Engineered Biosynthesis of Gilvocarcin Analogues with Altered Deoxyhexopyranose Moieties

ABSTRACTA combinatorial biosynthetic approach was used to interrogate the donor substrate flexibility of GilGT, the glycosyltransferase involved inC-glycosylation during gilvocarcin biosynthesis. Complementation of gilvocarcin mutantStreptomyces lividansTK24 (cosG9B3-U−), in which the biosynthesis of the natural sugar donor substrate was compromised, with various deoxysugar plasmids led to the generation of six gilvocarcin analogues with altered saccharide moieties. Characterization of the isolated gilvocarcin derivatives revealed five new compounds, including 4-β-C-d-olivosyl-gilvocarcin V (d-olivosyl GV), 4-β-C-d-olivosyl-gilvocarcin M (d-olivosyl GM), 4-β-C-d-olivosyl-gilvocarcin E (d-olivosyl GE), 4-α-C-l-rhamnosyl-gilvocarcin M (polycarcin M), 4-α-C-l-rhamnosyl-gilvocarcin E (polycarcin E), and the recently characterized 4-α-C-l-rhamnosyl-gilvocarcin V (polycarcin V). Preliminary anticancer assays showed thatd-olivosyl-gilvocarcin and polycarcin V exhibit antitumor activities comparable to that of their parent drug congener, gilvocarcin V, against human lung cancer (H460), murine lung cancer (LL/2), and breast cancer (MCF-7) cell lines. Our findings demonstrate GilGT to be a moderately flexibleC-glycosyltransferase able to transfer bothd- andl-hexopyranose moieties to the unique angucyclinone-derived benzo[d]naphtho[1,2b]pyran-6-one backbone of the gilvocarcins.

Synthetic study of ravidomycin, a hybrid natural product

Strategies and tactics associated with the total synthesis of hybrid natural products are discussed. The target is ravidomycin (2), one of the gilvocarcin-class antitumor antibiotics with an aryl C-glycoside structure. The first total synthesis of 2, which was achieved along similar lines of that of gilvocarcin V (1), served for the determination of the relative as well as the absolute stereochemistry of 2. Also revealed was a limitation of the synthetic scheme so long as the amino sugar congener was concerned. A preliminary result is discussed on the [2+2+2] approach that relies on the ready availability of various benzocyclobutene derivatives via regioselective [2+2] cycloaddition of α-alkoxybenzynes and ketene silyl acetals.