var MainView = Mn.LayoutView.extend({
    template: $(html).filter('#main')[0].outerHTML,
    regions: {...},
    ....
});
return MainView;

define([script.js], function(MainView) {
     $.ajax({
          url: url,
          success: function(data){
            this.main.show(
                 new MainView({ select_options: options });
             )
          }
      });
   }