Concise synthesis and biological assessment of (+)-neopeltolide and a 16-member stereoisomer library of 8,9-dehydroneopeltolide: identification of pharmacophoric elements.

We describe herein a concise synthesis of (+)-neopeltolide, a marine macrolide natural product that elicits a highly potent antiproliferative activity against several human cancer cell lines. Our synthesis exploited the powerful bond-forming ability and high functional group compatibility of olefin metathesis and esterification reactions to minimize manipulations of oxygen functionalities and to maximize synthetic convergency. Our findings include a chemoselective olefin cross-metathesis reaction directed by H-bonding, and a ring-closing metathesis conducted under non-high dilution conditions. Moreover, we developed a 16-member stereoisomer library of 8,9-dehydroneopeltolide to systematically explore the stereostructure-activity relationships. Assessment of the antiproliferative activity of the stereoisomers against A549 human lung adenocarcinoma, MCF-7 human breast adenocarcinoma, HT-1080 human fibrosarcoma, and P388 murine leukemia cell lines has revealed marked differences in potency between the stereoisomers. This study provides comprehensive insights into the structure-activity relationship of this important antiproliferative agent, leading to the identification of the pharmacophoric structural elements and the development of truncated analogues with nanomolar potency.

Stereoselective synthesis of 2,6-cis-substituted tetrahydropyrans: Brønsted acid-catalyzed intramolecular oxa-conjugate cyclization of α,ß-unsaturated ester surrogates.

Intramolecular oxa-conjugate cyclization (IOCC) of α,ß-unsaturated carbonyl compounds, triggered by deprotonation with a base, represents a straightforward method for the synthesis of tetrahydropyrans. However, it has been known that stereochemical outcome of IOCC depends on the local structure of substrates and sometimes requires harsh reaction conditions and/or prolonged reaction times for selective formation of 2,6-cis-substituted tetrahydropyrans. These shortcomings limit the feasibility of IOCC in the context of complex natural product synthesis. In this paper, we describe Brønsted acid-catalyzed IOCC of α,ß-unsaturated ester surrogates (e.g., α,ß-unsaturated thioesters, oxazolidinone imides, and pyrrole amides) under mild reaction conditions, which affords a series of synthetically versatile 2,6-cis-substituted tetrahydropyran derivatives with good to excellent stereoselectivity (dr from 7:1 to >20:1). These α,ß-unsaturated carbonyl compounds were found to be more reactive than the corresponding oxoesters that are generally unreactive toward Brønsted acid-catalyzed intramolecular oxa-conjugate additions. The product tetrahydropyrans could be transformed into various derivatives in an efficient manner, highlighting the usefulness of our methodology.

Tandem catalysis in domino olefin cross-metathesis/intramolecular oxa-conjugate cyclization: concise synthesis of 2,6-cis-substituted tetrahydropyran derivatives.

Herein, we describe the concise synthesis of 2,6-cis-substituted tetrahydropyran derivatives based on a domino olefin cross-metathesis/intramolecular oxa-conjugate cyclization (CM/IOCC) reaction. We have found that the domino CM/IOCC of δ-hydroxy olefins with α,ß-unsaturated carbonyl compounds (e.g., trans-crotonaldehyde or N-acryloyl-2,5-dimethylpyrrole) could be efficiently achieved in the presence of the second-generation Hoveyda-Grubbs catalyst under elevated temperature conditions, directly affording 2,6-cis-substituted tetrahydropyrans in excellent yields with synthetically useful diastereoselectivity ("auto-tandem catalysis"). In addition, we have found that the domino CM/IOCC of δ-hydroxy olefins with α,ß-unsaturated carbonyl compounds could be achieved simply by performing CM in the presence of a Brønsted acid in CH(2)Cl(2) at 25-35 °C, which delivered 2,6-cis-substituted tetrahydropyrans in good yields with excellent diastereoselectivity ("orthogonal-tandem catalysis"). To understand the mechanism of auto-tandem catalysis in the domino CM/IOCC reaction, we have investigated the role of ruthenium hydride complexes in the IOCC of a ζ-hydroxy α,ß-unsaturated ketone as a model case.

Biosynthesis-inspired intramolecular oxa-conjugate cyclization of α,ß-unsaturated thioesters: stereoselective synthesis of 2,6-cis-substituted tetrahydropyrans.

Intramolecular oxa-conjugate cyclization of α,ß-unsaturated thioesters under Brønsted acid catalysis, inspired by biosynthesis of polyketide natural products, provides a variety of 2,6-cis-substituted tetrahydropyran derivatives with excellent diastereoselectivities. An added bonus of this methodology is that the product tetrahydropyrans could be readily elaborated to various derivatives by exploiting the unique reactivity of the thioester group.

Stereoselective synthesis of substituted tetrahydropyrans via domino olefin cross-metathesis/intramolecular oxa-conjugate cyclization.

A novel strategy for the stereoselective synthesis of substituted tetrahydropyrans has been developed on the basis of a domino olefin cross-metathesis/intramolecular oxa-conjugate cyclization catalyzed by the Hoveyda-Grubbs second-generation catalyst.