(1R)-(+)-camphor and acetone derived α′-hydroxy enones in asymmetric diels-alder reaction: Catalytic activation by Lewis and brønsted acids, substrate scope, applications in syntheses, and mechanistic studies

Journal of Organic Chemistry

Volumn 75, Issue 5, 2010, Pages 1458-1473

  Bañuelos, Patrcia ^b   Garcia, Jesús M ^b   Gómez Bengpa, Enrique ^a   Herrero, Ada ^b   Odriozola, José M ^b   Oiarbide, Mikel ^a   Palomo, Claudio ^a   Razkin, Jesús ^b  

^a UNIVERSITY OF THE BASQUE COUNTRY UPV EHU   (Spain)

^b PUBLIC UNIVERSITY OF NAVARRE   (Spain)

Author keywords

[No Author keywords available]

Indexed keywords

ASYMMETRIC INDUCTION; BRONSTED ACIDS; C-C BONDS; CATALYTIC ACTIVATION; CHEMICAL EQUATIONS; CYCLOHEXENES; CYCLOPENTADIENES; DIASTEREOSELECTIVE; DIELS-ALDER; DIELS-ALDER REACTION; DIENOPHILES; ENANTIO; FACIAL SELECTIVITY; FIRST-ORDER RATES; HYDROXY ENONES; KINETIC EXPERIMENT; KINETIC MEASUREMENT; LEWIS ACID; MECHANISTIC STUDIES; NATURAL PRODUCTS; QUANTUM CALCULATION; REACTION PARTNERS; STEREOCONTROL;

ACETONE; ACIDS; CHEMICAL BONDS; COMPLEXATION; OLEFINS; STEREOSELECTIVITY; SYNTHESIS (CHEMICAL);

COORDINATION REACTIONS;

ACETONE; ACRYLIC ACID DERIVATIVE; ALDEHYDE; ALKADIENE; BRONSTED ACID; CAMPHOR; CARBOXYLIC ACID DERIVATIVE; COPPER ION; CYCLOHEXENE DERIVATIVE; CYCLOPENTADIENE DERIVATIVE; KETONE DERIVATIVE; LEWIS ACID; NICOLAIOIDESIN C; UNCLASSIFIED DRUG;

ARTICLE; CATALYSIS; CATALYST; CHIRALITY; CONTROLLED STUDY; DIELS ALDER REACTION; ENANTIOSELECTIVITY; NONHUMAN; OXIDATION; QUANTUM CHEMISTRY; STEREOCHEMISTRY; SYNTHESIS;

EID: 77949294774     PISSN: 00223263     EISSN: 15206904     Source Type: Journal    
DOI: 10.1021/jo9023039     Document Type: Article

References (125)

1. .) (a) Jurczak, J.; Bauer, T.; Chapuis, C. In Stereoselective Synthesis (Houben-Weyl); Heimchen, G., Hoffmann, R. W., Mulzer, J., Schaumann, E., Eds.; Thieme: Stuttgart, 1996; Vol. 21/5, pp 2735-2871.
2. b) Fringuelli, F.; Taticchi, A. The Diels-Alder Reaction. Selected Practical Methods; John Wiley: New York, 2002.
3. c) Cycloaddition Reactions in Organic Synthesis; Kobayashi, 5., Jørgensen, K. A., Eds.; Wiley-VCH: Weinheim, 2002.
4. Nicolaou, K. C.; Snyder, S. A.; Montagnon, T.; Vassilikogiannakis, G. Angew. Chem., Int. Ed. 2002, 41, 1668-1698.
5. For pertinent information, see: (a) Seyden-Penne, J. Chiral Auxiliaries and Lìgands in Asymmetric Synthesis; Wiley: New York, 1995.
6. b) Rück-Braun, K.;Kunz, H. Chiral Auxiliaries in Cycloadditions, Wiley: New York, 1999. For a recent review on chiral auxiliaries, see:
7. c) Gnas, Y.; Glorious, F. Synthesis 2006, 1899-1930.
8. For recent reviews, see: (a) Evans, D. A.; Johnson, J. S. In Comprehensive Asymmetric Catalysis; Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer: Berlin, 1999; Vol. III, pp 1177-1235.
9. b) Carmona, D.; Lamata, M. P.; Oro, L. A. Coord. Chem. Rev. 2000, 200 (202), 717-772.
10. c) Corey, E. J. Angew. Chem., Int. Ed. 2002, 41, 1650-1667.
11. For some leading examples of metal-catalyzed enantioselective Diels-Alder reactions, see the following. Relying on bis(oxazoline)-Cu(II) complexes: (a) Evans, D. A.; Miller, S. J.; Lectka, T.; von Matt, P. J. Am. Chem. Soc. 1999, 121, 7559-7573.
12. b) Evans, D. A.; Wu, J. J. Am. Chem. Soc. 2003, 125, 10162-10163. Oxazaborolidine complexes:
13. c) Corey, E. J.; Shibata, T.; Lee, T. W. J. Am. Chem. Soc. 2002, 124, 3808-3809.
14. d) Liu, D.; Canales, E.; Corey, E. J. J. Am. Chem. Soc. 2007, 129, 1498-1499 and references therein.
15. e) Futatsugi, K.; Yamamoto, H. J. Am. Chem., Int. Ed. 2005, 44, 1484-1487.
16. f) Payette, J. N.; Yamamoto, H. J. Am. Chem. Soc. 2007, 129, 9536-9537 and references therein.
17. g) Balskus, E. P.; Jacobsen, E. N. Science 2007, 317, 1736-1740. Schiffbase-chromium(III) complexes:
18. h)Jarvo, E. R.; Lawrence, B. M.; Jacobsen, E.N. Angew. Chem., Int. Ed. 2005, 44, 6043-6046.
19. i) Roclfcs, G.; Feringa, B. L. Angew. Chem., Int. Ed. 2005, 44, 1365-1372.
20. j) Reetz, M. T.; Jiao, M. Angew. Chem., Int. Ed. 2006, 45, 2416-2419.
21. (a) Ahrendt, K. A.; Borths, C. J.; MacMillan, D. W. C. J. Am. Chem. Soc. 2000, 122, 4243-4244.
22. b) Northrup, A. B.; MacMillan, D. W. C. J. Am. Chem. Soc 2002,124, 2458-2460.
23. c) Thayumanavan, R.; Dhevalapall, B.; Sakthivel, K.;Tanaka, F.; BarbasC. F., III. Tetrahedron Lett. 2002, 43, 3817-3820.
24. For selected recent examples, see: (d) Wilson, R. M.; Jen, W. S.; MacMillan, D. W. C. J. Am. Chem. Soc 2005, 127, 11616-11617.
25. e) Gotoh, H.; Hayashi, Y. Org. Lett. 2007, 9, 2859-2862.
26. f) Satoh, N.; Akiba, T.; Yokoshima, S.; Fukuyama, T. Angew. Chem., Int. Ed. 2007, 46, 5734-5736.
27. g) Gilmour, R.; Prior, T. J.; Burton, J. W.; Holmes, A. B. Chem. Commun. 2007, 3954-3956.
28. h) Kano, T.; Tanaka, Y.; Maruoka, K. Chem. Asian J. 2007, 21. 1161-1165.
29. i) Zou, Y.; Wang, Q.; Goeke, A. Chem.-Eur. J. 2008, 14, 5335-5345.
30. j) Ishihara, K.; Nakano, K.; Akakura, M. Org. Lett. 2008, 10, 2893-2896.
31. k) Tano, K.; Tanaka, Y.; Osawa, K.; Yurino, T.; Maruoka, K. Chem. Commun. 2009, 1956-1958.
32. For recent reviews on iminium activation, see: (l) Leiais, G.; MacMillan, D. W. C. Aldrichimica Acta 2006, 39, 79-87.
33. m) Mukherjee, S.; Yang, J. W.; Hoffmann, S.; List, B. Chem. Rev. 2007, 107, 5471-5569.
34. (a) Huang, Y.; Unni, A. K.; Thadani, A. N.; Rawal, V. H. Nature 2003, 424, 146.
35. b) Thadani, A. N.; Stankovic, A. R.; Rawal, V. H. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 5846-5850.
36. c) Unni, A. K.; Takenaka, N.; Yamamoto, H.; Rawal, V. H. J. Am. Chem. Soc. 2005, 127, 1336-1337.
37. d) Rajaram, S.; Sigman, M. S. Org. Lett. 2005, 7, 5473-5475.
38. e) Nakashima, D.; Yamamoto, H. J. Am. Chem. Soc. 2006, 128, 9626-9627.
39. For recent reviews on chiral Brønsted acids, see: (f) Pihko, S. M. Lett. Org. Chem. 2005, 2, 398-403.
40. g) Taylor, M. S.; Jacobsen, E. N. Angew. Chem., Int. Ed. 2006, 45, 1520-1543.
41. h) Akiyama, T.; Itoh, J.; Fuchibe, K. Adv. Synth, Catal. 2006, 348, 999-1010.
42. i) Doyle, A. G.; Jacobsen, E. N. Chem. Rev. 2007, 107, 5713-5743.
43. For a review on Brønsted acid and base catalyzed Diels-Alder reactions, see:
44. j) Shen, J.; Tan, C. H. Org. Biomol. Chem. 2007, 6, 3229-3236.
45. For organocatalytic asymmetric Diels-Alder reactions involving a bifunctional activation mechanism, see the following. Cinchona alkaloid catalyst: (a) Singh, R. P.; Bartelson, K.; Wang, Y.; Su, H.; Lu, X.; Deng, L. J. Am. Chem. Soc 2008, 130, 2422-2423.
46. Cinchona-thiourea catalyst: (b) Gioia, C.; Hauville, A.; Bernardi, L.; Fini, F.; Ricci, A. Angew. Chem., Int. Ed. 2008, 47, 9236-9239.
47. Pyrrolidine-pyridine catalyst in seawater: (c) Xu, D.-a.; Xia, A.-B.; Luo, S.-P.; Tang, J.; Zhang, S.; Jiang, J.-R.; Xu, Z.-Y. Angew. Chem., Int. Ed. 2009, 48, 3821-3824.
48. Palomo, C.; Oiarbide, M.; García, J. M.; González, A.; Lecumberri, A.; Linden, A. J. Am. Chem. Soc. 2002, 124, 10288-10289.
49. Palomo, C.; Oiarbide, M.; García, J. M.; González, A.; Arceo, E. J. Am. Chem. Soc. 2003, 125, 13942-13943.
50. a) Reed, L. A., III.; Choy, W.; Masamune, S. J. Org. Chem. 1983, 48, 1137-1139.
51. b) Masamune, S.; Reed, L. A. III; Davis, J. T.; Choy, W. J. Org. Chem. 1983, 48,4441-4444.
52. a)Palomo, C.; González, A.; García, J. M.; Landa, C.; Oiarbide, M.; Rodríguez, S.; Linden, A. Angew. Chem., Int. Ed. 1998, 37, 180-182.
53. b) Palomo, C.; Oiarbide, M.; Aizpurua, J. M.; González, A.; Garcia, J. M.; Landa, C.; Odriozola, I.; Linden, A. J. Org. Chem. 1999, 64, 8193-8200.
54. c) Palomo, C.; Oiarbide, M.; González-Rego, M. C.; Sharma, A. K.; García, J. M.; González, A.; Landa, C.; Linden, A. Angew. Chem., Int. Ed. 2000, 39, 1063-1.065.
55. d) Palomo, C.; Oiarbide, M.; Landa, A.; González-Rego, M. C.; García, J. M.; González, A.; Odriozola, J. M.; Martín-Pastor, M.; Linden, A. GracíJ. Am. Chem. Soc. 2002, 124, 8637-8643.
56. For a review on copper-catalyzed Diels-Alder reactions, see: Reymond, S.; Cossy, J. Chem. Rev. 2008, 108, 5359-5406.
57. (a) Wassermann, A. J. Chem. Soc. 1942, 618-622.
58. b) Wassermann, A. J. Chem. Soc. 1942, 623-627.
59. a, of the added acid and the reaction acceleration has been previously documented for the case of Diels-Alder reaction between several acrylates or methyl vinyl ketone and cyclopentadiene using a series of acetic acids: (a) Kasper, F.; Zobel, H. J. Prak. Chem. 1975, 317, 510-514.
60. b) Kasper, F.; Bischoff, St. J. Prak. Chem. 1986, 328, 449-453.
61. For Diels-Alder reactions catalyzed by trimethylsilyl bis(trifluoromethanesulfonyl)imide, see: Mathieu, B.; Ghosez, L. Tetrahedron 2002, 58, 8219-8226.
62. A reversal, in chemical efficiency has been observed in the reaction of azachalcone dienophiles with cyclopentadiene. See: Mubofu, E. B.; Engberts, J. B. F. N. Phys. Org. Chem. 2004, 17, 180-186.
63. Review on the chemistry of enol phosphates: Linchtenthaler, F. W. Chem. Rev. 1961, 61, 607-649. Enol phosphates in cross-coupling reactions:
64. a)Miller, J. A. Tetrahedron Lett. 2002, 43, 7111-7114.
65. b)Cahiez, G.;Gager, O.; Habiak, V. Synthesis 2008, 2636-2644.
66. c) Guo, J.; Harling, J. D.; Steel, P. G.; Woods, T. M. Org- Biomol. Chem. 2008, 6, 4053-4058.
67. Also, see:, (d) Skowrońiska, A.; Koprowski, M.; Krawczyk, E. Phosphorus, Sulfur Silicon 2002, 177, 1877-1880.
68. In α-hydroxyl ketone synthesis: (e) Krawczyk, E.; Koprowski, M.; Skowrońska, A.; Luczak, J. Tetrahedron: Asymmetry 2004, 15, 2599-2602.
69. Liu, H. J.; Feng, W. M.; Kim, J. B.; Browne, E. N. C. Can, J. Chem. 1994, 72, 2163-2175.
70. For non-asymmetric Diels-Alder reactions involving O-dienyl phosphate esters, see: (a) Skowronska, A.; Dybowsky, P.; Koprowski, M.; Crawczyc, E. Tetrahedron Lett. 1995, 36, 8133-8136.
71. b) Tatsuta, K.; Inukai, T.; Itoh, S.; Kawarasaki, M.; Nakano, Y. J. Antibiot. 2002, 55, 1076-1080.
72. c) Koprowski, M.; Skowornska, A.; Glowka, M. L.; Fruzunski, A. Tetrahedron 2007, 63, 1211-1.228.
73. ) For selected examples of bidentate dienophiles, see the following. NHydroxyacrylamides: (a) Corminboeuf, O.; Renaud, P. Org. Lett. 2002, 4, 1735-1738.
74. α,β-Unsaturated 2-acylimidazoles: (b) Evans, D. A.; Fandrick, K. R.; Soug, H. J.; Scheldt, K. A.; Xu, R. J. Am. Chem. Soc. 2007, 129, 10029-10041.
75. c) Boersma, A. J.; Feringa, B. L.; Roelfes, G. Org. Lett. 2007, 9, 3647-3650.
76. d) Ishihara, K.; Fushimi, M. Org. Lett. 2006, 8, 1921-1924. α,β-Unsaturated N-acylpyrazolidinones:
77. e) Sibi, M. P.; Stanley, L. M.; Nie, X.; Venkatraman, L.; Liu, M.; Jasperse, C. P. J. Am, Chem. Soc. 2007, 129, 395-405.
78. f) Zhou, J.; Tang, Y. Org- Biomol. Chem. 2004, 2, 429-433.
79. Alkylidenemalonates: (g) Yamauchi, M.; Aoki, T.; Li, M.-Z.; Honda, Y. Tetrahedron Asymmetry 2001, 12, 3113-3118.
80. α'-Arylsulfonyl enones: (h) Barroso, S.; Blay, G.; Al-Midfa, L.; Muñoz, M. C.; Pedro, J. R. J. Org. Chem. 2007, 129, 395-405 and also ref 13.
81. (a) Evans, D. A.; Chapman, K.T.; Bisaha, J. J. Am. Chem. Soc. 1984, 106, 4261-4263.
82. b) Evans, D. A.; Chapman, K. T.; Bisaha, J. J. Am. Chem. Soc: 1985, 106, 4261-4263.
83. c) Evans, D. A.; Chapman, K. T.; Bisaha, J. J. Am. Chem. Am. Chem. Soc. 1988, 110, 1238-1256.
84. (a) Evans, D. A.; Miller, S. J.; Lectka, T. J. Am. Chem. Soc. 1993, 115, 6460-6461.
85. b) Evans, D. A.; Murry, J. A.; von Matt, P.; Norcross, R. D.; Miller, S. J. Angew. Chem., Int. Ed. 1995, 34, 798-800.
86. Reaction scope and applications to synthesis: (c) Evans, D. A,; Ripin, D. H. B.; Johnson, J. S.; Shaugnessy, E. A. Angew. Chem., Int. Ed. 1997, 36, 2119-21.21.
87. d) Evans, D. A.; Barnes, D. M.; Johnson, J. S.; Lectka, T.; von Matt, T.; Miller, S. J.; Murry, J. A.; Norcross, R. D.; Shaughnessy, E. A.; Campos, K. R. J. Am. Chem. Soc. 1999, 121, 7582-7594.
88. (a) Johnson, J. S.; Evans, D. A. Acc. Chem. Res. 2000, 33, 325-335.
89. For a recent review on bis(oxazolidine) ligands, see: (b) Desimoni, G.; Faita, G.; Jørgensen, K. A. Chem. Rev. 2006, 106, 3561-3651.
90. The starting enones may be easily prepared in multigram quantity from commercially available materials in a manner similar to that employed for camphor-based enones 12. For details, see Supporting Information.
91. For adducts bearing acid-sensitive groups, like acetais, or showing limited solubility in the acetonitrile/water system, the scission step could be carried out with improved results by using alternative oxidants like lead tetraacetate in benzene or sodium periodate in diethyl ether.
92. III catalysts). For asymmetric organocatalytic methods mainly involving enals, see refs 6 and 7.
93. a) For detailed information on the experimental procedures employed and conditions variation, see Supporting Information. For substrate dependence in Diels-Alder reactions via iminium activation, see:
94. b) Kinsman, A. C.; Kerr, M. A. J. Am. Chem. Soc 2003, 125, 14120-14125.
95. For detailed information, see: (a) Pfeiffer, M. W. B.; Phillips, A. J. J. Am. Chem. Soc. 2005, 127, 5334-5335.
96. For a recent case of α'-hydroxy dienones as diene components of asymmetric Diels-Alder cycloadditions in the context of biomimetic natural products synthesis, see: (b) Dong, S.; Hamel, E.; Bai, R.; Covell, D. G.; Beutler, J. A.; Porco, J. A., Jr. Angew. Chem. Int. Ed. 2009, 48, 1494-1497.
97. a) Tuntiwachwuttikul, P.; Pancharoen, O.; Reutrakul, V.; Byrne, L. T. Aust. J. Chem. 1984, 37, 449-453.
98. b) Alias, Y.; Awang, K.; Hadi, A. H. A. J. Nat. Prod. 1995, 58, 1160-1166.
99. f) Shibata, K.; Tatsukawa, A.; Umeoka, K.-i.;-.L; Lee, H. S.; Ochi, M. Tetrahedron 2000, 56, 8821-8824.
100. c) Gu, J.-Q.; Park, E. J.; Vigo, J. S.; Graham, J. G.; Fong, H. H. S.; Pezzuto, J. M..; Kinghorn, A, D. J. Nat. Prod. 2002, 65,1616-1620.
101. d) Cheenpracha, S.; Karalai, C.; Ponghmanont, C.; Subhadhirasakul, S.; Tewtrakul, S. Bioorg. Med. Chem. 2006, 14, 1710-1714.
102. e) Win, N. N.; Awale, S.; Esumi, H.; Tezuka, Y.; Kadota, S. J. Nat. Prod. 2007, 70, 1582-1587.
103. a) Cong, H.; Ledbetter, D.; Rowe, G. T.; Caradonna, J. P.; Porco, J. A. Jr. J. Am. Chem. Soc. 2008, 130, 9214-9215.
104. b) Jung, E. M.; Lee, Y. R. Bull. Korean Chem. Soc 2008, 29, 1199-1204.
105. For racemic syntheses of nicolaioidesin C based on an electrontransfer-initiated Diels-Alder cycloaddition and thermal Diels-Alder reaction, respectively, of 2′-hydroxychalcones, see refs 31a and 31b.
106. Absolute configuration of several extracted prenylchalcones, including (+) and (-)-panduratin A, has recently been determined by CD: Yoshikawa, M.; Morikawa, T.; Funakoshi, K.; Ochi, M.; Pongpiriyadacha, Y.; Matsuda, H. Heterocycles 2008, 75, 1639-1650. Note that these determinations are in conflict with the structures of (-)-panduratin A depicted in refs 30e and 31a.
107. Under our optimized conditions (10 mol% TtDH, 5 equiv of myrcene, -78 °C, 20 h) a minute amount of polymer formation was detected. However, an increase of the reaction temperature up to about -30 °C or the loading of the promoter TfOH up to about 30 mol% led to extensive production of the undesired polymer.
108. The use of the CAN/acetonitrile/water oxidation, system was not practical in this particular case due to the poor solubility of the substrate diol in the polar media.
109. DFT has been shown to reliably predict the results of Diels-Alder cycloaddition reactions; see: (a) Goldstein, E.; Beno, B.; Houk, K. N. J. Am. Chem. Soc. 1996, 118, 6036.
110. b) Wiest, O.; Montiel, D. C.; Houk, K. N. J. Phys. Chem. A 1997, 101, 8378.
111. c) Garcia, J. I.; Martínez-Merino, V.; Mayoral, J. A.; Salvatella, L. J. Am. Chem. Soc 1998, 120, 2415.
112. d) Birney, D. M. J. Am. Chem. Soc 2000, 122, 10917.
113. In the discussion that follows, the reactivity concerning the enone moiety is considered only in its s-cis conformation. TS involving the s-transenone lie much higher in energy ( > 3 kcal/mol) in all cases and will not be discussed, here.
114. a) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785-789.
115. b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
116. c) Kohn, W.; Becke, A. D.; Parr, R. G. J. Phys. Chem. 1996, 100,12974-12980.
117. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A. Jr.; Vreven, T.; Kudin, K. N.; Buran , J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M, W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision E.01; Gaussian, Inc.: Wallingford CT, 2004.
118. For details on computational methods, see Supporting Information.
119. For the full collection of H-bonded structures considered for first evaluation, see Supporting Information.
120. For relevant information on the CH- ⋯ -O hydrogen bond, see: (a) Desiraju, G. R. Chem. Comm. 2005, 2995-3001.
121. For the importance of dual activation of substrate acceptors and donors in asymmetric synthesis, see: (b) Ma, J.-A.; Cahard, D. Angew. Chem. 2004, 116, 4666-4683.
122. c) Angew . Int. Ed. 2004, 43, 4566-4583.
123. The possibility of such H-bonding network between the acid catalyst and both the diene and the dienophile being also established in other systems, especially those involving enones, should not be discarded. See Supporting Information.
124. One referee suggested that the observed selectivity trend could be rationalized on the basis of sterics alone, considering the triflate, the one that imparts the best, selectivity, is the largest counterion.
125. Trost, B, M.; Toste, F. D. J. Am. Chem. Soc, 2003, 125, 3090-3100.