Two new citric acid derivatives, a cyclic carbonate and a γ-lactone, and a new N,7-dimethylindole-3-carboxaldehyde, have been isolated from the marine cyanobacterium Lyngbya majuscula. Long known as a rich source of novel natural products, the marine cyanobacterium Lyngbya majuscula Harvey ex Gomont (Oscillatoriaceae) (1) continues to yield new structural classes of compounds, such as curacin A (2). This note further illustrates the organism's ability to produce diverse compounds by describing the isolation of a unique cyclic carbonate [1] and its possible lactone precursor [2], a new indole derivative [3], and a metabolite previously known from a red alga [4], all from a shallow water Okinawan collection of L. majuscula. These compounds were found in chromatographic fractions showing brine shrimp toxicity (3) that was most likely caused by debromoaplysiatoxin, the major component of the mixture (4). The structure of lyngbyacarbonate [1] was elucidated by nmr, ms, and ir data. Integration of the peaks in the simple ¹H nmr spectrum, together with DEFT-135 data, indicated a symmetrical molecule with three isolated spin-systems. Although the long relaxation time of C-7 initially prevented detection of this peak in the ¹³C nmr spectrum of 1, recording of its spectrum with a relaxation delay of 5 sec revealed the position of this hidden peak at δ 153.6, indicative of a cyclic carbonate. Moreover, the molecular formula from positive-ion hrfabms and the ir band at 1805 cm⁻¹ further substantiated the presence of this unusual functionality. HMBC nmr data provided final confirmation of structure 1. Lyngbyacarbonate [1] represents a new structural class of marine natural product. The only other naturally occurring cyclic carbonates have been found in filamentous bacteria. Streptomyces spp. produce aldgarose as the glycoside of the antibiotic aldgamycin E (S. lauendulae) (5) and the antibiotic neocarzinostatin chromophore (S. carzinostaticus) (6). In addition, an antifungal triynecarbonate has been isolated from Actinomycetes spp. fermentation broths (7). However, no biological activity has been associated with the cyclic carbonate structure to date. Lyngbyacarbonate is not cytotoxic against HCT-116 and KB cancer cell lines,* and the DNA strand-scission activity in neocarzinostatin chromophore is unaffected by the absence of the carbonate moiety (8). Characterization of γ-lactone 2 (hrcims molecular formula C₈H₁₁O₅) was accomplished by essentially the same strategy as used above for lyngbyacarbonate [1]. The broad ir band at 3455 cm⁻¹ indicated a hydroxyl group while bands at 1783 and 1727 cm⁻¹ suggested the presence of γ-lactone and ester moieties, respectively. Additional data from ¹H-, ¹³C-, and DEPT-135 nmr spectra showed the presence of three quaternary carbon atoms, three isolated CH₂'s, and a OMe group. These structural fragments could be assembled into only a single planar structure, yielding γ-lactone 2. The absolute configuration of 2, [α]²⁸D -3.0° (c 0.23, CHCl₃), remains unknown. Although structurally similar to the known cyanobacterium metabolite 3 hydroxybutyrolactone (9), compound 2 may also be considered as a lactone derivative of citric acid in which the central carboxyl group has been reduced to an alcohol. The open form of the lactone 2 could serve as precursor to lyngbyacarbonate [1] by reaction with bicarbonate in a manner analogous to the mechanism established for aldgarose (10). Nmr experiments were used for the structure determination of indole derivative 3 (C₁₁H₁₂NO). The chemical shifts from the ¹H-nmr spectrum indicated the presence of an aldehyde group, two methyl groups, and four aromatic protons, while the spin-coupling patterns suggested a trisubstituted indole system with three adjacent protons. The four quaternary carbons detected by ¹³C-nmr and DEPT-135 data were consistent with an N-methylindole derivative. Positional assignments for the aldehyde and methyl groups were based on HMBC cross-peaks between C-7a and H-8, -10; C-6 and H-10; C-2 and H-8; and C-3a and H-9. Verification was provided by NOESY cross-peaks between H-6 and H-10 and between H-2 and H-8. The highfield shift for H-8 (δ 4.13) was attributed to conjugation effects between the nitrogen and aldehyde groups. These data, plus the hrcims molecular formula, established compound 3 as the new indole derivative N,7-dimethylindole-3-carboxaldehyde. Metabolite 3 is unusual in several respects. Although indole-3-carboxaldehyde has been isolated from the red alga Botryocladia leptopoda (11), 3 is the first aldehyde of any kind to be found in a cyanobacterium (1,12,13). Furthermore, N-methylindoles are uncommon in algae, the only other examples being three bromo indoles isolated from the red alga Laurencia brongniartii (12,14). Finally, no other 7-methylindoles have been found in marine algae. Apparently compound 3 is a widespread minor metabolite of L. majuscula, as it has subsequently been detected in another collection of this alga from Curasao (private communication from Dr. Jimmy Orjala, College of Pharmacy, Oregon State University). The structure of monoterpene 4 was elucidated by ¹H-nmr, ¹³C-nmr, DEPT-135, ¹H-¹H COSY, ¹³C-¹H COSY, and HMBC experiments and was found to be identical to a compound isolated by Higa from the red alga Desmia hornemanni (=Ptilonia hornemanni) (15). Although the investigated sample of L. majuscula was visually homogeneous, the relatively low concentration of metabolite 4, as well as of metabolites 1-3, raises a question as to the true origin of these compounds. The occurrence of monoterpene 4 in L. majuscula is unusual since cyanobacteria are not generally known as a source of these compounds except as presumed subunits in larger molecules, such as lyngbyatoxin (16). To our knowledge, β-cyclocitral is the only other monoterpene known from a cyanobacterium (Microcystis wesenbergii) (13). The biological function, if any, of compounds 1-4 is presently unknown. The apparent rarity of cyclic carbonates in nature poses questions concerning their biosynthesis. In the case of aldgarose, radioactive labeling experiments indicate that this functionality arises from a carboxylation reaction involving bicarbonate ion (10). If a similar mechanism applies to lyngbyacarbonate, then an unusual reduced form of citric acid, perhaps related to γ-lactone 2, would most likely be involved. As cyanobacteria have been shown to possess an incomplete citric acid cycle unable to transform α-ketoglutarate to succinyl-CoA (17), it is tempting to speculate that metabolites 1 and 2 represent shunt metabolites of citrate.