Ivermectin Impurity 35

(2R,2a'E,2a1'S,4'E,5S,6'S,8'E,11'R,15'S,20'R,20a'R) -6- ((S) -sec-butyl) -5-ethyl-2a1',20'-dihydroxy-7'- (((2R,4S,5S,6S) -5- (((2S,4S,5S,6S) -5-hydroxy-4-methoxy-6-methyltetrahydro-2H-pyran-2-yl) oxy) -4-methoxy-6-methyltetrahydro-2H-pyran-2-yl) oxy) -6',8',19'-trimethyl-2a1',3,4,5,6,6',7',10',11',14',15',17a',20',20a'-tetradecahydro-2'H,17'H-spiro[pyran-2,13'-[11,15]methano[1,5]dioxacyclooctadecino[9,8,7-cd]benzofuran]-17'-one