Muhammad Shaiq Ali et al., Jour. of Sci. Res. i n Phar. 2012, 1(2), 1-5
J
ournal of
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cientific
R
esearch in
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harmacy
Review Article
Available online thr oug h
ISSN: 2277-9469
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A Bird’s-Eye view on Chemistry of Marine Algae from Karachi Coasts of North Arabian Sea
(
Pakistan
)
Muhammad Shaiq Ali*
H. E. J. Research Institute of Chemistry, International Center for Chemical and Biological Sciences, University of Karachi, Karachi-75270, Pakistan
Received on: 04-05-2012; Revised on: 04-05-2012; Accepted on: 09-05-2012
ABS TRACT
During the search of new marine natural products, we have chemically examined a number of seaweeds from various coastal areas of Karachi (Pakistan) bordering the North Arabian Sea. In order to highlight the significance of marine chemistry, phycochemical analysis of a green alga (Codium iyengarii), a brown alga (Dictyota dichotoma) and a red alga (Laurencia pinnatifida) which are the true representatives of Phyla Chlorophyta, Phaeophyta and Rhodoophyta, respectively, is briefly discussed in this article. Codium iyengarii (green alga) afforded a steroid (iyengadione) and three steroidal glycosides (iyengaroside-A, B and clerosterol galactoside). Dictyota dichotoma (brown alga) yielded three ring-A hydroxylated dolastane-diterpenoids (dichototetraol, dichotopentaol and 1(15)-dolastane-4, 8, 9, 14-tetraol), three enone moiety containing dolastanes (dichotenone-A, B and loliolide), three seco-dolastanes (dichotone, dichotodione and isolinearol acetate) and three C -16 oxidized seco-dolastanes (dichotenol-A, B and C). Eight sesquiterpenoids belonging to the chamigrane series have been obtained from Laurencia pinnatifida (red alga). Most of the obtained metabolites have been reported for the first time by us and their structures were elucidated by means of spectroscopic techniques including 2D NMR. Their brief spectral data are presented in the discussion section.
Keywords: North Arabian Sea, Karachi coasts, Marine algae, Red algae, Brown algae Green algae, Phycochemical analysis.
INTRODUCTION
About 70.8 % of the earth’s surface is occupied by seas and oceans and Pakistan has a coastline of a bout 885 km bordering the North Arabian Sea. Locations of cities and coastal areas of Pakistan are shown in fig. 1. These include: Jiwani, Gawadar, Pasni, Ormara, Karachi and Choohar Jamali.
Fig. 1 Cities and Coastal Areas of Pakistan (North Arabian Sea) The coastal areas of Kara chi from where algae may be collection include: Sand dunes, Clifton, Keamari, Manora, Sandspit, Hawks Bay, Buleji, Paradise Point, Cape Monze and Gadani. P.L. Anand was the first phycologist who thoroughly investigated the algal vegetation of Karachi. Then, M. Nizamuddin extended this work in Pakistan and produced many phycologists including M. Shameel who had compiled for the first time a check -list of marine algae of Pakistan, in1990 [1].
*Corresponding author:
Muhammad Shaiq AliH. E. J. Research Institute of Chemistry,
International Center for Chemical and biological Sciences, University of Karachi, Karachi-75270, Pakistan. Telephone: 92-21-992661768
Fax: 92-21-34819018
*E-mail: [email protected]
In the last few decades, re search on chemistry of seaweed (or more generally marine organisms) has experienced a tremendous increase, due to the need of compounds possessing bioactivities of potential pharmaceutical applications. Until now, a varie ty of species has been assayed for their activity and a number of biodynamic molecules including anthelmintic compounds [2], feeding deterrents [3], inhibitors of
mitosis [4], ichthyotoxic compounds [5], cytotoxic, antibacterial [6] and
antiviral agents [7] have been isolated having unique skeleta different
from those of terrestrial plants [8].
In Pakistan, research in the field of marine natural product chemistry has been carried out to a limited extent. The fact th at Pakistan has a long coast line and a rich marine flora and fauna has motivated us to start our research in this direction. Our results of chemical analysis of
Codium iyengarii (green alga), Dictyota dichotoma (brown alga) and
Laurencia pinnatifida ( red alga) are briefly presented in this chapter. Most of the obtained metabolites have been reported by us for the first time and their structures were elucidated with the aid of spectroscopic techniques including 2D NMR. Spectral data of i solated meta bolites are briefly described in the discussion section.
RESULTS AND DISCUSSION
Codium iyengariiBørg. (Green Alga):
Members of the marine green algae of the genus Codium are well known for anticancer activity [9]. The genus Codium is comprised of
more than 40 species while only five species are found in Pakistan. These include: C. coronatum, C. divaricatum, C. dwarkense, C. flabellatum and C. iyengarii. We have investigated C. iyengari collected in the month of March from Karachi coast (Bulejii) resulted the isolation of a new ste roid (iyengadione) and two new steroidal glycosides ( iyengaroside-A & B) along with clerosterol galactoside.
Iyengadione (1):
The EIMS spectrum of 1 showed molecular ion peak at m/z
424, which was further, confirmed through FDMS. Molecular formula C29H44O2 for 1 was established by HR-EIMS showing the molecular ion at m/z 424.33272. 1H NMR spectrum displayed five methyl signals, three of
which appeared as singlets at 0.70 (Me-19), 1.15 (Me-18) and 1.55 (Me-26), a secondary methyl resonated at 0.92 (Me-21) and a primary methyl at 0.86 (Me-29) as a triplet. An olefinic proton (H-6) appeared at 6.15 as a singlet. The downfield shift of this signal was due to the enone function. The terminal olefinic protons appeared at 4.72 and 4.63 (each broad singlet). The 13C NMR spectrum showed altogether 29
methylenes, seven methines and the remaining six signals in the broad -band spectrum were due to the quaternary carbons. Out of six quaternary carbons, two at 204.4 and 200.3 were attributed to the carbonyl carbons [10, 11]. The obtained information helped to esta blish the
structure of discussed compound as 1. This is a new addition in the natural steroids named iyengadione [12].
Iyengaroside-A (2):
The molecular mass was determined by the molecular ion at
m/z 572 was determined through positive and negative FAB mass spectra which showed peaks at m/z 573 [M+H]+ and 571 [M-H]-,
respectively. 1H and 13C NMR data of 2 was almost similar to that of 1
except some additional signals due to the sugar moiety. Compound 2 was absorbing in the UV region a t 240 nm. A broad singlet appeared at 4.26 due to H-7. The broadness of this signal instead of multiplet revealed the
-configuration of proton (H-7). This information (through molecular model) helped to place the sugar with -configuration. In conjugated 4
steroids, C-5 often appears at 170.0 1.5[12] and shifts upfield if
hydroxyl function is present at C-6 (due to -effect). In compound 2, C-5 resonated at 174.2 inferred that no oxygenated function was present at C-6. These facts helped to place the double bond between C -4–C-5 and sugar unit at C-7. Resonance of anomeric proton at 4.35 and carbon at 101.5 revealed the presence of a sugar unit. The 1H NMR of 2 displayed
an extra methyl doublet at 0.90 and the corresponding carbon resonated at 18.1. The chemical shift of this methyl (C-1`) and the coupling constant of the anomeric proton confirmed the identification of sugar unit as fucose. Ultimately, the structure of 2 was established as 7-O--D-fucosyl-3-oxo-4-en clerosterol and named iyengaroside-A [12]. This
is also another new addition in the natural steroidal glycosides.
Iyengaroside-B (3):
The 1H NMR of 3 was almost similar to that of 2. It was
showing two extra doublets of methyls instead of one (as was observed in 2) at 0.85 & 0.83 and a doublet of methylene at 2.34. The significant point was noted that the methylene of sugar unit was resonating at 4.47-4.27 as a multiplet, whereas, it normally appears at
3.20. The only reason for the downfield shift of this signal was due to the natural esterification of C-6` hydroxyl function. Presence of the ester function in the molecule was confirmed through 13C NMR and IR
spectroscopy. In 13C NMR a quaternary carbon was found to resonating
at 175.6 which was due to the ester carbonyl carbon showing in the IR spectrum at 1730 cm-1. Other than the usual signals of clerosterol
skeleton and sugar unit , 13C NMR of 3 displayed five extra carbon signals
which were distinguished through DEPT experiments as a quaternary (
175.6), one methylene ( 41.7), one methine ( 29.1) and two methyls ( 22.9, 21.4) carbons. These signals could only be due to an isoprene uni t, which was coupled in the form of ester function at C-6 of the sugar unit. The HMBC experiments helped to confirm the structure of 3. This is another new addition in the constituents of C. iyengarii containing prenylated sugar and named iyengaroside-B [12].
Clerosterol galactoside (4):
Galactoside of clerosterol (4) was also obtained from C. iyengarii [12]. The structure of 4 was elucidated with the help of
comparative literature data [10, 13]. Dictyota dichotomaLamour. (Brown Alga):
In Pakistan, the brown algal genus Dictyota represents by ten species. These include: D. bartayresiana, D. cervicornis, D. ceylanica, D. ciliate, D. dichotoma, D. divaricata, D. dumosa, D. hauckiana, D. indica and
D. maxima. We have investigated Dictyota dichotoma due to its availability in bulk. It was collected in March from Buliji near Karachi coast. The obtained results include: three ring-A hydroxylated dolastane-diterpenoids (dichototetraol, dichotopentaol and 1(15)-dolastane-4, 8, 9, 14-tetraol), three enone moiety containing dolastanes (dichotenone-A, B and loliolide), three seco-dolastanes (dichotone, dichotodione and isolinearol acetate) and three C-16 oxidized seco-dolastanes (dichotenol-A, B and C).
Dichototetraol (5):
A peak at m/z 320 in the field desorption (FD) mass spectrum appeared after the loss of a wa ter molecule and the formula linked with this peak was determined through HRMS as C20H32O3 showing the five
degrees of unsaturation. Thus the actual formula would be C20H34O4 with
four degrees of unsaturation.The 1H NMR spectrum of 5 displayed two
secondary methyls associated with an isopropyl moiety at 0.98 (J = 7.0 Hz) and 1.04 (J = 6.8Hz). Other two quaternary methyl singlets for Me-16 and Me-20 resonated at 1.00 and 1.20, respectively. Two singlets at 5.05 and 5.08 were found to be connected in the HMQC experiments with the carbon at 113.5 attested for H-15A/B and C-15. A carbinylic proton and a carbon appeared at 4.34 (J = 3.0, 2.8 Hz) and 75.9 assigned to H-2 and C-H-2, respectively. Presence of hydroxyl function at C-H-2 was depicted
via HMBC connectivities, whereas the stereochemistry as axial and was concluded on the basis of magnitude of coupling constant (J = 3.0 and 2.8 Hz) [14]. Three hydroxyl containing quaternary carbons showed their
resonance frequencies at 73.5, 80.4 and 76.0 assigned to 8, 9 and C-14, respectively. The isopropyl methyls appeared at 18.7. Signals at 17.8 and 21.9 were assigned to the two quaternary methyls Me-16 and Me-20, respectively. Finally, structure was assigned as 5 named dichototetraol [15]. This is a new entry in diterpenoids of Dictyota
belonging to the dolastane class.
1(15) -Dolastene-4, 8, 9, 14-tetraol (6):
This compound was previously isolated from D. cerrvicornis
and reported by Kelecom and Teixeira [16]. Dichotopentaol (7):
In comparison with compound 5, NMR and mass spectra of dichotopentaol (7) showed the disappearance of a quaternary me thyl and appearance of a -CH2OH moiety in the molecule. The molecular mass of 7
as 318 a.m.u. with the removal of two water molecules from molecular ion peak (exact molecular mass was 354 a.m.u.) and formula C20H32O4 with
the loss of a water molecule from molecular ion peak(exact molecular formula was C20H34O5) were determined with the aid of FDMS and HRMS,
respectively. The typical methyls of dolastane skeleton appeared at 0.80
R2 R1
OH
OH
R3
OH
R1 R2 R3
OH H CH3
H OH CH3
H OH CH2OH
1 2
3
4 5
6
8 9
10 12 13
14 15
16
17
18 19 20
7 11
Dichototetraol
1(15)-Dolastene- 4,8,9,14-tetraol Dichotopentaol
(5) (6) (7)
O
O-Fuc 2 O
O 1
1 2
3 4 5
6
7
8 9 10
11 12
13
14 15
16 17 18
19
20 21
22
23 24
25 26
27
Gal-O 4
O
O OH
OH OH
O O
(d, J = 6.9 Hz, Me-18), 0.84 (d, J = 6.8 Hz, Me-19) and at 1.03 (s, Me-20) except a singlet of Me -16. However, an extra set of doublets with the coupling constant of 11.5 Hz was due to the –CH2OH moiety confirming
the oxidation of missing methyl (Me-16) as –CH2OH moiety. The carbon
due to this newly appeared moiety displayed its resonance frequency at
60.3 in the 13C NMR spectrum. Signals of olefinic protons and carbons
exhibited in the NMR spectra at 4.60 (H-15A), 4.66 (H-15B) as singlets, 108.6 (C-15) and 151.9 (C-1). The three quaternary hydroxyl containing carbons resonated at 74.4(C-8), 76.4(C-9) and 78.2 (C-14). Compound 7
is another new addition in the secondary metabolites extracted from D. dichotoma and it was named dichotopentaol (7) [15].
Dichotenone-A (8)
The diterpenic nature of dichotenone-A was stated by two secondary methyls at 1.01(d, J = 7.0 Hz, Me-18), 1.00 (d, J = 6.9 Hz, Me-19) due to an isopropyl moiety and two tertiary methyls at 0.55 (Me-16), 1.46 (Me-20) together with a converted methyl (Me-15) as an exo -cyclic double bond at 4.60 (s, H-15A) and 4.76( s, H-15B) in the proton NMR spectrum. Further, the presence of twenty carbonsin the formula (C20H30O4)of molecular ion peak (depicted from HREI-MS) showing six
degrees of unsaturation. The IR spectrum of 8 displayed absorption bands at 3373, 1674 and 1600 cm-1 due to hydroxyl, , -unsaturated
ketone and olefinic functions, respectively. The existence of an enone function was further supported via UV spectrum, which showed absorption at 241.7 nm (log = 6.7). The exo and endo-cyclic double bonds displayed their signals in the carbon spectrum at 109.2 (C-15), 151.2 (C-1) and 176.6 (C-8), 145.9 (C-9), respectively. The , -unsaturated ketone appeared in the broad band spectrum at 209.4. The two secondary and two tertiary methyls exhibited their signals in the carbon spectrum at 19.7 18), 19.4 19), 17.2 16) and 25.7 (C-20). On the basis of above spectral informa tion and arguments, the discussed compound was characterized as 8 and named dichotenone-A
[17].
Dichotenone-B (9)
The mass and NMR spectra of 9 suggest that a tertiary methyl has converted into a –CH2OAc moiety. The molecular ion peak was
determined through FDMS at m/z 392 and the formula associated with this peak was C22H32O6 (obtained from HRMS) suggesting the presence of
seven degrees of unsaturaton. In comparison of 9, the NMR spectra showed additional signals for –CH2OAc moiety. Instead a methyl singlet, 1H NMR spectrum exhibited a pair of doublets with the coupling constant
12.1 Hz at 3.37 and 3.74 due to H-16A and B. The carbon signal associated with these doublets resonated at 63.7. The methyl protons of acetoxyl and its carbons exhibited their resonances in the 1H and 13C
NMR spectra at 2.07 and 20.1 (CH3), 170.7 (CH3CO), respectively. This
new compound was named dichotenone-B (9)[17]. Loliolide (10)
A momoterpene, loliolide (10) was also isolated from the same source (D. dichotoma). This has never been isolated so far from any algal source. However, it was reported by Okada et al. in 1994 from a terrestrial plant Eucommia ulmoides [18].
Dichotone (11):
Presence of hydroxyl, olefinic and ketonic functions in 11
were attested by their signals at 3400, 1648 and 1708 cm-1, respectively,
in the IR spectrum. The molecular mass was determined through field desorption (FD) spectrum as 336 a.m.u. The first fragment appeared in the EI and HR spectra in the same manner at m/z 318 due to the loss of water molecule from the molecular ion peak. The formula associated with this peak (m/z 318) was determined via HRMS as C20H30O3
confirming the presence of six degrees of unsaturation in the fragment appeared after the removal of a water molecule revealed that 11 should have five degrees of unsaturation. The 1H NMR spectrum of 11 displayed
two secondary methyls associated with isopropyl moiety at 1.01, 1.02 (J = 6.9Hz) and two tertiary methyls a t 0.56, 0.91 due to 20 and Me-16, respectively. Signals in the 1H NMR spectrum a t 4.72 and 4.86 as
singlets were assigned to exo-cyclic double bond derived from Me-15 in
seco-dolastane skeleton. A characteri stic carbon resonance of seco -dolastane skeleton having an oxygen bridge across C-8 and C-14 appeared at 105.5 due to the C-8. The above spectral information had led to the conclusion that the discussed compound having structure 11
and was named dichotone. This is again a new entry in the list of diterpenoids obtained from D. dichotoma [19].
Dichotodione (12):
In comparison with compound 11, 13C NMR of dichotodione
(12) had an extra signal at 211.8 attributable to a ketone and simultaneously with the absence of CH carbon and proton signals in the NMR spectra. The molecular mass of 12 was determined through FDMS as 334 a.m.u. and the formula C20H30O4 of this mass was obtained from
HREI-MS (m/z 334.21444). A broad absorption at 1707 cm-1 in the IR
spectrum was due to the ketone functions. Broadness of this band was due to the merger of two ketone functions in the molecule. Compound 12
(fig.10) had usual two secondary [ 1.08 (d, J = 6.9 Hz, H-18), 1.09 (d, J = 6.9, H-19) ; 18.3 (C-18 & C-19)] and two tertiary methyls [ 0.85 (H-16); 21.5 (C-16) & 0.99 (H-20); 22.9 (C-20)] with olefinic signals [ 4.93 (s, H-15A), 5.06 (s, H-15B); 110.9 (C-15) and 145.1 (C-1)]. The characteristic signal of C-8 resonated at 105.1, whereas the signal of other carbon associated with the oxygen bridge was found at 85.1 (C-14) in the spectrum. Signals at 211.8 and 214.5 in the broad band spectrum were assigned to two ketone functions due to C-4 and C-9, respectively. The structure was thus concluded as 12 which is a new entry in the family of secondary metabolite s of the genus Dictyota and was named dichotodione [19].
Isolinearol acetate (13):
This compound has been already isolated from D. cervicornis
by Teixeira and Tomassini [20].
Dichotenol-A (14):
In addition to hydroxyl and olefinic functions, the presence of ketone and ester carbonyl groupswere revealed by their absorption at 3444, 1649, 1712 and 1741 cm-1, respectively, in the IR spectrum of 14.
O
OH OH
OH
R
(8) R = CH3 (dichotenone-A) (9) R = CH2OAc (dichotenone-B)
O
O
R
1OH
R
2R1 R2
H OH
H OAc H Ketone
(Dichotone) (Dichotodione) (Isolinearol acetate)
1 2
3 4
5
6 7 8
9
10 11 12 13
14 15
17
18 19 20
16
(11) (12) (13)
O
R4 R5
R2
R1
O
R3
20
R1 R2 R3 R4 R5
H H OH
OAc OAc OH
H H
OH OH
H OH
H
(dichotenol-A) (dichotenol-B) (dichotenol-C) O
(14) (15) (16)
O O
HO
1 2 3
4 5 6
7
8
9
10 11
The molecular mass was determined through field desorption (FD) spectrum as 378 a.m.u. and the formula (C22H34O5) associated with this
peak (at m/z 378) was determined through HR-EIMS revealing the presence of six degrees of unsaturation in the molecule. The 1H NMR
spectrum of 14 displayed two secondary methyls of isopropyl moiety at
1.08 and 1.09 (J = 6.9 Hz), a methyl assigned to acetoxyl unit at 2.01 along with another methyl singlet at 0.98 assigned to H-20. However, instead of a signal for Me-16, a set of doublets (J =11.3 Hz) at 3.58 and 4.04 appeared and were assigned to H-16A & B. To decide which methyl (Me-16 or Me-20) had undergone oxidation, a clear-cut distinction could be made via HMBC experiments depicted that the –CH2OAc moiety must
be attached at C-5 and not to C-12 and thus the oxidized methyl was Me-16 not Me-20. A characteristic carbon resonance of a seco-dolastane skeleton containing an oxygen bridge across C-8 and C-14 appeared at 105 assigned to C-8. The other significant signals in the carbon spectrum were at 147 (C-1) and 108.9 (C-15) associated with olefinic function, 66.3 (C-16) due to methyl oxidation (Me-16), 20.8 (CH3CO) and 171.1
(CH3CO) related to the acetoxyl moiety at C-16, and 214.5 (C-9) assigned
to ketonic function. The presence of an exo-cyclic double bond, a ketonic function and an acetoxyl moiety revealed that 14 had seco-dolastane skeleton with an oxygen bridge to balance the six degrees of unsaturation. Thus the compound was characterized as 14 and named dichotenol-A. This new addition in the metabolites of Dictyota can also be classified as a
seco-dolastane diterpenoid [21]. Dichotenol-B (15):
In comparison with compound 14, dichotenol-B had only an extra hydroxyl function which was confirmed through FDMS (m/z 394), HR-EIMS (376.23147, C22H32O5, M+-H2O), signal at 73.7 (C-13) in 13C
NMR and a broad singlet at 3.89 due to the H-13 in the proton NMR spectrum. However, the justification of position of hydroxyl function at C -13 and streochemistry as was based on HMBC connectivities plus n.O.e. experiments. This compound is another new addition in the seco -dolastanes of Dictyota named dichotenol-B (15) [21].
Dichotenol-C (16):
Compound 16 having the same skeleton as described above. The molecular formula of 16 obtained from HR-EIMS after re moval of a water molecule (C20H28O4) suggested the absence of acetoxyl moiety at
C-16. This was further supported by the absence of ester carbonyl absorption in IR, a cetoxyl me thyl signal in the proton NMR and the methyl plus an ester carbonyl resonances in the 13C NMR spectra. Al though, the
acetoxyl moiety was missing in the molecule, a set of doublets with same coupling constants (J = 11.5 Hz) at 3.06 and 3.50 in the 1H NMR
spectrum was still observed, indicating the presence of a hydroxyl function instead of an acetoxyl moiety at the same carbon (C -16). Indications for the presence of hydroxyl at C-13 were not observed in 16. However, a downfield CH proton signal appeared at 3.73 as a broad singlet assigned to and equatorially oriented H-4. Hence, the stereochemistry of hydroxyl function to this carbon should be axial and [20]. As far as position of C-4 hydroxyl is concerned, it was again depicted with the aid of HMBC connectivities. The upfield signal of one proton at 2.77 as a double-doublet (J = 3.6, 1.0 Hz) was due to the H-7. The low chemical shift of this signal suggested the only possibility of an epoxy ring across C-7 and C-8.The magnitude of coupling constant of this signal as well as the molecular models suggested the orientation of epoxy-ring as O-7 - O-8. Hence, the structure was elucidated as 16. This new addition among the natural products from D. dichotoma is named dichotenol-C (16) [21].
Laurencia pinnatifida (Huds.) Lamour. (Red Alga):
Most of the algal research effort todate has been concentra ted on the constituents of red algae from Mediterranean Sea. Therefore, it is interesting to investigate the chemical constituents of red algae from Arabian Sea. The rhodophytes (red algae) comprising about 4000 species. Among the red algae, the genus Laurencia is represented in Pakistan by only six species. These include: L. filiformis, L.hypnoides, L. obtuse, L. pinnatifida, L. platyclada and L. virgate. Our chemical investigation was carried out on Laurencia pinnatifida collected from Buleji near Karachi coast. Laurencia is the only genus among all red algal genera which is investigated all over the world due to its halogenated metabolites. Eight sesquiterpenoids belonging to the chamigrane series have been isolated.
4, 10-Dibromo-3-chloro-7, 8-epoxy--chamigrene (17):
The trihalo-compound 17 was isolated from the chloroform extract of L. Pinnatifida, which was eluted with hexane:ether (17:3) in crystalline form, M.P. 142 0C. Previously this compound has been isolated
from unidentified species of Laurencia [22] but the 13C-NMR data were not
being reported. The structure of -epoxy-chamigrene [23] (17) was
elucidated by comparing with the reported data [22].
4, 10-Dibromo-3-chloro-7, 8-epoxy--chamigrene(18):
The -epoxy compound [24] (18) was isolated from the
ethylacetate part of me thanolic extract. It was eluted with hexane:benzene (19:1) and further purified by preparative layer chromatography. This compound has been isolated only once before from L. akamurai [25] but 13C-NMR data were not reported. The same
compound was also synthesized by Howard and Fenical [22]. This
compound is an isomer of compound 17.
1, 4, 10-Tribromo-3-chloro-7 (14)-en--chamigrene(19): The tribromo chamigrene 19 was isolated from the chloroform extract as a gum [24]. It was eluted with hexane:ether (3:1)
and the final purification was performed by preparative layer chromatography. This tetrahalo-chamigrene derivative is a new addition in Laurencia metabolites having an exocyclic double bond at C-7, instead of the epoxy ring between C-7 and C-8. These vinylic protons appeared at
5.05 and 5.69 as a pair of broad dublets in the 1H-NMR spectrum.
Biogenetically the third bromine atom could be placed at C-1 position.
Pinnatifenol(20):
Another new halogenated sesquiterpenoide has been isolated from the ethyl acetate extract named pinnatifenol(20) [26]. The presence
of two halogen atoms in the molecule was confirmed by the appearance of the isotopic peaks in the EIMS of 20 at m/z 350 (M+2) and 352 (M+4). The presence of bis-equatorial dihalides at C-3 and C-4 was confirmed by comparing the reported 13C-NMR data of several halo-chamigrenes
isolated from genus Laurencia. The hydroxyl group was appeared in the IR spectrum at 3400 cm-1 which was further confirmed by making the
acetate derivative of 20. The two AB-X syste ms (C-1and C-4) found in the molecule were differentiated by hetero-nuclear 2D-NMR (hetero-COSY) experiments. The carbinylic and vinylic protons were appeared in proton NMR spectrum at 4.50(m) and 5.07(d, J = 2.3 Hz), 5.64 (d, J = 2.3 Hz), respectively. Pinnatifenol is a new addition in the Laurencia metabolites having an ether bridge between C-1 and C-10. The bridge containing carbons were resonated at 86.0 and 77.99, respectively.
Pinnatazane(21):
Pinnatazane [27] was obtained from hexane extract of L. pinnatifida a s needles, M.P. 190-192 oC. There were no absorptions found
for hydroxyl or carbonylic functionalities in the IR spectrum of 21. The HR-EIMS showed the molecular formula C15H22O2BrCl with four degrees
of unsaturation, which showed the two oxygen atoms in the molecule which were engaged in the form of ether linkages. The structure of 21
was elucidated with the aid of 2D-NMR spectroscopy and finally confirmed by single crystal X-ray diffraction te chnique. This compound is a new addition in the sesquiterpenoids from Laurencia having two ether bridges.
Pinnatifidone(22):
Pinnatifdone was isolated as semi-crystalline compound from the ethyl acetate extract [28]. The molecular formula, obtained from mass
spectrum as well as the signals in the 1H and 13C-NMR spectra indicated O
Br
Br Cl
7 8
1
2 3
4 5 6
9 10
11 12 13
14 15
17
O Br
BrCl
18
O OH
Cl Br
20
Cl Br Br
Br
19
Cl
Br O
O
21
O Br
Cl O
that 22 belongs to the chamigrane class of sesquiterpene with an ether bridge between C-1 and C-10. In addition to this, a ketonic function was found instead of the epoxy ring between C-7 and C-8, appeared at 212.4 in 13C-NMR spectrum. The position of ketonic function was confirmed
with the help of homodecoupling experiments. The 1H NMR spectrum
showed four methyl signals, out of which one secondary methyl doublet appeared at 1.34 (J = 6.4 Hz). This doublet must be due to the methyl attached at C-7 and the -orientation was concluded by n.O.e. experiments. The absolute configuration of the molecule was depicted with the help of CD technique. The magnitude of the cotton effect clearly rules out twist conformation of the cyclohexanone ring. According to the octant rule, the CD within the n transition of the carbonyl group will be + ve regardless of the details of cyclohexanone conformation if the absolute configuration is that as given in structure 22.
Pinnatifinone(23):
Another halogenated sesquiterpenoid having enone function was isolated from ethylacetate fraction of L. pinnatifida and named as pinnatifinone [29] (23). The presence of two halogen atoms as confirmed
by EIMS of 23, which showed two isotopic peaks at m/z 350 (M +2) and 352 (M +4). The IR spectrum showed the hydroxyl absorption a t 3600 cm-1 and an absorption due to the , -unsaturated ketone at 1680 cm-1,
which was further supported by the UV spectrum which showed the band at 230 nm characteristic for the enone function. The ketonic and olefinic carbons appeared at 200.7, 157.0 (C-9) and 124.7 (C-10), respectively, in the carbon spectrum. The proton spectrum showed a methyl doublet at 1.27 (J =7.6 Hz) which must be due to the methyl adjacent to the ketonic function, this moiety could be derived from the oxidation of genuine epoxide situated at C-7 and C-8. Compound 23 is a new addition in the Laurencia metabolites having enone function.
Rearranged Chamigrene(24):
The chamigrene 24 was eluted with 12% ethylacetate in benzene from the silica gel column loaded by ethylacetate fraction of methanolic extract. It was further purified by flash chromatography. This rearranged chamigrene [26] in which one methyl has been migrated from
C-11 to C-10 appeared at 1.02 as a doublet (J = 6.5 Hz) in the proton NMR spectrum. The other methyl at C-11 has been converted into an exomethylene function which appeared at 4.88 and 4.98 as a pair of broad singlets in the 1H NMR spectrum. The chemical shifts of all the
signals appeared in carbon spectrum were exactly matched with reported data [30]. This compound has already been reported by Fenical [30] from an unidentified species of the genus Laurencia in which the
methyl migration was confirmed by the signals of secon dary methyl and
exo-methylene function in the NMR spectra.
CONCLUSION
It can be concluded that seaweeds are also a rich source of secondary metabolites like higher plants. However, proper biological screening is required to explore their utility in drug designing.
ACKNOWLEDGEMENTS:
Author is very much thankful to all who were involved directly or indirectly in the work presented in this chapter.
REFERENCES:
1. Shameel M. A Preliminary Check-list of Marine Algae from the Coast and Inshore Waters of Pakistan, 1990: Seaweed Biological Lab., A.H.Q. Biological Re search Center, University of Karachi, Pakistan.
2. Murakami S, Takemoto T, Shimizu Z, Daigo K. Jpn. J. Pharm. Chem., 1953: 25; pp 57.
3. Paul VJ, Sun HH, Fenical W. Phytochemistry,1982: 21; pp 468-469.
4. Sun HH, Ferrara NM, McConnell, OJ, Fenical W. Tetrahedron Lett., 1980: 21; pp 3123.
5. Gerwick WH, Fenical W. J. Org. Chem., 1981: 46; pp 2233-2241.
6. Hornsey IS, Hide D. Br. Phycol. J.,1974: J9; pp 353.
7. Starr TJ, Piferrer M, Kajima M. Tex. Rep. Biol. Med., 1966: 24; pp 208.
8. Faulkner DJ. Nat. Prod. Rep., 1996: 13; pp 75.
9. Sheu J, Liaw CC, Chang-yih D. J. Nat. Prod., 1995: 58; pp 1521. 10. Ahmed VU, Rahman A, Perveen S, Shameel M.
Phytochemistry,1993: 33; pp 1189-1192.
11. Ahmad VU, Memon AH, Ali MS, Perveen S, Shameel M.
Phytochemistry,1996: 42; pp 1141-1143.
12. Ali MS, Saleem M, Yamdagni R, Ali MA. Nat. Prod. Lett., 2002: 16; pp 407-413.
13. Ahmed VU, Rahman A, Perveen S, Shameel M. Phytochemistry,
1992: 31; pp 1429-1431.
14. Teixeira VL, Tomassini T, Kelecom A. Bull. Soc. Chim. Belg.,
1986: 95; pp 263.
15. Ali MS, Pervez M K. Nat. Prod. Res., 2003: 17; pp 281-286. 16. Kelecom A, Teixeira VL. Phytochemistry,1988: 27; pp
2907-2909.
17. Ali MS, Pervez MK, Saleem M, Ahmed F. Nat. Prod. Res., 2003: 17; pp 301-306.
18. Okada N, Shirata K, Niwano M, Kiroyuki H, Uramoto M.
Phytochemistry, 1994: 3; pp 281.
19. Ali MS, Pervez MK. Z. Naturforsch, 2003: 58b; pp 438-442. 20. Teixeira VL, Tomassini T. J. Nat. Prod., 1986: 49; pp 570. 21. Ali MS, Pervez, MK, Ahmed F, Saleem M. Nat. Prod. Res., 2004:
18; pp 543-549.
22. Howard BM, Fenical W. Tetrahedron Lett., 1975:pp 1687. 23. Bano S, Ali MS, AhmadVU. Planta Medica,1987: 53; pp 508. 24. Bano S, Ali MS, AhmadVU. Sci. Pharm., 1988: 56,;pp 125-127. 25. Ojika M, Shizuri Y, Yamada K. Phytochemistry,1982: 21; pp
2410.
26. Ahmad VU, AliMS. Phytochemistry,1991: 30; pp 4172-4174. 27. Atta-ur-Rahman, Ahmad VU, Bano S, Abbas SA, Alvi KA, Ali
MS, Lu H SM.Phytochemistry,1988: 27; pp 3879-3880. 28. Bano S, Ali MS, Ahamd VU. Z. Naturforsch., 1988: 43b; pp
1347-1350.
29. Ahmad VU, Ali MS. Sci. Pharm., 1991: 59; pp 243-246. 30. BittnerM, Silva VJ, FenicalW. Phytochemistry,1985: 24; pp
987.
Source of suppor t: Nil, Conflict of interest: None Declare d
O Br
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23
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