Self-Replication of Oligonucleotides in Reverse Micelles

The coupling between the tri(deoxynucleotides) d[(MeO)C-G-Ap] ( 1 ) and d[(NH₂)T_d5′-C-G-] ( 2 ) to yield the phosphoramidate-linked (hexadeoxy-nucleotide) d[(MeO)C-G-Anh⁵′T_d5′-C-G] ( 3 ) was investigated both in aqueous solution and in reverse micelles constituted of CTAB (cetyl(trimethyl)ammonium bromide) in hexane/pentan-1-ol 9:1. No siginificant difference was found concerning the yield and the kinetics of the reaction in the two systems. The coupling between 1 and 2 was also carried out in the presence of the template d[(MeO)C-G-A-T-C-G] ( 4 ), an analogue of 3 , so as to reproduce the conditions of template-directed self replication. It was shown that the trinucleotide coupling in the presence of a template obeys the so-called square-root law both in H₂O and in reverse micelles. No significant difference of the time course of the reaction in H₂O and in reverse micelles was observed. This shows that self-replication of oligonucleotides occurs within geometrically bounded structures, which represents a step forward in the mimicking of minimal life processes.     

Chiral Amplification of Oligopeptides in the Polymerization of α‐Amino Acid N‐Carboxyanhydrides in Water

This article is concerned with the chiral amplification of oligopeptides formed in the polymerization of chiral, nonracemic mixtures of the N‐carboxyanhydride (NCA) of Leu and Glu in aqueous solution. Labeling (deuteration) of one enantiomer and reversed‐phase and normal‐phase high‐performance liquid chromatography mass spectrometry (RP‐ and NP‐HPLC/MS, respectively) were used to determine the product distribution, both with respect to oligopeptide chain length and stereoisomer distribution. Starting the polymerization with an enantiomeric excess (ee) of 20% of the L ‐enantiomer (L ‐amino acid/D ‐amino acid 6 : 4) leads to an ee of 73% at the level of the homochiral enantiomeric (Leu)₅, and of 71% at the level of the homochiral enantiomeric (Glu)₇. For the Leu system and in the presence of a solid support (quartz), the ee reached values of up to 100%. We argue that such amplification processes could be relevant for the chemical evolution towards single‐handedness.     

Metalation of 2-chloromethyl-2-oxazolines: synthesis of 1,2,3-tris(oxazolinyl)cyclopropanes and derivatives

2-Chloromethyl-2-oxazoline converts cleanly into trans-1,2,3-tris(oxazolinyl)cyclopropane upon treatment with strong bases such as LDA or KN(SiMe(3))(2). Deprotonation of the above cyclopropane followed by the addition of electrophiles allows the preparation of more functionalized tris(oxazolinyl)cyclopropanes.     

A stereospecific synthesis of oxazolinyloxiranes

Lithiooxiranes 3a and 3b, generated by deprotonation of oxiranes 2a and 2b with s-BuLi at -100 degrees C in Et(2)O, were found to be chemically very stable. trans-Lithiooxirane 3a was also configurationally stable and reacted stereospecifically with electrophiles to give 4a--k. In contrast, cis-lithiooxirane 3b was found to be configurationally much less stable and reacted with electrophiles affording mixtures of diastereomers 4, 7, and 8. After only a very short reaction time, 3b too reacted with electrophiles highly stereospecifically. Deprotonation--deuteration and deprotonation--alkylation of chiral oxazolinyloxiranes 12a and 12b to give oxiranes 12c and 12d were also examined. Semiempirical and ab initio calculations were carried out in an effort to explain the observed stereochemistry.     

Synthetic approach to substituted cyclopropanes based on the coupling reaction of lithiated chloroalkyloxazolines with Fischer carbene complexes

Regioselective addition of lithiated oxazoline 2a, easily available from 2-(1-chloroethyl)-4,4-dimethyl-2-oxazoline 1a (LDA, THF, -98 degrees C), to alpha,beta-unsaturated Fischer carbene complexes 3 afforded cyclopropylcarbene complexes 4 as sole diastereoisomers. Exposure of carbene complexes 4a-c (M = Cr) to air and sunlight gave cyclopropane carboxylate derivatives 5a-c. A plausible mechanistic explanation is proposed. Moreover, when lithiated oxazoline 2b was generated from 1b in the presence of the carbene complex 3a,b, the oxazolinylcyclopropane carboxylates 6a,b formed as a 1:1 mixture of diastereoisomers. Chiral lithiated oxazoline 2c added regioselectively and diastereoselectively to chromium complexes 3a,b and to tungsten complexes 3d,e, leading, after oxidation of the metal fragment, to esters 7a,b with good diastereoselectivity (dr = 4:1). The reaction of lithiated oxazoline 2d with chromium complex 3b and tungsten complex 3e proceeded less diastereoselectively, furnishing, in both cases, after oxidation, the ester 7c as a 3:2 diastereoselective mixture.     

Michael addition of chloroalkyloxazolines to electron-poor alkenes: synthesis of heterosubstituted cyclopropanes

Lithiated 2-(1-chloroethyl)-4,4-dimethyl-2-oxazoline 7 adds to electron-poor alkenyl heterocycles to afford substituted cyclopropanes in excellent yields. A route to chiral nonracemic heterosubstituted cyclopropanes, starting from optically active 2-chloromethyl-2-oxazolines, is highlighted as well.     

Amide-Bond Syntheses Catalyzed by Penicillin Acylase

The enzymatic synthesis of amide bonds catalyzed by penicillin acylase is investigated both in H₂O solution and in organic solvents containing reverse micelles. The specificity of the reaction is rather high on the side of the acyl component, practically only phenylacetic acid gives sizeable yields. On the contrary, a variety of amino-acid esters, dipeptides, and tripeptides can be used as amino component, e.g., serine methyl ester, methionine ethyl ester, tyrosine ethyl ester, Gly–Asp, Ala–Tyr, Gly–Tyr–Gly etc. However, Many other amino-acid residues do not react, and the possible reasons for this are discussed. Yields varyin rang the 10–80%. A. systematic study to optimize yields by varying the solvent composition is presented for one model reaction. The enzyme is also able to couple certain D-amino-acid residues (e.g. D-methionine ethyl ester or Gly-D -Asp) though at lower rate. Reverse micelles formed by the cationic surfactant cetyltrimethylammonium bromide (CTAB) in CHCl₃/isooctane are used to host penicillin acylase and to perform amide synthesis in which the product is perferentially soluble in the organic solvent mixture. The reaction is studied as a function of pH and certain micellar parameters, e. g. w_o (w_o = [H₂O]/[CTAB]). A new membrane enzyme reactor is utilized to separate the product from the enzyme-containing micelles. The adavantges and the limits of this approach are discussed.     

Oxazoline-mediated highly stereoselective synthesis of α,β-substituted-β-aminoalkanamides, potential precursors of unnatural β-amino acids

The reaction of α-lithiated-2-alkyl-2-oxazolines 1-Li with aliphatic, aromatic and heteroaromatic N-cumyl nitrones results in the stereoselective formation of N-cumyl-1,6-dioxa-2,9-diazaspiro[4,4]nonanes 3 which equilibrate with the hydroxylamino derivatives 4. Such equilibrating mixtures can be easily transformed into β-amino alkanamides 5 under reductive conditions, whereas acidic hydrolysis with trifluoroacetic acid (TFA) furnishes high yields of β-phenylamino alkanamides 6 via a cumene hydroperoxide-type rearrangement. Derivatives 5 and 6 provide a backbone of potentially useful unnatural β^2,2,3-amino acids.     

Synthesis of α-oxazolinylalkanamides

The synthesis of α-oxazolinylalkanamides 2, based on the reaction of α-chloroalkyloxazolines 1 with hexacarbonyltungsten [W(CO)₆] and lithium amides, has been developed. A plausible mechanism involving the ketene 5 as the intermediate is also proposed.