Execute UNION and after LEFT JOIN in MySQL
You are on the right way but just messed up with the syntax. 

SELECT t1.*
  FROM ( SELECT CLAVE_ART
              , DESCRIPCION_ART
              , EXISTENCIA_ART
              , MARCA_ART
              , exi_num_alm 
           FROM cdc_vw_Marbetes_Catalogo_Exis
          WHERE EXISTENCIA_ART &gt; 0 
            AND exi_num_alm = 25

          UNION

         SELECT clave
              , descripcion
              , existencia
              , marca
              , almacen 
           FROM tblMarbetesAdicionales 
          WHERE almacen = 25
       ) AS t1
  LEFT JOIN tblmarbetescongeladas AS t2 ON t1.CLAVE_ART = t2.clave 
 WHERE t2.clave IS NULL
;