Bextra, a COX-2 inhibitor once used in the treatment of arthritis, contains an isoxazole ring. Why is the ring aromatic?

Bextra

The structure bextra contains an isoxazole ring is a five-membered ring that has nitrogen and oxygen in the ring. It has three carbon atoms that are ${\mathbf{sp}}^2$hybridized and contributes the three electrons from the p-orbital.

The nitrogen atom is ${\mathrm{sp}}^2$hybridized and contributes one electron and the oxygen atom is ${\mathrm{sp}}^3$hybridized which has two lone pairs of electrons and from that one lone pair of electrons participates in the aromatic sextet due to the lone pair of electrons on oxygen in a p-orbital perpendicular to the ring. So, there are six pi electrons in the ring and the isoxazole ring is aromatic.

The nitrogen in the isoxazole ring is ${\mathrm{sp}}^2$hybridized and has one lone pair of electrons. But these lone pair electrons won’t participate in the resonance. Because the lone pair of electrons on nitrogen is on ${\mathrm{sp}}^2$orbital in the plane of the ring.

Isoxazole ring (aromatic)