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Avermectin stereochemistry

The polyketides are a family of natural products containing many important pharmaceutical agents that are synthesized through the multienzyme complex, polyketide synthase, which can display substantial molecular diversity with respect to chain length, monomer incorporated, reduction of keto groups, and stereochemistry at chiral centers (189). This variability, together with the existence of several discrete forms of polyketide synthase, allows the generation of diverse structures like erythromycin, avermectin, and rapamycin. This biochemical diversity has been considerably expanded by the introduction of new sub strate species that were used by the enzymes to produce new or unnatural polyketides (190). [Pg.350]

An instance of pathway manipulation with potentially high commercial significance was the production of epirubidn in S.peucetius [13]. The (4S)-TDP-4-keto-6-deoxyhexose reductase gene, dnmV, was disrupted by insertional inactivation, and the gene for a homologous reductase from erythromycin (EryBIV) or avermectin (AvrE) biosynthesis, with different product stereochemistry (4R), was introduced to the production strain (Scheme 9). [Pg.92]

Modifications at the 4a position starting with the 4a-hydroxy derivative [1, 57], with suitable protection of the remaining hydroxy groups lead via a 4a-phenylselenide to a 3-hydro-4a-dehydro-3-hydroxy derivative containing the 4-exocyclic double bond [58]. Further manipulation via the 4a-bromide leads to avermectin B with the correct stereochemistry of the crucial hexahydrobenzo-furan moiety, thus adding to the choice of suitable intermediates for total... [Pg.74]

As with all complex target molecules, there are many possible approaches to the construction of the highly functionalized southern half of the avermectins. Due to the tendency of the C-2 position to undergo epimerization or conjugation several of the approaches published to date have been directed at target molecules with a C-2,3 double bond rather than the C-3,4 double bond of the avermectins. This strategy eliminates several problems but demands a reliable method for eventual deconjugation and establishment of the correct stereochemistry at C-2. A solution to this problem was recently demonstrated by Hanessian et al. (vide infra). [Pg.82]

For structure elucidation, spectroscopy and X-ray crystallography offer not only speed and efficiency, but also a degree of structural certainty, unknown even a few years ago. Chief among these techniques is nuclear magnetic resonance, as illustrated throughout this volume for example, by Komis filal- for verifying stereochemistry of complex avermectin analogs. [Pg.4]


See other pages where Avermectin stereochemistry is mentioned: [Pg.451]    [Pg.391]    [Pg.578]    [Pg.578]    [Pg.575]    [Pg.97]    [Pg.691]    [Pg.67]    [Pg.578]   
See also in sourсe #XX -- [ Pg.6 , Pg.12 ]




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