Natural products of marine sponges. 7. The constitution of weakly basic guanidine compounds, dibromophakellin and monobromophakellin

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“…1 H and 13 C NMR data (Table 2) were very similar to those of the dibromophakellins. 39d, 43,44 The numbering scheme followed that of midpacamide (1) to indicate the cyclisation of the alkyl chain to form the central aliphatic ring system in the phakellin analogues and to conveniently apply the similar numbering to the other analogues described in this paper. Comparison of the 1 H NMR data to those of the known dibromophakellins helped in assigning the proton signals and in detecting obvious differences such as the emergence of a broad singlet at d H 7.19 and the disappearance of the doublet proton at d H 6.29 previously assigned to H-12 in compound 4.…”

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“…However, the X-ray data rather show that the C-8, N-9 bond is longer than the other two C-N bonds (34), implying insufficient participation of N-9 in the resonance (41). As the cause of the depressed pKa, the interaction of two neighboring guanidinium groups was once considered but excluded (41), and there is an indication that such anomaly may be intrinsic to condensed five-membered 2-aminoimidazolines as exemplified by another anomalous guanidinium compound phakellin (23) with a pKa of 7.7 (42,43).…”

“…Condensation of the vinylogous carbamate (40) with acetaldehyde and isocyanic acid afforded the pyrimidine derivative (41) which was converted to the urea derivative (42) by Curtius rearrangement in 65% overall yield. Treatment of (42) with acetic acid at 50° afforded exclusively the desired product (39), whereas treatment with trifluoroacetic acid gave a mixture of (39) a.!J.d stereoisomer (43).…”

“…The resulting bicyclic thiourea (48) was then converted to thiourea urea (49) in a sequence analogous to that used for the preparation of (42). Because the acidic conditions used for the cyclization of (42) were not applicable to the ketal (49), it was converted to the thioketal (50).…”

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“…Therefore, a lot of secondary metabolites have already been described in the literature. The most prominent natural products isolated from these sponges include the agelastatines [5], axinellamines 0932-0776 / 07 / 0400-0600 $ 06.00 © 2007 Verlag der Zeitschrift für Naturforschung, Tübingen · http://znaturforsch.com [6], hymenialdisin [7], phakellins [8], hymenamides and phakellistatins [9]. It is therefore very important to know at a very early stage of the structure elucidation process whether a sponge contains novel natural products or not.…”