The multicomponent reaction we presented in publication II expands the scope of our approach to N-arylated cyclic carbamates. Substitution elsewhere in the ring, however, cannot be achieved with this method. For example, the 3-aryl, 5-alkyl substituted 2-oxazolidinone structures required for Linezolid-type pharmaceuticals are inaccessible in a single step. We therefore looked into the cyclization of CO2 and amino alcohols in an effort to further enhance the substrate scope and the sustainability of cyclic carbamate synthesis.
Our approach involves the in situ conversion of the amino alcohol’s hydroxyl group into a pseudohalide with TsCl, which makes it more susceptible to an intramolecular nucleophilic attack by the carbamate moiety formed by the amino group and carbon dioxide. However, the obvious challenge in this approach is the competing N-tosylation reaction, which prevents the formation of the carbamate moiety and subsequently inhibits the cyclization reaction (Scheme 25).
Scheme 25 Tosyl-assisted cyclization of carbon dioxide and amino alcohols in the presence of an external base. The carbamate species must be formed first and stabilized to avoid the competing N-tosylation reaction, which prevents the formation of the desired product.
The reaction optimization process was thus selectivity-oriented: we wished to find the conditions in which the irreversible N-tosylation is minimized while O-tosylation is maximized. Using the simple, inexpensive and commercially available 2-(phenylamino)ethan-1-ol as a model substrate, we first investigated the reaction conditions carried over from publication II (Cs2CO3
as a base, MeCN as a solvent, 60 °C and 1 barg CO2). However, under these conditions, N-tosylation is heavily favoured over cyclization (product ratio 84:16). Reasoning that the carbamate moiety is too labile to sufficiently protect
temperature (RT) and an elevated pressure of 5 barg CO2. This indeed resulted in a jump to 65% selectivity towards cyclic carbamate formation. However, further decrease in temperature or increase in pressure gave no additional improvement. We thus settled with these conditions, and turned our attention to screening other solvents and bases.
Polar protic solvents (e.g. EtOH) are unsuitable for this reaction, as they can react with TsCl. Nonpolar solvents, meanwhile, are not capable of sufficiently solvating the reagents, and hence utilizing e.g. dichloromethane leads to low conversion. Polar aprotic solvents are therefore the best option;
compared to the initially used MeCN, both acetone and N,N-dimethylformamide offer higher selectivity (100%), with acetone also boosting overall conversion (50%).
The originally employed Cs2CO3 proved to be the most efficient base for this transformation. Et3N, which is commonly used in other sulfonylation reactions, gave here only 18% conversion and ca. 1:1 ratio of 2-oxazolidinone:N-tosylated product. Nucleophilic bases such as TMG or KOH, meanwhile, favour reaction with TsCl over deprotonating the AA. K2CO3, on the other hand, is less soluble in acetone than Cs2CO3, and consequently gives lower conversion. It follows that the best results (50% conversion and 100%
selectivity) are obtained by conducting the reaction at RT and under 5 barg CO2 for 20 hours, with Cs2CO3 as the base and acetone as the solvent.
Having found the optimal reaction conditions, we then explored the substrate scope of our method (Scheme 26). First focusing on substituents near the hydroxyl group, we discovered that secondary alcohols are more reactive (85% conversion, 98% selectivity). Tertiary hydroxyl groups, meanwhile, are too sterically hindered to undergo cyclization, and mainly starting material is recovered from the reaction mixture.
Unlike in publication II, substituents on the aromatic ring now play a role in reaction efficiency, and yields appear to weakly correlate with the pKa of the corresponding aniline. The presence of a weakly activating alkyl group leads to a slightly enhanced yield, while the more strongly activating alkoxy moiety affords quantitative conversion at the cost of selectivity, as the oxazolidinone and the N-tosylated amino alcohol form in a ca. 1:1 ratio. However, the reaction with 2-((4-methoxyphenyl)amino)ethan-1-ol actually marks the only occasion in which any N-tosylation was detected under the optimized reaction conditions. A deactivating halogen substituent, meanwhile, leads to a decrease in conversion, but full selectivity is maintained.
The method is also applicable to the synthesis of 6-membered carbamate rings and bicyclic fused rings. The 6-membered rings follow the same trend as 2-oxazolidinones in that a secondary hydroxyl group is more reactive than the primary 3(phenylamino)propan-1-ol. The bicyclic fused rings, which are found in many pharmaceuticals such as the antiretroviral Efavirenz, are also formed with good selectivities and in yields comparable to phosgene-based methods.144
Scheme 26 Conversions of various AAs and (isolated yields) of the corresponding cyclic carbamates. Reaction conditions: 0.5mmol substrate, 1.1 equiv. TsCl, 3 equiv. Cs2CO3, 5 barg CO2, 5mL acetone, RT, 20h.
Next, we probed the chemo- and stereoselectivity of the reaction by screening difunctionalized or chiral amino alcohols as substrates (Scheme 27). In publication I, we showed that the cyclization of carbon dioxide and 2,3-dichloropropylamine strongly favors the formation of the more stable 5-membered ring (81% 5-ring, 17% 6-ring): here, however, cyclizing CO2 and 1-chloro-3-(phenylamino)propan-2-ol gives a mixture of the two possible products in a ca. 5:6 ratio, as the thermodynamic stability of the 5-membered ring is offset by the better leaving group nature of -Cl. 3-(phenylamino)propane-1,2-diol, meanwhile, shows an expected preference towards the formation of the 5-membered ring, which can be isolated in 85%
yield. Intriguingly, this makes the selectivity towards the 5-membered ring in this case higher than what we observed in publication I.
Scheme 27 Chemo- and stereoselectivity in the TsCl-assisted cyclization of various amino alcohols and CO2. All yields are isolated yields. Reaction conditions: 0.5mmol substrate, 1.1 equiv. TsCl, 3 equiv. Cs2CO3, 5 barg CO2, 5mL acetone, RT, 20h.
Upon subjecting enantiopure AAs to this method, we discovered that the reaction’s stereochemistry is highly predictable. A >99% ee is consistently observed, with the exception of the amino diol, which undergoes some racemization. In all cases, the products have an inverted configuration at the stereocenter, which hints at a SN2-type mechanism. Computational studies conducted by our collaborators further supported this view, and the mechanism seen in Scheme 25 indeed seems to take place.
4.5 SUMMARY
Over the previous pages, three novel methods for the carbon dioxide-based synthesis of cyclic carbamates have been outlined. The underlying theme in these approaches is the formation of a carbamate species by an amine group and CO2, followed by an intramolecular substitution reaction with a (pseudo)halide leaving group to close the ring.
In publication I, we demonstrated the cyclization of β- and γ-halogenated amines with carbon dioxide to yield 5-membered and 6-membered carbamate rings. The reaction is highly efficient – as efficient as non-CO2-based methods – and quantitative yields are observed in very short reaction times. Only the scarcity of commercially available haloamines limits the power of this approach.
In publication II, the scope was expanded to N-arylated cyclic carbamates in an organocatalyzed, multicomponent reaction between an aniline, a dihaloalkane, and CO2. The method shows high functional group tolerance, and a wide range of variously substituted anilines may be employed. In addition to the preparation of 3-aryl-2-oxazolidinones, this process can also be used in the synthesis of 6- and 7-membered carbamate rings in yields surpassing isocyanate-based methods.
In publication III, cyclic carbamates were synthesized by cyclizing amino alcohols and carbon dioxide. This was achieved by converting the hydroxyl group in situ into a better pseudohalide leaving group with TsCl. Under the optimized reaction conditions, no competing N-tosylation is observed, and the method allows access to 5- and 6-membered carbamate rings with a variety of substitution patterns. Bicyclic fused rings can also be prepared in yields comparable to phosgene-based methods. Furthermore, the reaction displays high regioselectivity and predictable stereochemistry.
Seen as a whole, publications I-III provide a toolbox for CO2-based cyclic carbamate synthesis. A reader looking to prepare these types of compounds may refer to the checklist given in Table 5 to help decide which of the three methods to use.
Table 5. A checklist for comparing the synthetic methods presented in publications I-III.
Publication
I II III
Feature
Efficient Yes Yes Yes
Autoclave required Yes No Yes
Solvent type Polar protic Polar aprotic Polar aprotic
Amine source Haloamine Aniline Amino alcohol
Product types
5-rings Yes Yes Yes
- unsubstituted Yes No No
- with 3-substitution No Yes Yes
- with 4-substitution No No No
- with 5-substitution Yes No Yes
6-rings Yes Yes Yes
- unsubstituted Yes No No
- with substitution Yes Yes Yes
7-rings No Yes No
Fused rings No No Yes
Stereoselective No No Yes
5 CONCLUSIONS
Carbon dioxide is often considered to be a relatively unreactive molecule.
Despite this, it has been successfully employed in organic synthesis through nearly the whole history of the field.
The key to CO2 chemistry is the molecule’s electron-deficient carbon center, which is susceptible to a nucleophilic attack. Should the nucleophile be an amine, a carbamate species is formed. This reactivity can be employed in the synthesis of cyclic carbamates, a structural motif found in a plethora of pharmaceuticals and other high-value added chemicals. Since these compounds are historically synthesized by utilizing toxic reagents such as phosgene and various isocyanides, research into more sustainable CO2-based cyclic carbamate synthesis has been extensive. Literature reports on the topic fall into four reaction types: cycloaddition of CO2 into aziridines; epoxide-based syntheses; cyclization of unsaturated compounds; and cyclization of amino alcohols.
The reaction between aziridine and carbon dioxide is facilitated by a Lewis pair, which can either be an external catalyst, or a carbamate salt formed by the starting material. If a 2-substituted aziridine is used as a starting material, either a 4-substituted or a 5-substituted 2-oxazolidinone is formed. The regioselectivity is mainly substrate-controlled.
In the epoxide-based syntheses, CO2 first undergoes cycloaddition to give a cyclic carbonate, which then reacts with an amine species to give cyclic carbamates. This approach gives access to 3,5-disubstituted 5-membered carbamate rings.
Unsaturated compounds such as propargylic amines and alcohols, allylamines, alkenes, and alkynes will readily react with carbon dioxide. A large scope of products is accessible with these methods, as variously substituted 5-membered rings, as well as bicyclic fused rings and 6-5-membered rings can be obtained.
The dehydrative cyclization of CO2 and amino alcohols is seen as one of the more sustainable approaches. The cyclization can be achieved in the absence of any additional reagents by utilizing harsh reaction conditions. However, such conditions facilitate several undesired side reactions. More selective syntheses in milder conditions can be conducted by utilizing various catalysts, sacrificial reagents, or electrochemical methods. This approach allows for an extensive and versatile substrate scope.
Most commonly cited applications for cyclic carbamates are various pharmaceuticals and chiral auxiliaries, which commonly feature a stereocenter. Despite this, only a handful of reports on stereoselective, carbon dioxide-based synthesis of cyclic carbamates exist. These mainly include the utilization of chiral starting materials, but some methods featuring prochiral
This thesis features three novel contributions to the field of cyclic carbamate syntheses. The common feature in each work is the formation of a carbamate moiety by CO2 and an amine group, followed by an intramolecular substitution reaction with a nearby leaving group to close the ring. These (pseudo)halide-based methods are efficient and have a broad substrate scope; thus, the work outlined in publications I-III could be seen as the first examples of a fifth major pathway from CO2 to cyclic carbamates.
REFERENCES
1. A. S. Lindsey and H. Jeskey, Chem. Rev., 1957, 57, 583-620.
2. X. Yang, R. J. Rees, W. Conway, G. Puxty, Q. Yang and D. A. Winkler, Chem. Rev., 2017, 117, 9524-9593.
3. D. L. Nelson and M. M. Cox, Lehringer Principles of Biochemistry, W.
H. Freeman and Company, USA, 4 edn., 2005.
4. G. T. Rochelle, Science, 2009, 325, 1652-1654.
5. Y. Chen, C. A. Hone, B. Gutmann and C. O. Kappe, Org. Process Res.
Dev., 2017, 21, 1080-1087.
6. V. Famiglini and R. Silvestri, Molecules, 2016, 21, 1-18.
7. R. Schaadt, D. Sweeney, D. Shinabarger and G. Zurenko, Antimicrob.
Agents Chemother., 2009, 53, 3236-3239.
8. S. J. Brickner, Curr. Pharm. Des., 1996, 2, 175-194.
9. H. Ucar, K. Van derpoorten, S. Cacciaguerra, S. Spampinato, J. P.
Stables, P. Depovere, M. Isa, B. Masereel, J. Delarge and J. H. Poupaert, J. Med. Chem., 1998, 41, 1138-1145.
10. M. E. Dyen and D. Swern, Chem. Rev., 1967, 67, 197-246. A. R. Pinhas, Green Chem., 2011, 13, 3224-3229.
17. M. T. Hancock and A. R. Pinhas, Tetrahedron Lett., 2003, 44, 5457-5460.
18. A. Sudo, Y. Morioka, E. Koizumi, F. Sanda and T. Endo, Tetrahedron Lett., 2003, 44, 7889-7891.
19. Y. Wu and G. Liu, Tetrahedron Lett., 2011, 52, 6450-6452.
20. W.-M. Ren, Y. Liu and X.-B. Lu, J. Org. Chem., 2014, 79, 9771-9777.
21. X.-Z. Lin, Z.-Z. Yang, L.-N. He and Z.-Y. Yuan, Green Chem., 2015, 17, 795-798.
22. A. W. Miller and S. T. Nguyen, Org. Lett., 2004, 6, 2301-2304.
23. A. C. Kathalikkattil, R. Roshan, J. Tharun, R. Babu, G.-S. Jeong, D.-W.
Kim, S. J. Cho and D.-W. Park, Chem. Commun., 2016, 52, 280-283.
24. Y. Wu, L.-N. He, Y. Du, J.-Q. Wang, C.-X. Miao and W. Li, Tetrahedron, 2009, 65, 6204-6210.
25. H.-F. Jiang, J.-W. Ye, C.-R. Qi and L.-B. Huang, Tetrahedron Lett., 2010, 51, 928-932.
26. Y.-M. Shen, W.-L. Duan and M. Shi, Eur. J. Org. Chem., 2004, 2004, 3080-3089.
27. A. Ueno, Y. Kayaki and T. Ikariya, Green Chem., 2013, 15, 425-430.
28. H. Zhou, Y.-M. Wang, W.-Z. Zhang, J.-P. Qu and X.-B. Lu, Green Chem., 2011, 13, 644-650.
29. V. B. Saptal and B. M. Bhanage, ChemSusChem, 2016, 9, 1980-1985.
30. Y.-N. Zhao, Z.-Z. Yang, S.-H. Luo and L.-N. He, Catal. Today, 2013, 200, 2-8.
31. A. C. Kathalikkattil, J. Tharun, R. Roshan, H.-G. Soek and D.-W. Park, Appl. Catal., A, 2012, 447-448, 107-114.
32. Z.-Z. Yang, L.-N. He, S.-Y. Peng and A.-H. Liu, Green Chem., 2010, 12, 1850-1854.
33. Z.-Z. Yang, Y.-N. Li, Y.-Y. Wei and L.-N. He, Green Chem., 2011, 13, 2351-2353.
34. R. A. Watile, D. B. Bagal, K. M. Deshmukh, K. P. Dhake and B. M.
Bhanage, J. Mol. Catal. A: Chem., 2011, 351, 196-203.
35. X. Chen, J. Sun, J. Wang and W. Cheng, Tetrahedron Lett., 2012, 53, 2684-2688.
36. C. Qi, J. Ye, W. Zeng and H. Jiang, Adv. Synth. Catal., 2010, 352, 1925-1933.
37. J. Gao, Q.-W. Song, L.-N. He, C. Liu, Z.-Z. Yang, X. Han, X.-D. Li and Q.-C. Song, Tetrahedron, 2012, 68, 3835-3842.
38. S. Kumar and S. L. Jain, Ind. Eng. Chem. Res., 2014, 53, 541-546.
Houghton, D. R. Sidler, U.-H. Dolling, L. M. DiMichele and T. J. Novak, J. Org. Chem., 2005, 70, 7479-7487.
44. M. Lv, P. Wang, D. Yuan and Y. Yao, ChemCatChem, 2017, 9, 4451-4455.
45. B. Wang, Z. Luo, E. H. M. Elageed, S. Wu, Y. Zhang, X. Wu, F. Xia, G.
Zhang and G. Gao, ChemCatChem, 2016, 8, 830-838.
46. B. Wang, E. H. M. Elageed, D. Zhang, S. Yang, S. Wu, G. Zhang and G.
Gao, ChemCatChem, 2014, 6, 278-283.
47. U. R. Seo and Y. K. Chung, Green Chem., 2017, 19, 803-808.
48. S. Norio, H. Kiyotaka, I. Shota, A. Takashi and T. Takashi, Bull. Chem.
49. Y. Song, C. Cheng and H. Jing, Chem. - Eur. J., 2014, 20, 12894-12900.
50. Y. P. Patil, P. J. Tambade, S. R. Jagtap and B. M. Bhanage, J. Mol. Catal.
A: Chem., 2008, 289, 14-21.
51. B. Xu, P. Wang, M. Lv, D. Yuan and Y. Yao, ChemCatChem, 2016, 8, 2466-2471.
52. J. Rintjema, R. Epping, G. Fiorani, E. Martín, E. C. Escudero-Adán and A. W. Kleij, Angew. Chem. Int. Ed., 2016, 55, 3972-3976.
53. J. A. H. Inkster, I. Ling, N. S. Honson, L. Jacquet, R. Gries and E.
Plettner, Tetrahedron: Asymmetry, 2005, 16, 3773-3784.
54. J. Rintjema, W. Guo, E. Martin, E. C. Escudero-Adán and A. W. Kleij, Chem. - Eur. J., 2015, 21, 10754-10762.
55. J. X. Xu, J. W. Zhao and Z. B. Jia, Chin. Chem. Lett., 2011, 22, 1063-1066.
56. H. Jiang, J. Zhao and A. Wang, Synthesis, 2008, 2008, 763-769.
57. J. Xu, J. Zhao, Z. Jia and J. Zhang, Synth. Commun., 2011, 41, 858-863.
58. Y. Gu, Q. Zhang, Z. Duan, J. Zhang, S. Zhang and Y. Deng, J. Org.
Chem., 2005, 70, 7376-7380.
59. Q.-W. Song, B. Yu, X.-D. Li, R. Ma, Z.-F. Diao, R.-G. Li, W. Li and L.-N.
He, Green Chem., 2014, 16, 1633-1638.
60. H.-F. Jiang and J.-W. Zhao, Tetrahedron Lett., 2009, 50, 60-62.
61. Q.-W. Song, P. Liu, L.-H. Han, K. Zhang and L.-N. He, Chin. J. Chem., 2018, 36, 147-152.
62. J. Fournier, C. Bruneau and P. H. Dixneuf, Tetrahedron Lett., 1990, 31, 1721-1722.
63. H. Liu and R. Hua, Tetrahedron, 2016, 72, 1200-1204.
64. N. D. Ca, B. Gabriele, G. Ruffolo, L. Veltri, T. Zanetta and M. Costa, Adv.
Synth. Catal., 2011, 353, 133-146.
65. X.-D. Li, Q.-W. Song, X.-D. Lang, Y. Chang and L.-N. He, ChemPhysChem, 2017, 18, 3182-3188.
66. Q.-W. Song, Z.-H. Zhou, M.-Y. Wang, K. Zhang, P. Liu, J.-Y. Xun and L.-N. He, ChemSusChem, 2016, 9, 2054-2058.
67. K.-i. Fujita, J. Sato, A. Fujii, S.-y. Onozawa and H. Yasuda, Asian J. Org.
Chem., 2016, 5, 828-833.
68. K.-i. Fujita, A. Fujii, J. Sato, S.-y. Onozawa and H. Yasuda, Tetrahedron Lett., 2016, 57, 1282-1284.
69. B. Yu, D. Kim, S. Kim and S. H. Hong, ChemSusChem, 2017, 10, 1080-1084.
70. S. M. Sadeghzadeh, Appl. Organomet. Chem., 2016, 30, 835-842.
71. S. Hase, Y. Kayaki and T. Ikariya, ACS Catal., 2015, 5, 5135-5140.
72. S. Hase, Y. Kayaki and T. Ikariya, Organomet., 2013, 32, 5285-5288.
73. A. Fujii, J.-C. Choi and K.-i. Fujita, Tetrahedron Lett., 2017, 58, 4483-4486.
74. J. Hu, J. Ma, Z. Zhang, Q. Zhu, H. Zhou, W. Lu and B. Han, Green Chem., 2015, 17, 1219-1225.
75. J. Hu, J. Ma, Q. Zhu, Z. Zhang, C. Wu and B. Han, Angew. Chem. Int.
79. Y. Kayaki, M. Yamamoto, T. Suzuki and T. Ikariya, Green Chem., 2006, 8, 1019-1021.
80. D. Zhao, X.-H. Liu, C. Zhu, Y.-S. Kang, P. Wang, Z. Shi, Y. Lu and W.-Y. Sun, ChemCatChem, 2017, 9, 4598-4606.
81. M.-Y. Wang, Q.-W. Song, R. Ma, J.-N. Xie and L.-N. He, Green Chem., 2016, 18, 282-287.
82. P. Garcia-Dominguez, L. Fehr, G. Rusconi and C. Nevado, Chem. Sci., 2016, 7, 3914-3918.
83. T. Ishida, S. Kikuchi, T. Tsubo and T. Yamada, Org. Lett., 2013, 15, 848-851.
84. K. Sekine, R. Kobayashi and T. Yamada, Chem. Lett., 2015, 44, 1407-1409.
85. M. Yoshida, T. Mizuguchi and K. Shishido, Chem. - Eur. J., 2012, 18, 15578-15581.
86. R. Maggi, C. Bertolotti, E. Orlandini, C. Oro, G. Sartori and M. Selva, Tetrahedron Lett., 2007, 48, 2131-2134.
91. S. M. Sadeghzadeh, J. Mol. Catal. A: Chem., 2016, 423, 216-223.
92. S. M. Sadeghzadeh, R. Zhiani and S. Emrani, Appl. Organomet. Chem., 2018, 32, e3941-n/a.
93. Q.-W. Song and L.-N. He, Adv. Synth. Catal., 2016, 358, 1251-1258.
94. M. Feroci, M. Orsini, G. Sotgiu, L. Rossi and A. Inesi, J. Org. Chem., 2005, 70, 7795-7798.
95. R. Nicholls, S. Kaufhold and B. N. Nguyen, Catal. Sci. Technol., 2014, 4, 3458-3462.
96. M. Shi and Y.-M. Shen, J. Org. Chem., 2002, 67, 16-21.
97. S. Kikuchi, S. Yoshida, Y. Sugawara, W. Yamada, H.-M. Cheng, K.
Fukui, K. Sekine, I. Iwakura, T. Ikeno and T. Yamada, Bull. Chem. Soc.
Jpn., 2011, 84, 698-717.
98. S. Yoshida, K. Fukui, S. Kikuchi and T. Yamada, Chem. Lett., 2009, 38, 786-787.
99. N. Sugiyama, M. Ohseki, R. Kobayashi, K. Sekine, K. Saito and T.
Yamada, Chem. Lett., 2017, 46, 1323-1326.
100. P. Brunel, J. Monot, C. E. Kefalidis, L. Maron, B. Martin-Vaca and D.
Bourissou, ACS Catal., 2017, 7, 2652-2660.
101. E. García‐Egido, I. Fernández and L. Muñoz, Synth. Commun., 2006, 36, 3029-3042.
102. Y. Takeda, S. Okumura, S. Tone, I. Sasaki and S. Minakata, Org. Lett., 2012, 14, 4874-4877.
103. I. Fernández and L. Muñoz, Tetrahedron: Asymmetry, 2006, 17, 2548-2557. and D.-G. Yu, Angew. Chem. Int. Ed., 2016, 55, 10022-10026.
108. K. Yamashita, S. Hase, Y. Kayaki and T. Ikariya, Org. Lett., 2015, 17, 2334-2337.
109. Y. Kayaki, N. Mori and T. Ikariya, Tetrahedron Lett., 2009, 50, 6491-6493.
110. D.-L. Kong, L.-N. He and J.-Q. Wang, Catal. Commun., 2010, 11, 992-995.
111. J. Zhao and H. Jiang, Tetrahedron Lett., 2012, 53, 6999-7002.
112. B. Yu, B.-B. Cheng, W.-Q. Liu, W. Li, S.-S. Wang, J. Cao and C.-W. Hu, Adv. Synth. Catal., 2016, 358, 90-97.
113. W.-J. Yoo and C.-J. Li, Adv. Synth. Catal., 2008, 350, 1503-1506.
114. B.-B. Cheng, B. Yu and C.-W. Hu, RSC Adv., 2016, 6, 87179-87187.
115. T. Djamila, H. Smain and L. Chao-Jun, Lett. Org. Chem., 2012, 9, 585-593.
120. J. R. Khusnutdinova, J. A. Garg and D. Milstein, ACS Catal., 2015, 5, 2416-2422.
121. J. Paz, C. Pérez-Balado, B. Iglesias and L. Muñoz, J. Org. Chem., 2010,
122. J. Paz, C. Pérez-Balado, B. Iglesias and L. Muñoz, Synlett, 2009, 2009,
126. H. Kawanami and Y. Ikushima, J. Jpn. Pet. Inst., 2002, 45, 321-324.
127. A. Sarkar, S. Bhattacharyya, S. K. Dey, S. Karmakar and A. Mukherjee, New J. Chem., 2014, 38, 817-826.
131. R. Juarez, P. Concepcion, A. Corma and H. Garcia, Chem. Commun., 2010, 46, 4181-4183. Sustainable Chem. Eng., 2013, 1, 309-312.
135. K.-i. Tominaga and Y. Sasaki, Synlett, 2002, 2002, 0307-0309.
136. H. Matsuda, A. Baba, R. Nomura, M. Kori and S. Ogawa, Ind. Eng.
Chem. Res., 1985, 24, 239-242.
137. M. A. Casadei, M. Feroci, A. Inesi, L. Rossi and G. Sotgiu, J. Org. Chem., 2000, 65, 4759-4761.
138. M. Feroci, A. Gennaro, A. Inesi, M. Orsini and L. Palombi, Tetrahedron Lett., 2002, 43, 5863-5865.
139. J. Vaitla, Y. Guttormsen, J. K. Mannisto, A. Nova, T. Repo, A. Bayer and K. H. Hopmann, ACS Catalysis, 2017, 7, 7231-7244.
140. D. Adhikari, A. W. Miller, M.-H. Baik and S. T. Nguyen, Chem. Sci.,
143. A. Baba, I. Shibata, M. Fujiwara and H. Matsuda, Tetrahedron Lett., 1985, 26, 5167-5170.
144. US 20110172419, 2011.