Identification and Total Synthesis of Novel Fatty Acids from the Caribbean Sponge Calyx podatypa Ne ´stor M. Carballeira,* Mayra Paga ´ n, and Abimael D. Rodrı ´guez Department of Chemistry, University of Puerto Rico, P.O. Box 23346, San Juan, Puerto Rico 00931 Received April 10, 1998 The phospholipid fatty acid composition of the Caribbean sponge Calyx podatypa was studied, and 85 different fatty acids were identified, in particular the 11-methylpentadecanoic acid and 10-tricosenoic acid, which have no literature precedence. Structural characterization was accomplished by means of gas chromatography-mass spectrometry on their corresponding methyl esters and dimethyl disulfide derivatives. The structure of 11-methylpentadecanoic acid was further confirmed by total synthesis (17% overall yield) starting from commercially available 10-hydroxydecanoic acid. Calyx podatypa Van Soest (Phloeodictyidae) is a common Caribbean sponge rich in cyanobacteria. 1 Sev- eral unusual metabolites have been isolated from C. podatypa, including proline-derived diketopiperazines, 1 1,3-diphenylbutanoid compounds, 2 and antimicrobial N-methylpyridinium salts related to the xestamines. 3 Just recently, some unusual 2,2,4,6,6-pentasubstituted piperidine alkaloids were also reported from C. po- datypa. 4 Despite all of these studies, there is only one report on the lipid composition of C. podatypa, namely, the isolation and identification of novel sterols with cyclopropene-containing side chains, a probable common feature among Calyx species. 5 Motivated by this inter- esting finding, and under the possibility that unusual sterols might interact with unusual fatty acids in sponge membranes, we studied the complete phospholipid fatty acid composition of C. podatypa and found two new fatty acids: 11-methylpentadecanoic acid and 10-tricosenoic acid. Short-chain fatty acids such as 11-methylpenta- decanoic acid are of interest because other methyl- branched hexadecanoic acids, such as 13-methylpenta- decanoic acid, display antimicrobial activity (MIC ) 1.56 μg/mL) against the cariogenic bacterium Streptococcus mutans. 6 C. podatypa presented a typical sponge phospholipid profile. By means of careful 31 P NMR on the phospho- lipid fraction from C. podatypa, we identified phosphati- dylethanolamine (-1.32 ppm) and phosphatidylserine (-2.05 ppm) as the two most abundant phospholipids. Acid methanolysis of the phospholipids yielded around 85 identifiable fatty acids, which are shown in Table 1. Fatty acid chain lengths ranged between C 12 and C 26 , and a wide variety of structural types was identified. In particular, methyl-branched fatty acids were par- ticularly abundant in this sponge, inasmuch as they made up 50% of the total fatty acid composition. For example, a diversified methyl-branched hexadecanoic acid family was identified in C. podatypa and included the 3-, 9-, 10-, 11-, and 14-methylpentadecanoic acids. All of these acids were characterized as methyl esters by GC-MS. Moreover, the 11-methylpentadecanoic acid has no literature precedence. Initial characteriza- tion of this acid was accomplished through HRMS of its methyl ester 1. Upon HREIMS the title compound displayed a molecular ion peak at m/z 270.2554, corre- sponding to a molecular formula of C 17 H 34 O 2 . In addi- tion, 1 presented an equivalent chain-length (ECL) value of 15.54 in nonpolar capillary GC, indicating unusual methyl branching. A careful inspection of the MS of 1 revealed higher than normal fragmentation peaks at m/z 185.1542 [C 11 H 21 O 2 ] + and at m/z 213.1844 [C 13 H 25 O 2 ] + , as well as a diminished peak at m/z 199, indicating methyl branching at C-11. Methyl 11-meth- ylpentadecanoate is, therefore, the most probable struc- ture for 1. Final structural confirmation for 1 was achieved through total synthesis (Scheme 1). Commercially available 10-hydroxydecanoic acid (Aldrich) was con- verted to methyl 10-hydroxydecanoate in a 99% yield through acid-catalyzed methanolysis. 7 Reaction of the methyl ester with phosphorus tribromide resulted in a 96% yield of methyl 10-bromodecanoate, which was subsequently reacted with triphenylphosphine in ben- zene, affording the phosphonium salt methyl (9-meth- oxycarbonylnonyl)triphenyl phosphonium bromide as previously reported (Scheme 1). 7 Subsequent Wittig coupling with 2-hexanone, using potassium t-butoxide as base and THF as solvent, afforded a 2:1 cis-trans mixture of methyl 11-methyl-10-pentadecenoate in a low 19% yield. The product ratio was determined by GC, because, in our nonpolar capillary column, fatty acid methyl esters with the trans stereochemistry elute after their cis counterparts, as we previously demonstrated for methyl 7-methyl-6-octadecenoate. 8 Final hydroge- nation over PtO 2 resulted in the desired methyl 11- methylpentadecanoate (1) in an overall 17% yield, which coeluted on capillary GC with the natural methyl ester from C. podatypa. This reaction sequence represents the first synthesis of 1. Monounsaturated fatty acids also predominated, up to as much as 11% of the total, in the phospholipid fatty acid mixture from C. podatypa (Table 1). One particular monounsaturated fatty acid, 10-tricosenoic acid, became of interest because it has no precedent in the literature. Methyl 10-tricosenoate presented a molecular ion peak * To whom correspondence should be addressed. Tel.: (787) 764- 0000 ext. 4791. Fax: (787) 751-0625. E-mail: ncarball@ upracd.upr.clu.edu. 1049 J. Nat. Prod. 1998, 61, 1049-1052 S0163-3864(98)00141-4 CCC: $15.00 © 1998 American Chemical Society and American Society of Pharmacognosy Published on Web 07/18/1998