Nonaketide

The derivation of aflatoxin B can be traced back to the xanthone sterigmatocystin (693), which originates from a decaketide (692), whilst the xanthone bikaverin (694) is derived from a nonaketide. 

ZEA2. ZEA1 produces a nonaketide by using the hexaketide as a starter unit. The nonaketide is easily converted into zearalenone by aromatization and lactonization. 

LNKS formally should synthesize a fully elaborated nonaketide such as 47, which could undergo a biological Diels-Alder reaction to form dihydromonacolin L 48—which is the observed hrst PKS-free intermediate. Note that it is possible that the Diels Alder reaction actually occurs at the more activated hexaketide stage. When lovB was expressed in the heterologous fungal host A. nidulans (4) however, the polyunsaturated compounds... 

Feeding studies to Nocardia argentine sis with - C labelled NAC thioesters of di-, tri-, tetra- and pentaketide intermediates demonstrated regiospecific incorporations into nargenicin 37 (Scheme 18) which are consistent with an intramolecular Diels-Alder cyclisation of a linear nonaketide intermediate (Scheme 19) [72,73]. 

Transformations of cyclic nonaketides by Aspergillus terreus mutants blocked for lovastatin biosynthesis, J.L. Sorensen, K. Auclair, J. Kennedy, C.R. Hutchinson and J.C. Vederas, Org. Biomol. Chem., 2003,1, 50. 

Since we reported the enzymatic activity of SPS in 1995 as the first Diels-Alderase, two additional Diels-Alderases, lovastatin nonaketide synthase (LNKS) and macrophomate synthase (MPS), have been purified and characterized. Two of these catalyze intramolecular Diels-Alder reactions while the third catalyzes an intermolecular Diels-Alder reaction. We have recently reported the detailed reaction pathway ° ° of MPS and its catalytic mechanism based on the crystal structure. " In this section, we describe three natural Diels-Alderases and discuss the mechanism of their catalysis. 

A new nonaketide metabolite, obionin A (28), was isolated as a brownish-red solid from the air-dried mycelium of a halotolerant Ascomycotina, Leptospaeha obiones (Crouan et Crouan) Saccoro (Leptosphaeriaceae), collected from the coastal salt-marsh grass Spartina alternifolia. It constitutes the first reported lH-naphto[2,3-c]pyran 8,9-dione system. The stereochemistry of the side-chain has not yet been determined [76]. 

Later, an extensive study of the biosynthetic pathway involving the incorporation of [ C]acetates and [ C]- and [ C]malonates by cultures of P. terrestris revealed that phomazarin contains nine acetate moieties and must be formed from a single nonaketide chain folded, as shown in Scheme 6 (24). 

---