Hydroxylation pattern anthraquinones

The symmetrical substimtion pattern in the foregoing starting anthraquinones greatly simplified the chemistry. The presence of but a single hydroxyl substituent... 

The basic anthraquinone structure is depicted in Fig. (6). In the literature two different methods are used for the numbering of the carbon atoms of the parent molecule (Fig. (6)). In this review the method that numbers the carbon atoms from 1 till 14 is chosen. The anthraquinones found in Rubia tinctorum differ in the nature of the substituents and the substitution pattern. These substituents are only found on carbon atoms 1, 2, 3 and 4. Hydroxyl and methoxyl groups are frequently encountered as substituent... 

Due to the laxative properties of A., a number of drugs, such as Radix Rhei, Cortex Frangulae, Fructus and Folia Sermae are used pharmaceutically. A few A., such as carminic acid and alizarin, are still used to some extent as dyes. [J.Schripsema D.Dagnino Anthraquinones Elucidation of the substitution pattern of 9,10-Anthraquinones through chemical shifts of peri-hydroxyl protons Phytochemistry 42 (1996) 177-184]... 

ScHRiPSEMA, J., and D. Dagnino Elucidation of the Substitution Pattern of 9,10-Anthraquinones Through the Chemical Shifts of pen-Hydroxyl Protons. Phyto-chem., 42, 177 (1996). 

Itokawa et al 74) reported the NMR spectra of some anthraquinones isolated from Rubia species. For one of the compounds a 2-methyl-1,3,6 (or 7)-trihydroxy-9,10-anthraquinone structure was assumed based on various spectral data. Eventually it was stated that the hydroxyl group was in the 6-position rather than in the 7-position because of a 0.6-0.8 ppm upfield shift observed for C-9 if compared with compounds having a 1,3-dihydroxy-2-hydroxymethyl substitution pattern. For a better insight in the validity of using NMR spectrometry for distinguishing 6/7 or 2/3 substitution in anthraquinones bearing several substituents, further NMR studies of such multiply-substituted anthraquinones seem desirable. 

---