Achiral Units Research Articles

Incorporation of Achiral Peptoid-Based Trimeric Sequences into Collagen Mimetics

This report represents initial studies of collagen mimetics with achiral peptoid-based trimeric sequences. The incorporation of achiral units into collagen-like structures is of considerable interest for the structural simplification of collagen-like biomaterials. The achiral unit Gly-Nleu-Nleu (where Nleu represents N-isobutylglycine) was positioned between Gly-Pro-Hyp trimeric repeats in collagen-like structures in order to examine the effect of an achiral block on triple helicity. A series of single chain structures, Ac-(Gly-Pro-Hyp)n-(Gly-Nleu-Nleu)n-(Gly-Pro-Hyp)n-NH2 (where n = 1−3), and a template-assembled structure, KTA-[Gly-(Gly-Pro-Hyp)2-(Gly-Nleu-Nleu)2-(Gly-Pro-Hyp)2-NH2]3 (where KTA represents cis,cis-1,3,5-trimethylcyclohexane-1,3,5-tricarboxylic acid), were investigated. Biophysical studies were carried out in both H2O and ethylene glycol (EG)/H2O (2:1, v/v) solvents, using circular dichroism and optical rotation measurements. Highly cooperative melting curves from optical rotation determi...

Cooperative chiral order in copolymers of chiral and achiral units

Polyisocyanates can be synthesized with chiral and achiral pendant groups distributed randomly along the chains. The overall chiral order, measured by optical activity, is strongly cooperative and depends sensitively on the concentration of chiral pendant groups. To explain this cooperative chiral order theoretically, we map the random copolymer onto the one-dimensional random-field Ising model. We show that the optical activity as a function of composition is well-described by the predictions of this theory.

Backbone Modification of Chirasil-Val, Part II: Introduction of a Rigid Cyclohexyl Spacer

The polysiloxane backbone of the gas chromatographic phase Chirasil-Val is modified by introducing a rigid spacer. Thus, improved shielding of the chiral moiety from the achiral polysiloxane units is achieved. The new phase, Chirasil-Nova-CH, is prepared by coupling 4-exomethylen cyclohexyl carboxylic acid to L-valine tert-butylamide, followed by hydrosilylation with a copolymer containing Si-H groups. As compared with ChirasilNova bearing the linear spacer (CH 2 ) 3 , the new phase, Chirasil-Nova-CH with a cyclohexyl spacer, shows an advantageous shortening of retention times without an apparent loss in the resolution factors for the enantiomers of amino acid derivatives

Total Synthesis of Myxovirescins, 2 Assembly of the “Northwestern” Part [C(15)–C(28)

AbstractThe part of the target molecules myxovirescins A and M containing the atoms C(15)–C(28) is described in this paper (for retrosynthetic analysis see Scheme 1). There are three stereogenic centers which are incorporated by using (S)‐2‐hydroxymethyl‐butanoic acid and the appropriate enantiopure diastereoisomeric 2,4‐dimethyl‐glutaric acids as building blocks (Schemes 2–4). These are joined by the achiral unit 4‐oxo‐hex‐5‐enoic acid. The key steps of the assembly are a cuprate Michael addition (Scheme 5) and a nucleophilic addition of a Li derivative to an aldehyde (Scheme 6). In both cases the organometallic reagents are generated by I/Li exchange using two equiv. of tBuLi. The chiral building blocks are prepared by yeast reduction of ethyl 2‐formyl‐butanoate and by resolution of the 2,4‐dimethyl‐pentanedioic acid monomethyl ester with phenethylamine; both enantiomers derived from the meso‐2, 4‐dimethyl‐glutaric acid are converted to the same aldehyde (5a; “meso‐trick”, Schemes 3 and 4). The “northwestern” parts for the final synthesis are actually hydroxy sulfones (2 in Scheme 6), the termini of which are ready for Julia coupling and oxidation to a carboxylic acid group. The preparation of the intermediates on gram scales is described and all new compounds are fully characterized by their physical properties, by spectroscopy (IR, 1H‐ and 13C‐NMR spectra) and by elemental analysis. magnified image

Optically active vinyl polymers, 25. Chiroptical Properties and Conformation of Copolymers from 4‐Methyl‐1‐penten‐3‐one and (S)‐4‐Methyl‐1‐hexen‐3‐one

AbstractThe molar optical rotation at 589 nm, the UV‐, CD‐and ORD‐spectra of atactic and highly isotactic random copolymers of 4‐methyl‐1‐penten‐3‐one (4MP3O) and (S)‐methyl‐1‐hexen‐3‐one ((S)‐4MH3O) were measured in different solvents at different temperatures. The ellipticity observed in the isotactic copolymers between 240 and 320 nm is much larger than that calculated for a mixture of the two homopolymers, showing that in the isotactic (but not in the atactic) copolymers chiral conformations, giving contributions of negative sign to the CD, prevail also in the monomeric units arising from the non‐chiral monomer. The decrease of the ellipticity with increasing temperature for the copolymers is not linear; the average value between 20°C and 60°C is larger for the copolymers than for the homopolymers. The results indicate that in the isotactic copolymers main‐chain helical conformations of one handedness are favoured over those of the opposite screw sense, and that achiral units incorporated in helical sections are forced to assume chiral conformations similar to the conformations that chiral units assume in the same helical section.