248
structures despite this common biosynthetic origin. The hybrid phenylpropanoid/
polyketide pathway also produces another important class of polyphenols, the polyhydroxystilbenes. The trans-resveratrol is an archetypal member of this family. The
biosynthetic route to flavonoids is suspected to form condensed tannins (proanthocyanidins) and theatannins with flavanols as precursors, but the condensation and
polymerization phases are not yet fully elucidated (Quideau et al. 2011; BelščakCvitanović et al. 2018). Hydrolyzable tannins are produced from gallic acid or
hexahydroxydiphenic acid (Quideau et al. 2011). In algae, phlorotannins are essentially derived from oligomerization of phloroglucinol (dehydrogenative coupling).
In addition to this already large chemical diversity, polyphenols could be associated
with various carbohydrates and/or organic acids. Thus, many polyphenols exist as
glycosides with different sugar units, sometimes acylated sugars, at various positions onto the polyphenolic backbone.
Such a diversity and wide distribution of polyphenols have led to different ways
of classification, according to their chemical structure, natural distribution, source,
or biological functions (Belščak-Cvitanović et al. 2018). As chemists, we were logically more interested by the chemical classification. The most popular one follows
the chemical structure of aglycone polyphenols. However, this could lead to several
different ways, depending on consideration of the skeleton (giving 16 major classes)
or on consideration of the number of phenyl rings along with their structural connection to each other. This latter classification leads generally to five major classes
for plant polyphenols: phenolic acids, flavonoids, stilbenes, lignans, and others
(Manach et al. 2004; Belščak-Cvitanović et al. 2018). Note that this classification
includes monophenolic acids that cannot be really considered as true polyphenols
while the “others” category involves polymers like condensed or hydrolyzable tannins. On the other hand, phlorotannins are systematically classified into six subclasses: phlorethols, fucols, fuhalols, fucophlorethols, isofuhalols, and eckols
(Wang et al. 2014). Fucols incorporate phloroglucinol units that are linked with C-C
(aryl-aryl) bonds and have linear and branched structure. In phlorethols, the phloroglucinol units are only coupled with aryl-ether bonds (C-O-C). Fucophlorethols are
hybrid structures having both aryl-aryl and aryl-ether linkages. Fuhalols contain
only ether bonds, arranged in a regular sequence of para- and ortho-bridges and
exhibit additional hydroxyl groups on some units. Eckols exhibit at least one threering moiety with a dibenzo-1,4-dioxin element substituted by a phenoxy group at
C4-position. Eckols are quite specific, being isolated from some specific genera of
brown algae (Ecklonia, Eisenia, and Alarieae) (Wang et al. 2014). Similarly, isofuhalols are a specialized group isolated from Chorda filum and are generally of low
molecular weight (Wang et al. 2014). Moreover, phlorotannins can be also sulfated,
and such sulfated polyphenols are widely distributed among brown algae.
Chlorinated or bromated phlorethols and fucophlorethols have been also detected,
but the origin of some of these halogenated polyphenols is suspected to be an artifact (Wang et al. 2014). By taking into account the generally admitted chemical
classification, we have finally summarized the classification of natural polyphenols
from algae or plants in some large classes (Scheme 7.1).
L. Percevault et al.
structures despite this common biosynthetic origin. The hybrid phenylpropanoid/
polyketide pathway also produces another important class of polyphenols, the polyhydroxystilbenes. The trans-resveratrol is an archetypal member of this family. The
biosynthetic route to flavonoids is suspected to form condensed tannins (proanthocyanidins) and theatannins with flavanols as precursors, but the condensation and
polymerization phases are not yet fully elucidated (Quideau et al. 2011; BelščakCvitanović et al. 2018). Hydrolyzable tannins are produced from gallic acid or
hexahydroxydiphenic acid (Quideau et al. 2011). In algae, phlorotannins are essentially derived from oligomerization of phloroglucinol (dehydrogenative coupling).
In addition to this already large chemical diversity, polyphenols could be associated
with various carbohydrates and/or organic acids. Thus, many polyphenols exist as
glycosides with different sugar units, sometimes acylated sugars, at various positions onto the polyphenolic backbone.
Such a diversity and wide distribution of polyphenols have led to different ways
of classification, according to their chemical structure, natural distribution, source,
or biological functions (Belščak-Cvitanović et al. 2018). As chemists, we were logically more interested by the chemical classification. The most popular one follows
the chemical structure of aglycone polyphenols. However, this could lead to several
different ways, depending on consideration of the skeleton (giving 16 major classes)
or on consideration of the number of phenyl rings along with their structural connection to each other. This latter classification leads generally to five major classes
for plant polyphenols: phenolic acids, flavonoids, stilbenes, lignans, and others
(Manach et al. 2004; Belščak-Cvitanović et al. 2018). Note that this classification
includes monophenolic acids that cannot be really considered as true polyphenols
while the “others” category involves polymers like condensed or hydrolyzable tannins. On the other hand, phlorotannins are systematically classified into six subclasses: phlorethols, fucols, fuhalols, fucophlorethols, isofuhalols, and eckols
(Wang et al. 2014). Fucols incorporate phloroglucinol units that are linked with C-C
(aryl-aryl) bonds and have linear and branched structure. In phlorethols, the phloroglucinol units are only coupled with aryl-ether bonds (C-O-C). Fucophlorethols are
hybrid structures having both aryl-aryl and aryl-ether linkages. Fuhalols contain
only ether bonds, arranged in a regular sequence of para- and ortho-bridges and
exhibit additional hydroxyl groups on some units. Eckols exhibit at least one threering moiety with a dibenzo-1,4-dioxin element substituted by a phenoxy group at
C4-position. Eckols are quite specific, being isolated from some specific genera of
brown algae (Ecklonia, Eisenia, and Alarieae) (Wang et al. 2014). Similarly, isofuhalols are a specialized group isolated from Chorda filum and are generally of low
molecular weight (Wang et al. 2014). Moreover, phlorotannins can be also sulfated,
and such sulfated polyphenols are widely distributed among brown algae.
Chlorinated or bromated phlorethols and fucophlorethols have been also detected,
but the origin of some of these halogenated polyphenols is suspected to be an artifact (Wang et al. 2014). By taking into account the generally admitted chemical
classification, we have finally summarized the classification of natural polyphenols
from algae or plants in some large classes (Scheme 7.1).
L. Percevault et al.