Self-Regeneration of Chirality with l-Cysteine through 1,3-Dipolar Cycloadditions between Diazoalkanes and Enantiomerically Pure Thiazolines: Experimental and Computational Studies.

Enantiopure 2-thia-4-azabicyclo[3.1.0]hexanes, which can be considered constrained cysteines, have been obtained from l-cysteine by application of the "self-regeneration of chirality" concept. The key intermediates are homochiral thiazolines that can be prepared in multigram scale and react smoothly with a series of diazoalkanes providing Δ1-pyrazolines except for phenyldiazomethane that yield the isomeric Δ2-pyrazolines. Nitrogen extrusion in Δ1-pyrazolines and further reduction of the sulfinyl group yielded the target compounds in good overall yield. Computational studies of the cycloaddition reaction were used for determining the polarity of the process and explaining the observed stereoselectivity. Additional topological studies were employed for determining the influence of noncovalent interactions in the stereochemical course of the reaction, which showed to be a highly asynchronous concerted process.

Study of the conformational profile of the cyclohexane analogs of L-phenylalanine.

The conformational profile of the conformationally constrained cyclohexane analogs of phenylalanine (1-amino-2-phenylcyclohexanecarboxylic acids, c6Phe) was assessed using computational methods. For this purpose, the conformational space of the N-acetyl methylamide derivatives of the stereoisomers (2S,3R)c6Phe and (2S,3S)c6Phe was explored by computing their respective Ramachandran maps, and low-energy minima were characterized at molecular mechanics level by means of the AMBER program, using the parm94 force field set of parameters. In order to assess the performance of the molecular mechanics calculations, each of the low-energy conformations was also investigated further at the ab initio level. Accordingly, the molecular mechanics geometries were used as starting conformations to perform full geometry optimizations at the Hartree-Fock level, using a 6-31G(d) basis set. Analysis of the results revealed that the cyclohexane structure directly induces some restrictions on the backbone, and constrains the orientation of the aromatic side-chain to two narrow regions for each stereoisomer. The conformational profile of these amino acids is then explained on the grounds of the interaction between the rigidly held phenyl ring and the main chain NH and CO groups. The results obtained are in good accordance with the experimental observations.

Preparative HPLC resolution of the CIS cyclohexane analogs of phenylalanine.

The analytical resolution of different derivatives of cis c(6)Phe (cyclohexane analogs of phenylalanine) was tested by HPLC using the mixed 10-undecenoate/3,5-dimethylphenylcarbamate of amylose bonded on allylsilica gel as the chiral stationary phase. The same chiral support has allowed the enantioseparation of racemate cis-4 on a semipreparative column (150 x 20 mm ID) with a mixture of n-hexane/2-propanol/chloroform as the mobile phase. Some 200-300 mg of the optically pure enantiomers were isolated and transformed into the N-benzyloxycarbonyl amino acids. The X-ray diffraction structure of (1R;2R)-4 is reported.

(alphaMe)Nva: stereoselective syntheses and preferred conformations of selected model peptides.

Using different stereoselective chemical and chemoenzymatic approaches we synthesized the chiral, Calpha-methylated alpha-amino acid L-(alphaMe)Nva with a short, linear side-chain. A set of terminally protected model peptides to the pentamer level containing either (alphaMe)Nva or Nva in combination with Ala and/or Aib was prepared using solution methods and characterized fully. Two (alphaMe)Nva peptides were also synthesized using side-chain hydrogenation of the corresponding Calpha-methyl, Calpha-allylglycine (Mag) peptides. A detailed solution and crystal-state conformational analysis based on FT-IR absorption, 1H NMR and X-ray diffraction techniques allowed us to define that: (i) (alphaMe)Nva is an effective beta-turn and 3(10)-helix former; and (ii) the relationship between (alphaMe)Nva chirality and the screw sense of the turn/helix formed is that typical of protein amino acids, i.e. L-(alphaMe)Nva induces the preferential formation of right-handed folded structures. In more general terms, this study reinforced previous conclusions that peptides based on alpha-amino acids with a Calpha-methyl substituent and a Calpha-linear alkyl substituent are characterized by a strong tendency to fold into turn and helical structures.

Synthesis of conformationally constrained hydroxy-alpha-amino acids by intramolecular conjugate addition.

An efficient and easily applicable method for the synthesis of a variety of hydroxy-alpha-amino acids analogues of serine and phenylalanine has been established. The method involves the stereoselective intramolecular conjugate addition of the benzamide group to cyclohexenone promoted by Lewis acid. Subsequent transformations of functional groups provide the conformationally constrained 2-hydroxy- and 2,4-dihydroxy-6-phenylcyclohexane-alpha-amino acids.

Atmospheric pressure chemical ionization multi-stage mass spectrometry in the characterization of stereoisomeric synthons of cyclopropane amino acids.

The mass spectrometric behaviour of (1S,2R)-, (1R,2R)-, (1R,2S)- and (1S,2S)-2-[(S)-2,2-dimethyl-1, 3-dioxolan-4-yl]-1-spiro-¿4'[2'-phenyl-5'(4'H)-oxazolone]¿ cyclopropane (2) and (1S,2R)-, (1R,2R)-, (1R,2S)- and (1S, 2S)-methyl-1-benzamido-2-[(S)-2,2-dimethyl-1, 3-dioxolan-4-yl]cyclopropanecarboxylate (3) was studied under atmospheric pressure ionization conditions and by multi-stage mass spectrometric (MS(n)) experiments performed with an ion trap. Interestingly, by using methanol as solvent, compounds 2 lead to [M + H + CH(3)OH](+) ions which, as proved by collisional experiments, exhibit the same structure of the corresponding compound 3. MS/MS of [MH](+) ions allows a clear characterization of the different stereoisomers, which give rise to specific fragmentation pathways, rationalized with respect to the structure of the neutral molecules.

Study of the conformational profile of selected unnatural amino acid residues derived from L-phenylalanine.

The present work reports the results of a conformational study performed on seven unnatural amino acid residues and on its natural precursor, investigated by means of computational methods at the molecular mechanics level. Amino acid residues selected for the present study are derivatives of L-phenylalanine substituted at the alpha and/or beta carbons. This series is composed of different linear analogs, including alpha-methyl, beta-methyl and beta-phenyl substituted with different stereochemistry. Analysis of the Ramachandran maps of the corresponding dipeptides in vacuo reveals their conformational preferences, to be used as guidance for the synthesis of constrained peptide analogs with desired conformational propensities. The available conformational space for every dipeptide is also analysed.

Efficient access to all four stereoisomers of phenylalanine cyclopropane analogues by chiral HPLC

Bonded polysaccharide-derived chiral stationary phases were found to be useful for the preparation of the four stereoisomers of the cyclopropane analogue of phenylalanine (c(3)Phe) as well as for the direct determination of the enantiomeric purity of c(3)Phe derivatives by HPLC. Three chiral stationary phases, consisting of cellulose and amylose derivatives chemically bonded on allylsilica gel, were tested. The mixed 10-undecenoate/3, 5-dimethylphenylcarbamate of cellulose, 10-undecenoate/3, 5-dimethylphenylcarbamate of amylose and 10-undecenoate/p-methylbenzoate of cellulose were the starting polysaccharide derivatives for CSP-1, CSP-2, and CSP-3, respectively. Using mixtures of n-hexane/chloroform/2-propanol as mobile phase on a semi-preparative column (150 mm x 20 mm ID) containing CSP-2, we separated about 1.7 g of racemic cis-methyl 1-tert-butoxycarbonylamino-2-phenylcyclopropanecarboxylate (cis-6) and 1.2 g of racemic trans-methyl-1-tert-butoxycarbonylamino-2-phenylcycloprop-anecarboxyl ate (trans-6) by successive injections. Copyright 1999 Wiley-Liss, Inc.

Molecular electrostatic potential-anthelmintic activity relationships of 5H-mebendazole and some related heterocyclic carbamates.

5H-Mebendazole and some related heterocyclic methyl carbamates were synthesized and their anthelmintic activity against Caenorhabditis elegans was determined. In order to study the influence of the heteroaromatic region with regard to the carbamate moiety on biological activity, the molecular electrostatic potentials (MEP) of all structures were calculated and a structure-activity relationship (SAR) was established. The electrostatic pattern of activity includes two minima of the carbamate moiety, a third heterocyclic minimum, and a pi-electronic region.