Rotate a cube by 90 degree

Hi I am trying to rotate a cube by 90 horizontally or vertically depending upon user input. But its all confusing the cube's rotation is behaving awkwardly.

Currently i am using this messed up code...

 void Update ()
         {
                 if (Direction != SwitchDirection.None) {
                         SwitchMap (Direction);            
                 }
                     
 
         }
 
         void OnGUI ()
         {
                 if (GUI.Button (new Rect (Screen.width / 2, Screen.height - Screen.height / 10, 100, 100), "D")) {
                         Direction = SwitchDirection.Down;
                 }
                 if (GUI.Button (new Rect (Screen.width / 2, Screen.height / 40, 100, 100), "U")) {
                         Direction = SwitchDirection.Up;
                 }
                 if (GUI.Button (new Rect (0, Screen.height / 2, 100, 100), "L")) {
                         Direction = SwitchDirection.Left;
                 }
                 if (GUI.Button (new Rect (Screen.width - Screen.width / 10, Screen.height / 2, 100, 100), "R")) {
                         Direction = SwitchDirection.Right;
                 }
 
         }
 
         void SwitchMap (SwitchDirection switchDirection)
         {
 
                 string direction = string.Empty;
                 float directionValue = 0;                    
 
                 switch (switchDirection) {
                 case SwitchDirection.Up:
                         direction = "x";                    
                         directionValue = transform.rotation.eulerAngles.x - 90;
                         
                         break;
                 case SwitchDirection.Right:
                         direction = "z";
                         directionValue = transform.rotation.eulerAngles.z + 90;
                         break;
                 case SwitchDirection.Down:
                         direction = "x";
                         directionValue = transform.rotation.eulerAngles.x + 90;
                         break;
                 case SwitchDirection.Left:
                         direction = "z";
                         directionValue = transform.rotation.eulerAngles.z - 90;
                         break;
                 }                                
 
                 if (switchDirection != SwitchDirection.None) {
                         iTween.RotateTo (gameObject, 
                              iTween.Hash (direction, directionValue,
                                                   "time", 0.5,
                                                   "easetype", iTween.EaseType.easeInBack
                         ));
                 }    
 
                 Direction = SwitchDirection.None;
         }