Martin Ernst

Synthesis and Biological Evaluation of a Cyclo--tetrapeptide as a Somatostatin Analogue

Short β-peptides can mimic natural peptide hormones, as has been shown with a cyclo-β-tetrapeptide (1) that displays micromolar affinity to human somatostatin receptors. β-Peptides are thus a promising new class of peptidomimetics with potential high bioavailability due to their excellent resistance against proteases.

A Micellar Iodide‐Catalyzed Synthesis of Unprotected Aziridines from Styrenes and Ammonia

Aziridines are useful intermediates and pharmaceuticals. Therefore there is a growing need for their environmentally benign production. Many olefin aziridinations rely on the addition of nitrenes, which are generated by either thermal or photochemical azide decomposition or are formed in situ from tosylimino phenyliodinane, sulfonyl azides, or chloramine-T using metal catalysts. Halogens have also been proposed as catalysts in combination with chloramine-T, which is both a strong nucleophile and an oxidant. In this route, pioneered by Sharpless and Komatsu, reaction of the oxidized halogen (“Br”, “I”) with the double bond is followed by nucleophilic attack of chloramine-T and cyclization. The main drawback of all previous reactions is the use of complex nitrogen-containing sources, which lead to N-substituted aziridines that require a subsequent deprotection step. Direct routes from olefins to unprotected aziridines have only been described for a,b-unsaturated carbonyl compounds and often require complex NH donors. Ammonia, which is the most obvious nucleophilic nitrogen source, has barely been considered in aziridinations. Only the Gabriel–Cromwell aziridination uses NH3, but the scope of this reaction is restricted to a,b-unsaturated a-halocarbonyl compounds. The direct incorporation of ammonia into olefins is therefore justly recognized as a top priority for catalysis. Herein we describe the first successful catalytic synthesis of unprotected aziridines from NH3 and simple olefins. Our method resembles a halide-assisted epoxidation, in which the olefin is attacked by “Br” cation, which is formed in situ by oxidation of bromide, and then by water as the oxygen source. The resulting bromohydrin is then cyclized to the epoxide. As will be shown, a similar concept can be applied for the N-functionalization of styrenes by replacing water with ammonia as the nucleophile: unprotected aziridines are formed in a one-pot, micellar system using iodide as a catalyst, aqueous bleach as an oxidant, and ammonia as the nitrogen source [Eq. (1)]. Styrene was used as a model olefin in our reactions. The expected product, 2-phenylaziridine, was synthesized separately as a reference by cyclization of 2-bromo-2-phenylethylamine. Initial noncatalytic experiments were carried out with pre-oxidized halonium sources (“X”), such as Nhalosuccinimide (NXS) or X2 (X=Br, I). The reactions were performed under water-free conditions with NH3 in dioxane and one equivalent of halonium ion in the presence of an additional base. Remarkably, ammonia was incorporated to give 2-phenylaziridine in a yield of around 10% (Table 1, entries 1–3). While the use of NBS led mostly to bromohydrin and dibrominated by-products, the selectivities were encouragingly high with NIS or I2 (about 99%). Subsequent reactions were therefore performed with iodonium ion.