Synthesis of the C14−C25 Subunit of Bafilomycin A1

Efficient Synthesis of Differentiated syn-1,2-Diol Derivatives by Asymmetric Transfer Hydrogenation-Dynamic Kinetic Resolution of α-Alkoxy-Substituted β-Ketoesters

Asymmetric transfer hydrogenation was applied to a wide range of racemic aryl α-alkoxy-β-ketoesters in the presence of well-defined, commercially available, chiral catalyst Ru(II) -(N-p-toluenesulfonyl-1,2-diphenylethylenediamine) and a 5:2 mixture of formic acid and triethylamine as the hydrogen source. Under these conditions, dynamic kinetic resolution was efficiently promoted to provide the corresponding syn α-alkoxy-β-hydroxyesters derived from substituted aromatic and heteroaromatic aldehydes with a high level of diastereoselectivity (diastereomeric ratio (d.r.)>99:1) and an almost perfect enantioselectivity (enantiomeric excess (ee)>99 %). Additionally, after extensive screening of the reaction conditions, the use of Ru(II) - and Rh(III) -tethered precatalysts extended this process to more-challenging substrates that bore alkenyl-, alkynyl-, and alkyl substituents to provide the corresponding syn α-alkoxy-β-hydroxyesters with excellent enantiocontrol (up to 99 % ee) and good to perfect diastereocontrol (d.r.>99:1). Lastly, the synthetic utility of the present protocol was demonstrated by application to the asymmetric synthesis of chiral ester ethyl (2S)-2-ethoxy-3-(4-hydroxyphenyl)-propanoate, which is an important pharmacophore in a number of peroxisome proliferator-activated receptor α/γ dual agonist advanced drug candidates used for the treatment of type-II diabetes.

Iron- and bismuth-catalyzed asymmetric Mukaiyama aldol reactions in aqueous media

We have developed asymmetric Mukaiyama aldol reactions of silicon enolates with aldehydes catalyzed by chiral Fe(II) and Bi(III) complexes. Although previous reactions often required relatively harsh conditions, such as strictly anhydrous conditions, very low temperatures (-78 °C), etc., the reactions reported herein proceeded in the presence of water at 0 °C. To find appropriate chiral water-compatible Lewis acids for the Mukaiyama aldol reaction, many Lewis acids were screened in combination with chiral bipyridine L1, which had previously been found to be a suitable chiral ligand in aqueous media. Three types of chiral catalysts that consisted of a Fe(II) or Bi(III) metal salt, a chiral ligand (L1), and an additive have been discovered and a wide variety of substrates (silicon enolates and aldehydes) reacted to afford the desired aldol products in high yields with high diastereo- and enantioselectivities through an appropriate selection of one of the three catalytic systems. Mechanistic studies elucidated the coordination environments around the Fe(II) and Bi(III) centers and the effect of additives on the chiral catalysis. Notably, both Brønsted acids and bases worked as efficient additives in the Fe(II) -catalyzed reactions. The assumed catalytic cycle and transition states indicated important roles of water in these efficient asymmetric Mukaiyama aldol reactions in aqueous media with the broadly applicable and versatile catalytic systems.

Concise epoxide-based synthesis of the C14-C25 bafilomycin A(1) polypropionate chain

An efficient non-aldol convergent synthesis of the C14-C25 polyketide fragment of bafilomycin A(1) was completed in 16% overall yield and 8 steps in its longest linear sequence. This synthesis highlights the formation of the key fragments using a three-step sequence of epoxide cleavage, alkyne reduction, and epoxidation developed in our laboratory; starting from suitably protected enantiomeric epoxides of trans-2,3-epoxybutanol. This chemistry represents a quick asymmetric and diastereoselective construction of the polyketide chain of bafilomycin A(1), in which every stereogenic center was constructed using solely epoxide chemistry.

Highly diastereo- and enantioselective synthesis of monodifferentiated syn-1,2-diol derivatives through asymmetric transfer hydrogenation via dynamic kinetic resolution

The first enantio- and diastereoselective approach to alpha-alkoxy-substituted syn-beta-hydroxyesters through highly efficient catalytic asymmetric transfer hydrogenation via dynamic kinetic resolution reactions from the corresponding racemic beta-ketoesters is described. In this atom-economical process, two contiguous stereogenic centers are generated simultaneously with an excellent diastereoselectivity (up to 99/1) and enantioselectivity (up to 99%), allowing a rapid access to a wide variety of aromatic and heteroaromatic monodifferentiated syn-1,2-diols.

Reductive aldol coupling of divinyl ketones via rhodium-catalyzed hydrogenation: syn-diastereoselective construction of beta-hydroxyenones

Catalytic hydrogenation of divinyl ketones 1a and 1e in the presence of diverse aldehydes 2a-e at ambient temperature and pressure using cationic rhodium catalysts ligated by tri-2-furyl phosphine enables formation of aldol products 3a-e and 5a-e, respectively, with high levels of syn diastereoselection. Through an assay of counterions (Rh(COD)2X), Rh(COD)2SbF6 is identified as the optimum precatalyst for reductive aldol couplings of this type. For para-substituted styryl vinyl ketones 1b-e, a progressive increase in isolated yield is observed for electron-releasing para substituents. [reaction: see text].

Dynamic kinetic resolution of 2-oxo-3-aryl-succinates by organocatalyzed aldolization

The dynamic kinetic resolution of 2-oxo-3-arylsuccinates was achieved vial-proline-catalyzed addition of acetone in acetonitrile at room temperature, providing the desired adduct in good yield with up to 87 : 13 dr and high ee up to 99%.

Total Synthesis of Pteridic Acids A and B

Pteridic acid A (1) is a spirocyclic octaketide produced by the phytoepiphytic actinomycete Streptomyces hygroscopicus TP-A0451 and possesses potent plant-growth-promoting activity comparable to that of indole-3-acetic acid. The enantioselective total synthesis of this natural product was achieved by employing the Sn(OTf)(2)-mediated Evans aldol reaction and the Fukuyama acetylenic coupling reaction as the key C--C bond-forming steps producing 1 through a 14-step sequence in 22 % overall yield from a known oxazolidinone derivative. MgBr(2)-mediated equilibration of an anomerically favored spirocyclic intermediate used for the synthesis of 1 brought about partial epimerization of the spirocenter to give the corresponding anomerically disfavored epimer, which was converted into pteridic acid B (11-epi-1), another plant-growth promoter of the same microbial origin.