Synthesis of the C1–C27 Fragment of Stambomycin D Validates Modular Polyketide Synthase-Based Stereochemical Assignments

Abstract

The stambomycins are a family of bioactive macrolides isolated from Streptomyces ambofaciens . Aside from two stereocenters installed through cytochrome P450 oxidations, their stereochemistry has been predicted by sequence analysis of the polyketide synthase. We report a synthesis of the C1–C27 fragment of stambomycin D, the spectroscopic data of which correlates well with that of the natural product, further validating predictive sequence analysis as a powerful tool for stereochemical assignment of complex polyketide natural products.

Engineering the stambomycin modular polyketide synthase yields 37-membered mini-stambomycins

“…As in Fig. 1 , the indicated configurations have been extrapolated from those assigned to the stambomycins 1 77 . TE thioesterase, SDM site-directed mutagenesis.…”

“…As in Figs. 1 and 3 , the indicated configurations have been extrapolated from those assigned to the stambomycins 1 77 . KS ketosynthase (KS Q refers to replacement of the active site cysteine residue by glutamine), AT acyl transferase, ACP acyl carrier protein, DH dehydratase, ER enoyl reductase, KR ketoreductase, TE thioesterase, C DD C-terminal docking domain, N DD N-terminal docking domain.…”

“…The last KR domain of module 24 (gray) is inactive. b Structure of stambomycins 1 (A−F), which differ from each other in the alkyl functionality (R group) at position C-26 (the indicated stereochemistries 33 were predicted based on analysis of known domain stereochemical determinants 76 , and those for the C-1–C-27 fragment recently confirmed by total synthesis 77 ). The monomers are color-coded to match the modules responsible for their incorporation.…”

Synthesis of the C50 diastereomers of the C33–C51 fragment of stambomycin D

“…[25][26][27][28] We recently described a synthesis of the C1-C27 region of stambomycin D, in which the spectroscopic data of the synthetic truncate showed a good match with that of the natural product. 29 Continuing from that work, we report here the synthesis of two C50 diastereomers of the C33-C51 segment of the stambomycins, and comparison of the associated spectroscopic data with the natural product, which further supports the sequence analysis based stereochemical prediction. This study also identifies suitable conditions for deprotection of the target fragments, bearing in mind the potential sensitivities in these molecules (specifically the C50 allylic oxygen functionality) and, for future work, compatibility with the previously synthesized C10-C13 diene.…”

“…These conditions are particularly appealing, as they were also those optimized for deprotection of the C1-C27 fragment in our previous work. 29 Application of these conditions to the epimer 22b gave 1b in 52% yield. Purification of 1a and 1b proved non-trivial due to their extremely high polarity: normal phase chromatography was unsuitable, but high purity samples could be obtained using reversed phase HPLC.…”