21b reacted selectively with the 2-acylamino-1,3-cyclopentanedione unit A of moenomycin A (1) to give the corresponding 2-alkylated products 14a, 14b, 22a, and 22b. These products, depending on the pH of the solution, were either stable under the reaction conditions or they underwent retro-Claisen-type reactions. The method can be used for the attachment of reporter groups to moenomycin A for the synthesis