Structure of Pseudocerosine, an Indolic Azafulvene Alkaloid from the Flatworm Pseudoceros indicus

Abstract

The rim of the tunic of the flatworm Pseudoceros indicus is characterized by blue dots on a white background. The isolation and structure elucidation of the blue pigment is reported. It is shown by extensive analysis of spectroscopic data to be an indolic azafulvene, which has been named pseudocerosine.Marine biota continue to provide unique structures with compelling biological activity and/or structural interest. As part of our bioprospecting effort aimed at the discovery of new leads for antitumor compounds, we had the opportunity to sample the marine flatworm Pseudocerus indicus, which was collected by snorkeling in mangrove forests of Palau, Chuuk, and Pohnapei (Micronesia). 1 The animal was observed in high abundance in between the mangrove roots, often feeding on the white colonial ascidian Eudistoma toealensis. We previously have described a series of staurosporine-type indolocarbazole alkaloids from the flatworm and its ascidian prey. [2][3][4] In this paper, we describe the structure elucidation of the blue pigment, which forms the characteristic, well-defined blue spots along the body margin of the otherwise offwhite mantle. 5 Compound 1 was isolated as an intensely blue colored microcrystalline solid in 0.025% yield (dry weight). Optical activity could not be established because of the intense color of the compound (λ max (log ε) 218 (4.31), 260 (4.33), 310 (4.36), 405 (3.40), 569 (3.83), 602 (3.86) indicative of a molecular formula of C 22 H 22 N 3 O 4 S (−0.5 mmu error). This putative molecular formula was corroborated by the presence of 22 distinct signals in the 13 C NMR spectrum of 1 between 22.7 and 172.6 ppm (see Table 1). Most signals in both the proton and the 13 C domain displayed doubling, with a ratio of major to minor of approximately 5:2. On the basis of extensive NMR data analysis, the structure shown in Figure 1 was assigned to compound 1, which we named pseudocerosine.Interpretation of gHSQC data suggested that the 22 carbon atoms were present in the form of four methyl, one methylene, six methine, and 11 quaternary carbon atoms, which meant that two exchangeable protons were present. On the basis of gCOSY data recorded in MeOH-d 4 and MeOH-d 3 , respectively, three spin systems were readily assembled as shown in Figure 2. For fragment a, the coupling pattern of H-5, H-7 and H-8 suggested that they were located in a 1,2,4-trisubstituted aromatic ring.Furthermore, the deshielding evident in the chemical shifts of H-5/H-7 and of H-8 in conjunction with the 13 C chemical shifts of the attached carbon atoms (109.9, 127.6, 118.5 ppm) was suggestive of an indole system.Another two-proton spin system consisted of a vinyl group resonating at 8.22 ppm and 7.32 ppm, respectively, and the J-coupling suggested that these two protons were located ortho to each other (fragment b Figure 2). A third spin system revealed by the gCOSY spectrum consisted of three signals in the form of an ABX system (fragment c Figure 2), which showed an additional coupling to the X-spin when the spectrum wa...

Natural Products in Polyclad Flatworms

“…The alkaloid pseudocerosine (12) was isolated by Hemscheidt as a distinct blue pigment located around the margin of P. indicus [67]. It was originally incorrectly described as an azepinoindole molecule; however, a recent revision of its structure by Sperry identified it as a pyridoacridine, but a new branch of the pyridoacridine family tree [68].…”

“…They have been isolated from sponges, molluscs, tunicates, and cnidarians and it is suspected that 12 is trophically sequestered from the polyclad's prey [69,70]. It is unclear how 12 contributes to the defence of the polyclad considering several staurosporines were also isolated from this species [67,71]. Sperry reported a failed attempt to synthesise the incorrect structure and then subsequently synthesised the correct structure to show that its spectral data matched that of the natural product [71].…”

Synthetic Studies Toward the Flatworm‐Derived Alkaloid Pseudocerosine

“…The alkaloid pseudocerosine (1; Figure 1) is an intensely blue pigment responsible for the coloured spots along the rim of the tunic Pseudoceros indicus. [1] The fascinating molecular architecture present in 1 includes an azafulvene embedded within an azepinoindole possessing a quaternized nitrogen, a sulfoxide and an aminal unit. [2] The unique heteroaromatic structure of 1, along with our ongoing interest in azepinoindole alkaloids [3] led us to instigate a study towards its total synthesis.…”

“…We incorporated this methodology into the proposal shown in Scheme 1B; treating 3 with base would lead to a bis-cyclisation, simultaneously installing both the azepine ring and the lactam to give the pentacycle 4. Dehydrogenation would then lead to 5, the core of pseudocerosine (1).…”

“…The alkaloid pseudocerosine ( 1 ; Figure ) is an intensely blue pigment responsible for the coloured spots along the rim of the tunic Pseudoceros indicus . The fascinating molecular architecture present in 1 includes an azafulvene embedded within an azepinoindole possessing a quaternized nitrogen, a sulfoxide and an aminal unit .…”