Enantioselective Synthesis of (+)-Isobretonin A

Stereoselective Synthesis of Conjugated Polyenes Based on Tethered Olefin Metathesis and Carbonyl Olefination: Application to the Total Synthesis of (+)-Bretonin B

The combination of a highly stereoselective tethered olefin metathesis reaction and a Julia-Kocienski olefination is presented as a strategy for the synthesis of conjugated polyenes with at least one Z-configured C═C bond. The strategy is exemplified by the synthesis of the marine natural product (+)-bretonin B.

[3,3]-Sigmatropic rearrangement/5-exo-dig cyclization reactions of benzyl alkynyl ethers: synthesis of substituted 2-indanones and indenes

Substituted benzyl alkynyl ethers, prepared from the corresponding α-alkoxy ketones in a two-step sequence involving enol triflate formation and KOtBu-induced E2 elimination, undergo [3,3]-sigmatropic rearrangement/intramolecular 5-exo-dig cyclization at 60 °C to form substituted 2-indanones in good overall yields. 1,3-cis-Disubstituted-2-indanones are formed preferentially when the benzylic substituent R(1) is bulky. Substituted indenes may be prepared from 2-indanones in high yields by Horner-Wadsworth-Emmons reaction.

The First Synthesis of the ABC-Ring System of ‘Upenamide

The first synthetic route to the spirooxaquinolizidinone core (ABC core) of the macrocyclic marine alkaloid 'upenamide (1) has been developed. All five stereocenters were introduced with complete stereocontrol. The hydroxyl group at C-11 was introduced by a regio- and stereoselective SeO(2)-mediated allylic oxidation. The spirocyclic skeleton was formed by a stannous chloride induced deacetalization-bicyclization procedure. Further stereocenters were introduced by an enzymatic desymmetrization and by incorporation of an (S)-malic acid derived building block.

On the Biosynthetic Origin of Methoxymalonyl-Acyl Carrier Protein, the Substrate for Incorporation of “Glycolate” Units into Ansamitocin and Soraphen A

Feeding experiments with isotope-labeled precursors rule out hydroxypyruvate and TCA cycle intermediates as the metabolic source of methoxymalonyl-ACP, the substrate for incorporation of "glycolate" units into ansamitocin P-3, soraphen A, and other antibiotics. They point to 1,3-bisphosphoglycerate as the source of the methoxymalonyl moiety and show that its C-1 gives rise to the thioester carbonyl group (and hence C-1 of the "glycolate" unit), and its C-3 becomes the free carboxyl group of methoxymalonyl-ACP, which is lost in the subsequent Claisen condensation on the type I modular polyketide synthases (PKS). d-[1,2-(13)C(2)]Glycerate is also incorporated specifically into the "glycolate" units of soraphen A, but not of ansamitocin P-3, suggesting differences in the ability of the producing organisms to activate glycerate. A biosynthetic pathway from 1,3-bisphosphoglycerate to methoxymalonyl-ACP is proposed. Two new syntheses of R- and S-[1,2-(13)C(2)]glycerol were developed as part of this work.

Convergent highly stereoselective preparation of the C12-C24 fragment of macrolactin A

The convergent synthesis of the C12-C24 fragment (lower part) of macrolactin A is described. The adapted strategy allowed building up the lower moiety by the assembly of three key intermediates via organometallic addition. One hydroxylic stereogenic center was introduced by the application of chiral sulfoxides methodology on fragment C19-C24. The preparation of the versatile 1,3-anti diol synthon C12-C16 was achieved via opening of chiral epoxide and subsequent oxidation to a hydroxy ketone. Finally, reductive elimination of the appropriate allylic dibenzoate with Na/Hg introduced directly the C16-C19 (E,E)-diene unit, in a highly efficient stereoselective fashion.

A highly enantioselective benzothiepine synthesis

A highly enantioselective synthesis of benzothiepine 1a has been accomplished via an enantioenriched sulfoxide intermediate obtained by asymmetric oxidation with a chiral oxaziridine in 89:11 er. The key step is a thermodynamically controlled asymmetric cyclization reaction that produces two new stereogenic centers. The (4R,5R) isomer 1a was obtained in 98:2 er.