Two new tyrosine-derived metabolites, 14-debromoaraplysillin I [1] and 14-debromoprearaplysillin I [2], together with the known compound araplysillin I [3], have been isolated from the sponge Druinella (=Psammaplysilla) purpurea. Marine sponges of the order Verongida are characterized by their ability to synthesize brominated tyrosine derivatives, many of which possess antimicrobial activity. Chemical modification occurs both on the side chain and aromatic ring of the brominated tyrosine precursors, giving rise to a broad range of biosynthetically related compounds (1-5). In this paper we report the isolation and structural elucidation of two new antimicrobial metabolites, 14-debromoaraplysillin I [1] and its presumed biosynthetic precursor 14-debromoprearaplysillin I [2]. A specimen of Druinella (=Psammaplysilla) purpurea Carter (order Verongida, family Druinellae=Aplysinellidae) was collected by hand using scuba (-15 m) near Praslin, Seychelles. The MeOH extract of the lyophilized sponge showed moderate antimicrobial activity against Staphylococcus aureus and Bacillus subtilis. The MeOH extract was chromatographed on Sephadex LH-20 using CH₂Cl₂-MeOH (1:1) as eluent, and the active fractions were rechromatographed on Si gel. Araplysillin I [3] was isolated as one of the active constituents and identified by comparison of spectral data with literature values (6). 14-Debromoaraplysillin I [1] and 14-debromoprearaplysillin I [2] were also isolated from the active fraction and purified by hplc on an amino column using EtOAc-MeOH (9:1) as eluent. Both 14-debromoaraplysillin I [1] and 14-debromoprearaplysillin I [2] showed mild antimicrobial activity against S. aureus and B. subtilis at a loading of 100 µg/disk.14-Debromoaraplysillin I [1] was obtained as a clear glass that colored on standing. The hrfabms of 1 contained a cluster of peaks at m/z 635.9360, 638, 640, and 642 for the molecular formula C₂₇H₂₄Br₃N₃O₇. The ¹H- and ¹³C-nmr spectral data of 1 were very similar to those of araplysillin I [3], except that the ¹H-nmr spectrum of 1 contained signals due to a 1,2,4-trisubstituted benzene ring at δ 7.40 (d, 1H,J=2 Hz), 7.09 (dd, 1H, J=8.4, 2 Hz), 6.83 (d, 1H, J= 8.4 Hz) in place of the singlet ascribed to H-15 and H-17 in araplysillin [3}. These data showed good agreement with the spectral data reported for the monobrominated aromatic rings of psammaplin A [4] (7-9) and aplysamine 2 [5] (10). All other spectral data are in accord with a structure in which the bromine at C-14 (or C-18) in araplysillin I [3] is replaced by hydrogen.14-Debromoprearaplysillin I [2] was isolated as a clear glass. The high resolution mass spectrum of [2] contained a molecular ion at m/z 619.9377 for the formula C₂₇H₂₄Br₃N₃O₄. The ¹H-nmr data for the C-10-C-20 portion of 2 were almost identical to those of 1, but the signals assigned to the spirocyclooxazoline ring system of 1 were not present in 2 and were replaced by signals at δ 7.48 (s, 2H), 3.84 (s, 2H), and 3.82 (s, 3H), which were assigned to a 3,5-dibromo-4-methoxybenzyl moiety. However, comparison of the nmr spectral data of 2 with those of aplysamine 2 [5] (10) revealed that the position of the dibrominated and monobrominated aromatic rings could not be assured from these data alone, although biosynthetic considerations and the mass spectral fragmentation pattern strongly favored the proposed structure. The positions of the bromine atoms were confirmed by two NOEDS experiments (11); irradiation of the H-19 signal at δ 2.68 (t, 1H, J = 6.5 Hz) caused enhancements of the H-15 signal at 7.32 (d, 1H,J= 1.8 Hz) and the H-17 signal at 7.02 (dd, 1H, J = 8.4, 1.8 Hz), while irradiation of the H-1 and H-5 signals at 7.48 (s, 1H) caused an enhancement of the signals due to the H-7 methylene protons at 3.84 (s, 2H). The E geometry of the oxime was assigned on the basis of the chemical shift of the C-7 signal at δ 28.0 (t); the corresponding signal for the (Z) oxime is found at >35 ppm (7).The biosynthesis of the spirocyclic isoxazolines is thought to involve the intramolecular cyclization of the hydroxyl group of an oxime with an unstable intermediate arene oxide (12) as shown in Scheme 1. Spiroisoxazolines have been obtained by oxidation of benzyl oximes, although in low yields (13). The isolation of the isoxazole 1 and the oxime 2 from the same sponge is the first reported instance in which a spiroisoxazoline has been isolated together with its proposed precursor. These results lend further weight to the biosynthetic hypothesis shown in Scheme 1.