102
3. Benzenoids
discussion of the six classic echinoid pigments, the compounds that have
been characterized in this most recent phase of this research, since the mid1960's, will be described more briefly whenever a new compound presents
only a minor structural variation. Characterization of new compounds in this
group was aided greatly by a series of studies that were carried out by
Scheuer and his collaborators (Moore and Scheuer, 1966; Moore et al,
1966b; Becher et al, 1966; Piette et al, 1967; Moore et al, 1967; Singh et al
1968b). These studies were designed to correlate various physical and
chemical parameters that had become useful in the course of the spinochrome
research.
c. The Naphthazarins
Naphthazarin (16) has in recent years been the subject of a great deal
of structural scrutiny. The molecule is generally represented as 5,8-dihydroxy1,4-naphthoquinone 16a or 16b.* The tautomer 16c (4,8-dihydroxy-l,5-naphthalenedione), however, appeared to be the correct representation in the
crystalline state (Pascard-Billy, 1962). As a result of more refined threedimensional data (Cradwick and Hall, 1971) dissymmetry of the phenolic
hydrogen atoms is indicated and a representation such as 16a is favored. In
solution, naphthazarin is probably best formulated as the tautomers 16a,b
and not by the fully delocalized structure 16d since inter alia a dipole moment
of 1.1 D is reported for naphthazarin in dilute benzene solution (Dumas et al,
1969). A rapid tautomeric equilibrium (16a,b) is supported by the fact that at
temperatures from ambient to — 60° only a single nmr signal is observed for
the nuclear protons (Chang, 1964a; Brockmann and Zeeck, 1968).
For substituted naphthazarins in solution Moore and Scheuer (1966)
have shown that the nature of the substituents determines which of the
rings will have predominant quinoidal character. An X-ray crystallographic
study (Fehlmann and Niggli, 1965) of the plant pigment cordeauxiaquinone
(2-acetyl-7-hydroxy-6-methoxy-3-methylnaphthazarin) is in accord with the
X-ray work on naphthazarin (16), which showed that 16c is the predominant
tautomer in the crystalline state.
The naphthazarins (16) because of their symmetry present fewer structural
problems than do the juglones (15). Of the six classic echinoid pigments five
are derivatives of naphthazarin—trihydroxy- (24), tetrahydroxy- (17), a
monoacetyldihydroxy- (20), monoacetyltrihydroxy- (14), and ethyltrihydroxy(11) naphthazarin. It is not surprising then that among the newly isolated
echinoid pigments we find monohydroxynaphthazarin (naphthopurpurin), 40
and 2,7-dihydroxy- naphthazarin, (41). Naphthopurpurin (40) had long been
known as a synthetic compound, while 2,7-dihydroxynaphthazarin (41) had
been described under the trivial name mompain following its isolation from
the microorganism Helicobasidium mompa (Nishikawa, 1962; Natori et al,
* For structures 16a-d, see page 96.
3. Benzenoids
discussion of the six classic echinoid pigments, the compounds that have
been characterized in this most recent phase of this research, since the mid1960's, will be described more briefly whenever a new compound presents
only a minor structural variation. Characterization of new compounds in this
group was aided greatly by a series of studies that were carried out by
Scheuer and his collaborators (Moore and Scheuer, 1966; Moore et al,
1966b; Becher et al, 1966; Piette et al, 1967; Moore et al, 1967; Singh et al
1968b). These studies were designed to correlate various physical and
chemical parameters that had become useful in the course of the spinochrome
research.
c. The Naphthazarins
Naphthazarin (16) has in recent years been the subject of a great deal
of structural scrutiny. The molecule is generally represented as 5,8-dihydroxy1,4-naphthoquinone 16a or 16b.* The tautomer 16c (4,8-dihydroxy-l,5-naphthalenedione), however, appeared to be the correct representation in the
crystalline state (Pascard-Billy, 1962). As a result of more refined threedimensional data (Cradwick and Hall, 1971) dissymmetry of the phenolic
hydrogen atoms is indicated and a representation such as 16a is favored. In
solution, naphthazarin is probably best formulated as the tautomers 16a,b
and not by the fully delocalized structure 16d since inter alia a dipole moment
of 1.1 D is reported for naphthazarin in dilute benzene solution (Dumas et al,
1969). A rapid tautomeric equilibrium (16a,b) is supported by the fact that at
temperatures from ambient to — 60° only a single nmr signal is observed for
the nuclear protons (Chang, 1964a; Brockmann and Zeeck, 1968).
For substituted naphthazarins in solution Moore and Scheuer (1966)
have shown that the nature of the substituents determines which of the
rings will have predominant quinoidal character. An X-ray crystallographic
study (Fehlmann and Niggli, 1965) of the plant pigment cordeauxiaquinone
(2-acetyl-7-hydroxy-6-methoxy-3-methylnaphthazarin) is in accord with the
X-ray work on naphthazarin (16), which showed that 16c is the predominant
tautomer in the crystalline state.
The naphthazarins (16) because of their symmetry present fewer structural
problems than do the juglones (15). Of the six classic echinoid pigments five
are derivatives of naphthazarin—trihydroxy- (24), tetrahydroxy- (17), a
monoacetyldihydroxy- (20), monoacetyltrihydroxy- (14), and ethyltrihydroxy(11) naphthazarin. It is not surprising then that among the newly isolated
echinoid pigments we find monohydroxynaphthazarin (naphthopurpurin), 40
and 2,7-dihydroxy- naphthazarin, (41). Naphthopurpurin (40) had long been
known as a synthetic compound, while 2,7-dihydroxynaphthazarin (41) had
been described under the trivial name mompain following its isolation from
the microorganism Helicobasidium mompa (Nishikawa, 1962; Natori et al,
* For structures 16a-d, see page 96.