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

Transition metal catalysis—a unique road map in the stereoselective synthesis of 1,3-polyols

The present review summarizes recent diverse reactions employed in the formation of 1,3-polyols providing an overview of the mechanistic pathway and the enantioselectivity obtained, in terms of the properties of transition metals directly involved in the catalytic transformations and their interaction with various ligands.

Formal Synthesis of Premisakinolide A and C(19)-C(32) of Swinholide A via Site-Selective C-H Allylation and Crotylation of Unprotected Diols

Using stereo- and site-selective C-H allylation and crotylation of unprotected diols, an intermediate in the synthesis of premisakinolide A (bistheonellic acid B) that was previously made in 16-27 (LLS) steps is now prepared in only nine steps. This fragment also represents a synthesis of C(19)-C(32) of the actin-binding macrodiolide swinholide A.

Catalyst-directed diastereo- and site-selectivity in successive nucleophilic and electrophilic allylations of chiral 1,3-diols: protecting-group-free synthesis of substituted pyrans

The iridium-catalyzed, protecting group-free synthesis of 4-hydroxy-2,6-cis- or trans-pyrans through successive nucleophilic and electrophilic allylations of chiral 1,3-diols occurs with complete levels of catalyst-directed diastereoselectivity in the absence of protecting groups, premetallated reagents, or discrete alcohol-to-aldehyde redox reactions.

Total synthesis of the marine toxin phorboxazole A using palladium(II)-mediated intramolecular alkoxycarbonylation for tetrahydropyran synthesis

The potent antitumor agent phorboxazole A was synthesized from six subunits comprising C1-C2 (115), C3-C8 (98), C9-C19 (74), C20-C32 (52), C33-C41 (84) and C42-C46 (85). Tetrahydropyrans B and C containing cis-2,6-disubstitution were fabricated via palladium(II)-mediated intramolecular alkoxycarbonylation which, in the case of tetrahydropyran C, was carried out with catalytic palladium(II) and p-benzoquinone as the stoichiometric re-oxidant. Tetrahydropyran D was obtained by a stereoselective tin(IV)-catalyzed coupling of a C9 aldehyde with an allylsilane, and the C19-C20 connection was made using a completely stereoselective Wittig-Schlosser (E) olefination. Coupling of the oxazole C32 methyl substituent with the intact C33-C46 δ-lactone 3was accompanied by elimination of the vinyl bromide to a terminal alkyne, but the C32-C33 linkage was implemented successfully with 83 and C33-C41 lactone 84. The C42-C46 segment of the side chain was then appended via Julia-Kocienski olefination. The macrolide portion of phorboxazole A was completed by means of an Ando-Still-Gennari intramolecular (Z)-selective olefination at C2-C3 which required placement of a (dimethoxyphosphinyl)acetate moiety at C24. Final deprotection led to phorboxazole A via a route in which the longest linear sequence is 37 steps and the overall yield is 0.36%.

Stereoselective synthesis of cyclic ethers via the palladium-catalyzed intramolecular addition of alcohols to phosphono allylic carbonates

Cross metathesis of the acrolein-derived phosphono allylic carbonate and hydroxy alkenes using second generation Grubbs catalyst and copper(I) iodide gave the substituted phosphonates in good yield. Stereospecific palladium(0)-catalyzed cyclization gave tetrahydrofuran and tetrahydropyran vinyl phosphonates. Regioselective Wacker oxidation of the vinyl phosphonate gave the beta-keto phosphonate, which underwent HWE reaction with benzaldehyde to yield the unsaturated ketone. The utility of the cross metathesis/cyclization protocol was further demonstrated by a formal synthesis of centrolobine.

Total Synthesis of Phorboxazole A. 1. Preparation of Four Subunits

[Structure: see text] Four subunits of the potent antitumor agent phorboxazole A were constructed; fragments C20-C32 and C9-C19 containing tetrahydropyrans A and B, respectively, were assembled using palladium-catalyzed intramolecular alkoxycarbonylation.

Stereochemically General Approach to Adjacent Bis(tetrahydrofuran) Cores of Annonaceous Acetogenins

A series of six 2,5-disubstituted adjacent bis(tetrahydrofuran) stereoisomers with trans/erythro/cis, trans/threo/trans, or cis/threo/cis relative stereochemistry have been synthesized from known dihydroxycyclooctenes via ring opening/cross metathesis and Pd(0)-mediated asymmetric double cycloetherification. The stereochemistry of four of these isomers has been found in the biologically active annonaceous acetogenin natural products. [reaction: see text].

Divergent Synthesis of Complex Polyketide-Like Macrolides from a Simple Polyol Fragment

[structure: see text] Three stereochemically and structurally complex non-natural polyketide macrolides have been synthesized from the same simple polyol precursor. That this polyol fragment is rapidly available in quantity is key to the practicality of these syntheses and allows the targeting of more highly complex and "natural product-like" macrolides.

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.

(+)-Phorboxazole A Synthetic Studies. A Highly Convergent, Second Generation Total Synthesis of (+)-Phorboxazole A

[structure: see text] A second generation total synthesis of the potent antitumor agent (+)-phorboxazole A (1) has been achieved. The cornerstone of this approach comprises a more convergent strategy, involving late-stage Stille union of a fully elaborated C(1-28) macrocycle with a C(29-46) side chain. The second generation synthesis entails the longest linear sequence of 24 steps, with an overall yield of 4.2%.

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.

Synthetic Efforts toward the C22−C36 Subunit of Halichondrin B Utilizing Local and Imposed Symmetry

The C22-C34 portion (2) of halichondrin B was synthesized from meso-symmetric bis-silyl protected cyclopentenediol (7) in 20 steps and 7% overall yield. This was accomplished through a two-directional synthesis/terminus differentiation strategy that proceeded via achiral, meso-symmetric intermediates for eight steps and employed a Pd(0)-mediated asymmetric double cycloetherification to establish both tetrahydropyran rings. [Structure: see text]

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.

Synthesis of Long-Chain Polyketide Fragments by Reaction of 1,3-Dioxy-1,3-dienes with Allylsilanes: Umpolung with Sulfur Dioxide

[reaction: see text] In the presence of a Lewis or protic acid and at low temperature, 1,3-dioxy-1,3-dienes add to sulfur dioxide generating zwitterionic intermediates that can react with carbon nucleophiles such as allylsilanes. After a retro-ene elimination of SO(2), valuable polyketide precursors are obtained.