Palladium(II)-catalyzed oxidative ring cleavage of tert-cyclobutanols under oxygen atmosphere [25]

Journal of the American Chemical Society

Volumn 121, Issue 11, 1999, Pages 2645-2646

  Nishimura, Takahiro ^a   Ohe, Kouichi ^a   Uemura, Sakae ^a  

^a KYOTO UNIVERSITY   (Japan)

Author keywords

[No Author keywords available]

Indexed keywords

BICYCLO COMPOUND; BUTANOL; OXYGEN; PALLADIUM;

CATALYSIS; CATALYST; CHEMICAL REACTION; CHEMICAL STRUCTURE; CYCLIZATION; LETTER; OXIDATION; REACTION ANALYSIS;

EID: 0033599513     PISSN: 00027863     EISSN: None     Source Type: Journal    
DOI: 10.1021/ja984259h     Document Type: Letter

References (30)

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6. Nishimura, T.; Onoue, T.; Ohe, K.; Uemura, S. Tetrahedron Lett. 1998, 39, 6011.
7. The ring expansion reactions of siloxycyclopropanes, see: (a) Kuwajima, I.; Nakamura, E. Top. Curr. Chem. 1990, 155, 1. (b) Kirihara, M.; Ichinose, M.; Takizawa, S.; Momose, T. Chem. Commun. 1998, 1691 and references therein. The oxidative rearrangement reactions of cyclobutanols, see: Schlecht, M. F. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Ley, S. V., Eds.; Pergamon: Oxford, U.K., 1991; Vol.7, pp 824-826. The recent advance of β-carbon elimination in the reaction sequence using spiro cyclobutanones, see: Murakami, M.; Takahashi, K.; Amii, H.; Ito, Y. J. Am. Chem. Soc. 1997, 119, 9307.
8. The ring expansion reactions of siloxycyclopropanes, see: (a) Kuwajima, I.; Nakamura, E. Top. Curr. Chem. 1990, 155, 1. (b) Kirihara, M.; Ichinose, M.; Takizawa, S.; Momose, T. Chem. Commun. 1998, 1691 and references therein. The oxidative rearrangement reactions of cyclobutanols, see: Schlecht, M. F. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Ley, S. V., Eds.; Pergamon: Oxford, U.K., 1991; Vol.7, pp 824-826. The recent advance of β-carbon elimination in the reaction sequence using spiro cyclobutanones, see: Murakami, M.; Takahashi, K.; Amii, H.; Ito, Y. J. Am. Chem. Soc. 1997, 119, 9307.
9. The ring expansion reactions of siloxycyclopropanes, see: (a) Kuwajima, I.; Nakamura, E. Top. Curr. Chem. 1990, 155, 1. (b) Kirihara, M.; Ichinose, M.; Takizawa, S.; Momose, T. Chem. Commun. 1998, 1691 and references therein. The oxidative rearrangement reactions of cyclobutanols, see: Schlecht, M. F. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Ley, S. V., Eds.; Pergamon: Oxford, U.K., 1991; Vol.7, pp 824-826. The recent advance of β-carbon elimination in the reaction sequence using spiro cyclobutanones, see: Murakami, M.; Takahashi, K.; Amii, H.; Ito, Y. J. Am. Chem. Soc. 1997, 119, 9307.
10. The ring expansion reactions of siloxycyclopropanes, see: (a) Kuwajima, I.; Nakamura, E. Top. Curr. Chem. 1990, 155, 1. (b) Kirihara, M.; Ichinose, M.; Takizawa, S.; Momose, T. Chem. Commun. 1998, 1691 and references therein. The oxidative rearrangement reactions of cyclobutanols, see: Schlecht, M. F. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Ley, S. V., Eds.; Pergamon: Oxford, U.K., 1991; Vol.7, pp 824-826. The recent advance of β-carbon elimination in the reaction sequence using spiro cyclobutanones, see: Murakami, M.; Takahashi, K.; Amii, H.; Ito, Y. J. Am. Chem. Soc. 1997, 119, 9307.
11. The study on the strain energy, see: Schleyer, P. v. R.; Williams, J. E.; Blanchard, K. R. J. Am. Chem. Soc. 1970, 92, 2377.
12. For some previous works on the palladium-catalyzed ring expansion of cyclobutanols, see: (a) Boontanonda, P.; Grigg, R. J. Chem. Soc., Chem. Commun. 1977, 583. (b) Clark, G. R.; Thiensathit, S. Tetrahedron Lett. 1985, 26, 2503. (c) Liebeskind, L. S.; Mitchell, D.; Foster, B. S. J. Am. Chem. Soc. 1987, 109, 7908. (d) Demuth, M.; Pandey, B.; Wietfeld, B.; Said, H.; Viader, J. Helv. Chim. Acta 1988, 71, 1392. (e) de Almeida Barbosa, L.-C.; Mann, J. J. Chem. Soc., Perkin Trans. 1 1990, 177. (f) Mitchell, D.; Liebeskind, L. S. J. Am. Chem. Soc. 1990, 112, 291. (g) Nemoto, H.; Nagamochi, M.; Fukumoto, K. J. Chem. Soc., Perkin Trans. 1 1993, 2329. (h) Nemoto, H.; Nagamochi, M.; Ishibashi, H.; Fukumoto, K. J. Org. Chem. 1994, 59, 74. (i) Nemoto, H.; Shiraki, M.; Fukumoto, K. Synlett 1994, 599. (j) Nemoto, H.; Miyata, J.; Fukumoto, K. Tetrahedron 1996, 52, 10363. (k) Nemoto, H.; Yoshida, M.; Fukumoto, K. J. Org. Chem. 1997, 62, 6450. (l) Nemoto, H.; Miyata, J.; Yoshida, M.; Raku, N.; Fukumoto, K. J. Org. Chem. 1997, 62, 7850.
13. For some previous works on the palladium-catalyzed ring expansion of cyclobutanols, see: (a) Boontanonda, P.; Grigg, R. J. Chem. Soc., Chem. Commun. 1977, 583. (b) Clark, G. R.; Thiensathit, S. Tetrahedron Lett. 1985, 26, 2503. (c) Liebeskind, L. S.; Mitchell, D.; Foster, B. S. J. Am. Chem. Soc. 1987, 109, 7908. (d) Demuth, M.; Pandey, B.; Wietfeld, B.; Said, H.; Viader, J. Helv. Chim. Acta 1988, 71, 1392. (e) de Almeida Barbosa, L.-C.; Mann, J. J. Chem. Soc., Perkin Trans. 1 1990, 177. (f) Mitchell, D.; Liebeskind, L. S. J. Am. Chem. Soc. 1990, 112, 291. (g) Nemoto, H.; Nagamochi, M.; Fukumoto, K. J. Chem. Soc., Perkin Trans. 1 1993, 2329. (h) Nemoto, H.; Nagamochi, M.; Ishibashi, H.; Fukumoto, K. J. Org. Chem. 1994, 59, 74. (i) Nemoto, H.; Shiraki, M.; Fukumoto, K. Synlett 1994, 599. (j) Nemoto, H.; Miyata, J.; Fukumoto, K. Tetrahedron 1996, 52, 10363. (k) Nemoto, H.; Yoshida, M.; Fukumoto, K. J. Org. Chem. 1997, 62, 6450. (l) Nemoto, H.; Miyata, J.; Yoshida, M.; Raku, N.; Fukumoto, K. J. Org. Chem. 1997, 62, 7850.
14. For some previous works on the palladium-catalyzed ring expansion of cyclobutanols, see: (a) Boontanonda, P.; Grigg, R. J. Chem. Soc., Chem. Commun. 1977, 583. (b) Clark, G. R.; Thiensathit, S. Tetrahedron Lett. 1985, 26, 2503. (c) Liebeskind, L. S.; Mitchell, D.; Foster, B. S. J. Am. Chem. Soc. 1987, 109, 7908. (d) Demuth, M.; Pandey, B.; Wietfeld, B.; Said, H.; Viader, J. Helv. Chim. Acta 1988, 71, 1392. (e) de Almeida Barbosa, L.-C.; Mann, J. J. Chem. Soc., Perkin Trans. 1 1990, 177. (f) Mitchell, D.; Liebeskind, L. S. J. Am. Chem. Soc. 1990, 112, 291. (g) Nemoto, H.; Nagamochi, M.; Fukumoto, K. J. Chem. Soc., Perkin Trans. 1 1993, 2329. (h) Nemoto, H.; Nagamochi, M.; Ishibashi, H.; Fukumoto, K. J. Org. Chem. 1994, 59, 74. (i) Nemoto, H.; Shiraki, M.; Fukumoto, K. Synlett 1994, 599. (j) Nemoto, H.; Miyata, J.; Fukumoto, K. Tetrahedron 1996, 52, 10363. (k) Nemoto, H.; Yoshida, M.; Fukumoto, K. J. Org. Chem. 1997, 62, 6450. (l) Nemoto, H.; Miyata, J.; Yoshida, M.; Raku, N.; Fukumoto, K. J. Org. Chem. 1997, 62, 7850.
15. For some previous works on the palladium-catalyzed ring expansion of cyclobutanols, see: (a) Boontanonda, P.; Grigg, R. J. Chem. Soc., Chem. Commun. 1977, 583. (b) Clark, G. R.; Thiensathit, S. Tetrahedron Lett. 1985, 26, 2503. (c) Liebeskind, L. S.; Mitchell, D.; Foster, B. S. J. Am. Chem. Soc. 1987, 109, 7908. (d) Demuth, M.; Pandey, B.; Wietfeld, B.; Said, H.; Viader, J. Helv. Chim. Acta 1988, 71, 1392. (e) de Almeida Barbosa, L.-C.; Mann, J. J. Chem. Soc., Perkin Trans. 1 1990, 177. (f) Mitchell, D.; Liebeskind, L. S. J. Am. Chem. Soc. 1990, 112, 291. (g) Nemoto, H.; Nagamochi, M.; Fukumoto, K. J. Chem. Soc., Perkin Trans. 1 1993, 2329. (h) Nemoto, H.; Nagamochi, M.; Ishibashi, H.; Fukumoto, K. J. Org. Chem. 1994, 59, 74. (i) Nemoto, H.; Shiraki, M.; Fukumoto, K. Synlett 1994, 599. (j) Nemoto, H.; Miyata, J.; Fukumoto, K. Tetrahedron 1996, 52, 10363. (k) Nemoto, H.; Yoshida, M.; Fukumoto, K. J. Org. Chem. 1997, 62, 6450. (l) Nemoto, H.; Miyata, J.; Yoshida, M.; Raku, N.; Fukumoto, K. J. Org. Chem. 1997, 62, 7850.
16. For some previous works on the palladium-catalyzed ring expansion of cyclobutanols, see: (a) Boontanonda, P.; Grigg, R. J. Chem. Soc., Chem. Commun. 1977, 583. (b) Clark, G. R.; Thiensathit, S. Tetrahedron Lett. 1985, 26, 2503. (c) Liebeskind, L. S.; Mitchell, D.; Foster, B. S. J. Am. Chem. Soc. 1987, 109, 7908. (d) Demuth, M.; Pandey, B.; Wietfeld, B.; Said, H.; Viader, J. Helv. Chim. Acta 1988, 71, 1392. (e) de Almeida Barbosa, L.-C.; Mann, J. J. Chem. Soc., Perkin Trans. 1 1990, 177. (f) Mitchell, D.; Liebeskind, L. S. J. Am. Chem. Soc. 1990, 112, 291. (g) Nemoto, H.; Nagamochi, M.; Fukumoto, K. J. Chem. Soc., Perkin Trans. 1 1993, 2329. (h) Nemoto, H.; Nagamochi, M.; Ishibashi, H.; Fukumoto, K. J. Org. Chem. 1994, 59, 74. (i) Nemoto, H.; Shiraki, M.; Fukumoto, K. Synlett 1994, 599. (j) Nemoto, H.; Miyata, J.; Fukumoto, K. Tetrahedron 1996, 52, 10363. (k) Nemoto, H.; Yoshida, M.; Fukumoto, K. J. Org. Chem. 1997, 62, 6450. (l) Nemoto, H.; Miyata, J.; Yoshida, M.; Raku, N.; Fukumoto, K. J. Org. Chem. 1997, 62, 7850.
17. For some previous works on the palladium-catalyzed ring expansion of cyclobutanols, see: (a) Boontanonda, P.; Grigg, R. J. Chem. Soc., Chem. Commun. 1977, 583. (b) Clark, G. R.; Thiensathit, S. Tetrahedron Lett. 1985, 26, 2503. (c) Liebeskind, L. S.; Mitchell, D.; Foster, B. S. J. Am. Chem. Soc. 1987, 109, 7908. (d) Demuth, M.; Pandey, B.; Wietfeld, B.; Said, H.; Viader, J. Helv. Chim. Acta 1988, 71, 1392. (e) de Almeida Barbosa, L.-C.; Mann, J. J. Chem. Soc., Perkin Trans. 1 1990, 177. (f) Mitchell, D.; Liebeskind, L. S. J. Am. Chem. Soc. 1990, 112, 291. (g) Nemoto, H.; Nagamochi, M.; Fukumoto, K. J. Chem. Soc., Perkin Trans. 1 1993, 2329. (h) Nemoto, H.; Nagamochi, M.; Ishibashi, H.; Fukumoto, K. J. Org. Chem. 1994, 59, 74. (i) Nemoto, H.; Shiraki, M.; Fukumoto, K. Synlett 1994, 599. (j) Nemoto, H.; Miyata, J.; Fukumoto, K. Tetrahedron 1996, 52, 10363. (k) Nemoto, H.; Yoshida, M.; Fukumoto, K. J. Org. Chem. 1997, 62, 6450. (l) Nemoto, H.; Miyata, J.; Yoshida, M.; Raku, N.; Fukumoto, K. J. Org. Chem. 1997, 62, 7850.
18. For some previous works on the palladium-catalyzed ring expansion of cyclobutanols, see: (a) Boontanonda, P.; Grigg, R. J. Chem. Soc., Chem. Commun. 1977, 583. (b) Clark, G. R.; Thiensathit, S. Tetrahedron Lett. 1985, 26, 2503. (c) Liebeskind, L. S.; Mitchell, D.; Foster, B. S. J. Am. Chem. Soc. 1987, 109, 7908. (d) Demuth, M.; Pandey, B.; Wietfeld, B.; Said, H.; Viader, J. Helv. Chim. Acta 1988, 71, 1392. (e) de Almeida Barbosa, L.-C.; Mann, J. J. Chem. Soc., Perkin Trans. 1 1990, 177. (f) Mitchell, D.; Liebeskind, L. S. J. Am. Chem. Soc. 1990, 112, 291. (g) Nemoto, H.; Nagamochi, M.; Fukumoto, K. J. Chem. Soc., Perkin Trans. 1 1993, 2329. (h) Nemoto, H.; Nagamochi, M.; Ishibashi, H.; Fukumoto, K. J. Org. Chem. 1994, 59, 74. (i) Nemoto, H.; Shiraki, M.; Fukumoto, K. Synlett 1994, 599. (j) Nemoto, H.; Miyata, J.; Fukumoto, K. Tetrahedron 1996, 52, 10363. (k) Nemoto, H.; Yoshida, M.; Fukumoto, K. J. Org. Chem. 1997, 62, 6450. (l) Nemoto, H.; Miyata, J.; Yoshida, M.; Raku, N.; Fukumoto, K. J. Org. Chem. 1997, 62, 7850.
19. For some previous works on the palladium-catalyzed ring expansion of cyclobutanols, see: (a) Boontanonda, P.; Grigg, R. J. Chem. Soc., Chem. Commun. 1977, 583. (b) Clark, G. R.; Thiensathit, S. Tetrahedron Lett. 1985, 26, 2503. (c) Liebeskind, L. S.; Mitchell, D.; Foster, B. S. J. Am. Chem. Soc. 1987, 109, 7908. (d) Demuth, M.; Pandey, B.; Wietfeld, B.; Said, H.; Viader, J. Helv. Chim. Acta 1988, 71, 1392. (e) de Almeida Barbosa, L.-C.; Mann, J. J. Chem. Soc., Perkin Trans. 1 1990, 177. (f) Mitchell, D.; Liebeskind, L. S. J. Am. Chem. Soc. 1990, 112, 291. (g) Nemoto, H.; Nagamochi, M.; Fukumoto, K. J. Chem. Soc., Perkin Trans. 1 1993, 2329. (h) Nemoto, H.; Nagamochi, M.; Ishibashi, H.; Fukumoto, K. J. Org. Chem. 1994, 59, 74. (i) Nemoto, H.; Shiraki, M.; Fukumoto, K. Synlett 1994, 599. (j) Nemoto, H.; Miyata, J.; Fukumoto, K. Tetrahedron 1996, 52, 10363. (k) Nemoto, H.; Yoshida, M.; Fukumoto, K. J. Org. Chem. 1997, 62, 6450. (l) Nemoto, H.; Miyata, J.; Yoshida, M.; Raku, N.; Fukumoto, K. J. Org. Chem. 1997, 62, 7850.
20. For some previous works on the palladium-catalyzed ring expansion of cyclobutanols, see: (a) Boontanonda, P.; Grigg, R. J. Chem. Soc., Chem. Commun. 1977, 583. (b) Clark, G. R.; Thiensathit, S. Tetrahedron Lett. 1985, 26, 2503. (c) Liebeskind, L. S.; Mitchell, D.; Foster, B. S. J. Am. Chem. Soc. 1987, 109, 7908. (d) Demuth, M.; Pandey, B.; Wietfeld, B.; Said, H.; Viader, J. Helv. Chim. Acta 1988, 71, 1392. (e) de Almeida Barbosa, L.-C.; Mann, J. J. Chem. Soc., Perkin Trans. 1 1990, 177. (f) Mitchell, D.; Liebeskind, L. S. J. Am. Chem. Soc. 1990, 112, 291. (g) Nemoto, H.; Nagamochi, M.; Fukumoto, K. J. Chem. Soc., Perkin Trans. 1 1993, 2329. (h) Nemoto, H.; Nagamochi, M.; Ishibashi, H.; Fukumoto, K. J. Org. Chem. 1994, 59, 74. (i) Nemoto, H.; Shiraki, M.; Fukumoto, K. Synlett 1994, 599. (j) Nemoto, H.; Miyata, J.; Fukumoto, K. Tetrahedron 1996, 52, 10363. (k) Nemoto, H.; Yoshida, M.; Fukumoto, K. J. Org. Chem. 1997, 62, 6450. (l) Nemoto, H.; Miyata, J.; Yoshida, M.; Raku, N.; Fukumoto, K. J. Org. Chem. 1997, 62, 7850.
21. For some previous works on the palladium-catalyzed ring expansion of cyclobutanols, see: (a) Boontanonda, P.; Grigg, R. J. Chem. Soc., Chem. Commun. 1977, 583. (b) Clark, G. R.; Thiensathit, S. Tetrahedron Lett. 1985, 26, 2503. (c) Liebeskind, L. S.; Mitchell, D.; Foster, B. S. J. Am. Chem. Soc. 1987, 109, 7908. (d) Demuth, M.; Pandey, B.; Wietfeld, B.; Said, H.; Viader, J. Helv. Chim. Acta 1988, 71, 1392. (e) de Almeida Barbosa, L.-C.; Mann, J. J. Chem. Soc., Perkin Trans. 1 1990, 177. (f) Mitchell, D.; Liebeskind, L. S. J. Am. Chem. Soc. 1990, 112, 291. (g) Nemoto, H.; Nagamochi, M.; Fukumoto, K. J. Chem. Soc., Perkin Trans. 1 1993, 2329. (h) Nemoto, H.; Nagamochi, M.; Ishibashi, H.; Fukumoto, K. J. Org. Chem. 1994, 59, 74. (i) Nemoto, H.; Shiraki, M.; Fukumoto, K. Synlett 1994, 599. (j) Nemoto, H.; Miyata, J.; Fukumoto, K. Tetrahedron 1996, 52, 10363. (k) Nemoto, H.; Yoshida, M.; Fukumoto, K. J. Org. Chem. 1997, 62, 6450. (l) Nemoto, H.; Miyata, J.; Yoshida, M.; Raku, N.; Fukumoto, K. J. Org. Chem. 1997, 62, 7850.
22. For some previous works on the palladium-catalyzed ring expansion of cyclobutanols, see: (a) Boontanonda, P.; Grigg, R. J. Chem. Soc., Chem. Commun. 1977, 583. (b) Clark, G. R.; Thiensathit, S. Tetrahedron Lett. 1985, 26, 2503. (c) Liebeskind, L. S.; Mitchell, D.; Foster, B. S. J. Am. Chem. Soc. 1987, 109, 7908. (d) Demuth, M.; Pandey, B.; Wietfeld, B.; Said, H.; Viader, J. Helv. Chim. Acta 1988, 71, 1392. (e) de Almeida Barbosa, L.-C.; Mann, J. J. Chem. Soc., Perkin Trans. 1 1990, 177. (f) Mitchell, D.; Liebeskind, L. S. J. Am. Chem. Soc. 1990, 112, 291. (g) Nemoto, H.; Nagamochi, M.; Fukumoto, K. J. Chem. Soc., Perkin Trans. 1 1993, 2329. (h) Nemoto, H.; Nagamochi, M.; Ishibashi, H.; Fukumoto, K. J. Org. Chem. 1994, 59, 74. (i) Nemoto, H.; Shiraki, M.; Fukumoto, K. Synlett 1994, 599. (j) Nemoto, H.; Miyata, J.; Fukumoto, K. Tetrahedron 1996, 52, 10363. (k) Nemoto, H.; Yoshida, M.; Fukumoto, K. J. Org. Chem. 1997, 62, 6450. (l) Nemoto, H.; Miyata, J.; Yoshida, M.; Raku, N.; Fukumoto, K. J. Org. Chem. 1997, 62, 7850.
23. For some previous works on the palladium-catalyzed ring expansion of cyclobutanols, see: (a) Boontanonda, P.; Grigg, R. J. Chem. Soc., Chem. Commun. 1977, 583. (b) Clark, G. R.; Thiensathit, S. Tetrahedron Lett. 1985, 26, 2503. (c) Liebeskind, L. S.; Mitchell, D.; Foster, B. S. J. Am. Chem. Soc. 1987, 109, 7908. (d) Demuth, M.; Pandey, B.; Wietfeld, B.; Said, H.; Viader, J. Helv. Chim. Acta 1988, 71, 1392. (e) de Almeida Barbosa, L.-C.; Mann, J. J. Chem. Soc., Perkin Trans. 1 1990, 177. (f) Mitchell, D.; Liebeskind, L. S. J. Am. Chem. Soc. 1990, 112, 291. (g) Nemoto, H.; Nagamochi, M.; Fukumoto, K. J. Chem. Soc., Perkin Trans. 1 1993, 2329. (h) Nemoto, H.; Nagamochi, M.; Ishibashi, H.; Fukumoto, K. J. Org. Chem. 1994, 59, 74. (i) Nemoto, H.; Shiraki, M.; Fukumoto, K. Synlett 1994, 599. (j) Nemoto, H.; Miyata, J.; Fukumoto, K. Tetrahedron 1996, 52, 10363. (k) Nemoto, H.; Yoshida, M.; Fukumoto, K. J. Org. Chem. 1997, 62, 6450. (l) Nemoto, H.; Miyata, J.; Yoshida, M.; Raku, N.; Fukumoto, K. J. Org. Chem. 1997, 62, 7850.
24. Cyclobutanols are readily accessible from the corresponding cyclobutanones and Grignard reagent or alkyllithium. Typical methods for cyclobutanones, see: (a) Krepski, L. R.; Hassner, A. J. Org. Chem. 1978, 43, 2879. (b) Greene, A. E.; Luche, M.-J.; Serra, A. A. J. Org. Chem. 1985, 50, 3957.
25. Cyclobutanols are readily accessible from the corresponding cyclobutanones and Grignard reagent or alkyllithium. Typical methods for cyclobutanones, see: (a) Krepski, L. R.; Hassner, A. J. Org. Chem. 1978, 43, 2879. (b) Greene, A. E.; Luche, M.-J.; Serra, A. A. J. Org. Chem. 1985, 50, 3957.
26. The byproduct, 7-methylenebicyclo[4.3.0]nonan-8-one (2a′) was also obtained in 21% yield. The formation of 2a′ might stem from bond b breaking as indicated in Scheme 3 or palladium catalyzed rearrangement as shown in Scheme 4. (Equation Presented)
27. This effect of ethyl acrylate is not clear at present, but it might accelerate the β-carbon and hydrogen elimination. In the reaction of 1-vinylcyclobutanols the addition of ethyl acrylate did not show significant difference in the yield of the products.
28. 1H NMR for compound 16, see: Thompson, H. W.; Long, D. J. J. Org. Chem. 1988, 53, 4201. Ring juncture was isomerized in some stage during the reaction. The stereochemistry of 16 was confirmed by X-ray crystallographic analysis in our hands: see Supporting Information.
29. Blackburn, T. F.; Schwartz, J. J. Chem. Soc., Chem. Commun. 1977, 157.
30. Kravtsova, S. V.; Romm, I. P.; Stash, A. I.; Belsky, V. K. Acta Crystallogr. 1996, C52, 2201.