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11.13.1.1.1.2.3.2 Using Carboxylic Acids

DOI: 10.1055/sos-SD-111-00417

Schnürch, M.; Hämmerle, J.; Stanetty, P.Science of Synthesis Knowledge Updates, (20101165.

2-Aminophenols can cyclize to benzoxazoles in the presence of carboxylic acids upon heating to high temperatures, either neat or in a solvent, in the presence of a dehydrating agent (e.g., PPA, PPE, H3BO3). When equimolar amounts of 2-aminophenols 25 and aryl- or alkylcarboxylic acids are heated, temperatures of 140–220 °C are required[‌34‌,‌64‌-‌68‌] for the formation of the corresponding 2-aryl- or 2-alkylbenzoxazoles 26 (Scheme 20). The above-mentioned methods can also be applied to the synthesis of oxazolo[5,4-b]pyridines [using PPA,[‌69‌,‌70‌] polyphosphoric acid trimethylsilyl ester (PPSE),[‌71‌,‌72‌] or H3BO3[‌73‌]]. In some cases, solvents (xylene[‌74‌]) can be used and the formed water can be removed by azeotropic distillation.

Meeeee 88 Meeeeeeeeeee eeee 8-Meeeeeeeeeee eee Meeeeeeeee Meeee[‌88‌,‌88‌-‌88‌,‌88‌]

Meeeeeeeeee 88

M8 M8 M8 Meeeeeeeee Meeee (%) ee ( °M) ee ee ( °M/Meee) Mee
Me M Me 888–888 °M, 8 e 88 88–88 [‌88‌]
M M e-Me e-MeMM8M (eeeeee), 888 °M, 88 e 88 888/88.88 [‌88‌]
M M (MM8)8Me 8. eeeeee
8. eeeeeee
88 888–888 [‌88‌]
e-Me e-Me M eeeeee, 8.8 e 88 88–88 [‌88‌]
MM8M M M 8. eeeeee, 8 e
8. 888 °M, 8 e
88 888 (eee) [‌88‌]

Meeee eeee eeeeeeeeeeee eee eeeeeeeee eee eeee eeeeeee, eeeeeeeee-eeeeeeee eeeeeeeee ee eeeee eeeeeee eee eeee eeee ee eeeeeeeeee eeee eeeeeeeeeeeeee.[‌88‌] Me eee eeeeeeee ee eeeeeeeeeeeeeeeee eeeeeee ee Meeeeeee‘e eeeeeee, e eeeeee ee eeeeeeeeeeee 88 eee eeeeee eeeeee 8–8 eeeeeee ee 88–88% eeeee (Meeeee 88).[‌88‌]

Meeeee 88 Meeeeeeee-Meeeeeee Meeeeeeee ee Meeeeeeeeeee[‌88‌]

Meeeeeeeeee 88

M8 Meee (eee) Meeee (%) Mee
Me 8 88 [‌88‌]
8,8-(MeM)8M8M8 8 88 [‌88‌]
8,8,8-(MeM)8M8M8 8 88 [‌88‌]
8-Mee 8 88 [‌88‌]
8-MeM8M8 8 88 [‌88‌]
8-eeeeeeee 8 88 [‌88‌]
8-eeeeeee 8 88 [‌88‌]
8-eeeeeee 8 88 [‌88‌]
MM═MMMe 8 88 [‌88‌]
Me 8 88 [‌88‌]
8,8-(MeM)8M8M8MM8 8 88 [‌88‌]
(MM8)8Me 8 88 [‌88‌]
(MM8)88Me 8 88 [‌88‌]

8-Meeeeeeeeeee eeeeeee eeee eeeeeeeeee eeeee ee eeeeeeeeeeeeee eeee ee 888–888 °M.[‌88‌,‌88‌] Meee eeeeeeeeeeeee eeeee ee eeee eeeeeee, eeeeee eeeeeeee eeeeeeeeee (eeee eeeeeeeeeee 888–888 °M, 88 eee) eee ee eeeeeee (Meeeee 88). 8-Meeee- eee 8-eeeeeeeeeeeeeeee 88 eee ee eeeeeeee eee eeee eeeeee.[‌88‌-‌88‌]

Meeeee 88 Meeeeeeeeeeee Meeee Meeeeeee Meeeeeeee ee Meeeeeeeeeee eeee 8-Meeeeeeeeeee eee Meeeeeeeee Meeee[‌88‌,‌88‌,‌88‌]

Meeeeeeeeee 88

M8 Meeee (%) ee ( °M) ee ee ( °M/Meee) Mee
Me 88 888/88 [‌88‌]
Me 88 888–888 [‌88‌]
8-MeM8M8 88 888–888 [‌88‌]
8-M8MM8M8 88 888–888 [‌88‌]
8-MMM8M8MM8 88 88–88.8 [‌88‌]
8-MeMM8M8M8 88 888–888 [‌88‌]
8-MeMMM8M8 88 888 [‌88‌]

8-Meee- eee 8-eeeeeeeeeeeeeeeeeee 88 eee eeeeeeee eeee eeeeeee eeeeeeeee eeeeeee ee 8-eeeeeeeeeeee eee eeeeeeeeee eeeee ee eeee-eeeeeee eeeeeeee ee eee eeeeeeee ee eeeeeeeee[‌88‌,‌88‌] ee eeeeeeeeeeeeee eeeeeee[‌88‌] ee eeeee eeee (Meeeee 88). Mee eeeeee eeeee eee ee ee eeeeeee eeeeeeeeeeee. Meeee eeeee eeee ee eeeeee ee e eeeeeeeeee eeee eeee eeeeeeee ee eeeeeeeeeeeeee eeee, eeeeeeeeeeeeeee eeeee eee eeeeeeeeee eeeeeeeeeeeeee eee eeee ee eeeeeee eeeeeee eeeee eeeeeeeeeeeee. Meeeeeeeeeee, eee eeeeee eeeeeeeee ee eeee eeeeeeeee ee eee eeee ee eeeee eeee eeeeeeee ee eeeeeeeeeeeeee eeee ee eeeeeeeeeeeee eeeee.[‌88‌]

Meeeee 88 Meeee Meee Meeeeeee Meeeeeeee ee Meeeeeeeeeee eeee 8-Meeeeeeeeeee eee Meeeeeeeee Meeee[‌88‌-‌88‌]

Meeeeeeeeee 88

M8 M8 M8 Meeeeeeeee Meeee (%) ee ( °M) Mee
M M 8-MeM8MM8M8 eeeeee, eeeeee, 88 e 88 888–888 [‌88‌]
M M eeeeeeee eeeeee, eeeeee, 8 e 88 >888 [‌88‌]
M M 8-Mee [MeM(MM8)8]8M, [MeM(MM8)8]8M, 888 °M, 8.8 e 88 888–888 [‌88‌]
M Me 8-e-MeM8M8 [MeM(MM8)8]8M, [MeM(MM8)8]8M, 888 °M, 8.8 e 88 88.8–88 [‌88‌]
M Me Me [MeM(MM8)8]8M, [MeM(MM8)8]8M, 888 °M, 8.8 e 88 88.8–88 [‌88‌]
Me Me 8-Mee [MeM(MM8)8]8M, [MeM(MM8)8]8M, 888 °M, 8.8 e 88 888–888.8 [‌88‌]
M Me 8-Mee [MeM(MM8)8]8M, [MeM(MM8)8]8M, 888 °M, 8.8 e 88 888.8–888 [‌88‌]
e-Me M 8-Mee [MeM(MM8)8]8M, [MeM(MM8)8]8M, 888 °M, 8.8 e 88 888.8–888 [‌88‌]
Me M 8-Mee eeeeeeee, eeeeeeeeeeeeeeee, 888–888 °M 88 888.8–888 [‌88‌]

M eeee eeeeeeeee eeeeeeee eeee Meeee-Meeee {M8MM[(MM8)8MMe]8} eee eee eeeeeeeee ee e eeeeee ee 8-eeeeeeeeeee eeeeeeeeeeee, e.e. 88, eeee 8-eeeeeeeeeeee eee eeeeeeeeee eeeee (Meeeee 88).[‌88‌] Meee eeeeeeee eeeeee ee ee eeeeeeeee eeee eeeeeeee, eeee eeeeee ee >88% eee eee eee eeeeeeee eeeeeeee.

Meeeee 88 Meeee-Meeee Meeeeeee Meeeeeeee ee (M)-8-(Meeeeeee-8-eeee)eeeeeeeeeee[‌88‌]

Meeeeee eeeeeeee/eeeeeeeeeeeeeeeee eee eeee eeee eeee ee eee eeeeeeee ee e eeee-eeeeeeee eee-eee eeeeeeeee ee 8-eeeeeeeeeeeeeeee eeee 8-eeeeeeeeeee eee eeeeeee eeeee.[‌88‌]

Meeeeeeeeeee Meeeeeeee

8-Meeeee-8-eeeeeeeeeeeeeeeee (88, M8 = Me; M8 = M; M8 = Me); Meeeeee Meeeeeeee:[‌88‌]

8-Meeee-8-eeeeeeeeeeee (88 e, 8.88 eee) eee MeMM8M (88 e, 8.88 eee) eeee eeeeee eeeeeeee ee 888–888 °M eee 8 e. Mee eeeeeee eee eeeeeee eeee ee MeMM eee eee eeeeeee eee eeeeeeeee eeee MMMe8. Meeeeee ee eee MMMe8 eeee ee eee, eeeee eee eeeeeeeeeeee [eeeeee (Meeee eeeeee)] ee eeee eeeee eeeeeee; eeeee: 88 e (88%); ee 88–88 °M.

8-(8-Meeee)eeeeeeeeeee (88, M8 = 8-Mee); Meeeeee Meeeeeeee:[‌88‌]

M eeeeeee ee 8-MeeMM8M (888 ee, 8 eeee), 8-eeeeeeeeeee (888 ee, 8 eeee), eee Meeeeeee‘e eeeeeee (888 ee, 8.88 eeee) eee eeeeeeeeee ee ee eeee eeeeee eeee eeeeeeeeee ee e eeeeeeee eeee (Meeeeeee MMM, 888 M, eeeeeeeeeee eeeeeee eee ee 888 °M, eeeeeeee eeee ee MM eeeeee) eee 8 eee. Mee eeeee eeeeeee eee eeeeeeeee ee MM8Me8 (88 eM), eee eee eeee eee eeeeee eeee 88% ee MeMM (8 × 88 eM), eeeee (Me8MM8), eee eeeeeeeeeeee ee eeee eeee (ee MMM) 8-(8-eeeee)eeeeeeeeeee. Meeeeee eeeeeeeeeeee ee eeeee eeeeeeeeeeeeee eeee e eeeee eeeee; eeeee: 888 ee (88%); ee 88–88 °M (MeMM).

8-Meeeeeeeeee Meeeeeeeeeee 88; Meeeeee Meeeeeeee:[‌88‌]

Me e eeeeeee ee 8-eeeeeeeeeee (8.8 e, 88 eeee) eee MMM (88 e) eee eeeee eee eeeeeeeeee eeee (88 eeee) ee 888 °M (eeee eeeeeeeeeee) eee eee eeeeeee eee eeee ee eeee eeeeeeeeeee eee eeeeeee 88 eee. Meeee eeeeeee, eee eeeee (88 eM) eee eeeee eee eee eeeeeee eee eeeeeeeeeee eeee ee eeeeee ee MeMMM8. Mee eeeeeeeee eeeeeeeeeee eee eeeeeeeee ee eeeeeeeeee, eeeeee, eee eeeee. Mee eeeee eeeeeee eee eeeeeeee eeeeee ee eeeeeeeeee eeee eeeeee, eeeeeee (MMMMMMM: eeeeeeeeee), ee Me8M, ee ee eeeeeeeeeeeeeeeee.

8-(8-Meeee)eeeeeeeeeee (88, M8 = M8 =M; M8 = 8-Mee); Meeeeee Meeeeeeee:[‌88‌]

8-MeeMM8M (88.8 e, 8.8 eee), 8-eeeeeeeeeee (88.8 e, 8.8 eee), eee M8MM8 (8.8 e, 8 eeee) eeee eeeeee ee 888 °M ee e eeeeeee ee eee[8-eeeeeeeeeee] eeeee (88 eM) eee eee[8-eeeeeeeeeee] eeeee (88 eM) eeeee eeeeeeee eee e M8 eeeeeeeeee eee 8 e. Mee M8M eeee eeeeee eee eee eeeee-eeeeeee eeeeeee eeee eeeeeeeeeeee eeeeeee ee eeeeeeeeeeee. Mee eeeeeee eee eeee ee 888 °M eee 88 eee. Meee, ee eee eeeeee eee eeeeeee (88 eM) eee eeee MeMM (88 eM) eeee eeeee. Mee eeeeeee eee eeeeee ee 8–8 °M eee eee eeeeeeeeeee eee eeeeeeeee ee eeeeeeeeee, eeeeee eeee MeMM, eee eeeee; eeeee: 88.8 e (88%); ee 888–888 °M (MeMM).

(M)-8-(Meeeeeee-8-eeee)eeeeeeeeeee (88); Meeeeee Meeeeeeee:[‌88‌]

(M)-Meeeeee-8-eeeee eeee (88.8 ee, 8.888 eeee), eMe8MMe (88 μM, 8.88 eeee, 8.8 eeeee), 8-eeeeeeeeeee (88.88 ee, 8.888 eeee, 8.8 eeeee), eee M8MM8 (eeeeee) eeee eeeeeeeee ee MM8Me8 (8 eM). Mee eeeeeee eee eeeeee ee 8 °M, eee Meeee-Meeee (88 μM, 8.888 eeee, 8.8 eeeee) eee eeeee eeeeeeee. Meeee 8 e, eee eeeeeeee eee eeeeeeee eeee eee. ee MeMMM8 ee 8 °M. Meeee eeeeeee ee ee, eee eeeeeeee eeeeeee eee eeeeeeeee eeee MM8Me8. Mee eeeeeeee eeeeeee eeeeeeee eeee eeeee (MeMM8), eeeeeeee, eee eeeeeeeeeeee eeeee eeeeeee eeeeeeee. Meeee eeeeee eeeeeeeeeeeeee eeeeeeee e eeeee eeeeee eeeee; eeeee: 88.8 ee (88%).

References