[20662-89-9]  · C9H7NO  · 4-Phenyloxazole  · (MW 145.17)

(Diels-Alder reaction with alkynes leads to 3,4-disubstituted furans3 and with alkenes leads to 2-phenylpyridines;4 the N-methylide derivative leads to 3-pyrrole esters5)

Physical Data: mp 6 °C; bp 220-222 °C, 60-61 °C/0.1 mmHg; lmax 243 nm (ε 17 420).6

Preparative Methods: two different approaches have been described (eqs 1 and 2).7,8 Four detailed experimentals have been published for the phenacyl bromide route, including a multikilo preparation.7c

General Discussion.

4-Phenyloxazole (1) undergoes Diels-Alder reactions with activated3a,b and unactivated3c-e alkynes. The cycloadduct (eq 3) is unstable to the reaction temperatures and undergoes a retro-Diels-Alder reaction with expulsion of benzonitrile, forming a 3-mono- or 3,4-disubstituted furan in good to excellent yield (eqs 4 and 5).

Alkenes also undergo a Diels-Alder reaction with (1), and with reactive dienophiles the initial adduct (e.g. (2), largely endo) can be isolated (eq 6).4 Adduct (2) dehydrates with traces of acid to yield pyridines (eq 7).

Trimethylsilylmethyl Trifluoromethanesulfonate readily N-alkylates (1) (eq 8) and the resulting salt, on treatment with fluoride, generates ylide (4). When (4) is generated in the presence of an alkynic ester, an unstable cycloadduct is formed that fragments to give pyrroles such as (5) (eq 9).5

1. Turchi, I. J. In Oxazoles; Turchi, I. J., Ed. (The Chemistry of Heterocyclic Compounds; Weissberger, A.; Taylor, E. C., Eds.; Vol. 45); Wiley: New York, 1986; p 1.
2. Lakhan, R.; Ternai, B. Adv. Heterocycl. Chem. 1974, 17, 99.
3. (a) Ansell, M. F.; Caton, M. P. L.; North, P. C. TL 1981, 22, 1727. (b) Reich, H. J.; Kelly, M. J.; Olson, R. E.; Holtan, R. C. T 1983, 39, 949. (c) Liotta, D.; Saindane, M.; Ott, W. TL 1983, 24, 2473. (d) Traylor, T. G.; Hill, K. W.; Tain, Z.-Q.; Rheingold, A. L.; Peisach, J.; McCracken, J. JACS 1988, 110, 5571. (e) Ho, M. S.; Wong, H. N. C. CC 1989, 1238.
4. Usui, Y.; Hara, Y.; Shimamoto, N.; Yurugi, S.; Masuda, T. H 1975, 3, 155.
5. Padwa, A.; Chiacchio, U.; Venkatramanan, M. K. CC 1985, 1108.
6. Brown, D. J.; Ghosh, P. B. JCS(B) 1969, 270.
7. (a) Lewy, M. CB 1887, 20, 2576. (b) Bredereck, H.; Gompper, R. CB 1954, 87, 700. (c) Hutton, J.; Potts, B.; Southern, P. F. SC 1979, 9, 789. (d) Whitney, S. E.; Winters, M.; Rickborn, B. JOC 1990, 55, 929.
8. Schröder, R.; Schöllkopf, U.; Blume, E.; Hoppe, I. LA 1975, 533.

Scott McN. Sieburth

State University of New York at Stony Brook, NY, USA

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