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The molecular formula of 2 is proposed to be C22H34N2OS,also on the basis of HRESIMS data. The proposed molecular formula indicates that one of the oxygens present in the structure of 1 is replaced by a sulfur. This is evident as the IR absorption band of the isocyanate functionality of 1 (ν 2250 cm−1) is replaced by that of an isothiocyanate (ν 2075 cm−1) in the spectrum of 2. The 13C chemical shift of the isocyanate carbon(δ 123.0; 8-NCO) of 1 also shifted downfield, resonating in the characteristic range for the isothiocyanate (δ 131.0; 8-NCS). The NMR spectra of 1 and 2, apart from the resonances assigned for the isocyanate and isothiocyanate carbons, were almost identical (Table 1). This included the characteristic features of a 3:2 rotameric mixture. The structure of 2, determined in a similar fashion to that of 1 (Figure 1), was proposed to be a new 8-isothiocyanate analogue of 1. Also similar to that of 1 was the relative configuration of 2, which was determined by means of the same experiments and assumption as described earlier. Bifunctionalized diterpenes possessing two different isonitrilerelated functionalities, although readily documented,2,4b,5b,c are rare. Recent reports from Garson’s group proved that sponges had the ability to incorporate cyanide and thiocyanate as sources of isonitrile and isothiocyanate functionalities as well as to interconvert the two groups biosynthetically,7 whereas the formamide was shown to be derived chemically from hydrolysis of the isonitrile,7b,8 but has never been proven to be formed biosynthetically. The biogenetic origin of the least common isocyanate has not been addressed experimentally. The coexistence of 1−3, each of which bears the identical structural scaffold and differs from each other only at the nitrogenous substitutions on C-8, extends the biosynthetic connection from the isonitrile and isothiocyanate to the isocyanate, presumably through peroxidase in a related manner to that mentioned by Simpson and Garson.7b The presence of the bifunctionalized diterpenes possessing a formamide group along with other nitrogenous functionalities either indicates that the formamide functional group genuinely exists in the sponge or is merely evidence for the chemical transformation from a precursor that is more sterically accessible at C-15 on the side chain rather than on the sterically hindered C-8. Sponges of the genus Stylissa from various collecting sites have been reported as sources of biologically active metabolites, among which cyclic peptides9 and bromoimidazole and bromopyrrole alkaloids10 were common. However, to our knowledge, the report by Mitome et al. on the Okinawan specimen was the one that focused on the isonitrile-containing amphilectane diterpenes and related derivatives isolated from this genus.3 Bearing the same skeletons, diterpenes isolated from the Thai specimen reported here and in our previous work6 have extended the range of nitrogenous diterpenes from the Stylissa sponges toward multifunctionalized derivatives. The antimalarial activities of all four isolated compounds were assessed to confirm that the isonitrile functionality of 3, through the linear nonbonded sp orbital, is the most crucial for such bioactivity (IC50 0.52 μM). The isocyanate- and isothiocyanate containing compounds, although presumably reacting through their linear functionalities but with the less active lone-pair electrons, were approximately 10-fold less effective (IC50's 8.85 and 8.07 μM for 1 and 2, respectively). Compound 4, with only a single formamide group, was inactive at the highest concentration tested (10 mg/mL). Such observations agreed well with a previous report by Wright and Lang-Unnasch4b and complemented the heme-isonitrile coordination mechanism also proposed by Wright et al.5d Cytotoxicity against MCF-7 breast adenocarcinoma cells was also examined. All of the isolated compounds were inactive, inhibiting less than 50% of the tested cells at the highest concentration of 25 mg/mL. |
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爱与雨下: 金币-1, 请勿在求助内容灌水！ 2012-03-18 10:48:53
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