Synthesis and Solid State Conformation of Tetrapeptide Amides Containing two Aib and two (αMe)Phe Residues - Use of Enantiomerically Pure 2-Benzyl-2-methyl-2H-azirin-3-amines as (αMe)Phe-Synthons

A series of tetrapeptide amides containing two aminoisobutyric acids (Aib) and two α-methylphenylalanine ((αMe)Phe) units were prepared through the 'azirine/oxazolone method'. New 2-benzyl-2-methyl-2H-azirin-3-amines have been used for the selective introduction of (S)- and (R)-(αMe)Phe, respectively. The solid-state conformations of five tetrapeptide amides were determined by X-ray crystallography. In all cases, two β-turns stabilize 3₁₀ -helical conformations and it was confirmed that, in contrast to proteinogenic amino acids, the configuration of (αMe)Phe does not determine the screw sense of the helix.

Exploiting the MeDbz Linker To Generate Protected or Unprotected C-Terminally Modified Peptides

C-terminally modified peptides are important targets for pharmaceutical and biochemical applications. Known methods for C-terminal diversification are limited mainly in terms of the scope of accessible modifications or by epimerization of the C-terminal amino acid. In this work, we present a broadly applicable approach that enables access to a variety of C-terminally functionalized peptides in either protected or unprotected form. This chemistry proceeds without epimerization of C-terminal Ala and tolerates nucleophiles of varying nucleophilicity. Finally, unprotected peptides bearing nucleophilic side chain groups can be selectively functionalized by strong nucleophiles, whereas macrocyclization is observed for weaker nucleophiles. The potential utility of this method is demonstrated through the divergent synthesis of the conotoxin conopressin G and GLP-1(7-36) and analogs.

Synthesis of Aib-containing cyclopeptides via the ‘azirine/oxazolone method’

The cyclic pentapeptide cyclo[Gly-Phe(2Me)-Aib-Aib-Gly], containing three α,α-disubstituted α-amino acids, was prepared by cyclization of H-Gly-Phe(2Me)-Aib-Aib-Gly-OH with diphenyl phosporazidate (DPPA) in 60% yield. The synthesis of the linear precursor was performed by using the ‘azirine/oxazolone method’, in which the α,α-disubstituted α-amino acids were introduced by coupling of the corresponding amino or peptide acid with a 2,2-disubstituted 3-amino-2H-azirine. Whereas the cyclization of an analogous hexapeptide afforded the cyclopeptide in 42% yield, the corresponding tetrapeptide yielded a 1:2 mixture of the monomeric and dimeric cyclopeptide. In the case of the tripeptide H-Gly-Phe(2Me)-Aib-OH, the cyclization with DPPA led to the dimeric cyclohexapeptide exclusively.

Natural cyclopeptaibiotics and related cyclic tetrapeptides: structural diversity and future prospects

Linearity is not considered a prerequisite anymore, and extension of the current definition of 'peptaibiotics' to cyclic, Aib-containing peptides is proposed. Sequences and bioactivities, together with ecophysiological importance of cyclopeptaibiotics and related cyclic tetrapeptides, and their fungal-taxonomic relationships, are discussed.

New 21- and 24-atom Aib-containing cyclopeptides

The synthesis of two Aib-containing cyclic octapeptides, cyclo(Leu-Aib-Phe-Aib-Gly-Aib-Phe-Aib) (1) and cyclo(Gly-Aib-Aib-Val-Aib-Leu-Aib-Phe) (2), and a cyclic heptapeptide cyclo(Gly-Aib-Val-Aib-Leu-Aib-Phe) (3), is described. The linear precursors of 1-3 were prepared using solution-phase techniques, and the cyclisation was also accomplished in solution. Among the coupling reagents examined in the final macrolactamisation step, PyAOP, HATU and DEPC/DEPBT efficiently yielded cyclised products. However, the success of the cyclisation was found to be dependent on the coupling reagent used. The two octapeptides 1 and 2 were obtained in much better yields (up to 63%) than the cycloheptapeptide 3 (30-37%). In addition, crystal-state conformational analysis of 2 was performed by X-ray crystallography. Six intramolecular hydrogen bonds stabilise the structure characterised by two consecutive type II'/I beta-turns, two consecutive type II/III' beta-turns, one gamma-turn, and one inverse gamma-turn.

The ‘Azirine/Oxazolone Method’ in Peptaibol Synthesis: Preparation of a Derivative of Trichotoxin A-50 (G)

The synthesis of a mixture of epimeric derivatives of the peptaibol trichotoxin A-50 (G) is described. The 'azirine/oxazolone method' has been used as a superior method for the introduction of the Aib as well as the Iva units into the peptide chain. In this protocol, 2,2-disubstituted 2H-azirin-3-amines are the synthons for 2,2-disubstituted glycines, which undergo coupling with N-protected amino or peptide acids in high yield, and without any need of coupling reagents. The problem of the instability of the amide function of the Gln side chain under the conditions of the acid-catalyzed hydrolysis of Z-Gln-(Aib)(n)-N(Me)Ph was solved by using an appropriate protecting group for the amide function of the Gln side chain, e.g., the triphenylmethyl (trityl; Tr) group. The structures of two intermediate peptides, i.e., the segments comprising residues 1-5 and 10-13, resp., were established by X-ray crystallography.

The first total synthesis of the peptaibol hypomurocin A1 and its conformation analysis: an application of the 'azirine/oxazolone method'

The first total synthesis of Hypomurocin A1 (HM A1) in solution phase is described. As members of the peptaibol family, hypomurocins are constituted by two groups of peptides: six undecapeptides (undecamers) in the HM A group and six octadecapeptides (18-mers) in the HM B group. The synthesis presented has been successfully achieved by the 'azirine/oxazolone method' to introduce the two Aib-Pro sequences included in this undecapeptaibol in one step with methyl 2,2-dimethyl-2H-azirine-3-prolinate as the building block. The coupling reactions of the Z-protected amino acids or peptide acids involved the use of N,N,N',N'-tetramethyluronium tetrafluoroborate (TBTU) and 1-hydroxybenzotriazole (HOBt), and led to the peptides in good-to-very-good yields. The peptides were purified by reverse-phase HPLC and characterized by NMR spectroscopy (1H, 13C, COSY, TOCSY, HSQC, HMBC, ROESY), ESI-MS, IR, elemental analysis, optical rotation, and X-ray crystallography. An NMR analysis of HM A1 was also carried out in deuterated micelles to perform a structural comparison of the helix in solution and in membranes.

Synthetic Studies on Stevastelins. 1. Total Synthesis of Stevastelins B and B3

[Chemical reaction: See text] The synthesis of stevastelin B3 (2) and B (5) are described. In a first approach, epoxy cyclodepsipeptide 8 was considered as a promising candidate for the synthesis of the [15]-membered ring members of the stevastelins; however, the oxirane ring opening, required for the completion of the natural stevastelin synthesis, failed. Thus, we synthesized stevastelin B (5), carrying out the oxirane ring opening earlier in the synthesis and following a synthetic scheme capable of delivering analogues. On the other hand, a translactonization reaction of the [15]-membered ring derivative 59 led to the total synthesis of the natural [13]-membered ring component of the stevastelins family, stevastelin B3 (2).