mass spectrometry was performed using a vg model 7070f mass spectrometer the m e values for the principal fragments were 321 303 261 257 237 219 209 191 179 167 145 127 113 111 95 and 87 25 cyclobutyl avermectin a2 r 1 oh r h r 3 ch 3 the combined fractions 5 to 9 from example 8 were evaporated to dryness and the residue was rechromatographed twice on a c18 zorbax ods trademark dupont column 21 mm u00d7 25 cm eluting with a methanol and water mixture 77 23 at a flow rate of 9 ml per minute suitable fractions were combined and evaporated to yield the compound of formula i wherein r is h r 1 is oh r 2 is cyclobutyl r 3 is ch 3 and r 4 is 4 39 alpha l oleandrosyl l oleandrosyloxy as a white powder m p 135 u00b0 140 u00b0 c the structure of the product was confirmed by mass spectrometry as follows fast atom bombardment mass spectrometry was performed on a vg model 7070e mass spectrometer using a sample matrix of triethylene glycol with solid sodium chloride m na observed at m e 925 theoretical 925 electron impact mass spectrometry was performed using a vg model 7070f mass spectrometer the m e values for the.