. "A preferred class of photoremovable protecting groups has the general formula: where R1, R2, R3 and R4 independently are a hydrogen atom, a lower alkyl, aryl, benzyl, halogen, hydroxyl, alkoxyl, thiol, thioether, amino, nitro, carboxyl, formate, formamido or phosphido group, or adjacent substituents (i.e., R1-R2, R2-R3, R3-R4) are substituted oxygen groups that together form a cyclic acetal or ket" . . . . .