunt, M. J. J. Am. Chem. Soc. 2008, 130, 404−405. (c) Leon, R.; Jawalekar, A.; Redert, T.; Gaunt, M. J. Chem. Sci. 2011, 2, 1487−1490. (d) Corbett, M. T.; Johnson, J. S. Chem. Sci. 2013, 4, 2828−2832. For other selected references, see: (e) Liu, Q.; 16


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Rovis, T. J. Am. Chem. Soc. 2006, 128, 2552−2553. (f) Gu, Q.; You, S.-L. Chem. Sci. 2011, 2, 1519−1522. (g) Jia, M.-Q. ; You, S.-L. Chem. Commun. 2012, 48, 6363−6365. (h) Takizawa, S.; Nguyen, T. M.-N. ; Grossmann, A.; Enders, D.; Sasai, H. Angew. Chem. Int. Ed. 2012, 51, 5423−5426. (i) Wu, W.; Li, X.; Huang, H.; Yuan, X.; Lu, J.; Zhu, K.; Ye, J. Angew. Chem. Int. Ed. 2013, 52, 1743–1747. (j) Martín-Santos, C.; Jarava-Barrera, C.; del Pozo, S.; Parra, A.; DíazTendero, S.; Mas-Ballesté, R.; Cabrera, S. J. Alemán, Angew. Chem. Int. Ed. 2014, 53, 8184– 8189. (k) Yao, W.; Dou, X.; Wen, S.; Wu, J.; Vittal, J. J.; Lu, Y.; Nat. Commun. 2016, 7, 13024. (4) For Pd-catalyzed enantioselective desymmetrization of cyclohexadienones to access cishydrobenzofuranones, see: (a) Takenaka, K.; Mohanta, S. C.; Sasai, H. Angew. Chem. Int. Ed. 2014, 53, 4675–4679. For other Pd-catalyzed desymmetrizations, see: (b) Kondo, K.; Sodeoka, M.; Mori, M.; Shibasaki, M. Tetrahedron Lett. 1993, 34, 4219–4222. (c) Imbos, R.; Minnaard, A. J.; Feringa, B. L. J. Am. Chem. Soc. 2002, 124, 184–185. (d) Tello-Aburto, R.; Harned, A. M. Org. Lett. 2009, 11, 3998–4000. (e) He, C.; Zhu, C.; Dai, Z.; Tseng, C.-C.; Ding, H. Angew. Chem. Int. Ed. 2013, 52, 13256–13260. (5) For Cu-catalyzed enantioselective desymmetrization of cyclohexadienones to access cishydrobenzofuranones, see: (a) Liu, P.; Fukui, Y.; Tian, P.; He, Z.-T.; Sun, C.-Y.; Wu, N.-Y.; Lin, G.-Q. J. Am. Chem. Soc. 2013, 135, 11700–11703. (b) He, Z.-T.; Tang, X.-Q.; Xie, L.-B.; Cheng, M.; Tian, P.; Lin, G.-Q.; Angew. Chem., Int. Ed. 2015, 54, 14815–14818. For other Cu-catalyzed desymmetrizations, see: (c) Feringa, B. L.; Pineschi, M.; Arnold, L. A.; Imbos, R.; de Vries, A. H. M. Angew. Chem. Int. Ed. 1997, 36, 2620–2623. (d) Imbos, R.; Brilman, M. H. G.; Pineschi, M.; Feringa, B. L. Org. Lett. 1999, 1, 623–626. (e) Imbos, R.; Minnaard, A. J.; Feringa, B. L. Tetrahedron 2001, 57, 2485–2489. (f) Meister, A. C.; Sauter, P. F.; Bräse, S. Eur. J. Org. Chem. 2013, 7110–7116. 17


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(6) For Rh-catalyzed enantioselective desymmetrization of cyclohexadienones to access cishydrobenzofuranones, see: (a) Keilitz, J.; Newman, S. G.; Lautens, M. Org. Lett. 2013, 15, 1148– 1151. (b) He, Z.-T.; Tian, B.; Fukui, Y.; Tong, X.; Tian, P.; Lin, G.-Q. Angew. Chem. Int. Ed. 2013, 52, 5314–5318. For other Rh-catalyzed desymmetrizations, see: (c) Guo, F.; Konkol, L. C.; Thomson, R. J. J. Am. Chem. Soc. 2011, 133, 18–20. (d) Kress, S.; Johnson, T.; Weisshar, F.; Lautens, M. ACS Catal. 2016, 6, 747–750. (e) Fukui, Y.; Liu, P.; Liu, Q.; He, Z.-T.; Wu, N.-Y.; Tian, P.; Lin, G.-Q. J. Am. Chem. Soc. 2014, 136, 15607–15614. (7) For other transition metal catalyzed desymmetrization of cyclohexadienones, see: (a) Cai, S.Y.; Liu, Z.; Zhang, W.-B.; Zhao, X.-Y.; Wang, D. Z. Angew. Chem. Int. Ed. 2011, 50, 11133– 11137. (b) Clarke, C.; Incerti-Pradillos, C. A.; Lam, H. W. J. Am. Chem. Soc. 2016, 138, 8068– 8071. (c) Kumar, R.; Hoshimoto, Y.; Tamai, E.; Ohashi, M.; Ogoshi, S. Nat. Commun. 2017, 8, 32. (8) (a) Murthy, A. S.; Donikela, S.; Reddy, C. S.; Chegondi, R. J. Org. Chem. 2015, 80, 5566−5571. (b) Kallepu, S.; Gollapelli, K. K.; Nanubolu, J. B.; Chegondi, R. Chem. Commun. 2015, 51, 16840−16843. (c) Gollapelli, K. K.; Kallepu, S.; Govindappa, N.; Nanubolu, J. B.; Chegondi, R. Chem. Sci. 2016, 7, 4748–4753. (d) Chavan, L. N.; Gollapelli, K. K.; Chegondi, R.; Pawar, A. B. Org. Lett. 2017, 19, 2186–2189. (e) Anugu, R. R.; Chegondi, R. J. Org. Chem. 2017, 82, 6786−6794. (9) (a) Breit, B. Acc. Chem. Res. 2003, 36, 264−275. (b) Breit, B.; Seiche, W. Synthesis 2001, 1−36. (10) (a) Herrmann, W. A. Angew. Chem. Int. Ed. Engl. 1982, 21, 117−130. (b) Rofer-Depoorter, C.-K. Chem. Rev. 1981, 81, 447−474. 18


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(11) For recent reviews, see: (a) Jang, H.-Y.; Krische, M. J. Acc. Chem. Res. 2004, 37, 653−661. (b) Kim, S. W.; Zhang, W.; Krische, M. J. Acc. Chem. Res. 2017, 50, 2371−2380. (12) For selected reference, see: (a) Jang, H.-Y.; Krische, M. J. J. Am. Chem. Soc. 2004, 126, 7875–7880. (b) Jang, H.-Y.; Hughes, F. W.; Gong, H.; Zhang, J.; Brodbelt, J. S.; Krische, M. J. J. Am. Chem. Soc. 2005, 127, 6174–6175. (c) Kong, J.-R.; Cho, C.-W.; Krische, M. J. J. Am. Chem. Soc. 2005, 127, 11269–11276. (d) Kong, J.-R.; Ngai, M.-Y.; Krische, M. J. J. Am. Chem. Soc. 2006, 128, 718–719. (e) Cho, C.-W.; Krische, M. J. Org. Lett. 2006, 8, 3873–3876. (f) Rhee, J.U.; Krische, M. J. J. Am. Chem. Soc. 2006, 128, 10674–10675. (g) Komanduri, V.; Krische, M. J. J. Am. Chem. Soc. 2006, 128, 16448–16449. (13) Hexum, J. K.; Tello-Aburto, R.; Struntz, N. B.; Harned, A. M.; Harki, D. A. ACS Med. Chem. Lett. 2012, 3, 459−464. (14) Jung, M. E.; Piizzi, G. Chem. Rev. 2005, 105, 1735−1766. (15) CCDC-1550556 (compound 2g) contains the supplementary crystallographic data for this paper which can be obtained free of charge from the Cambridge Crystallographic Data Centre via https://summary.ccdc.cam.ac.uk/structure-summary-form. (16) Huddleston, R. R.; Jang, H.-Y.; Krische, M. J. J. Am. Chem. Soc. 2003, 125, 11488−114890.

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Rhodium-Catalyzed Highly Regio- and Enantioselective Reductive Cyclization of AlkyneTethered Cyclohexadienones Krishna Kumar Gollapelli,a, ‡ Sangeetha Donikela,b, ‡ Nemali Manjula,b Rambabu Chegondi*,a

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and Enantioselective Reductive Cyclization of Alkyne- Tethered

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