Nikkomycins

Nikkomycins. The nikkomycins (141—159), isolated from S. tendae are nucleoside-peptide antibiotics (1,4,244,245) as shown in Table 8. Nikkomycins X and Z are stmcturaHy identical to neopolyoxins A and C, respectively. Compound (141) is a competitive inhibitor of chitin synthetase. Two new nikkomycins, nikkomycin pseudo-Z and pseudo-J (158, 159), contain a C-glycosidic bond between C-5 of uracil and C-1 of... 

However, not all nucleophiles show the same bias as shown in Scheme 4 5 on addidon to the nirroalkene The product of the addidon of potassium phthalimide has 5(R) stereochemistry fEq 4 38 This stereoselecdve addidon is applied for the synthesis of other related andbiodcs, such as nikkomycine B ... 

Enzyme preparations from liver or microbial sources were reported to show rather high substrate specificity [76] for the natural phosphorylated acceptor d-(18) but, at much reduced reaction rates, offer a rather broad substrate tolerance for polar, short-chain aldehydes [77-79]. Simple aliphatic or aromatic aldehydes are not converted. Therefore, the aldolase from Escherichia coli has been mutated for improved acceptance of nonphosphorylated and enantiomeric substrates toward facilitated enzymatic syntheses ofboth d- and t-sugars [80,81]. High stereoselectivity of the wild-type enzyme has been utilized in the preparation of compounds (23) / (24) and in a two-step enzymatic synthesis of (22), the N-terminal amino acid portion of nikkomycin antibiotics (Figure 10.12) [82]. 

Figure 10.12 Stereoselective synthesis ofthe amino acid portion of nikkomycin antibiotics and hexulosonic acids using KDPGIc aldolase.

Antimycin A is effective against Altemaria solani spores(42)5 karumin is effective against Rhizoctonia solani ( iD. Nikkomycin is being used to cure trees of Dutch elm disease in Hamden, Connecticut(44). It inhibits the formation of chitin and stops the myceTia of Ceratocy-stis ulmi from growing normally. 

The tandem transesterification/[3 + 2]-cycloaddition methodology is be a powerful synthetic tool, since it guarantees high diastereoselectivity even under thermal conditions. It has been successfully apphed to synthetic work of the N-terminal amino acid component of Nikkomycin Bz (Scheme 11.53) (173). Thus, the sugar-based oxime is condensed with a glyoxylate hemiacetal to produce a chiral nitrone ester, which is then reacted with ( )-p-niethoxycinnamyl alcohol in the presence of a catalytic amount of TiCU at 100 °C. After the intramolecular cycloaddition, the... 

With the same protocol, a heterocyclic dibenzoate 86 derived from furan in one step has been efficiently desymmetrized to provide facile entry to either D or L nucleosides (see Scheme 8E.10). As depicted in Scheme 8E.10, the catalyst derived from ligand 71 gave rise to high enantioselectivities in the alkylation with both a purine 83 and a pyrimidine 87 [62], Subsequent allylic alkylations with an achiral ligand introduced the tartronate and aminomalonate moieties to furnish enantiomerically pure Ci s-2,5-disubstituted-2,5-dihydrofurans 89 and 91, respectively. Only six steps from furan were required to synthesize the alio and talo isomers of the nucleoside skeleton of the polyoxin-nikkomycin complexes. It should be noted that the corresponding enzymatic desymmetrization of substrate 86 is impossible because the product is labile. 

An application of the deracemization strategy has provided efficient entry to a novel amino acid substituent of the antifungal agents, polyoxins and nikkomycins, as shown in Scheme 8E.20. The versatile five-carbon building block was obtained from phthalimidation of the hydroxymethyl-substituted epoxide in 87% yield and 82% ee. Straightforward synthesis of polyoxamic acid was then accomplished by subsequent dihydroxylation and selective oxidation of the alkylation product. 

A further interesting example in the context of natural products is shown, in the transformation of the p-lactam 102 into the p-amino ketone 103, Scheme 34, which upon carbonyl reduction provides the amino lactone 104, the cyclized form of the Ai-terminal amino acid residue found in the antibiotic family of nikkomycins, Fig. 6 [101]. 

Scheme 34 Opening of P-lactams with a Grignard reagent as an entry to precursors of nikkomycins...

Asymmetric induction in the aldol reaction of enolsilane and metal enolate nucleophiles with yS-substituted aldehydes gives rise to both excellent yields and good diastereoselectivities (equation 128)507. The best diastereoselectivity was obtained using a trimethylsilyl enolate in the presence of boron trifluoride-etherate (92 8 anti. syn). The key step in the synthesis of the N-terminal amino acid analogue of nikkomycin B and Bx (nucleoside peptide antibiotics) has been performed using this type of methodology508. 

A few other biologically interesting and naturally occurring peptides and amino acids of rather simple structure were synthesized using the Ugi four-component reaction (Figure 12.5) the phosphonic acid antibiotics plumbemycin A 249 and B 250 [126], both epimers of the polychlorinated antihypertensive peptide (+)-demethyldysidenin 251 [127], and the nucleoside antibiotic nikkomycin 252 [128]. 

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