The convergent synthesis of bicyclo[2.2.2]diazaoctane structures using an intermolecular Diels-Alder cycloaddition between a pyrazinone and commercially available fumarate or maleate precursors is reported. High reactivity and stereoselection is observed with both dienophile substrates. Structure validation was achieved by conversion of cycloadducts into known [2.2.2]diazabicyclic compounds or into crystalline derivatives suitable for X-ray analysis. The cycloadduct derived from reaction of pyrazinone and maleic anhydride underwent selective anhydride ring opening and intersected an established precursor in the synthesis of brevianamide B.
Infrared multiple photon dissociation action spectroscopy was performed on the AlaOrn b 2 + and AlaAlaOrn b 3 + fragment ions from ornithine-containing tetrapeptides. Infrared spectra were obtained in the fingerprint region (1000-2000 cm −1 ) using the infrared free electron lasers at the Centre Laser Infrarouge d'Orsay (CLIO) facility in Orsay, France, and the free electron lasers for infrared experiments (FELIX) facility in Nijmegen, the Netherlands. A novel terminal ornithine lactam AO + b 2 + structure was synthesized for experimental comparison and spectroscopy confirms that the b 2 + fragment ion from AOAA forms a lactam structure. Comparison of experimental spectra with scaled harmonic frequencies at the B3LYP/6-31+G(d,p) level of theory shows that AO + b 2 + forms a terminal lactam protonated either on the lactam carbonyl oxygen or the N-terminal nitrogen atom. Several low-lying conformers of these isomers are likely populated following IRMPD dissociation. Similarly, a comparison of the experimental IRMPD spectrum with calculated spectra shows that AAO + b 3 + -ions also adopt a lactam structure, again with multiple different protonation sites, during fragmentation. This study provides spectroscopic confirmation for the lactam cyclization proposed for the Bornithine effect^and represents an alternative b n + structure to the oxazolone and diketopiperazine/macrocycle structures most often formed.
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