The invention relates to penicillin derivatives of Formula I, and a method of synthesis; the derivatives having high antimicrobial activity against gram-positive and gram-negative microorganisms. ##STR1## R is selected from the group consisting of ##STR2## R.sub.1 is selected from the group consisting of hydrogen and a hydroxyl group;R.sub.2 is selected from the group consisting of hydrogen, an alkaline metal, and a carboxy protective group;R.sub.3 is selected from the group consisting of hydrogen, a lower alkyl, and a phenyl residue;R.sub.4, R.sub.5, and R.sub.6 are each selected from the group consisting of hydrogen, a halogen, a lower alkyd, and a lower alkoxy group;A is selected from the group consisting of oxygen and a N-(lower alkyl) residue; andn is 0 or 1.