public AnimationCurve rotationCurve;
public float rotationAngle;
public float rotationSpeed;

private float curvePosition = 0f;
private Quaternion startRot;

private void Start()
{
    startRot = transform.rotation;
}

private void Update()
{
    if (rotationCurve == null) return;

    curvePosition += rotationSpeed * Time.deltaTime;

    curvePosition %= 1f;

    float curveValue = rotationCurve.Evaluate(curvePosition);

    transform.rotation = startRot * Quaternion.Euler(0f, curveValue * rotationAngle, 0f);
}