Synthesis of chiral and bioactive fluoroorganic compounds

Indium-mediated diastereoselective allylation of D- and L-glyceraldimines with 4-bromo-1,1,1-trifluoro-2-butene: highly stereoselective synthesis of 4,4,4-trifluoroisoleucines and 4,4,4-trifluorovaline

A practical and efficient route for the stereoselective synthesis of (2R,3S)- and (2S,3R)-4,4,4-trifluoroisoleucines and (2R,3S)-4,4,4-trifluorovaline was developed. Indium-mediated allylation of (R)-N-benzyl-2,3-O-isopropylideneglyceraldimine 7 with 4-bromo-1,1,1-trifluoro-2-butene 4 gave the desired homoallylic amine 8 in high diastereoselectivity (>95% de) with moderate yield. The Cbz-protected (2R,3S)-4,4,4-trifluoroisoleucine 14 and Boc-protected (2R,3S)-4,4,4-trifluorovaline 21 were then readily prepared from 8. In addition, following the same procedure, Cbz-protected (2S,3R)-4,4,4-trifluoroisoleucine 28, the enantiomer of 14, was prepared starting from (S)-N-benzyl-2,3-O-isopropylideneglyceraldimine 24.

Organocatalyzed highly enantioselective direct aldol reactions of aldehydes with hydroxyacetone and fluoroacetone in aqueous media: the use of water to control regioselectivity

An organocatalyst prepared from (2R,3R)-diethyl 2-amino-3-hydroxysuccinate and L-proline exhibited high regio- and enantioselectivities for the direct aldol reactions of hydroxyacetone and fluoroacetone with aldehydes in aqueous media. It was found that water could be used to control the regioselectivity. The presence of 20-30 mol% of the catalyst afforded the direct aldol reactions of a wide range of aldehydes with hydroxyacetone to give the otherwise disfavored products with excellent enantioselectivities, ranging from 91 to 99% ee, and high regioselectivities. Aldolizations of fluoroacetone with aldehydes mediated by 30 mol% of the organocatalyst in aqueous media preferentially occurred at the methyl group, yielding products with high enantioselectivities (up to 91% ee); however, these additions took place dominantly at the fluoromethyl group in THF. Optically active 3,5-disubstituted tetrahydrofurans and (2S,4R)-dihydroxy-4-biphenylbutyric acid were prepared by starting from the aldol reaction of hydroxyacetone. Theoretical studies on the role of water in controlling the regioselectivity revealed that the hydrogen bonds formed between the amide oxygen of proline amide, the hydroxy of hydroxyacetone, and water are responsible for the regioselectivity by microsolvation with explicit one water molecule as a hydrogen-bond donor and/or an acceptor.

New routes to organofluorine compounds based on ketenes and the radical transfer of xanthates

New routes to organofluorine derivatives based mostly on the powerful xanthate radical transfer technology are described. A special emphasis is placed on the synthesis of trifluoromethyl-substituted structures, including trifluromethyl ketones and fluorinated aromatic and heteroaromatic substances of interest to the pharmaceutical and agrochemical industries.

Stabilization of bzip peptides through incorporation of fluorinated aliphatic residues

Two fluorinated amino acids, 5,5,5-trifluoroisoleucine (5TFI) and (2S,3R)-4,4,4-trifluorovaline (4TFV), which have been shown to serve as isoleucine surrogates in protein synthesis in Escherichia coli, have been incorporated in vivo into basic leucine zipper (bzip) peptides derived from GCN4. The extents of residue-specific incorporation of 5TFI and 4TFV were 90 and 88 %, respectively, of the encoded isoleucine residues, as evidenced by MALDI mass spectrometry and amino acid analysis. Both circular dichroism and equilibrium sedimentation studies of the fluorinated bzip peptides indicated preservation of secondary and higher-order protein structure. Thermal-denaturation experiments showed an increase of 27 degrees C in melting temperature when isoleucine was replaced by 5TFI. However, the T(m) of the peptide containing 4TFV was increased by only 4 degrees C over that of the peptide containing valine. Similar trends were observed in chemical denaturation studies in which DeltaDeltaG(unfold) in water was determined to be 2.1 or 0.3 kcal mol(-1) upon incorporation of 5TFI or 4TFV, respectively. When the fluorinated peptides were tested for DNA binding, both their affinity and specificity were similar to those of the respective hydrogenated peptides. These results suggest that fluorinated amino acids, even when introduced into the same positions, can have markedly different effects on the physical properties of proteins, while having little impact on secondary and higher-order structure.

2005 Emil Thomas Kaiser Award

Collagen is the most abundant protein in animals. The conformational stability of the collagen triple helix is enhanced by the hydroxyl group of its prevalent (2S,4R)-4-hydroxyproline residues. For 25 years, the prevailing paradigm had been that this enhanced stability is due to hydrogen bonds mediated by bridging water molecules. We tested this hypothesis with synthetic collagen triple helices containing 4-fluoroproline residues. The results have unveiled a wealth of stereoelectronic effects that contribute markedly to the stability of collagen, as well as other proteins. This new understanding is leading to synthetic collagens for a variety of applications in biotechnology and biomedicine.

Synthesis and antiproliferative activity of benzo[d]isothiazole hydrazones

Several benzo[d]isothiazole hydrazones have been evaluated for their potential antiretroviral activity. Since a number of these compounds were found to be inactive against viruses, but showed cytotoxicity at micromolar concentrations against the human CD4+ lymphocytes (MT-4) that were used to support HIV-1 growth, they were further tested for antiproliferative activity. The compounds resulted as being cytotoxic for MT-4 cells and new derivatives which were rationally designed and synthesized, were tested for antiproliferative activity against several leukaemia and solid tumour cell lines. In addition, these compounds were evaluated against "normal" cell lines. Compound 2h proved to be the most active compound and the fragment -CO-NH-N=CH-2-hydroxyphenyl was identified as being very important for biological activity, suggesting intramolecular hydrogen bond formation or favourable mutual disposition between two important centres in the pharmacophore. 1H-NMR spectra have been explained with the support of a conformational analysis.

Palladium-Catalyzed Regio- and Stereoselective Formate Reduction of Fluorine-Containing Allylic Mesylates. A New Entry for the Construction of a Tertiary Carbon Attached with a Fluoroalkyl Group

The regioselective palladium-catalyzed formate reduction of gamma-fluoroalkylated allylic esters is described. Reduction of the allylic esters under the influence of palladium with a monodentate phosphine ligand proceeded preferentially at the gamma position, the corresponding reduction products with a fluoroalkyl group at the tertiary carbon being afforded in high yields. When the chiral allylic ester was employed, complete chirality transfer was observed, leading to the optically active materials in high yields.

Metal enolates ofα-CF3 ketones: theoretical guideline, direct generation, and synthetic use

The difficulty as well as the significance of the direct generation of metal enolates of alpha-CF(3) ketones cannot be easily understood by chemists unfamiliar with F. In sharp contrast to the sunny side of non-F, hydrocarbon chemistry, F chemistry has long been overshadowed. Metal enolates of carbonyl compounds are synthetically important in C-C bond-forming reactions. However, the metal enolates of fluorinated carbonyl compounds have been severely limited to alpha-F metal enolates. Alpha-CF(3) metal enolates have generally been recognized as unstable and difficult to prepare because of the facile beta-M-F elimination. However, we have developed a direct generation and synthetic application of alpha-CF(3) metal enolates. Therefore, the present results regarding the direct generation and synthetic use of metal enolates of alpha-CF(3) ketones might be recognized as a real breakthrough for the general use of metal enolates in F chemistry.

Stereoelectronic and Steric Effects in the Collagen Triple Helix: Toward a Code for Strand Association

Collagen is the most abundant protein in animals. The protein consists of a helix of three strands, each with sequence X-Y-Gly. Natural collagen is most stable when X is (2S)-proline (Pro) and Y is (2S,4R)-4-hydroxyproline (4R-Hyp). We had shown previously that triple helices in which X is (2S,4S)-4-fluoroproline (4S-Flp) or Y is (2S,4R)-4-fluoroproline (4R-Flp) display hyperstability. This hyperstability arises from stereoelectronic effects that preorganize the main-chain dihedral angles in the conformation found in the triple helix. Here, we report the synthesis of strands containing both 4S-Flp in the X-position and 4R-Flp in the Y-position. We find that these strands do not form a stable triple helix, presumably because of an unfavorable steric interaction between fluoro groups on adjacent strands. Density functional theory calculations indicate that (2S,3S)-3-fluoroproline (3S-Flp), like 4S-Flp, should preorganize the main chain properly for triple-helix formation but without a steric conflict. Synthetic strands containing 3S-Flp in the X-position and 4R-Flp in the Y-position do form a triple helix. This helix is, however, less stable than one with Pro in the X-position, presumably because of an unfavorable inductive effect that diminishes the strength of the interstrand 3S-FlpC=O...H-NGly hydrogen bond. Thus, other forces can counter the benefits derived from the proper preorganization. Although (Pro-Pro-Gly)7 and (4S-Flp-4R-Flp-Gly)7 do not form stable homotrimeric helices, mixtures of these two peptides form stable heterotrimeric helices containing one (Pro-Pro-Gly)7 strand and two (4S-Flp-4R-Flp-Gly)7 strands. This stoichiometry can be understood by considering the cross sections of the two possible heterotrimeric helices. This unexpected finding portends the development of a "code" for the self-assembly of determinate triple helices from two or three strands.

Novel Synthesis of α-Trifluoromethylated α-Amino Acid Derivatives from γ-Hydroxy-α-fluoro-α-trifluoromethyl Carboxamides

On treatment with an organoaluminum reagent such as trimethylaluminum or DIBAL-H, gamma-hydroxy-alpha-fluoro-alpha-trifluoromethyl carboxamides (1) give a single diastereomer of alpha-amino-alpha-trifluoromethyl-gamma-lactones (2), which are a ring-closed form of gamma-hydroxy-alpha-trifluoromethyl-alpha-amino acids. This intriguing reaction results from intramolecular replacement of the fluorine atom on the alpha-carbon atom with the nitrogen atom of the amide group, which occurs in an SN2 manner. [reaction: see text]

Construction of Optically Active CF3-Containing Quaternary Carbon Centers via Stereospecific SN2‘ Reaction

[reaction: see text] Phosphates from 3-substituted 4,4,4-trifluorobut-2-en-1-ols were found to be effective for construction of CF3-containing quaternary carbon centers by way of Cu(I)-catalyzed Grignard reactions in the presence of catalytic amounts of CuCN and trimethylsilyl chloride (TMSCl) in an anti S(N)2' manner.

Preparation and Regioselective SN2‘ Reaction of Novel gem-Difluorinated Vinyloxiranes with RLi

A series of hitherto unknown 3,4-epoxy-1,1-difluorobutenes were prepared from the readily accessible alpha,beta-epoxy ketones and these compounds were found to undergo regioselective S(N)2' reactions with hard RLi nucleophiles occurring at the highly positively charged terminal fluorine-possessing sp(2) carbon atom in quite sharp contrast to the cases of the corresponding nonfluorinated vinyloxiranes which only attained a low level of regioselectivity. Addition of HMPA substantially improved the products' olefinic stereoselectivity. Theoretical calculations were used to qualitatively explore the nature of selectivity in these reactions.