Use of a Catalytic Chiral Leaving Group for Asymmetric Substitutions at sp3‐Hybridized Carbon Atoms: Kinetic Resolution of β‐Amino Alcohols by p‐Methoxybenzylation

A catalytic strategy was developed for asymmetric substitution reactions at sp³‐hybridized carbon atoms by using a chiral alkylating agent generated in situ from trichloroacetimidate and a chiral phosphoric acid. The resulting chiral p‐methoxybenzyl phosphate selectively reacts with β‐amino alcohols rather than those without a β‐NH functionality. The use of an electronically and sterically tuned chiral phosphoric acid enables the kinetic resolution of amino alcohols through p‐methoxybenzylation with good enantioselectivity.     

A direct entry to substituted piperidinones from α,β-unsaturated amides by means of aza double Michael reaction

An intermolecular aza-double Michael reaction has been developed leading to functionalized piperidin-2-ones from simple starting materials. The method allows to utilize α,β-unsaturated amides as a synthon of piperidine nucleus. The short synthesis of anti-depressant paroxetine was demonstrated by using the new methodology.     

Novel intramolecular (4 + 1) and (4 + 2) annulations of halopolyenes by cascade radical reaction

Free radical reaction of vinyl iodides having dinenoate function in the presence of tributyltin hydride or tris(trimethylsilyl)silane caused a sequential cyclization reaction to produce (4 + 1) and (4 + 2) annulated compounds by means of a cascade radical reaction. Stereogenic centers of the cascade reaction were highly controlled. On the contrary, the cathodic electrolysis of the vinyl iodides afforded monocyclic compounds.     

Noncovalent domino effect on helical screw sense of chiral peptides possessing C-terminal chiral residue

Recently, a novel chiral intermolecular interaction was found in an N-deprotected achiral nonapeptide that undergoes the predominance of one-handed screw sense through the addition of chiral small carboxylic acid (Inai, Y.; Tagawa, K.; Takasu, A.; Hirabayashi, T.; Oshikawa, T.; Yamashita, M. J. Am. Chem. Soc. 2000, 122, 11731). We here clarify to what extent such noncovalent chiral domino effect affects the helical screw sense of an N-deprotected chiral peptide. Two chiral peptides consisting of C-terminal L-Leu (1) or L-Leu(2) (2) and the preceding achiral helical octapeptide segment were employed. NMR and IR spectroscopy, and energy calculation indicated that both peptides adopt a helical conformation in chloroform. Peptide 1 showed a small excess of a left-handed screw sense for the achiral helical octapeptide, but peptide 2 strongly preferred a right-handed screw sense. The addition of chiral Boc amino acid to a chloroform solution of peptide 1, depending on its chirality, underwent a unique helix-to-helix transition or led to remarkable stabilization of the original left-handed screw sense. Peptide 2 retained the original right-handed screw sense on addition of chiral Boc-amino acid, but its helical stability changed to some extent depending on its added chirality. Therefore, the importance of noncovalent domino effect for controlling the helical screw sense or helical stability of a chiral peptide has been demonstrated here for the first time. In addition, we here have presented a unique system that both N-terminal noncovalent and C-terminal covalent domino effects operate simultaneously on the helical screw sense of a single achiral segment and have compared both powers for inducing the screw sense bias.     

Asymmetric total synthesis of fredericamycin A: an intramolecular cycloaddition pathway

The asymmetric total synthesis of the potent antitumor antibiotic fredericamycin A ((S)-1) was achieved by the intramolecular [4+2] cycloaddition of the silylene-protected styrene derivative (S)-7 followed by the aromatic Pummerer-type reaction of the sulfoxide (S)-5. Although we had already succeeded in the total synthesis of racemic 1 by the same approach, synthesis of its asymmetric version was more complicated than we had expected due to the difficulties involved in constructing the quaternary carbon center and the tendency of this center to undergo facile racemization. Racemization of this center during the installation of the acetylene moiety on the dione (R)-8 was the most serious aspect. Systematic studies of its DE-ring analogue (R)-25 revealed that racemization of the quaternary carbon center proceeded by a retro-aldol-aldol reaction of the initial adduct, (1R)-39 a-Li, and that the degree of racemization was dependent on the reaction temperature. The racemization process could be completely depressed by keeping the reaction temperature at -78 degrees C. The construction of the stereogenic quaternary carbon center was achieved by the lipase-catalyzed desymmetrization of the prochiral 1,3-diol 9 a bearing the DEF-ring moiety. These studies enabled us to attain the asymmetric total synthesis of (S)-1 while completely retaining the chiral integrity created by the enzymatic reactions.     

A practical catalytic method for preparing highly substituted cyclobutanes and cyclobutenes

Tf2NH, an organic acid, efficiently catalyzes (2 + 2) cycloaddition reactions of silyl enol ethers with acrylates or propiolate. The process affords highly substituted cyclobutanes in high yield with high stereoselectivity under practical and environmentally benign conditions. Application for the formation of terpene skeletons and multigram synthesis of four-membered carbocycles were demonstrated.     

3′-Selective modification of a 4′,5′-didehydro-5′-deoxy-2′,3′-epoxyuridine using nucleophiles

1-(2,3-Anhydro-5-deoxy-4,5-didehydro-α-l-erythro-pent-4-enofuranosyl)uracil 4 was obtained by the treatment of 5′-iodo-2′,3′-epoxyuridine 5 with LiHMDS in excellent yield. The pyrimidine nucleoside 4 possesses quite unique vinyl epoxide moiety within the molecules. The reactions of 4 with a variety of nucleophiles gave 3′-substituted pyrimidine nucleosides without the formation of the corresponding 2′-substituted isomers. In the case of NaN₃ or PhSH, the corresponding 5′-adduct was obtained as a minor product together with the expected 3′-adduct.     

Novel intramolecular [4 + 1] and [4 + 2] annulation reactions employing cascade radical cyclizations

Tributyltin hydride and tris(trimethylsilyl)silane promote sequential/cascade free radical cyclization reactions of dienoate tethered vinyliodides or alkynes. These processes produce [4 + 1] and [4 + 2] annulated products. In contrast, the electrochemical reductions of the vinyliodides afford monocyclic compounds. Both the regiochemical and stereochemical courses of the sequential radical cyclizations strongly depend on substrate structure. Especially important is the balance between steric and stereoelectronic (Baldwin's rules) factors that serve to control cyclization regiochemistry.     

Preparation and biodegradation of sugar-containing poly(vinyl acetate) emulsions

To accelerate the biodegradability of poly(vinyl acetate)-based emulsions, emulsion copolymerizations of vinyl sugars, including triacetylated N-acetyl-D-glucosamine (GlcNAc)-substituted 2-hydroxyethyl methacrylate (GlcNAc(Ac)3-substituted HEMA), glucose-substituted HEMA (GEMA) and 6-O-vinyladipoyl-D-glucose (6-O-VAG) with vinyl acetate (VAc), were carried out using poly(vinyl alcohol) as an emulsifying agent in the presence of poly[(butylene succinate)-co-(butylene adipate)] [poly(BS-co-BA)]. Copolymerization with GEMA produced a stable emulsion and that with 6-O-VAG also produced a homogeneous emulsion. Their biodegradation tests indicated that PVAc main chain scission was accelerated by copolymerization with vinyl sugars.