Synthetic studies towards phorboxazole A. Stereoselective synthesis of the C3–C19 bis-oxane oxazole portion of the phorboxazole macrolide

Assembly of Tetrahydropyran Derivatives from Aldehydes, Allylboronates, and Syngas by Asymmetric Relay Catalytic Cascade Reaction

An efficient synthesis of highly enantio-enriched tetrahydropyrans from readily available aldehydes, allylboronates, and syngas has been established by multiply relay catalysis of rhodium and chiral phosphoric acid. The cascade reaction integrates the asymmetric allylboration of aldehydes and alkene hydroformylation, providing a structurally diverse range of products with different workup procedures. The concise synthesis of key chiral building blocks to access herboxidiene and leucascandrolide A demonstrates the high synthetic utility of this method. The cascade reaction employing alkenes to replace aldehydes was also successful.

ent-Kaurane-based regio- and stereoselective inverse electron demand hetero-Diels-Alder reactions: synthesis of dihydropyran-fused diterpenoids

A mild and concise approach for the construction of a 3,4-dihydro-2H-pyran ring integrated into the A-ring of the natural product oridonin using an optimized inverse electron demand hetero-Diels-Alder (IED HDA) reaction is reported herein. A self-dimerization of the exocyclic enone installed in the A-ring through a homo-HDA reaction was identified to exclusively give a dimeric ent-kaurane diterpenoid with the spirochroman core. Moreover, efficient cross-HDA cycloadditions of this enone with various vinyl ethers or vinyl sulfides, instead of its own homo-HDA dimerization, were achieved in a regio- and stereoselective manner, thus providing access to novel dihydropyran-fused diterpenoids as potential anticancer agents to overcome chemoresistance.

Synthesis and evaluation of heteroarylalanine diacids as potent and selective neutral endopeptidase inhibitors

Heteroarylalanine derivatives 4 were designed as potential inhibitors of neutral endopeptidase (NEP EC 3.4.24.11). Selectivity over other zinc metalloproteinases was explored through occupation of the S2' subsite within NEP. Structural optimisation led to the identification of 5-phenyl oxazole 4f, a potent and selective NEP inhibitor. A crystal structure of the inhibitor bound complex is reported.

Total Synthesis of Phorboxazole B

An efficient and highly convergent total synthesis of the potent antitumor agent phorboxazole B has been achieved. The synthetic strategy of this synthesis features: 1) a highly efficient substrate-controlled hydrogenation to construct the functionalized cis-tetrahydropyrane unit; 2) iterative crotyl addition to synthesize the segment that contains alternating hydroxyl and methyl substituents; 3) Hg(OAc)2/I2-induced cyclization to establish the cis-tetrahydropyrane moiety; 4) 1,3-asymmetric induction in the Mukaiyama aldol reaction to afford the stereogenic centers at C9 and C3; and 5) the exploration of the Still-Gennari olefination reaction to complete the macrolide ring of phorboxazoloe B.

Synthesis of the C1-C17 segment of phorboxazole B

The C1-C17 bis-oxane subunit 22 of phorboxazole B is efficiently synthesized by exploiting differential reactivities between similar substituents on the hydropyran rings in 4. Selective dihydroxylation of the equatorial vinyl group, hydroboration of the axial vinyl group, and intramolecular Mitsunobu lactonization serve to fully differentiate the similar hydropyrans.

A Catalytic Enantioselective Hetero Diels−Alder Approach to the C20−C32 Segment of the Phorboxazoles

[reaction: see text] An efficient synthesis of the C20-C32 segment of the phorboxazoles has been achieved using an enantioselective hetero Diels-Alder reaction catalyzed by Jacobsen's Cr(III) amino indanol Schiff base catalyst.

Marine natural products: synthetic aspects

An overview of marine natural products synthesis during 2003 is provided. The emphasis on total syntheses of molecules of contemporary interest, new total syntheses, and syntheses that have resulted in structure conformation or stereochemical assignments.

Complex Peptidyl Nucleoside Antibiotics: Efficient Syntheses of the Glycosyl Nucleoside Amino Acid Cores

Employing an amino acid chiral template strategy, the present research describes a general and highly efficient protocol for the rapid construction of enantiopure furanosyl and pyranosyl nucleoside amino acid cores as present in various complex peptidyl nucleoside antibiotics. Starting from easily available d-serine, the strategy and the approach involve rapid and efficient stereoselective synthesis of five- or six-membered lactone amino alcohols, followed by incorporation of the required functionalities of the target molecules on these strategically functionalized chiral templates.

Toward the Total Synthesis of Phorboxazole B: An Efficient Synthesis of the C20−C46 Segment

An efficient synthesis of the C20-C46 segment of phorboxazole B is described. The key steps involved Hg(OAc)(2)/I(2)-induced cyclization to construct the cis-tetrahydropyran moiety, the coupling of the metalated 2-methyloxazole 7 with lactone 6, and Julia olefination to furnish the conjugated diene moiety.

Studies of stereocontrolled allylation reactions for the total synthesis of phorboxazole A

A highly convergent total synthesis of the potent anticancer agent (+)-phorboxazole A (1) is accomplished. Four components (3-6) are assembled with considerations for control of absolute and relative stereochemistry. Iterative asymmetric allylation methodology addresses key stereochemical features in the preparation of the 2,6-cis- and 2,6-trans-tetrahydropyranyl rings of the C3-C19 component (3). The stereocontrolled asymmetric allylation process is also used for development of the C28-C41 fragment (4). Novel Barbier coupling reactions of alpha-iodomethyl oxazoles and related thiazoles are described with samarium iodide. The convergent assembly of components 4 and 5 features formation of the fully substituted C22-C26 pyran by intramolecular capture of an allyl cation intermediate with high facial selectivity, and further efforts lead to E-C19/C20 olefination. The synthesis culminates with use of a modified Julia olefination for attachment of the C42-C46 segment and subsequent late-stage macrocyclization by installation of the (Z)-C2/C3 alpha,beta-unsaturated lactone.

Palladium(0)-Mediated Desymmetrization of Meso Tetraols: An Approach to the C3−C17 Bis-oxane Segment of Phorboxazoles A and B

[reaction: see text] meso-Tetraol bis(allylic acetates) 2 and 5 were synthesized via two-directional chain elongation. A palladium-mediated, ligand-controlled desymmetrization provided the desired bis-oxanes in greater than 98% ee. Bis-oxanes 1 and 4 represent potential synthetic intermediates for the C3-C17 subunits of phorboxazoles A and B.

Total synthesis of (+)-phorboxazole A, a potent cytostatic agent from the sponge Phorbas sp

A convergent total synthesis of phorboxazole A (1a), from the C(3-19), C(20-27) and C(33-46) fragments 5, 4 and 91, respectively, concentrating on stereocontrolled formation of the bonds at C(2-3), C(19-20) and C(27-28), is described. Although a coupling reaction between a macrolide ketone and the side chain substituted sulfone, at C(27-28) was not successful, a Wadsworth-Emmons olefination involving the oxane methyl ketone 4 and an oxazole produced the oxane 90 which was next coupled to 91 leading to the C(20-46) unit 100. A further coupling of 100 to 71c at C(19-20) then led to 105, ultimately, and the synthesis was completed by a macrocyclisation reaction from 105, at the C(2-3) alkene bond, followed by deprotection of 106.

Total synthesis of (+)-phorboxazole A exploiting the Petasis-Ferrier rearrangement

A highly convergent, stereocontrolled total synthesis of the potent antiproliferative agent (+)-phorboxazole A (1) has been achieved. Highlights of the synthesis include: modified Petasis-Ferrier rearrangements for assembly of both the C(11-15) and C(22-26) cis-tetrahydropyran rings; extension of the Julia olefination to the synthesis of enol ethers; the design, synthesis, and application of a novel bifunctional oxazole linchpin; and Stille coupling of a C(28) trimethyl stannane with a C(29) oxazole triflate. The longest linear sequence leading to (+)-phorboxazole A (1) was 27 steps, with an overall yield of 3%.

Synthetic studies toward phorboxazole A. stereoselective synthesis of the C(28)-C(46) side chain fragment

A stereoselective synthesis of the C(28)-C(46) fragment (3) of phorboxazole A is described. Key advances include an enantioselective allylation to establish the stereochemistry of the tetrahydropyran unit and a useful SmI(2)-mediated modification of the Barbier reaction of iodomethyloxazole 15 with aldehyde 14.