**Takeshi Yamada 1,\*, Asumi Tanaka 1, Tatsuo Nehira 2, Takumi Nishii <sup>1</sup> and Takashi Kikuchi <sup>1</sup>**


Received: 25 March 2019; Accepted: 9 April 2019; Published: 11 April 2019

**Abstract:** In order to find out the seeds of antitumor agents, we focused on potential bioactive materials from marine-derived microorganisms. Marine products include a number of compounds with unique structures, some of which may exhibit unusual bioactivities. As a part of this study, we studied metabolites of a strain of *Alternaria* sp. OUPS-117D-1 originally derived from the sea urchin *Anthocidaris crassispina*, and isolated five new decalin derivatives, altercrasins A–E (**1**–**5**). The absolute stereostructure of altercrasins A (**1**) had been decided by chemical transformation and the modified Mosher's method. In this study, four decalin derivatives, altercrasins B–E (**2**–**5**) were purified by silica gel chromatography, and reversed phase high-performance liquid chromatography (RP HPLC), and their structures were elucidated on the basis of 1D and 2D nuclear magnetic resonance (NMR) spectroscopic analyses. The absolute configuration of them were deduced by the comparison with **1** in the NMR chemical shifts, NOESY correlations, and electronic circular dichroism (ECD) spectral analyses. As a result, we found out that compound pairs of **1**/**2** and **4**/**5** were respective stereoisomers. In addition, their cytotoxic activities using murine P388 leukemia, human HL-60 leukemia, and murine L1210 leukemia cell lines showed that **4** and **5** exhibit potent cytotoxicity, in especially, the activity of **4** was equal to that of 5-fluorouracil.

**Keywords:** altercrasins; *Alternaria* sp.; *Anthocidaris crassispina*; decalin derivatives; cytotoxicity
