100 SOME MISCELLANEOUS UNSAPONIFIABLE LIPIDS 



However, none of the natural naphthoquinones have the oxygenation pattern that would be 

 predicted by either of the condensations shown. If a polyacetate pathway is followed in 

 nature, loss and migration of hydroxyl groups must occur at some stage in the process. 

 In spite of this discrepancy, and for want of any better theory, natural naphthoquinones 

 are included in this chapter (except for vitamin K which because of its isoprenoid nature 

 is found in Chapter 8). Ideas on the biosynthesis of naphthoquinones are reviewed by 

 Neelakantan and Seshadri (24). 



A few examples of naphthoquinones are given in Table 3 with their natural occurrence. 



All of these quinones are crystalline materials ranging in color from yellow to red 

 and easily soluble in such organic solvents as benzene. Some of them are toxic and anti- 

 microbial; plants containing them have been used as drugs and poisons since prehistoric 

 times (e. g. chimaphilin, plumbagin, eleutherin). Others have been equally important 

 as dyestuffs. Lawsone is the chief ingredient of henna; lapachol is extracted from various 

 woods and used for dyeing cotton; alkannin is the coloring matter obtained by alkaline 

 treatment of the root of Alkanna tinctoria (dyer's bugloss). At least a few of these com- 

 pounds do not exist as such in plants but are formed during the extraction process. Thus 

 the native form of alkannin is an ester of angelic acid with the hydroxyl group of the side 

 chain; juglone is formed by hydrolysis and oxidation of l-hydrojuglone-4-/3-D-glucoside. 

 Plumbagin is formed by hydrolysis and oxidation of dianellin, a yellow naphthol glycoside 

 of Dianella laevis (25). Although we have grouped these napthoquinones as unsaponifiable 

 lipids, they should not be subjected to the procedure of saponification, since treatment 

 with alkali in the presence of air frequently brings about oxidative decomposition. 



The naphthoquinones may be extracted from plant tissues with benzene or other non- 

 polar solvents. The 1, 4 -quinones are often steam distillable and may be removed from 

 many other lipids by this procedure. Another property which may be used in their separa- 

 tion from other lipids is their solubility in weakly basic aqueous solutions such as sodium 

 carbonate or bicarbonate. 1, 2-quinones are not steam distillable, but are soluble in solu- 

 tions of sodium bisulfite. Final purification may be achieved with chromatography on in- 

 activated alumina or weaker adsorbents. 



The properties of naphthoquinones used in their isolation may also be put to good 

 use in their characterization. Thus, a steam distillable, yellow-red solid which is soluble 

 in benzene or sodium carbonate solution but insoluble in water is very likely to be a 1, 4- 

 naphthoquinone. Additional indications are given by color reactions and spectra. 1, 4- 

 naphthoquinones give yellow solutions in benzene, changing to red in alkali. 1, 2-quinones 

 are usually red rather than yellow when crystalline or dissolved in benzene; in alkali they 

 become blue -violet. If a double bond in a side chain of a 1, 4-quinone is conjugated with 

 bonds in the quinone ring, the color reactions shown are like those of 1, 2-quinones. 

 Other characteristic color reactions are given with concentrated sulfuric acid. Measure- 

 ment of absorption spectra shows maxima at about 250 m^ for 1, 4 -quinones and one or 

 more longer wavelength bands depending on what substituents are present. The basic 

 nucleus absorbs at about 330 m^ . When oxygen substitution is present there are other 

 maxima toward the red, sometimes nearly to 600 m\i; and the 330 maximum may not be 

 apparent. 1, 2 -naphthoquinone has the same ultraviolet absorption bands as the 1, 4-qui- 

 none but additional bands at about 400 and 530 mjix. As with the 1, 4-quinones, the positions 

 of all bands except the lowest are greatly influenced by oxygen substitution on the rings. 

 Characterization of 1, 4 -naphthoquinones is discussed by Sawicki and Elbert (25). If hy- 

 droxyl groups are not present at C-2, the naphthoquinones react with o-aminothiophenol 

 to yield red-blue colors. If C-2 hydroxyl groups are present, there is no reaction with 

 this reagent, but a color reaction is given with o-phenylenediamine. 



