Aldehydes Alizarine

Prenyl-naphthohydroquinone-2-carboxyhc acid is the key intermediate in the formation of anthraquinones of the alizarin type. (In contrast the A-ring-substituted anthraquinone derivatives found in fungi and certain higher plants, e.g., Rhamnaceae and Polygonaceae, are polyketides, D 3.3.5). In the biosynthesis of alizarin one C-atom of the isoprene residue is lost. In structurally related compounds, however, this C-atom may still be present in the form of a methyl, hydroxymethyl, aldehyde, or carboxy group. 

Nitrophenols and nitro-acids, (yellow to orange) salicyl-aldehyde (yellow) derivatives of alizarin JV-bromosucdn-imide phthaleins. 

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