References and Notes
1 For a recent review, see: Palmer DC.
Venkatraman S.
Oxazoles: Synthesis, Reactions and Spectroscopy, In The Chemistry of Heterocyclic Compounds, Part A
Vol 60:
Palmer DC.
John Wiley and Sons;
New York:
2003.
p.1-390 ; and references therein
2a
Lindquist N.
Fenical W.
Van Duyne GD.
Clardy J.
J. Am. Chem. Soc.
1991,
113:
2303
2b
Ichiba T.
Yoshida WY.
Scheuer PJ.
Higa T.
Gravalos DG.
J. Am. Chem. Soc.
1991,
113:
3173
2c
Fusetani N.
Yasumuro K.
Matsunaga S.
Hashimoto K.
Tetrahedron Lett.
1989,
30:
2809
2d
Egawa Y.
Umino K.
Tamura Y.
Shimizu M.
Kaneko K.
Sakurazawa M.
Awataguchi S.
Okuda T.
J. Antibiot.
1969,
22:
12
2e
Shinya K.
Wierzba K.
Matsuo K.
Ohtani T.
Yamada Y.
Furihata K.
Hayakawa Y.
Seto H.
J. Am. Chem. Soc.
2001,
123:
1262
2f
Sohda K.
Nagai K.
Yamori T.
Suzuki K.
Tanaka A.
J. Antibiot.
2005,
58:
27
2g
Sohda K.
Hiramoto M.
Suzumura K.
Takebayashi Y.
Suzuki K.
Tanaka A.
J. Antibiot.
2005,
58:
32
3
Undheim K. In
Handbook of Organopalladium Chemistry for Organic Synthesis
Vol 1:
Negishi E.
John Wiley and Sons;
New York:
2002.
p.409-492 ; and references therein
4
Dondoni A.
Fantin G.
Fogagnolo M.
Medici A.
Pedrini P.
Synthesis
1987,
693
5a
Barrett AGM.
Kohrt JT.
Synlett
1995,
415
5b
Collins I.
Castro JL.
Street LJ.
Tetrahedron Lett.
2000,
41:
781
5c
Clapham B.
Sutherland AJ.
J. Org. Chem.
2001,
66:
9033
6a
Anderson BA.
Harn NK.
Synthesis
1996,
583
6b
Anderson BA.
Becke LM.
Booher RN.
Flaugh ME.
Harn NK.
Kress TJ.
Varie DL.
Wepsiec JP.
J. Org. Chem.
1997,
62:
8634
6c
Vedejs E.
Luchetta LM.
J. Org. Chem.
1999,
64:
1011
7a
Kelly TR.
Lang F.
J. Org. Chem.
1996,
61:
4623
7b
Boto A.
Ling M.
Meek G.
Pattenden G.
Tetrahedron Lett.
1998,
39:
8167
7c
Liu C.-M.
Chen B.-H.
Liu W.-Y.
Wu X.-L.
Ma Y.-X.
J. Organomet. Chem.
2000,
598:
348
7d
Hodgetts KJ.
Kershaw MT.
Org. Lett.
2002,
4:
2905
7e
Hodgetts KJ.
Kershaw MT.
Org. Lett.
2003,
5:
2911
7f
Maekawa T.
Sakai N.
Tawada H.
Murase K.
Hazama M.
Sugiyama Y.
Momose Y.
Chem. Pharm. Bull.
2003,
51:
565
7g
Young GL.
Smith SA.
Taylor RJK.
Tetrahedron Lett.
2004,
45:
3797
8a
Miyaura N.
Suzuki A.
Chem. Rev.
1995,
95:
2457
8b
Suzuki A.
J. Organomet. Chem.
1999,
576:
147
9
Tyrrell E.
Brookes P.
Synthesis
2004,
469 ; and references therein
10a
Ishiyama T.
Murata M.
Miyaura N.
J. Org. Chem.
1995,
60:
7508
10b
Ishiyama T.
Itoh Y.
Kitano T.
Miyaura N.
Tetrahedron Lett.
1997,
38:
3447
11
Schaus JV.
Panek JS.
Org. Lett.
2000,
2:
469
12
Preparation of 2-Phenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)oxazole (
2).
A suspension of Pd2(dba)3·CHCl3 (120 mg, 0.13 mmol) and tricyclohexylphosphine (220 mg, 0.78 mmol) in 1,4-dioxane (60 mL) was stirred for 30 min at r.t. under Ar. Bis(pinacolato)diboron (1.46 g, 5.7 mmol), 2-phenyl-4-tri-fluoromethanesulfonyloxyoxazole (1, 1.53g, 5.2 mmol) and KOAc (769 mg, 7.8 mmol) were successively added to the resulting solution. After being at reflux for 2 h, the reaction mixture was diluted with Et2O (300 mL). The resulting mixture was washed with H2O (100 mL) and then dried over Na2SO4. Concentration of the solvent in vacuo gave a residue, which was purified by silica gel column chromato-graphy (hexane-EtOAc, 20:1 to 1:1) to give 2-phenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)oxazole (2). After recrystallization from hot hexane, pure 2 (1.06 g, 75%) was obtained as colorless needles; mp 124-125 °C. 1H NMR (500 MHz, CDCl3): δ = 1.38 (12 H, s), 7.44 (3 H, m), 8.07 (1 H, s), 8.14 (2 H, m). 13C NMR (125 MHz, CDCl3): δ = 24.8, 84.3, 126.9, 127.3, 128.6, 130.4, 148.0, 162.8; IR (neat) 2996, 2973, 1567, 1363, 1319, 1081, 692 cm-1. HRMS (EI): m/z calcd for C15H18BNO3 [M+]: 271.1380; found: 271.1374.
13a
Prager RH.
Smith JA.
Weber B.
Williams CM.
J. Chem. Soc., Perkin Trans. 1
1997,
2665
13b The method reported in ref. 13a is inconvenient to synthesize 3. We developed the alternative method to prepare bromide 3 from benzoyl chloride and propargylamine as follows (Scheme
[3]
).
14
Preparation of 5-Methyl-2-phenyl-4-(4,4,5,5-tetra-methyl-1,3,2-dioxaborolan-2-yl)oxazole (
4).
n-BuLi in n-hexane (1.58 M, 0.42 mL, 0.67 mmol) was added to a stirred solution of 4-bromo-5-methyl-2-phenyl-oxazole (3, 144 mg, 0.65 mmol) in THF (3 mL) at -78 °C under Ar. After 30 min, triisopropylborate (0.17 mL, 0.73 mmol) was added to the resulting solution and stirred for 1 h at the same temperature. The reaction mixture was allowed to warm to r.t. and stirred for 1 h. Pinacol (86 mg, 0.73 mmol) and glacial AcOH (42 µL, 0.73 mmol) were added to the resulting solution and the resulting mixture was stirred for 1 h. The reaction mixture was diluted with Et2O (30 mL) and washed with H2O (10 mL) and then dried over Na2SO4. Concentration of the solvent in vacuo gave a residue, which was purified by silica gel column chromatography (hexane-EtOAc, 1:1) to give 5-methyl-2-phenyl-4-(4,4,5,5-tetra-methyl-1,3,2-dioxaborolan-2-yl)-oxazole (4). After recryst-allization from hot hexane, pure 4 (95 mg, 55%) was obtained as colorless needles; mp 108-109 °C. 1H NMR (500 MHz, CDCl3): δ = 1.36 (12 H, s), 2.56 (3 H, s), 7.41 (3 H, m), 8.10 (2 H, m). 13C NMR (125 MHz, CDCl3): δ = 11.8, 24.9, 83.9, 126.5, 127.6, 128.5, 129.9, 160.1, 161.0. IR (neat): 2979, 1593, 1407, 1382, 1317, 1141, 1085, 1050, 696 cm-1. HRMS (EI): m/z calcd for C16H20BNO3 [M+]: 285.1536; found: 285.1532.
15 Although both K2CO3 and K3PO4 worked well in the coupling reaction of 2 with bromobenzene, we chose K2CO3 as a base for further applications due to its milder basicity.
16
General Procedure of the Suzuki Coupling [Synthesis of 2,4-Diphenyloxazole (
5)].
A solution of 2 (60 mg, 0.22 mmol), bromobenzene (23 µL, 0.22 mmol), tetrakis(triphenylphosphine)palladium(0) (13 mg, 11 µmol) and K2CO3 (92 mg, 0.66 mmol) in DMF (1 mL) was heated at 100 °C for 30 min under Ar. The reaction mixture was diluted with Et2O (30 mL) and washed with H2O (10 mL) and then dried over Na2SO4. Concentration of the solvent in vacuo gave a residue, which was purified by silica gel column chromatography (hexane-EtOAc, 20:1) to give 2,4-diphenyloxazole (5, 43 mg, 88%) as a white solid. All characterization data of 5 were compatible with the literature.
[13]
17 Self-coupling reaction of the borane reagent 2 occurred to give 2,2′-diphenyl-4,4′-bioxazole in ca. 20% yield. This undesired event sometimes happened in slow Suzuki reaction, see: Moreno-Mañas M.
Pérez M.
Pleixats R.
J. Org. Chem.
1996,
61:
2346