Isolation and structure elucidation of three metabolites from verticillium intertextum: sorbicillin, dihydrosorbicillin and bisvertinoquinol

From the culture medium of Verticillium inlertexlum three metabolitcs have been isolated, namely the hexaketidcs sorbicillin (1) and 2'3'-dihydrosorbicillin (2), and the dimeric hexaketide bisvertinoquinol (3). 1 has previously been isolated from Penicillium chrysogenum and also synthesised. Hydrogenation of 1 yielded tetrahydrosorbicillin (8), 2'5'-dihydrosorbicillin (9) and 2',3'-dihydrosorbicillin (2). 2 was also obtained by a BF₃-catalysed condensation of 2,4-dimethyl-resorcinol (5) with (4R^*, 5S^*)-4,5-dibromo-hexanoic acid (6), followed by debromination with zinc and acetic acid. The structure of the dimeric hexaketide 3 (without absolute configuration) was obtained by X-ray structure analysis. It may be considered to be a Diels-Alder adduct of the quinols 11 and 12, the latter being related to 1 and 2, respectively, by simple hydroxylation at C(5). The ¹H and ¹³C NMR signals of 3 and its mono-methyl ether 10 are interpreted and compared with the corresponding properties of 1, 2, vertinolide (4), 3-methoxy-2-methyl-2-cyclohexenone (14), and 2-[(E,E)-2,4-hexadienoyl)]-cyclohexanone (13). The latter was prepared by acylation of the lithium enolate of cyclohexanone with sorbyl chloride. From the spectra of the hexaketides from V. intertextum several patterns have been extracted which are characteristic for some common and some distinguishing substructures in these natural products.