By stepwise catenation of bicyclo[1.1.1]pentasilane units persila[n]staffanes (n=1, 2, and 3) were synthesized as air‐stable colorless crystals. A remarkable red‐shift of the UV/Vis absorption bands with increasing number of bicyclo[1.1.1]pentasilane units suggests remarkable interactions between bridgehead SiSi σ orbitals and between cage SiSi σ orbitals (see picture).
An enantioselective electrophilic amination of a-substituted cyanothioacetates with azodicarboxylate is demonstrated using an axially chiral guanidine as a chiral Brønsted base catalyst. The corresponding product, having a quaternary stereogenic center at the a-carbon atom, is formed in excellent enantioselectivity.Keywords: amination; amino acids; asymmetric catalysis; organocatalysis; quaternary stereocenters a-Amino acids having a quaternary stereogenic center at the a-position are attractive compounds owing to their potent biological activities as non-proteinogenic analogues of natural a-amino acids and their increasing importance as constituents for preparing conformationally restricted peptides.[1] Catalysis of the enantioselective electrophilic amination of a,adisubstituted carbonyl compounds using azodicarboxylates has emerged as an efficient strategy to address the challenging issue of constructing a nitrogen-substituted quaternary stereogenic center at the a-position of a carbonyl compound. [2][3][4] Recently, several excellent enantioselective catalytic reactions for electrophilic amination of a-substituted b-keto esters and their analogues have been developed using chiral metal-based catalysts [3] or organocatalysts. [2,4] Among these, the enantioselective amination of a-substituted a-cyanoacetates is a powerful method to construct a densely functionalized quaternary stereogenic center, and hence is a substantial step toward the enantioselective synthesis of a-amino acid derivatives having a quaternary a-carbon atom. [3d,4a,c,d,f,g,5] However, the methods are applicable to a-cyanoacetates with an aromatic group at the a-position. General approaches to the highly enantioselective amination of alkyl-substituted a-cyanoacetates have not been established to date, [4a,c,6] despite the potential for further elaboration of the corresponding amination products.Recently, we successfully developed the novel axially chiral guanidines 1 [4e,7] as highly efficient Brønsted base catalysts for promotion of enantioselective transformations.[4e,7-9] Specifically, we reported on the enantioselective electrophilic amination of unsymmetrical 1,3-dicarbonyl compounds with di-tert-butyl azodicarboxylate catalyzed by a chiral guanidine catalyst (1a), which afforded excellent enantioselectivity and catalytic efficiency for cyclic b-keto esters; however, for the acyclic b-keto esters, the enantioselectivities were highly dependent on the alkyl substituent at the a-position of the keto ester.[4e] In this context, we aimed to develop the enantioselective electrophilic amination of a-alkyl-substituted a-cyanoacetate derivatives 2 with azodicarboxylate 3. Herein, we report that the guanidine catalyst 1 exhibited excellent performance for the amination of a-cyanothioacetates 2 having a variety of primary alkyl substituents at the a-position, giving the products 4 in excellent enantioselectivity and chemical yield (Scheme 1).Our investigation began by using thioacetate 2 as a substrate in the present electrophilic amin...
Acute resistance exercise (RE) increases muscle protein synthesis (MPS) via activation of mechanistic target of rapamycin complex (mTORC), and chronic resistance exercise training (RT) results in skeletal muscle hypertrophy. Although MPS in response to RE is blunted over time during RT, no effective restorative strategy has been identified. Since eccentric muscle contraction (EC) has the potential to strongly stimulate mTORC1 activation and MPS, changing the muscle contraction mode to EC might maintain the MPS response to RE during chronic RT. Male rats were randomly divided into RE (1 bout of RE) and RT (13 bouts of RE) groups. Additionally, each group was subdivided into isometric contraction (IC) and EC subgroups. The RE groups performed acute, unilateral RE using IC or EC. The RT groups performed 12 bouts of unilateral RE using IC. For bout 13, the RT-IC subgroup performed a further IC bout, while the RT-EC subgroup changed to EC. All muscle contractions were induced by percutaneous electrical stimulation. Muscle samples were obtained at 6 h post exercise in all groups. After the 1st RE bout, the EC group showed significantly higher p70S6K Thr389 phosphorylation than the IC group. However, the phosphorylation of other mTORC1-associated proteins (4E-BP1 and ribosomal protein S6) and the MPS response did not differ between the contraction modes. After the 13th bout of RE, mTORC1 activation and the MPS response were significantly blunted as compared with the 1st bout of RE. Changing from IC to EC did not improve these responses. In conclusion, changing the contraction mode to EC does not reinvigorate the blunted mTORC1 activation and MPS in response to RE during chronic RT.
By stepwise catenation of bicyclo[1.1.1]pentasilane units persila[n]staffanes (n=1, 2, and 3) were synthesized as air‐stable colorless crystals. A remarkable red‐shift of the UV/Vis absorption bands with increasing number of bicyclo[1.1.1]pentasilane units suggests remarkable interactions between bridgehead SiSi σ orbitals and between cage SiSi σ orbitals (see picture).
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