Complex sentence: Phenylalanine and tyrosine are the precusors used in the phenylpropanoids biosynthesis, then to produce the flavonoids, coumarins, tannins and lignin.
Split into simpler sentences: Phenylalanine and tyrosine are the precusors used in the phenylpropanoids biosynthesis. The phenylpropanoids are then used to produce the flavonoids, coumarins, tannins and lignin.