Definition: In this task, you're given a paragraph from the research paper and your task is to generate a suitable title for the research paper based on the given paper. Under 100 words is a good title length.
Input: Investigation of the secondary metabolites of the ascidian Herdmania momus led to the isolation and characterization of four new nucleoside derivatives (1) (2) (3) (4) . Structural studies showed that these derivatives represent a series of rare methylsulfinyladenosine derivatives of interconvertible transesterification isomers and/or sulfinyl epimers. The antiviral activities of these rare nucleosides were evaluated against a series of human pathogenic viruses. Marine ascidians are a rich source of chemically diverse secondary metabolites, with remarkable biological activities. 1,2 such as amino acid derivatives and complex alkaloids. [3] [4] [5] [6] [7] [8] [9] In our previous study on bioactive compounds from the solitary tunicate Herdmania momus, a series of amino acid derivatives (herdmanines A-M) were isolated. 3,4 As a part of continuing study on the metabolites of the same tunicate, four new complex nucleosides (1-4) containing the rare methylsulfinyladenosine were isolated. Details of the isolation, structure elucidation, and of the results of the antiviral screening of these metabolites are described herein. The combined MeOH and CH 2 Cl 2 extracts were sequentially partitioned between CH 2 Cl 2 and H 2 O, and n-BuOH and H 2 O. The n-BuOH-soluble fraction was subjected to chromatographic separation and purification to yield a series of nucleoside derived metabolites (1-4). 10 Compounds 1 and 2 were found to be interconvertible, and on standing at room temperature after HPLC separation, each compound afforded the same equilibrium ratio of compound 1-2 (56:44). Compounds 3 and 4 were also interconvertible, and on standing at room temperature produced an identical equilibrium mixture (56:44) as that of 1 and 2. Spectral analysis of these interconvertible compounds was performed on equilibrium mixtures. The equilibrium mixture of compounds 1 and 2 was a white powder. The (+)-FABMS spectrum of this mixture showed isotopic peaks of [M+2H] + and [M+3H] + at m/z 554/552 and 555/553, respectively, with an isotopic peak ratio of 1:1. This pseudomolecular ionic configuration is shared with various other nucleosides N N N N NH 2

Epimeric methylsulfinyladenosine derivatives from the marine ascidian Herdmania momus