Is it possible to lerp to a negative euler angle?

Try lerping the forward vector instead

You can use Quaternions.

 t += Time.deltaTime;
 transform.rotation = Quaternion.Lerp(transform.rotation, new Quaternion (endRotation), t);

You can find the desired endRotation quaternion in the inspector. Rotate the object to the desired end rotation and right-click on the inspector, click Debug. Now you can see the corresponding quaternion. (for example x=0, y=0.7, z=0.7, w=0). Substitute endRotation with this value.