Discodermolide is a potentially important antitumor agent that stabilizes microtubules and blocks cells at the G2/M phase of the cell cycle in a manner similar to that of Taxol. Discodermolide also has unique properties that distinguish it from Taxol. In the present study, photoaffinity-labeled discodermolide analogues are used to investigate their binding site in tubulin. Three photoaffinity-labeled discodermolide analogues were synthesized, all of which promoted microtubule polymerization in the absence of GTP. The analogue, C19-[4-(4-(3)H-benzoyl-phenyl)-carbamate]-discodermolide (C19-[3H]BPC-discodermolide), was selected for photolabeling studies because it had the highest extent of photoincorporation, approximately 1%, of the three radiolabeled discodermolide analogues explored. Although compared to discodermolide, C19-BPC-discodermolide revealed no hypernucleation effect in the in vitro microtubule polymerization assay, it was more cytotoxic than discodermolide, and, like discodermolide, demonstrated synergism with Taxol. These results suggest that the hypernucleation effect of discodermolide is not involved in its cytotoxic activity. Similar to discodermolide, C19-BPC-discodermolide can effectively displace [3H]Taxol from microtubules, but Taxol cannot effectively displace C19-[3H]BPC-discodermolide binding. Discodermolide can effectively displace C19-[3H]BPC-discodermolide binding. Formic acid hydrolysis, immunoprecipitation experiments, and subtilisin digestion indicate that C19-BPC-discodermolide labels amino acid residues 305-433 in beta-tubulin. Further digestion with Asp-N and Arg-C enzymes suggested that C19-BPC-discodermolide binds to amino acid residues, 355-359, in beta-tubulin, which is in close proximity to the Taxol binding site. Molecular modeling guided by the above evidence led to a putative binding model for C19-BPC-discodermolide in tubulin.

A plausible origin of biomolecular homochirality is advanced, where alpha-methyl amino acids found on meteorites transfer their chirality in the synthesis of normal amino acids. This asymmetry can be amplified to nearly homochiral levels, thus providing the necessary prerequisite for life to start on this planet and elsewhere in the universe.

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A convergent synthesis of (-)-2-epi-Peloruside A has been achieved. Highlights include implementation of multicomponent Type I Anion Relay Chemistry (ARC) to unite 2-TBS-1,3-dithiane with two epoxides to construct the eastern hemisphere, a late-stage dithiane union to secure the complete, fully functionalized carbon backbone, and Yamaguchi macrolactonization, which led to (-)-2-epi-peloruside A via an unexpected epimerization at C(2).*E-mail: smithab@sas.upenn.edu. Supporting Information Available Spectroscopic and analytical data for compounds 6-28 and selected experimental and computational procedures. This material is available free of charge via the internet at http://pubs.acs.org. In 2000 Northcote and co-workers reported the isolation and relative stereochemistry of (+)-peloruside A (1), 1 an architecturally complex marine metabolite produced by the sponge Mycale (Carmia). Although a microtubule-stabilizing agent with potency similar to Taxol, 2 recent studies reveal that (+)-peloruside A competes competitively for the laulimalide binding site, at a newly discovered microtubule site. 3 NIH Public AccessOur interest in (+)-peloruside A (1) emanated from the synthetic challenge, in conjunction with the opportunity to showcase the synthetic utility of dithiane linchpin tactics, in particular the use of the three-component union of trialkylsilyl dithianes with diverse electrophiles, a synthetic tactic we now recognize as Type I Anion Relay Chemistry (ARC). 4Structurally (+)-peloruside A (1) is comprised of 10-stereogenic centers, a Z-trisubstituted olefin, and a six-membered hemi-ketal ring, inscribed in a 16-membered macrolactone. Not surprisingly, the structural complexity, interesting biological activity, and scarcity, has led to considerable interest from both the chemical 5 and biological communities. 6In 2003, De Brabander and co-workers 7 achieved an elegant total synthesis of unnatural (−)-peloruside A, thus permitting assignment of the absolute configuration. Shortly thereafter (2005), the Taylor group 8 reported the first total synthesis of natural (+)-peloruside A, followed in 2008 by a second total synthesis from the Ghosh laboratory. 9 We report here completion of the total synthesis of (−)-2-epi-peluroside A (28, Scheme 5), the result of a surprising, late stage epimerization (vide infra) that procluded access to (+)-peloruside A (1).Shortly after the report by Northcote and co-workers, 1 we initiated a synthetic venture directed toward the total synthesis of (+)-peloruside A (1). 10 Our endgame strategy called for formation of the inscribed tetrahydropyran ring after macrocyclization (Scheme 1). Central to this scenario was a flexible route that would permit either acid or alcohol activation to achieve macrolactonization. Taken together, (+)-peloruside A (1) was envisioned to arise from macrolide 2 upon removal of the dithiane and isopropylidene protecting groups. To construct the macrolactone precursor, we would employ union of a dithiane 3 with aldehyde 4, followed by appropriate funct...