40
1. Isoprenoids
of compounds, most of which consist of a central branched nonaene chain
of 22 carbon atoms with various nine-carbon moieties at each end of the
central chain. Or, from an isoprenoid point of view, one can describe the
carotenoids as compounds that are constructed of two diterpenoid chains
connected tail to tail, with both chains being made up of four isoprene units
joined head to tail. Since carotenoids possess these common structural
features and since these compounds, unlike many other terpenoids, have
biochemical significance that is understood, carotenoid chemistry represents
a well-defined area of terpenoid chemistry that is intensively studied by
relatively few research groups and the literature of which is characterized by
timely reviews. A recent general review of the field is that by Davis (1968).
Isler's (1971) comprehensive monograph is the standard reference for all
aspects of carotenoid research.
The carotenoids are among the most widely distributed naturally occurring
organic pigments, generally yellow or red in color, depending on the length
and stereochemistry of the conjugated polyene chain. In spite of the relative
structural uniformity of the carotenoids mentioned earlier, their isolation and
structural elucidation have been formidable tasks: They occur in low concentrations, usually 0.02-0.1% of dry weight of extracted material, and
normally as mixtures; they are sensitive to light, heat, and air; and they must
be isolated from fresh material.
In a recent progress report on carotenoid chemistry Liaaen-Jensen (1971)
plotted the number of known naturally occurring carotenoids versus the
year beginning at 1900; the resulting curve is virtually smooth and exponential. The steepest slope of the curve was reached less than ten years ago,
approximately coinciding with the wide use of mass spectrometry. Not
surprisingly, the rapid discovery of novel structural features has taken
place only during the past few years since the full development of many
physical methods of structure elucidation. At the time of Liaaen-Jensen's
review (1971) about three hundred natural carotenoids were recognized,
about two thirds of them with correct structures assigned.
Carotenoids are generally subdivided (Davis, 1968) into the hydrocarbons
or carotenes; the xanthophylls, carotenoids that possess oxygen functions
other than carboxyl groups ; the natural xanthophyll esters, often of palmitic
acid; and the carotenoid acids. An additional group of compounds that
possess fewer than forty carbon atoms and are considered degraded carotenoids are referred to as the apocarotenoids. And finally, there have been
isolated a number of carotenoids with one or two extra isoprene units, but
so far all of these have been found in microorganisms. Our discussion will
follow this generally accepted outline; there is, however, no need from a
structural viewpoint to treat the xanthophyll esters apart from the xanthophylls.
1. Isoprenoids
of compounds, most of which consist of a central branched nonaene chain
of 22 carbon atoms with various nine-carbon moieties at each end of the
central chain. Or, from an isoprenoid point of view, one can describe the
carotenoids as compounds that are constructed of two diterpenoid chains
connected tail to tail, with both chains being made up of four isoprene units
joined head to tail. Since carotenoids possess these common structural
features and since these compounds, unlike many other terpenoids, have
biochemical significance that is understood, carotenoid chemistry represents
a well-defined area of terpenoid chemistry that is intensively studied by
relatively few research groups and the literature of which is characterized by
timely reviews. A recent general review of the field is that by Davis (1968).
Isler's (1971) comprehensive monograph is the standard reference for all
aspects of carotenoid research.
The carotenoids are among the most widely distributed naturally occurring
organic pigments, generally yellow or red in color, depending on the length
and stereochemistry of the conjugated polyene chain. In spite of the relative
structural uniformity of the carotenoids mentioned earlier, their isolation and
structural elucidation have been formidable tasks: They occur in low concentrations, usually 0.02-0.1% of dry weight of extracted material, and
normally as mixtures; they are sensitive to light, heat, and air; and they must
be isolated from fresh material.
In a recent progress report on carotenoid chemistry Liaaen-Jensen (1971)
plotted the number of known naturally occurring carotenoids versus the
year beginning at 1900; the resulting curve is virtually smooth and exponential. The steepest slope of the curve was reached less than ten years ago,
approximately coinciding with the wide use of mass spectrometry. Not
surprisingly, the rapid discovery of novel structural features has taken
place only during the past few years since the full development of many
physical methods of structure elucidation. At the time of Liaaen-Jensen's
review (1971) about three hundred natural carotenoids were recognized,
about two thirds of them with correct structures assigned.
Carotenoids are generally subdivided (Davis, 1968) into the hydrocarbons
or carotenes; the xanthophylls, carotenoids that possess oxygen functions
other than carboxyl groups ; the natural xanthophyll esters, often of palmitic
acid; and the carotenoid acids. An additional group of compounds that
possess fewer than forty carbon atoms and are considered degraded carotenoids are referred to as the apocarotenoids. And finally, there have been
isolated a number of carotenoids with one or two extra isoprene units, but
so far all of these have been found in microorganisms. Our discussion will
follow this generally accepted outline; there is, however, no need from a
structural viewpoint to treat the xanthophyll esters apart from the xanthophylls.