On the use of N-dicyclopropylmethyl aspartyl-glycine synthone for backbone amide protection.

To prevent aspartimide formation and related side products in Asp-Xaa, particularly Asp-Gly-containing peptides, usually the 2-hydroxy-4-methoxybenzyl (Hmb) backbone amide protection is applied for peptide synthesis according to the Fmoc-protocols. In the present study, the usefulness of the recently proposed acid-labile dicyclopropylmethyl (Dcpm) protectant was analyzed. Despite the significant steric hindrance of this bulky group, N-terminal H-(Dcpm)Gly-peptides are quantitatively acylated by potent acylating agents, and alternatively the dipeptide Fmoc-Asp(OtBu)-(Dcpm)Gly-OH derivative can be used as a building block.

Dicyclopropylmethyl peptide backbone protectant.

The N-dicyclopropylmethyl (Dcpm) residue, introduced into amino acids via reaction of dicyclopropylmethanimine hydrochloride with an amino acid ester followed by sodium cyanoborohydride or triacetoxyborohydride reduction, can be used as an amide bond protectant for peptide synthesis. Examples which demonstrate the amelioration of aggregation effects include syntheses of the alanine decapeptide and the prion peptide (106-126). Avoidance of cyclization to the aminosuccinimide followed substitution of Fmoc-(Dcpm)Gly-OH for Fmoc-Gly-OH in the assembly of sequences containing the sensitive Asp-Gly unit.

Segment Coupling to a Highly Hindered N-Terminal, Alamethicin-Related alpha-Aminoisobutyric Acid (Aib) Residue.

A model [6 + 5] segment coupling process involving a C-terminal valine hexapeptide acid and a resin-attached pentapeptide amide which N-terminated in a hindered Aib unit was examined using a variety of HOAt-derived coupling reagents. Best results were observed with HAPyU in DCM solvent in which loss of configuration amounted to 5.8%.

1,1-dioxonaphtho[1,2-b]thiophene-2-methyloxycarbonyl (alpha-Nsmoc) and 3,3-dioxonaphtho[2,1-b]thiophene-2-methyloxycarbonyl (beta-Nsmoc) amino-protecting groups.

Of the three theoretically possible, Bsmoc-related, naphthothiophene sulfone-based amino-protecting groups, the two most readily available derivatives, the alpha- and beta-Nsmoc analogues, have been examined as substitutes for the Bsmoc residue in cases where the latter lead to oily protected amino acids or amino acid fluorides. All of the naphtho systems gave easily handled solid amino acid derivatives. The intermediate sulfone alcohol 11 used as the key reagent for introduction of the alpha-Nsmoc protecting group was readily made from alpha-tetralone (Scheme 1).

Depsipeptide methodology for solid-phase peptide synthesis: circumventing side reactions and development of an automated technique via depsidipeptide units.

The depsipeptide technique is a recently developed method for peptide synthesis which is applicable to difficult sequences when the synthetic difficulty arises because of aggregation phenomena. In the present work, application of the depsipeptide method to extremely difficult sequences has been demonstrated and a serious side reaction involving diketopiperazine formation uncovered and subsequently avoided by the appropriate use of the Bsmoc protecting group. Many other aspects of the technique have been investigated, such as the stability of the depsi units during assembly and workup procedures, the completeness of the O-acylation step, the occurrence of epimerization of the amino acid activated during O-acylation, and the nature of side products formed.

Organophosphorus and nitro-substituted sulfonate esters of 1-hydroxy-7-azabenzotriazole as highly efficient fast-acting peptide coupling reagents.

Organophosphorus esters 9, 10, 14, and 15 prepared via reaction of diethyl- and diphenylphosphoryl chloride, di(o-tolyl)phosphinyl chloride, and 2,8-dimethylphenoxaphosphinyl chloride with HOAt are excellent coupling reagents for peptide synthesis which are generally superior to their uronium/guanidinium analogues and HOBt- or HODhbt-derived phosphate ester counterparts in minimizing loss of configuration during segment coupling. The phosphinyl analogues are more shelf-stable than the phosphoryl systems. The new reagents have been tested in segment couplings leading to two tripeptides (20, 21) and a hexapeptide 22.

3-Hydroxy-4-oxo-3,4-dihydro-5-azabenzo-1,2,3-triazene.

The known but long-neglected compound HODhat was shown to be in certain situations a useful peptide coupling additive. Uronium and phosphonium salts with HODhat built into the system were also useful stand-alone coupling reagents. Comparisons with related additives and coupling reagents showed that the new systems were sometimes more and sometimes less effective than previously described systems in the case of stepwise and segment couplings.

Complex polyfluoride additives in Fmoc-amino acid fluoride coupling processes. Enhanced reactivity and avoidance of stereomutation.

[reaction: see text] Isolated Fmoc amino acid fluorides have previously been shown to be among the most efficient reagents for peptide bond formation. Now, it has been found that anionic, polyhydrogen fluoride additives are capable of diverting many of the classical peptide coupling processes to acid fluoride couplings. Examples include the use of N-HBTU or N-HATU and the carbodiimide technique.

Cyclization of all-L-Pentapeptides by Means of 1-Hydroxy-7-azabenzotriazole-Derived Uronium and Phosphonium Reagents.

Due to their restricted conformational flexibility, cyclic peptides are of great interest in connection with structure-activity relationships, especially the elucidation of bioactive conformations. For linear peptides that do not contain turn structure-inducing amino acid residues, the cyclization reaction may be an inherently improbable or slow process, and side reactions, such as cyclodimerization and epimerization at the C-terminal residue, may dominate. A number of 1-hydroxy-7-azabenzotriazole-based onium salts were examined for cyclization of thymopentin-derived pentapeptides and the results compared with data from more conventional coupling reagents.