Pyrazine Derivatives with Osteoclast Differentiation Inhibitory Activities from the Endophytic Fungus <i>Alternaria</i> sp. HJT-Y7

Four new pyrazine derivatives, namely (<i>S</i>,<i>E</i>)-1-((5-(3-hydroxy-2-(hydroxymethyl)styryl)-3,6-dimethylpyrazin-2-yl)methyl)-1,3-dihydroisobenzofuran-4-ol (<b>1</b>), (<i>S</i>,<i>E</i>)-1-((5-(3-hydroxy-2-(methoxymethyl)styryl)-3,6-dimethylpyrazin-2-yl)methyl)-1,3-dihydroisobenzofuran-4-ol (<b>2</b>), (<i>S</i>,<i>Z</i>)-1-((5-(3-hydroxy-2-(methoxymethyl)styryl)-3,6-dimethylpyrazin-2-yl)methyl)-1,3-dihydroisobenzofuran-4-ol (<b>3</b>), and (<i>S</i>)-1-((5-(3-hydroxy-2-(methoxymethyl)phenethyl)-3,6-dimethylpyrazin-2-yl)methyl)-1,3-dihydroisobenzofuran-4-ol (<b>4</b>), together with one new 1,3-dihydroisobenzofuran derivative, (<i>S</i>)-2-(4-hydroxy-1,3-dihydroisobenzofuran-1-yl)acetamide (<b>5</b>), were isolated from the endophytic fungus <i>Alternaria</i> sp. HJT-Y7 collected from the leaves of <i>Rhodiola tibetica</i>. Their structures were determined based on extensive spectroscopic analysis and comparison of experimental and calculated electronic circular dichroism (ECD) curves. <b>1</b>, <b>4</b>, and <b>5</b> significantly inhibited osteoclast differentiation in RANKL-induced RAW264.7 cells and markedly restored bone mineralization in a dexamethasone-induced zebrafish osteoporosis model. These results highlight the potential of <b>1</b>, <b>4</b>, and <b>5</b> as promising candidates for osteoporosis therapy.