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Ring-A Hydroxylated Dolastanesfrom the Marine Brown Alga DictyotaDichotoma (HUDS.) LamourMuhammad Shaiq Ali a & Muhammad Kashif Pervez aa H.E.J. Research Institute of Chemistry, University of Karachi,Karachi, 75270, Pakistan

Version of record first published: 27 Oct 2010.

To cite this article: Muhammad Shaiq Ali & Muhammad Kashif Pervez (2003): Ring-A HydroxylatedDolastanes from the Marine Brown Alga Dictyota Dichotoma (HUDS.) Lamour, Natural ProductResearch: Formerly Natural Product Letters, 17:4, 281-286

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Natural Product Research, 2003, Vol. 17, No. 4, August, pp. 281–286

RING-A HYDROXYLATED DOLASTANES FROM

THE MARINE BROWN ALGA DICTYOTADICHOTOMA (HUDS.) LAMOUR

MUHAMMAD SHAIQ ALI* and MUHAMMAD KASHIF PERVEZ

H.E.J. Research Institute of Chemistry, University of Karachi, Karachi-75270, Pakistan

(Received 14 September 2002; In final form 20 November 2002)

Marine brown alga Dictyota dichotoma (Dictyotaceae) collected from Karachi coast of Arabian Sea yieldedtwo new dolastane–diterpenoids named: dichototetraol (1) and dichotopentaol (3) along with 2 of the sameskeleton as a new source. Their structures have been characterized with the aid of 2D-NMR spectroscopictechniques.

Keywords: Dictyota dichotoma; Dictyotaceae; Brown alga; Arabian Sea; Karachi coast; Diterpenoids;Dolastanes; Dichototetraol and dichotopentaol; 2D-NMR

INTRODUCTION

Chemical studies have shown that marine algae produce a variety of unique secondarymetabolite, which are quite different from those of terrestrial plants [1]. Perhaps theyfunction as defensive compounds in the natural habitat [2] and possibly act as feedingdeterrents against herbivorous marine animals [3]. Members of the brown algal familyDictyotaceae have received considerable attention from marine natural productchemists in recent years as a fruitful source of novel secondary metabolites.Out of 18 genera of this family, eight occur at the coasts of Pakistan. Among them

Dictyota Lamour. occurs quite commonly. This is the most extensively studied genus ofthe family Dictyotaceae. Hundreds of diterpenoids belonging to more than 16 skeletalclasses have been isolated from several species [2,4]. The cosmopolitan genus Dictyotahas been shown to produce a number of bioactive bi- and tri-cyclic diterpenes [1,5,6].They include: anthelmintic compounds [7], feeding deterrents [8], inhibitors of mitosis[9], ichthyotoxic compounds [10], cytotoxic, antibacterial [11] and antiviral activitiescontaining compounds [12]. Most of them are based on the dolastane ring system[13,14].

Dictyota dichotoma (Hudson) Lamour. is the only species of the genus Dictyotareported in all the oceans. Studies on D. dichotoma have noted a wide range of variation

*Corresponding author.

ISSN 1478-6419 print: ISSN 1029-2349 � 2003 Taylor & Francis Ltd

DOI: 10.1080/1057563031000072514

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among its constituents depending upon time and locations of collection [15]. Diterpenesof many structural types have been isolated from collections of D. dichotoma from theIndian Ocean [16]. In this communication, we wish to report the isolation and charac-terization of dichototetraol (1) and dichotopentaol (3) from D. dichotoma along with 2

of the same skeleton as a new source [17].

RESULTS AND DISCUSSION

The ethanol soluble part of fresh brown alga D. dichotoma collected from Karachi coastof Arabian Sea yielded two new dolastanes [dichototetraol (1) and dichotopentaol (3)]along with 2 as were expected from Dictyota species. Their skeletal characterizationis as follows:The diterpenic nature of 1 and 3 was depicted by the exhibition of two

secondary (Me-18, Me-19) and two tertiary (Me-16, Me-20) methyls together witha converted methyl as an exo-cyclic double bond (Me-15). The key points for thecharacterization of dichototetraol and dichotopentaol are summarized below.

Dichototetraol (1)

Presence of hydroxyl and olefinic functions in IR spectrum of 1 were attested bytheir absorptions at 3358 and 1652 cm�1, respectively. A peak at m/z 320 in the fielddesorption (FD) spectrum appeared after the loss of a water molecule and the formulalinked with this peak was determined through HRMS as C20H32O3 showing the fivedegrees of unsaturation. Thus the actual formula would be C20H34O4 with four degreesof unsaturation.The 1H-NMR spectrum of 1 displayed two secondary methyls associated with

isopropyl moiety at � 0.98 (J¼ 7.0Hz) and 1.04 (J¼ 6.8Hz). Other two quaternarymethyl singlets for Me-16 and Me-20 resonated at � 1.00 and 1.20, respectively. Twosinglets at � 5.05 and 5.08 were found to be connected in the HMQC spectrumwith the carbon at � 113.5 attested for H-15A/B and C15. A carbinylic proton anda carbon appeared at � 4.34 (J¼ 3.0 and 2.8Hz) and � 75.9 assigned to H-2 andC-2, respectively. Situation of hydroxyl function at C-2 was depicted via HMBC

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connectivities (Fig. 1), whereas the stereochemistry as axial and � was concluded on thebases of magnitude of coupling constant (J¼ 3.0 and 2.8Hz) [18].Three hydroxyl containing quaternary carbons showed their resonances at � 73.5,

80.4 and 76.0 assigned to C-8, C-9 and C-14, respectively. The isopropyl methylsappeared at � 18.7. Signals at � 17.8 and 21.9 were due to the two quaternary methylsMe-16 and Me-20, respectively.The above given arguments and spectral information have led to the conclusion that

the discussed compound has Structure 1 and named dichototetraol. This would be anew addition in diterpenoids of Dictyota belonging to the dolastane class.

Dichotopentaol (3)

In comparison with Compound 1, NMR and mass spectra of dichotopentaol (3)showed the disappearance of a quaternary methyl and appearance of an –CH2OHmoiety in the molecule.The molecular mass of 3 as 318 a.m.u. with the removal of two water molecules from

molecular ion peak (exact molecular mass would be 354 a.m.u.) and formula C20H32O4

with the loss of a water molecule from molecular ion peak ( exact molecular formulawould be C20H34O5) were determined with the aid of FDMS and HRMS, respectively.The normal methyls of dolastane class appeared at � 0.80 (d, J¼ 6.9Hz, Me-18), 0.84

(d, J¼ 6.8Hz, 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 11.5Hz was due to the –CH2OHmoiety confirming the oxidation of missing methyl (Me-16) as –CH2OH moiety. Todistinguish which methyl underwent oxidation could be possible via HMBC experi-ments (Fig. 2) and with the guidance of literature [19]. The carbon due to this newlyappeared moiety displayed its existence at � 60.3 in the 13C-NMR spectrum. Anothercarbinylic proton appeared at � 3.18 as a broad singlet and its associated carbonresonated at � 74.3 assigned to H-4 and C-4. Broadness of this signal inferringthe � and equatorial orientation of H-4. Hence, the stereochemistry of hydroxylfunction to this carbon should be axial and � [18]. As far as position of hydroxylat C-4 is concerned, it was again decided with the aid of HMBC connectivities(Fig. 2). Signals of olefinic protons and carbons exhibited in the NMR spectraat � 4.60 (H-15A), 4.66 (H-15B) as singlets, 108.6 (C-15) and 151.9 (C-1). The threequaternary hydroxyl containing carbons resonated at � 74.4 (C-8), 76.4 (C-9) and78.2 (C-14).This is another new addition in the D. dichotoma metabolites and named

dichotopentaol (3).

FIGURE 1 HMBC connectivities in 1.

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Compound 2

Compound 2 was isolated from D. cerrvicornis and published by Kelecom and Teixeira[17]. Spectral data of this compound are given in the experimental section.

EXPERIMENTAL

General

The 1H- and 13C-NMR spectra were recorded at 400 and 300MHz on Bruker AM 400and 300, respectively in CDCl3 and in a mixture of CDCl3þCD3OD.

Collection and Identification

Alga (wet. weight 12 kg) was collected in March 2001 from Buliji near Karachicoast (Arabian Sea) by hands and identified by Ms. Shaista Hameed, Institute ofMarine Biology, University of Karachi, where a voucher specimen (# MBB-55) hasbeen deposited.

Extraction and Isolation

The collected material was washed with plenty of tap water to remove the sea salts andimmediately soaked in ethanol (16L) for a period of eight days. After filtration, thefiltrate was concentrated at low pressure to avoid thermal decomposition. The crudeoily concentrated extract (321 g) was subjected to silica gel column chromatographyusing hexane, hexane–ethyl acetate, ethyl acetate, and ethyl acetate–methanol asmobile phase.Elution with 25% ethyl acetate in hexane, 1 was obtained after repeated CC as a gum

(10mg).

Dichototetraol (1)

[�]D24: �28.9� (CHCl3 , c 0.076); IR (CHCl3) �max: 3358 (OH), 1652 (C¼C) cm

�1; El-MS:m/z 302 [M�2H2O]

þ, 287 [302�Me]þ, 285 [302�OH]þ, 259 [302� isopropyl]þ, 284[287�H2O]

þ, 269 [284�CH3]þ; FD-MS: m/z 320 (M�H2O); HREI-MS: m/z 320.2328

(C20H32O3 requires 320.235131), 302.2215 (C20H30O2 requires 302.224568), 287.2019(C19H27O2 requires 287.201094), 285.2140 (C20H29O requires 285.221829), 284.2152(C20H28O requires 284.214004), 269.1895 (C19H25O requires 269.190530), 259.1737

FIGURE 2 HMBC connectivities in 3.

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(C17H23O2 requires 259.169795);1H-NMR (CDCl3, 300 MHz): � 0.98 (3H, d, J¼ 7.0Hz,

H-18), 1.00 (3H, s, H-16), 1.04 (3H, d, J¼ 6.8Hz, H-19), 1.20 (3H, s, H-20), 4.34 (1H,dd, J¼ 3.0, 2.8Hz, H-2�), 5.05 (1H, s, H-15A), 5.08 (1H, s, H-15B); 13C-NMR (CDCl3,

75 MHz): � 150.6 (C-1), 75.9 (C-2), 32.6 (C-3), 32.1 (C-4), 42.0 (C-5), 25.5 (C-6), 22.2(C-7), 73.5 (C-8), 76.0 (C-9), 29.7 (C-10), 43.2 (C-11), 44.9 (C-12), 37.6 (C-13), 80.4(C-14), 113.5 (C-15), 17.8 (C-16), 29.0 (C-17), 18.7 (C-18), 18.7 (C-19) and 21.9 (C-20).Elution with 25% ethyl acetate in hexane, 2 was obtained after repeated CC as

crystals (11mg).

Compound 2

EIMS: m/z 320 [M�H2O]þ, 305 [320�Me]þ, 302 [320�H2O]

þ, 287 [302-Me]þ; HREI-

MS: m/z 320.2359 (C20H32O3 requires 320.2351), 305.2126 (C19H29O3 requires305.2117), 302.2251 (C20H30O2 requires 302.2246); 1H-NMR (CDCl3 , 300 MHz): �0.95 (3H, s, H-16), 0.99 (3H, d, J¼ 6.9, 7.0Hz, H-18), 1.03 (3H, d, J¼ 6.9, 7.0Hz,H-19), 1.31 (3H, s, H-20), 3.47 (1H, br.s, H-4�) 4.89 (1H, s, H-15A), 4.89 (1H, s,H-15B); 13C-NMR (CDCl3, 75 MHz): � 152.1 (C-1), 31.0 (C-2), 26.8 (C-3), 81.0(C-4), 43.8 (C-5), 25.5 (C-6), 22.1 (C-7), 75.7 (C-8), 73.7 (C-9), 29.1 (C-10), 43.5(C-11), 44.6 (C-12), 38.2 (C-13), 81.0 (C-14), 109.3 (C-15), 18.0 (C-16), 29.2 (C-17),18.6 (C-18), 18.7 (C-19) and 21.6 (C-20).Elution with 35% ethyl acetate in hexane, 3 was obtained after repeated CC as a

white powder (9.3mg).

Dichotopentaol (3)

M.P.: 112–115 �C; [�]D24: �20� (pyridine, c 0.09); IR (CHCl3) �max: 3336 (OH), 1639

(C¼C) cm�1; El-MS: m/z 318 [M�2H2O]þ, 303 [318�CH3]

þ, 300 [318�H2O]þ

and 257 [300�C3H7]þ; FD-MS: m/z 318 (M�2H2O); HREI-MS: m/z 336.2259

(C20H32O4 requires 336.230045), 318.2147 (C20H30O3 requires 318.219482), 303.1860(C19H27O3 requires 303.196008), 300.2018 (C20H28O2 requires 300.208919), 257.1576(C17H21O2 requires 257.154146); 1H-NMR (CDCl3þCD3OD, 400 MHz): � 0.80 (3H,d, J¼ 6.9Hz, H-18), 0.84 (3H, d, J¼ 6.8Hz, H-19), 1.03 (3H, s, H-20), 3.24 (1H, d,J¼ 11.5Hz, H-16A), 3.70 (1H, d, J¼ 11.5Hz, H-16B), 3.18 (1H, br.s, H-4�), 4.60(1H, s, H-15A), 4.66 (1H, s, H-15B); 13C-NMR (CDCl3þCD3OD, 100 MHz): � 151.9(C-1), 26.2 (C-2), 24.3 (C-3), 74.3 (C-4), 46.8 (C-5), 25.0 (C-6), 21.8 (C-7), 74.4 (C-8),76.4 (C-9), 29.7 (C-10), 42.1 (C-11), 44.4 (C-12), 37.4 (C-13), 78.2 (C-14), 108.6(C-15), 60.3 (C-16), 28.6 (C-17), 17.9 (C-18), 18.1 (C-19) and 21.4 (C-20).

Dedication

This article is dedicated to Prof. Atta-ur-Rahman, the first scientist from Pakistan andthe Muslim world who has received the UNESCO Science Prize.

Acknowledgement

We are grateful to Ms. Shaista Hameed, Institute of Marine Biology, University ofKarachi, for identifying the algal material.

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