We describe measurements of the permittivity and Frank elastic constant in the nematic phase of a binary system displaying a transition between the nematic (N) and the recently discovered twist-bend nematic (NTB) phase. Among the salient features observed are (i) the existence of the NTB phase even when the system is loaded with a high concentration (∼64 mol %) of a rodlike component; (ii) a clear signature in permittivity of the N-NTB transition; and (iii) a lower value of the bend elastic constant compared to the splay over a large phase space, with the difference between the two becoming a maximum for an intermediate mixture. These studies further support the surprising idea that the elastic features associated with bent molecules can be further augmented by suitable rodlike additives.
Ubiquitousness of amide and ester functionality makes coupling reactions extremely important. Although numerous coupling reagents are available, methods of preparation of the common and efficient reagents are cumbersome. Those reagents generate a substantial amount of chemical waste and lack recyclability. Ethyl 2-cyano-2-(2-nitrobenzenesulfonyloxyimino)acetate (o-NosylOXY), the first member of a new generation of coupling reagents, produces byproducts that can be easily recovered and reused for the synthesis of the same reagent, making the method more environmentally friendly and cost-effective. The synthesis of amides, hydroxamates, peptides, and esters using this reagent is described. The synthesis of the difficult sequences, for example, the islet amyloid polypeptide (22-27) fragment (with a C-terminal Gly, H-Asn-Phe-Gly-Ala-Ile-Leu-Gly-NH2) and acyl carrier protein (65-74) fragment (H-Val-Gln-Ala-Ala-Ile-Asp-Tyr-Ile-Asn-Gly-OH), following the solid-phase peptide synthesis (SPPS) protocol and Amyloid β (39-42) peptide (Boc-Val-Val-IIe-Ala-OMe), following solution-phase strategy is demonstrated. Remarkable improvement is noticed with respect to reaction time, yield, and retention of stereochemistry. A mechanistic investigation and recyclability are also described.
Am etal-free and catalytic methodf or the synthesis of 2-substitutedb enzoxazoles and benzothiazoles from N-protecteda mino acids and carboxylic acids is described. The synthesis of benzoxazole on as olid support is demonstrated by the synthesis of benzoxazole attached to ah uman islet amyloid polypeptide, hIAPP [22][23][24][25][26][27] ,f ragment and benzoxazole derivatives of the side chain of aspartic acido n am odified fragment of Ab 18À21 (amyloid b)p eptide. The reactions proceeded through at wo-step mechanism.T he coupling step is realized with ar elativelyn ew reagent, ethyl 2-cyano-2-(2-nitrobenzenesulfonyloxyimino)acetate (ortho-NosylOXY), and the cyclization step takes place thanks to an inexpensive organocatalyst, para-toluenesulfonic acid, under microwavei rradiation. To the best of our knowledge,t his is the first report of the complete synthesis of benzoxazole derivatives of ap olypeptide on aresin solid support.
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