Double-chelation-assisted Rh-catalyzed intermolecular hydroacylation between salicylaldehydes and 1,4-penta- or 1,5-hexadienes

Intermolecular hydroacylation between salicylaldehydes 1, 26-40 and 1,4-penta- or 1,5-hexadienes 4-13 by Rh-catalyst proceeded under mild reaction conditions to give a mixture of iso- and normal-hydroacylated products 14-25, 41-55, and 57-60. In the hydroacylation reaction, chelation of both salicylaldehyde and diene to the Rh-complex plays a crucial role. The ratio of iso- and normal-hydroacylated products could be regulated by the addition of salicylic acid or amines. The effects of various Rh-complexes, solvents, and additives were examined, and the plausible mechanisms of the catalytic cycle were proposed on the basis of the deuterium-labeling salicylaldehyde experiments.     

The First Fully Planar C5‐Conformation of Homooligopeptides Prepared from a Chiral α‐Ethylated α,α‐Disubstituted Amino Acid: (S)‐Butylethylglycine (=(2S)‐2‐Amino‐2‐ethylhexanoic Acid)

An optically active α‐ethylated α,α‐disubstituted amino acid, (S)‐butylethylglycine (=(2S)‐2‐amino‐2‐ethylhexanoic acid; (S)‐Beg; (S)‐ 2 ), was prepared starting from butyl ethyl ketone ( 1 ) by the Strecker method and enzymatic kinetic resolution of the racemic amino acid. Homooligopeptides containing (S)‐Beg (up to hexapeptide) were synthesized by conventional solution methods. An ethyl ester was used for the protection at the C‐terminus, and a trifluoroacetyl group was used for the N‐terminus of the peptides. The structures of tri‐ and tetrapeptides 5 and 6 in the solid state were solved by X‐ray crystallographic analysis, and were shown to have a bent planar C₅‐conformation (tripeptide) and a fully planar C₅‐conformation (tetrapeptide) (see Figs. 1 and 2, resp.). The IR and ¹H‐NMR spectra of hexapeptide 8 revealed that the dominant conformation in CDCl₃ solution was also a fully planar C₅‐conformation. These results show for the first time that the preferred conformation of homopeptides containing a chiral α‐ethylated α,α‐disubstituted amino acid is a planar C₅‐conformation.     

Palladium‐Catalyzed Direct Thiolation of Aryl CH Bonds with Disulfides

A catalytic variant of the direct thiolation of arenes, bearing directing groups, with disulfides or thiols has been developed under palladium and copper co‐catalysis. Both sulfenyl moieties of the disulfide could be incorporated into the thiolated products, therefore, the reactions reached completion with only half an equivalent of disulfide, with respect to the starting arene. Experimental evidence suggested that the reaction proceeds through a Pd^II/Pd^IV mechanism.     

Swelling–shrinking behavior of hydrated noncross‐linked copolymer films in response to photoreversible isomerizaton and metal complexation of spiropyran units of the copolymers

The copolymer of hydroxypropyl methacrylate (HPMA) and photochromic spiropyran methacrylate (SPMA) has been synthesized. The films of the copolymer (P(HPMA‐SPMA)) in a hydrated state showed reversible swelling–shrinking behavior in response to photoreversible isomerization and metal complexation of SPMA units in spite of covalently noncross‐linked copolymers. In addition, the protonated open form of the SPMA units of the copolymer was possibly stabilized thermodynamically by the HPMA units from ultraviolet–visible absorption measurement of the hydrated P(HPMA‐SPMA) film. On the other hand, the difference in color of the hydrated films between P(HPMA‐AABMA) and P(NIPMA‐AABMA), which was a copolymer of N‐isopropyl methacryl amide (NIPMA) and azobenzene methacrylate (AABMA) as a pH indicator, was suggestive of the interference of the proximal hydroxyl groups of the immobilized HPMA units with protonation of the AABMA units. The HPMA units of the copolymers also contributed to improvement of thermodynamic stability of the metal complexes with the SPMA units. Copyright © 2013 John Wiley & Sons, Ltd.