12-11-2012, 04:44 PM
TERPENES AND TERPENOIDS
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INTRODUCTION
Formation and Classification of Terpenolds
Condensation of isopentenyl pyrophosphate an d
dimethylallyl pyrophosphate give rise to the corresponding
esters of,geraniol, farnesol . geranylgeraniol,
and geranylfarnesol. These acyclic alcohols constitute
in a biogenetic sense the simplest representatives of
the mono-, sesqui-, di-, and sesterterpenoids . Squalene.
derived from two farnesyl pyrophosphate units,
and lycopersene. formed by coupling of two geranylgeranyl
pyrophosphate entities, may similarly be
regarded as the simplest members of the triterpenoids
and carotenoids . Although these groups provide a
natural framework for this chapter, the fact that the
initial reaction products usually undergo further
transformations necessitates a more detailed classification
. Thus we have adopted the systems of naming
and numbering terpenoids outlined in Rodd's Chemistry
of Carbon Compounds (I) .
Identification and Structure Elucidatio n
The use of mass spectrometry for the purpose
of identification and structural elucidation of natural
products including terpenoids is outlined elsewhere
in this book . It is worth noting, however, that terpenoids
usually lack strong fragmentation-directing
groups and only in favorable cases such as those of
aromatic diterpenes (cf. p. I 1 1) and carotcnoids
(cf. p. I1 1 ) may complete structure assignments be
made by mass spectrometry alone . Much additional
information about the structure of a compound can
often be achieved, on submicrogram quantities . by
exchange of active hydrogen atoms for deuterium (7) .
A pertinent example is the location of the oxo group
in ketomanoyl oxide (I) : the molecular ion peak is
shifted by four units on base-catalyzed deuterium
exchange, and only position 2 in the molecule allows
such exchange of four hydrogens (8) .
Alcohols and Carboxylic Esters
The presence of oxygenated groups provides an
additional directing influence on the fragmentation
sequences and the spectra are often more complex
than those of the corresponding hydrocarbons .
Molecular ions of alcohols are generally of low
intensity and may elude detection . In such cases the
presence of a hydroxyl group may be demonstrated
by the co-occurrence of M - 15 and M - 18 fragments .
The outstanding feature of ester spectra is the ready
elimination of the carboxylic acid moiety .
SESQUITERPENOIDS
This group encompasses a large number of compounds
that exhibit extensive structural diversity,
and consequently it is almost impossible to make
generalizations concerning modes of decomposition
upon electron bombardment. This problem is enhanced
by the fact that comparatively few labeling
studies have been performed . Comprehensive studies
have been made on simple members of this class and
in particular on the ionones and related model compounds
(26, 27).
[attachment=39127]
INTRODUCTION
Formation and Classification of Terpenolds
Condensation of isopentenyl pyrophosphate an d
dimethylallyl pyrophosphate give rise to the corresponding
esters of,geraniol, farnesol . geranylgeraniol,
and geranylfarnesol. These acyclic alcohols constitute
in a biogenetic sense the simplest representatives of
the mono-, sesqui-, di-, and sesterterpenoids . Squalene.
derived from two farnesyl pyrophosphate units,
and lycopersene. formed by coupling of two geranylgeranyl
pyrophosphate entities, may similarly be
regarded as the simplest members of the triterpenoids
and carotenoids . Although these groups provide a
natural framework for this chapter, the fact that the
initial reaction products usually undergo further
transformations necessitates a more detailed classification
. Thus we have adopted the systems of naming
and numbering terpenoids outlined in Rodd's Chemistry
of Carbon Compounds (I) .
Identification and Structure Elucidatio n
The use of mass spectrometry for the purpose
of identification and structural elucidation of natural
products including terpenoids is outlined elsewhere
in this book . It is worth noting, however, that terpenoids
usually lack strong fragmentation-directing
groups and only in favorable cases such as those of
aromatic diterpenes (cf. p. I 1 1) and carotcnoids
(cf. p. I1 1 ) may complete structure assignments be
made by mass spectrometry alone . Much additional
information about the structure of a compound can
often be achieved, on submicrogram quantities . by
exchange of active hydrogen atoms for deuterium (7) .
A pertinent example is the location of the oxo group
in ketomanoyl oxide (I) : the molecular ion peak is
shifted by four units on base-catalyzed deuterium
exchange, and only position 2 in the molecule allows
such exchange of four hydrogens (8) .
Alcohols and Carboxylic Esters
The presence of oxygenated groups provides an
additional directing influence on the fragmentation
sequences and the spectra are often more complex
than those of the corresponding hydrocarbons .
Molecular ions of alcohols are generally of low
intensity and may elude detection . In such cases the
presence of a hydroxyl group may be demonstrated
by the co-occurrence of M - 15 and M - 18 fragments .
The outstanding feature of ester spectra is the ready
elimination of the carboxylic acid moiety .
SESQUITERPENOIDS
This group encompasses a large number of compounds
that exhibit extensive structural diversity,
and consequently it is almost impossible to make
generalizations concerning modes of decomposition
upon electron bombardment. This problem is enhanced
by the fact that comparatively few labeling
studies have been performed . Comprehensive studies
have been made on simple members of this class and
in particular on the ionones and related model compounds
(26, 27).