Stereoselective synthesis of substituted ketopyranose subunits of polyketide natural products by intramolecular alkoxycarbonylation of δ-alkynyl alcohols

Natural product syntheses via carbonylative cyclizations

This review summarizes the application of various transition metal-catalyzed/mediated carbonylative cyclization reactions in natural product total synthesis.

Palladium-catalyzed oxidative carbonylation reactions

Palladium-catalyzed coupling reactions have become a powerful tool for advanced organic synthesis. This type of reaction is of significant value for the preparation of pharmaceuticals, agrochemicals, as well as advanced materials. Both, academic as well as industrial laboratories continuously investigate new applications of the different methodologies. Clearly, this area constitutes one of the major topics in homogeneous catalysis and organic synthesis. Among the different palladium-catalyzed coupling reactions, several carbonylations have been developed and widely used in organic syntheses and are even applied in the pharmaceutical industry on ton-scale. Furthermore, methodologies such as the carbonylative Suzuki and Sonogashira reactions allow for the preparation of interesting building blocks, which can be easily refined further on. Although carbonylative coupling reactions of aryl halides have been well established, palladium-catalyzed oxidative carbonylation reactions are also interesting. Compared with the reactions of aryl halides, oxidative carbonylation reactions offer an interesting pathway. The oxidative addition step could be potentially avoided in oxidative reactions, but only few reviews exist in this area. In this Minireview, we summarize the recent development in the oxidative carbonylation 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%.

Synthesis of β-methoxyacrylate natural products based on box-Pd(II)-catalyzed intermolecular methoxycarbonylation of alkynoles

Bis(oxazoline)-palladium(II) catalyzed carbonylation of homopropargyl alcohols afforded acyclic methoxyacrylate 2 and 6-membered lactone 3a-k in good combined yield. In the case of propargyl alcohols, 5-membered lactones 3p, 3q, 16 were obtained in moderate yields. The one-pot synthesis of kawa lactones 3a, 3r, 3s and formal synthesis of dihydroxycystothiazole A and dihydroxycystothiazole C are presented. To elucidate the stereochemistry of (+)-annularin G and (-)-annularin H, the first asymmetric syntheses of these natural products were achieved.

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.

Synthesis of stereopentad analogues of the C14-C22 segment of callystatin A through additions of chiral allenylzinc reagents to stereotriads

The addition of (P)- and (M)-allenylzinc reagents, prepared in situ through Pd-catalyzed metalation of (R)- and (S)-3-butyn-2-ol mesylates, to diastereomeric stereotriad aldehydes 8, 13, 18, and 23 of syn,syn, syn,anti, anti,anti, and anti,syn stereochemistry was examined. Additions to the former two aldehydes afforded the four anti adducts with high diastereoselectivity and negligible mismatching. Significant mismatching was observed with the latter two aldehydes and the (M)-allenylzinc reagent. An evaluation of possible transition states is presented in consideration of steric and dipolar control elements.