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Revised structures for three isoetin glycosides, yellow flower pigments in Heywoodiella oligocephala
Phyrochemisrry, Vol. 30, No. 5, pp. 1677-1678, Printed in Great Britain.
1991
0031 9422/91$3.00+0.00 Pcrgamon Press plc
REVISED STRUCTURES FOR THREE ISOETIN GLYCOSIDES, YELLOW FLOWER PIGMENTS IN HEYWOODZELLA OLIGOCEPHALA JEFFREY B. HARBORNE Department of Botany, University of Reading, Reading, RG6 2AS, U.K. (Received 14 November 1990)
Key Word Index-Heywoodielh 7-glycosides.
Abstract-Re-examination
oligocephala;Compositae; flavones; yellow pigments; spectral properties; isoetin
of the UV spectral properties
of three isoetin ‘2’gfycosides isolated earlier from are, in fact, the ?-glucoside, 7-arabinoside and 7xylosylarabinosylglucoside respectively. Substitution of sugar at the 2’- or S-position of isoetin causes an unexpectedly large hypsochromic shift (- 12 nm) in the neutral spectrum. It presumably produces a distortion in the planar flavone structure comparable to that recorded for 3,2’-dioxygen substitution in the flavonol series. Heywoodiella oligocephala flowers shows that these compounds
lNTRODUCTlON
The first report of the natural occurrence of the flavone isoetin with its rare B-ring substitution pattern was from Isoetes delilei (Isoetaceae) in the Lycopsida [I]. Its structure as 5,7,2’,4’,5’-pentahydroxyflavone was established by synthesis, but although it was shown to occur probably as the corresponding 5’-glucoside, the position of attachment of the glucose was not fully established. Later the same aglycone was discovered to occur in the flowering plants in the tribe Cichorieae of the Compositae, in Hieracium, Hispidella and Heywoodiella flowers. In Hispidella hispanica, the glycoside of isoetin was identified as the ‘I-glucoside, whereas in Heywoodiella oligocephala, the three glucosides were reported to be the 2’-arabinoside, 2’-glucoside and 2’-xylosylarabinosylglucoside. The position of attachment of the stlgars was based on complete methylation and hydrolysis and UV spectral analysis of the resulting monohydroxytetramethoxyflavones [2]. In 1988, Marco et al. [3] reported the presence of isoetin 5’-glucoside from a different tribe (the Anthemideae) of the Compositae, from Artemisia hispanica. This structural elucidation was based on ‘H and “CNMR spectral analysis. Very recently, Gluchoff-Fiasson et al. [4] have found four isoetin glycosides in leaves of several Hypochoeris species and as a result of UV and NMR spectral analyses concluded that they are the 2’-xyloside, the 7-glucoside-2’-xyloside and their 4”-acetates respectively. They drew our attention to the discrepancy in UV spectral properties between the Heywoodiella 2’-glycosides and the 2’-xyloside in Hypochoeris. Re-examination of the evidence for the 2’-glycoside structures in Heywoodiella was therefore called for and it is now clear, as described here, that they must be reformulated as the 7-rather than as the 2’-glycosides. RESULTS
Comparison
of the neutral spectra of all the isoetin
glycosides so far described (Table I) shows that the location of the sugar on the A- or B-ring can be determined simply by the spectral shift compared with isoetin itself. It is clear that substitution of the sugar in the B-ring produces a profound hypsochromic shift; this differs from what happens in the luteolin (5,7,3’,4’-tetrahydroxyflavone) series, where the 7- and 3’-glucosides have nearly identical neutral spectra [S]. This shift is - 12 nm and is comparable to the shifts (AA- 16 nm) recorded by Fang and Mabry [6] for 3,2’-dioxygenated Bavonols. Presumably, substitution of the bulky sugar at either the 2’or S-positions is sufficient to hinder free rotation of the Bring, even in the absence of 3-hydroxylation, causing the otherwise planar molecule to be distorted. 3,2’Dioxygenation in the flavonol series also alters the relative intensities of the two main UV absorption bands, so that the long wave band is weaker than the short wave band. The relative intensities of these bands in flavonols with 3,2’-dioxygenation average about 2.70, while the normal value is 0.97. The value recorded for isoetin 2’xyloside is 1.40 [4]. The data in Table 1 indicate that isoetin 7-glycosides, by contrast, are spectrally identical to isoetin, not only in the position of the long wave band but also in the number and position of other maxima. Therefore, the ‘2’-glycosides’ of Heywoodiella must in fact be ‘I-glycosides. These glycosides were all unstable in the presence of sodium acetate, which made it impossible at the time to determine whether the 7-hydroxyl was free or substituted. This instability to sodium acetate would appear to be another distinctive property of ‘I-glycosides, since the 2’-xyloside is reported to give a stable spectral shift with this reagent c41. While most of the data recorded earlier for the Heywoodiella glycosides [2] agree with this reformulation, there is still a discrepancy in the R, values of the 7glucoside reported in Hispidella and the 7-glucoside (originally the 2’-glucoside) of Heywoodiella, but the simplest explanation for this is that the latter sample was not entirely pure. 1677
J. B. HARBORNE
1678 Table
Source
I. Spectral
properties
in methanol
Flavonoid Glycosides
Hispidella Heywoodiello Hppochoeris Artemisia
Isorres .._ *Shift in neutral
Neutral maxima
‘I-glucoside ‘2’-glycosides’ 2’-xyloside 5’-glucoside 5’-glucoside (‘?) lsoetin long wavelength
of isoetin and its glycosides
258, 258, 260, 256, 256, 256, maximum
the ‘2’~glycosides’ in Heywoodiella oligoare hereby reformulated as the 7-glucoside, 7arabinoside and 7-xylosylarabinosylglucoside. The latter two glycosides are still only known from this one source. This reformulation is much more logical for substances which contribute to yellow flower colour in this plant, since glycosylation in the A-ring, rather than in the B-ring, does not lead to a large hypsochromic shift lo shorter wavelengths.
spectral (nm)
266, 288, 265, 287, -, 290, -. -, 288, -. 265, 288, 264, 288. compared
AA*
306, 379 308, 375 362 362 -, 362 306, 374 to the aglycone.
REFERENCES
In summary,
cephala
Acknowledyement-The author is grateful to Dr Fiasson communicating his findings in advance of publication.
for
+5 +I -I2 -12 -12
1. Voirin, B., Jay, M. and Hauteville,
M. (1975) Phytochemistry
14, 257. 2. Harbome,
J. B. (1978) Phytochemisrry 17. 915. I. A.. Barbera. O., Rodriguez, S., Domingo, C. and Adell, J. (1988) Phyrochemistry 27, 3155. 4. GluchotT-Fiasson, K., Favre-Bouvin, J. and F&son, J. L. (1991) Phyrochemistry 30. 1673. 5. Harborne, J. B. (1967) Comparative Biochemistry of the Flauonoids. Academic Press, London. 6. Fang, N. and Mabry. T. J. (1989) in Studies in Natural 3. Marco,
Products
Chemistry
(Atta-ur-Rahman.
Volume
5, Srrucrure
ed.), pp. 673-694.
EIucidorion
(Part
Elsevier. Amsterdam.
B)
1991
0031 9422/91$3.00+0.00 Pcrgamon Press plc
REVISED STRUCTURES FOR THREE ISOETIN GLYCOSIDES, YELLOW FLOWER PIGMENTS IN HEYWOODZELLA OLIGOCEPHALA JEFFREY B. HARBORNE Department of Botany, University of Reading, Reading, RG6 2AS, U.K. (Received 14 November 1990)
Key Word Index-Heywoodielh 7-glycosides.
Abstract-Re-examination
oligocephala;Compositae; flavones; yellow pigments; spectral properties; isoetin
of the UV spectral properties
of three isoetin ‘2’gfycosides isolated earlier from are, in fact, the ?-glucoside, 7-arabinoside and 7xylosylarabinosylglucoside respectively. Substitution of sugar at the 2’- or S-position of isoetin causes an unexpectedly large hypsochromic shift (- 12 nm) in the neutral spectrum. It presumably produces a distortion in the planar flavone structure comparable to that recorded for 3,2’-dioxygen substitution in the flavonol series. Heywoodiella oligocephala flowers shows that these compounds
lNTRODUCTlON
The first report of the natural occurrence of the flavone isoetin with its rare B-ring substitution pattern was from Isoetes delilei (Isoetaceae) in the Lycopsida [I]. Its structure as 5,7,2’,4’,5’-pentahydroxyflavone was established by synthesis, but although it was shown to occur probably as the corresponding 5’-glucoside, the position of attachment of the glucose was not fully established. Later the same aglycone was discovered to occur in the flowering plants in the tribe Cichorieae of the Compositae, in Hieracium, Hispidella and Heywoodiella flowers. In Hispidella hispanica, the glycoside of isoetin was identified as the ‘I-glucoside, whereas in Heywoodiella oligocephala, the three glucosides were reported to be the 2’-arabinoside, 2’-glucoside and 2’-xylosylarabinosylglucoside. The position of attachment of the stlgars was based on complete methylation and hydrolysis and UV spectral analysis of the resulting monohydroxytetramethoxyflavones [2]. In 1988, Marco et al. [3] reported the presence of isoetin 5’-glucoside from a different tribe (the Anthemideae) of the Compositae, from Artemisia hispanica. This structural elucidation was based on ‘H and “CNMR spectral analysis. Very recently, Gluchoff-Fiasson et al. [4] have found four isoetin glycosides in leaves of several Hypochoeris species and as a result of UV and NMR spectral analyses concluded that they are the 2’-xyloside, the 7-glucoside-2’-xyloside and their 4”-acetates respectively. They drew our attention to the discrepancy in UV spectral properties between the Heywoodiella 2’-glycosides and the 2’-xyloside in Hypochoeris. Re-examination of the evidence for the 2’-glycoside structures in Heywoodiella was therefore called for and it is now clear, as described here, that they must be reformulated as the 7-rather than as the 2’-glycosides. RESULTS
Comparison
of the neutral spectra of all the isoetin
glycosides so far described (Table I) shows that the location of the sugar on the A- or B-ring can be determined simply by the spectral shift compared with isoetin itself. It is clear that substitution of the sugar in the B-ring produces a profound hypsochromic shift; this differs from what happens in the luteolin (5,7,3’,4’-tetrahydroxyflavone) series, where the 7- and 3’-glucosides have nearly identical neutral spectra [S]. This shift is - 12 nm and is comparable to the shifts (AA- 16 nm) recorded by Fang and Mabry [6] for 3,2’-dioxygenated Bavonols. Presumably, substitution of the bulky sugar at either the 2’or S-positions is sufficient to hinder free rotation of the Bring, even in the absence of 3-hydroxylation, causing the otherwise planar molecule to be distorted. 3,2’Dioxygenation in the flavonol series also alters the relative intensities of the two main UV absorption bands, so that the long wave band is weaker than the short wave band. The relative intensities of these bands in flavonols with 3,2’-dioxygenation average about 2.70, while the normal value is 0.97. The value recorded for isoetin 2’xyloside is 1.40 [4]. The data in Table 1 indicate that isoetin 7-glycosides, by contrast, are spectrally identical to isoetin, not only in the position of the long wave band but also in the number and position of other maxima. Therefore, the ‘2’-glycosides’ of Heywoodiella must in fact be ‘I-glycosides. These glycosides were all unstable in the presence of sodium acetate, which made it impossible at the time to determine whether the 7-hydroxyl was free or substituted. This instability to sodium acetate would appear to be another distinctive property of ‘I-glycosides, since the 2’-xyloside is reported to give a stable spectral shift with this reagent c41. While most of the data recorded earlier for the Heywoodiella glycosides [2] agree with this reformulation, there is still a discrepancy in the R, values of the 7glucoside reported in Hispidella and the 7-glucoside (originally the 2’-glucoside) of Heywoodiella, but the simplest explanation for this is that the latter sample was not entirely pure. 1677
J. B. HARBORNE
1678 Table
Source
I. Spectral
properties
in methanol
Flavonoid Glycosides
Hispidella Heywoodiello Hppochoeris Artemisia
Isorres .._ *Shift in neutral
Neutral maxima
‘I-glucoside ‘2’-glycosides’ 2’-xyloside 5’-glucoside 5’-glucoside (‘?) lsoetin long wavelength
of isoetin and its glycosides
258, 258, 260, 256, 256, 256, maximum
the ‘2’~glycosides’ in Heywoodiella oligoare hereby reformulated as the 7-glucoside, 7arabinoside and 7-xylosylarabinosylglucoside. The latter two glycosides are still only known from this one source. This reformulation is much more logical for substances which contribute to yellow flower colour in this plant, since glycosylation in the A-ring, rather than in the B-ring, does not lead to a large hypsochromic shift lo shorter wavelengths.
spectral (nm)
266, 288, 265, 287, -, 290, -. -, 288, -. 265, 288, 264, 288. compared
AA*
306, 379 308, 375 362 362 -, 362 306, 374 to the aglycone.
REFERENCES
In summary,
cephala
Acknowledyement-The author is grateful to Dr Fiasson communicating his findings in advance of publication.
for
+5 +I -I2 -12 -12
1. Voirin, B., Jay, M. and Hauteville,
M. (1975) Phytochemistry
14, 257. 2. Harbome,
J. B. (1978) Phytochemisrry 17. 915. I. A.. Barbera. O., Rodriguez, S., Domingo, C. and Adell, J. (1988) Phyrochemistry 27, 3155. 4. GluchotT-Fiasson, K., Favre-Bouvin, J. and F&son, J. L. (1991) Phyrochemistry 30. 1673. 5. Harborne, J. B. (1967) Comparative Biochemistry of the Flauonoids. Academic Press, London. 6. Fang, N. and Mabry. T. J. (1989) in Studies in Natural 3. Marco,
Products
Chemistry
(Atta-ur-Rahman.
Volume
5, Srrucrure
ed.), pp. 673-694.
EIucidorion
(Part
Elsevier. Amsterdam.
B)