especially preferred), trialkylsilyl, arylalkyl, arylalkenyl or arylalkynyl; wherein R6 is selected from hydrogen, chlorine, amino, alkyl or alkoxy (with hydrogen, methyl or methoxy being especially preferred); wherein R7 is absent or selected from hydrogen or methyl; and wherein R9 and R9a are each independently selected from hydrogen, lower alkyl, alkoxy or aryloxy.