180 degree player "quickturn"

So, i want to make a "psx survival horror game", and i need to make a quickturn button, that makes the player rotate 180 degrees smoothly, but i can't figure out how to add this to my "walk" script, i already tried something, but doesn't works. Here's the "quickturn" lines:

Code (CSharp):

1. if (Input.GetButton ("Quickturn")) {
2.             Vector3 smoothedRotation = Vector3.Lerp (transform.rotation, desiredAngle, quickTurnSpeed * Time.deltaTime);
3.             transform.rotation = smoothedRotation;
4.         }
5.  

"desiredAngle" need to be current player rotation + 180 degrees, ... i guess.

Also, sorry for the poor writing, i'm brazilian and obviously english aren't my primary language.

Edit #1: I've looking similar scripts, and i've wrote this:

Code (CSharp):

1. if (Input.GetButton ("Quickturn")) {
2.             rotation.y = transform.eulerAngles.y;
3.             transform.Rotate = Vector3.Lerp(rotation, rotationSet, quickTurnSpeed * Time.deltaTime);

But, i still getting a error "The left-hand side of an assignment must be a variable, a property or an indexer", i can't figure out what's going on...

 

If you would read the manual...
https://docs.unity3d.com/ScriptReference/Transform.Rotate.html

You would know that Rotate is a method, so you need to call it, not assigning value to it. Please visit the link, there is an example there.

 

I've managed to do it in another way, but now i can't turn the player whatever angle i want.

Code (CSharp):

1.     void Update (){
2.         if (Input.GetButton ("Right")) {
3.             transform.Rotate (Vector3.up, Time.deltaTime * rotSpeed);
4.         } else if (Input.GetButton ("Left")) {
5.             transform.Rotate (-Vector3.up, Time.deltaTime * rotSpeed);
6.         } else if (Input.GetButton ("Quickturn")) {
7.                 targetAngle.y += 180f;
8.             }
9.             currentAngle = transform.eulerAngles;
10.             currentAngle.y = Mathf.LerpAngle(currentAngle.y, targetAngle.y, lerpTime * Time.deltaTime);
11.             transform.eulerAngles = currentAngle;
12.  
13.         if (Input.GetButton ("Up")) {
14.             transform.Translate (Vector3.forward * Time.deltaTime * playerSpeed);
15.  
16.         }else if (Input.GetButton ("Down")) {
17.             transform.Translate (-Vector3.forward * Time.deltaTime * playerSpeed);
18.         }
19.     }

(Also, sorry for still making mistakes like this, I have not been programming for a long time, I'm still a beginner).