Adding a function to title close in jQuery dialog - event not firing up

Adding a function to title close in jQuery dialog - event not firing up

I'm assigning and extra class to Jquery modal .dialog() so I can custom
treat the title bar close (.ui-dialog-titlebar-close)
though it doesn't work for some reason
$(".extra_close .ui-dialog-titlebar-close").click(function () {
alert("hello");
});
no alert = just closes the dialog.
What I'm doing wrong?
Thank you Community