Smooth rotating by 90 degrees in x and y axis.

I want to rotate cube in one of x or y axis by 90 degrees. This code works in half. Sometimes cubes is taking bad rotating path and instead of rotating clearly one axis, it rotate in all axis.

  cubex.transform.rotation = Quaternion.RotateTowards(cubex.transform.rotation, to, speed * Time.deltaTime);    

 var touch = Input.GetTouch (0); 
 var phase:  TouchPhase = touch.phase;  
 
     switch(phase)
     {
     case TouchPhase.Began:
         apos = touch.position;
         break;  
     case TouchPhase.Ended:
         bpos = touch.position;
         var ang = Mathf.Atan2(apos.y - bpos.y  , apos.x - bpos.x) * Mathf.Rad2Deg + 180;
         
             if(ang <=360 && ang >270){                
                 to = Quaternion.Euler(cubex.transform.eulerAngles.x, cubex.transform.eulerAngles.y - 90 , 0);                                    
             }else if(ang <=270 && ang >180){    
                 to = Quaternion.Euler(cubex.transform.eulerAngles.x - 90, cubex.transform.eulerAngles.y , 0);
             }else if(ang <=180 && ang >90){
                 to = Quaternion.Euler(cubex.transform.eulerAngles.x, cubex.transform.eulerAngles.y + 90 , 0);
             }else if(ang <=90 && ang >=0){
                 to = Quaternion.Euler(cubex.transform.eulerAngles.x +90, cubex.transform.eulerAngles.y , 0);
             }
         break;      

      }