Organozirconocene-mediated polyene synthesis: Preparation of asukamycin and manumycin a side chains

Gold(I)-catalysed synthesis of conjugated trienes

Gold(I)-catalysed reaction between cyclopropenes and furans produces functionalised conjugated trienes. The reaction is mild, facile and proceeds with very low catalyst loadings.

Zirconium mediated total synthesis of crinitol, 9-hydroxyfarnesoic acid, 9-hydroxyfarnesol, 9-hydroxysargaquinone and the selectively-protected aglycone of moritoside and euplexide A

Tandem formation of an unsaturated zirconacycle, insertion of methallyl carbenoid, and addition of an aldehyde provides a rapid synthetic route to several linear terpenoid and terpene-polyketide natural products.

Further Studies on the Biosynthesis of the Manumycin-Type Antibiotic, Asukamycin, and the Chemical Synthesis of Protoasukamycin

Asukamycin (2), a metabolite of Streptomyces nodosus ssp. asukaensis ATCC 29757 and a member of the manumycin family of antibiotics, is assembled from three components, an "upper" polyketide chain initiated by cyclohexanecarboxylic acid, a "lower" polyketide chain initiated by the novel starter unit, 3-amino-4-hydroxybenzoic acid (3,4-AHBA), and a cyclized 5-aminolevulinic acid moiety, 2-amino-3-hydroxycyclopent-2-enone (C(5)N unit). To shed light on the order in which these components are assembled, we synthesized in labeled form various potential intermediates and evaluated their incorporation into 2. The assembly of the molecular framework of 2 from 3,4-AHBA and cyclohexanecarboxylic acid apparently does not involve free, unactivated intermediates. However, protoasukamycin (12), the total synthesis of which is reported, was efficiently converted into 2, demonstrating that the modification of the aromatic ring to the epoxyquinol structure is the terminal step in the biosynthesis. The results suggest that the two polyketide chains are synthesized separately and that the "upper" chain must be connected to the "lower" polyketide chain before the C(5)N unit.

On the role of chiral catalysts in the alkenyl zirconocene/zinc addition to aldehydes: a study of ligand loading and asymmetric amplification

Unusual nonlinear asymmetric amplification and chiral ligand loading effects were discovered for the use of catalytic quantities of chiral aminoalcohols in the in situ hydrozirconation-transmetalation-aldehyde addition processes. While the stereochemically most efficient aminothiol ligands demonstrated mechanistically conventional reaction parameters in excellent agreement with Kagan's ML(2) system, the asymmetric induction in the presence of a chiral aminoalcohol was found to vary greatly with loading and %ee of the ligand. Aminothiols remain the ligands of choice for the highly enantioselective formation of allylic alcohols and provide experimentally more predictable reaction variables. However, new, optimized conditions lead to a synthetically useful product %ee using the readily available and scalable aminoalcohol 2a.

Thiophilic ring-opening and rearrangement reactions of epoxyketone natural products

Thiol additions to the highly functionalized core structures of aranorosin- and manumycin-type antibiotics reveal the general reactivity patterns of epoxyketone natural products. Rapid hemiacetal and hydrate formations decrease the reactivity of the epoxyketone moiety in aqueous media toward the cellular scavenger glutathione, and secondary 1,2-shift, elimination, aromatization and intramolecular aldol reactions provide novel reaction pathways. In a hydrophobic environment, the thiol-capture function of the ketone moiety facilitates electrophilic attack.