Total synthesis of (+)-phorboxazole A

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.

MgI2-catalyzed halo aldol reaction: a practical approach to (E)-beta-iodovinyl-beta'-hydroxyketones

A novel generation of 1-iodo-3-siloxy-1,3-butadienes has been developed by reacting trimethylsilyl iodide (TMS-I) with alpha, beta-unsaturated ketones in dichloromethane at 0 degrees C without the use of any catalyst. The halo aldol reaction of these butadiene intermediates with aldehydes was efficiently carried out by using magnesium iodide as the catalyst. Twelve beta-iodo-alpha,beta-unsaturated-beta'-hydroxyketones (halo aldols) have been synthesized under the new condition with excellent geometric selectivity and good chemical yields (>80% chemical yields for 11 examples).

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.

Catalytic Asymmetric 1,4-Addition Reactions Using α,β-Unsaturated N-Acylpyrroles as Highly Reactive Monodentate α,β-Unsaturated Ester Surrogates

Synthesis and application of alpha,beta-unsaturated N-acylpyrroles as highly reactive, monodentate ester surrogates in the catalytic asymmetric epoxidation and Michael reactions are described. alpha,beta-Unsaturated N-acylpyrroles with various functional groups were synthesized by the Wittig reaction using ylide 2. A Sm(O-i-Pr)(3)/H(8)-BINOL complex was the most effective catalyst for the epoxidation to afford pyrrolyl epoxides in up to 100% yield and >99% ee. Catalyst loading was successfully reduced to as little as 0.02 mol % (substrate/catalyst = 5000). The high turnover frequency and high volumetric productivity of the present reaction are also noteworthy. In addition, a sequential Wittig olefination-catalytic asymmetric epoxidation reaction was developed, providing efficient one-pot access to optically active epoxides from various aldehydes in high yield and ee (96-->99%). In a direct catalytic asymmetric Michael reaction of hydroxyketone promoted by the Et(2)Zn/linked-BINOL complex, Michael adducts were obtained in good yield (74-97%), dr (69/31-95/5), and ee (73-95%). This represents the first direct catalytic asymmetric Michael reaction of unmodified ketone to an alpha,beta-unsaturated carboxylic acid derivative. The properties of alpha,beta-unsaturated N-acylpyrrole are also discussed. Finally, the utility of the N-acylpyrrole unit for further transformations is demonstrated.

Phorboxazole B synthetic studies: construction of C(1–32) and C(33–46) subtargets

The convergent syntheses of the C(1-32) and C(33-46) domains of phorboxazole B are described. An iterative cyclocondensation strategy exploited the Jacobsen hetero-Diels-Alder (HDA) reaction as a platform for the synthesis of both the C(5-9) and C(11-15) tetrahydropyran rings. The use of 2-silyloxydiene coupling partners bearing an increasing resemblance to the phorboxazole skeleton was found to lead to a reduction in diastereoselectivity, however, in the case of the C(11-15) ring. The coupling of aldehyde and 2-silyloxydiene by this route provided a C(1-32) fragment which was elaborated to the macrolide core of phorboxazole B. The synthesis of the C(33-46) domain involved a Nozaki-Kishi coupling of aldehyde 31 and vinyl iodide 39. The syntheses of 31 and 39 were highly diastereoselective: an Evans [Cu(Ph-pybox)](SbF6)2-catalysed Mukaiyama aldol reaction formed the cornerstone of the synthesis of 31 whilst a Nagao-Fujita acetate aldol reaction provided a convenient means of installing the sole stereogenic centre of 39.

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.

A new halo aldol reaction: three-component reaction via 1,4-robust activation of ethynyl alkyl ketones for stereoselective formations of versatile aldol adducts

[reaction: see text] A new three-component halo aldol reaction has been discovered for the tandem formations of I-C/C-C bonds by activating the alpha',beta-positions of alpha,beta-acetylenic ketones. The key intermediates, 1-iodo-3-siloxy-1,3-butadienes, were generated from allenolates and directly monitored by (1)H NMR spectroscopic analysis. Excellent geometric selectivity (>95%) and good yields (65-82%) have been achieved for 10 examples.

Total syntheses of (+)-zampanolide and (+)-dactylolide exploiting a unified strategy

The first total syntheses of (+)-zampanolide (1) and (+)-dactylolide (2), members of a new class of tumor cell growth inhibitory macrolides, have been achieved. Key features of the unified synthetic scheme included the stereocontrolled construction of the cis-2,6-disubstituted tetrahydropyran via a modified Petasis-Ferrier rearrangement, a highly convergent assembly of the macrocyclic domain, and, in the case of zampanolide, a Curtius rearrangement/acylation tactic to install the N-acyl hemiaminal. The complete relative and absolute stereochemistries for both (+)-zampanolide and (+)-dactylolide were also assigned, albeit tentatively in the case of (+)-zampanolide (1).

Pyran annulation: asymmetric synthesis of 2,6-disubstituted-4-methylene tetrahydropyrans

[reaction: see text] A reaction process for the asymmetric construction of a variety of cis or trans disubstituted pyrans is described. This sequences allows for the asymmetric convergent union of two aldehydes with silyl-stannane reagent 1 in a two-step process: catalytic asymmetric allylation of the first aldehyde using 1 with a BITIP catalyst, followed by reaction of the alcohol so obtained with a second aldehyde and TMSOTf.

The total synthesis of C-glycosides with completely resolved seven-carbon backbone polyol stereochemistry: stereochemical correlations and access to L-configured and other rare carbohydrates

The de novo synthesis of a full set of hydroxymethyl C-glycosides from only two precursors is described. The seven-carbon target molecules contain five stereocentres and bridge the stereochemical gap between natural D-configured and non-natural L-configured series of hexoses. Key steps include hydroxylation, differential protection, stereoselective reduction and desymmetrization of 8-oxabicyclo[3.2.1]oct-6-enes. C-Terminus differentiation and C-terminus excision of the seven-carbon polyol backbone lead to hexoses, including those of the L-series. A stereochemical and genetic classification of C-glycosides is presented.

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%.