Stereoselective synthesis of (Z)-fluoroalkenes directed to peptide isosteres: copper mediated reaction of trialkylaluminum with 4,4-difluoro-5-hydroxyallylic alcohol derivatives

Copper mediated alkyl-transfer reaction of trialkylaluminum (R₃Al) with (E)-4,4-difluoro-5-hydroxyallylic alcohol derivative smoothly proceeded to give the corresponding 2-alkylated 4-fluoro-5-hydroxyhomoallylic alcohol derivative with completely Z and 2,5-syn selective manner. Regio- and stereoselective conversion of the C5-hydroxyl group of the fluoroolefin thus obtained to amino group could be achieved through one-pot mesylation and azidation reaction.     

Fluoropropenoates as dienophiles in the Diels-Alder reaction

Several fluorinated alkenes were prepared from known ethyl (R)-2-fluoro-4,5-dihydroxyisopropylidine-2-pentenoate (1). The fluoroalkenoates were tested as dienophiles with several dienes and showed cycloaddition only with the very reactive diene, 1,3-diphenylisobenzofuran (5). Fluorobutenolide 2 reacted with 5 to produce the endo-syn adduct 9 in a highly stereoselective manner.     

Copper-free defluorinative alkylation of allylic difluorides through Lewis acid-mediated C-F bond activation

The reactions of difluorohomoallyl alcohols with trialkylaluminiums smoothly proceeded in CH₂Cl₂ at room temperature to give (Z)-fluoro-olefin products in excellent yields. On the basis of this chemistry, fluoro-olefinic dipeptide isostere of norvalinyl glycine was synthesized in stereoselective manner.     

Chromium mediated stereoselective synthesis of (Z)-1-fluoro-2-alkenyl alkyl and trialkylsilyl ethers from dibromofluoromethylcarbinyl ethers

Reactions of 2,2-dibromo-2-fluoroethyl alkyl ethers and dibromofluoromethylcarbinyl silyl ethers with CrCl₂ and Mn powder provided 2-fluorovinyl alkyl ethers and 1-fluoro-2-alkenyl enol silyl ethers in high yields and in Z selective manner.     

Diastereoselective synthesis of highly functionalized fluoroalkene dipeptide isosteres and its application to Fmoc-based solid-phase synthesis of a cyclic pentapeptide mimetic

A diastereoselective and divergent method for synthesis of a highly functionalized (Z)-fluoroalkene dipeptide isosteres has been developed. The key feature of this synthetic method is an efficient one-pot reaction involving reduction/asymmetric alkylation via transmetalation, which produces trans-amide type (Z)-fluoroalkenes flanking two stereogenic centers in high yields, with excellent (Z)-selectivity and diastereoselectivity. Practical Fmoc-based solid-phase synthesis of a specific CXCR4 antagonistic pseudopeptide 25 containing (Z)-fluoroalkene isostere is also described.     

Stereoselective synthesis of fluoroalkenes via (Z)-2-fluoroalkenyliodonium salts

Stereoselective synthesis of fluoroalkenes is described. (Z)-2-Fluoro-1-alkenyl(phenyl)iodonium tetrafluoroborates (1) were synthesized stereoselectively in good yields by Michael-type addition of HF to 1-alkynyl(phenyl)iodonium tetrafluoroborates (2) with a commercially available HF reagent, hydrofluoric acid or Et₃N-3HF. Pd-catalyzed cross-coupling reactions using 1 gave (Z)-2-fluoro-1-alkene derivatives in moderate yields. The treatment of 1 with KI in the presence of a catalytic amount of CuI gave (Z)-2-fluoro-1-iodo-1-alkenes (3). Pd-catalyzed cross-coupling reactions of 3 gave better results than that of 1, and a variety of (Z)-2-fluoro-1-alkene derivatives were synthesized in good yields.     

Diels-Alder reactions of trifluoromethyl alkenes with 5-ethoxyoxazoles: synthesis of trifluoromethylated pyridine derivatives

Cycloaddition reactions between 5-ethoxyoxazole derivatives [2,4-dimethyl 5-ethoxyoxazole and 4-methyl 5-ethoxyoxazole] and various trifluoromethyl alkenes [ethyl 4,4,4-trifluorocrotonate CF₃CHCHCO₂Et, 3,3,3-trifluoro 1-(phenylsulfonyl)-1-propene CF₃CHCHSO₂Ph and 2-(trifluoromethyl) propenoic acid CF₃(HO₂C)CCH₂] gave several trifluoromethyl-substituted pyridine systems in moderate yield (20-56%). Their structures were determined by multinuclear NMR techniques including ¹H-¹³C HETCOR spectroscopy.     

First enantioselective reductive amination of α-fluoroenones

From α-fluoroenones 2, a synthesis of (E) ketone oxime O-alkyl ethers 5 is reported with good to excellent yields. Then the first enantioselective reduction of these ketimines, via oxazaborolidine, is described with moderate to good enantiomeric excesses, leading to valuable chiral fluoroallylic amines 1.     

Copper mediated defluorinative allylic alkylation of difluorohomoallyl alcohol derivatives directed to an efficient synthetic method for (Z)-fluoroalkene dipeptide isosteres

Difluoroallylation of optically pure O-silylated (S)-2-methyl-3-hydroxypropanal 10a with bromodifluoropropene mediated by indium provided the corresponding difluorohomoallyl alcohol 11a with low diastereoselectivity, but without a decrease in optical purity. Defluorinative allylic alkylation of each diastereomer of the difluorohomoallyl alcohol efficiently proceeded by the reaction with trialkylaluminium and Cu(I) system or Grignard reagent and a catalytic amount of CuI system in THF to give the fluorine-substituted allylic alcohol 12 in an high yield and in an excellent Z selective manner. Subsequent imidate Claisen rearrangement of the allylic alcohol 12 proceeded with a complete 1,3-chirality transfer to give the fluoroalkene dipeptide isostere structure 14 after the final conversion of the primary alcohol 20 into the carboxylic acid form.