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The calix[4]arene monomethyl ether supported nitridomolybdenum complex [MoN(MeCalix)] (1) was obtained in high yield from the reaction of [MoN(OtBu)3] and calix[4]arene methyl ether (H3MeCalix). Contrary to the calix[4]arene dimethyl ether supported dioxomolybdenum complex [MoO2(paco-Me2Calix)] published earlier, the calix[4]arene ligand in 1 adopts a cone conformation. Complex 1 reacts smoothly with GaCl3 and GaEt3 to yield the compounds [Mo(NGaCl3)(MeCalix)] (2) and [Mo(NGaEt3)(MeCalix)] (3), respectively. Complex 2 is the first structurally characterized example of a nitrido-bridged dinuclear compound of a group 6 metal and a group 13 halide (EX3). The [Mo≡N–GaCl3] unit, in which both metal atoms are unsymmetrically bridged with a short Mo≡N and a long Ga–N contact, shows significant bending at the nitrido bridge. This deviation from a linear alignment is attributed to packing forces. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

The preparation of side-chain-to-side-chain-cyclised peptides through lactam bridge formation requires orthogonal protecting groups for side-chain amino and carboxylate functionalities. Use of the 4-{N-[1-(4,4-dimethyl-2,6-dioxocyclohexylidene)-3-methylbutyl]amino}benzyl ester (Dmab) group for this role in the protection of the glutamyl side-chain resulted in the formation of unexpected side-products. During synthesis of fully protected peptide targets, Nα-pyroglutamyl chain-terminated peptides were observed. Pyroglutamyl peptides were not observed in analogous peptides synthesised using the traditional tert-butyl ester protecting group. Selective removal of the Dmab group proceeds through a two-stage procedure, hydrazinolytic cleavage of the dimedone moiety followed by 1,6-elimination of the resulting peptide-glutamyl 4-aminobenzyl ester. The latter reaction is sufficiently slow to allow isolation of the transiently stable glutamyl derived 4-aminobenzyl ester peptide. Attempted side-chain-to-side-chain cyclisation (through orthogonally protected Glu and Lys residues) of peptides prepared via Glu(ODmab) failed and led to modification of the Lys Ne-amino group when 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU) was used as carboxy-activating reagent. Analogous peptides prepared utilising allyl side-chain protection for glutamyl residues were successfully cyclised using HBTU.

The cover picture shows the reaction of a calix[4]arene methyl ether stabilized nitridomolybdenum complex [MoN(MeCalix)] with GaCl3 to yield the compound [Mo(NGa-Cl3)(MeCalix)]. This complex represents the first structurally characterized example of a nitrido-bridged dinuclear compound of a group 6 metal and a group 13 halide [EX3]. Both metal atoms are unsymmetrically bridged with a short [Mo≡N] and a long [Ga−N] contact. Details on synthesis and structural characterization are discussed in the article by U. Radius et al. on p. 2037 ff. In the background there is a view from the University of Karlsruhe (TH) to the adjacent “Hardtwald” on a stormy day photographed by Bernhard Muhr (www.wolkenatlas.de).