3
5.10 Synthesis of 6-Hydroxyeuryopsin
216
5.11 Synthesis of 3β-Angeloyloxyfuranoeremophilane
217
5.12 Synthesis of Ligularone and Isoligularone
218
5.13 Synthesis of (−)-(R)-Ligularenolide and (−)-(R)-PF1092C
219
5.14 Synthesis of Nootkatone
220
5.15 Synthesis of Cacalol
220
5.16 Synthesis of Noreremophilanes
221
5.17 Synthesis of Bakkane-type Sesquiterpenoids
223
5.18 Synthesis of Bisabolane-type Sesquiterpenoids
226
5.19 Synthesis of Nelumol A
228
6 Biological Activities
229
References
230
1 Introduction
To understand diversification of secondary metabolites in plants is a major theme in
natural product chemistry. The genus Ligularia Cass., belonging to the family
Asteraceae tribe Senecioneae, is highly diversified in the Hengduan Mountains area
of China. More than a hundred species are recorded in the “Flora of China” [1, 2]
and the evolution and diversification is considered to be still ongoing [3]. Ligularia
species in this area occupy a great variety of habitats from streams to alpine meadows, ranging from 1000 to 5000 m in elevation [2]. Thus, Ligularia species in this
domain provide natural products scientists with very interesting materials for the
study of the diversity of their secondary metabolite profiles [4, 5].
Ligularia species have been studied with respect to secondary metabolites for a
long time, and many sesquiterpenoids have been isolated from them [6, 7]. It is well
known that Ligularia is a major source of eremophilane sesquiterpenoids, which have
also been isolated from other genera in the family Senecioneae, including Parasenecio
(Cacalia), Senecio, and Petasites [8–10]. Certain derivatives, such as rearranged- or
seco-compounds as well as dimers, have been recorded. Among various eremophilane
sesquiterpenoids, furanoeremophilanes and eremophilan- 12,8- olides constitute the
major class. During the 1960s and the 1970s, many furanoeremophilanes were isolated from roots of Japanese Ligularia species by the groups of Minato and Takahashi
[11, 12]. The structure of ligularol (= petasalbin) (161), the most commonly isolated
furanoeremophilane, was determined by Minato’s group. During this same period,
Bohlmann’s group also obtained a large number of eremophilanes and related compounds from European Ligularia species [13]. Many eremophilanes and other types
of sesquiterpenoids have been isolated and characterized from a number of other species in the family Senecioneae as well. Following these pioneering reports, especially
from around 2000 and over the last two decades, an abundance of related compounds
has been obtained from Chinese Ligularia species [14].
Over the last 20 years, the individual groups of the current authors have studied
the diversity of compounds present in the roots of Ligularia species growing in the
in northwestern Yunnan Province, western Sichuan Province, and, in part, in southern Qinghai and Gansu Provinces of (Hengduan Mointains area) (Plates 1 and 2). In
Chemical Constituents of Ligularia Species (Asteraceae) and Their Diversity…
5.10 Synthesis of 6-Hydroxyeuryopsin
216
5.11 Synthesis of 3β-Angeloyloxyfuranoeremophilane
217
5.12 Synthesis of Ligularone and Isoligularone
218
5.13 Synthesis of (−)-(R)-Ligularenolide and (−)-(R)-PF1092C
219
5.14 Synthesis of Nootkatone
220
5.15 Synthesis of Cacalol
220
5.16 Synthesis of Noreremophilanes
221
5.17 Synthesis of Bakkane-type Sesquiterpenoids
223
5.18 Synthesis of Bisabolane-type Sesquiterpenoids
226
5.19 Synthesis of Nelumol A
228
6 Biological Activities
229
References
230
1 Introduction
To understand diversification of secondary metabolites in plants is a major theme in
natural product chemistry. The genus Ligularia Cass., belonging to the family
Asteraceae tribe Senecioneae, is highly diversified in the Hengduan Mountains area
of China. More than a hundred species are recorded in the “Flora of China” [1, 2]
and the evolution and diversification is considered to be still ongoing [3]. Ligularia
species in this area occupy a great variety of habitats from streams to alpine meadows, ranging from 1000 to 5000 m in elevation [2]. Thus, Ligularia species in this
domain provide natural products scientists with very interesting materials for the
study of the diversity of their secondary metabolite profiles [4, 5].
Ligularia species have been studied with respect to secondary metabolites for a
long time, and many sesquiterpenoids have been isolated from them [6, 7]. It is well
known that Ligularia is a major source of eremophilane sesquiterpenoids, which have
also been isolated from other genera in the family Senecioneae, including Parasenecio
(Cacalia), Senecio, and Petasites [8–10]. Certain derivatives, such as rearranged- or
seco-compounds as well as dimers, have been recorded. Among various eremophilane
sesquiterpenoids, furanoeremophilanes and eremophilan- 12,8- olides constitute the
major class. During the 1960s and the 1970s, many furanoeremophilanes were isolated from roots of Japanese Ligularia species by the groups of Minato and Takahashi
[11, 12]. The structure of ligularol (= petasalbin) (161), the most commonly isolated
furanoeremophilane, was determined by Minato’s group. During this same period,
Bohlmann’s group also obtained a large number of eremophilanes and related compounds from European Ligularia species [13]. Many eremophilanes and other types
of sesquiterpenoids have been isolated and characterized from a number of other species in the family Senecioneae as well. Following these pioneering reports, especially
from around 2000 and over the last two decades, an abundance of related compounds
has been obtained from Chinese Ligularia species [14].
Over the last 20 years, the individual groups of the current authors have studied
the diversity of compounds present in the roots of Ligularia species growing in the
in northwestern Yunnan Province, western Sichuan Province, and, in part, in southern Qinghai and Gansu Provinces of (Hengduan Mointains area) (Plates 1 and 2). In
Chemical Constituents of Ligularia Species (Asteraceae) and Their Diversity…