3-Aza-6,8-dioxabicyclo[3.2.1]octane-7-carboxylic acids (named BTAa) derived from (R,R)-, (S,S)-, or meso-tartaric acid and natural (l), unnatural (d), or unusual α-amino acids are described as conformationally constrained dipeptide isosteres. The general strategy developed for their preparation has required the trasformation of the amino acids into the corresponding N-benzylamino alcohols, followed by the PyBroP-promoted condensation with the monomethyl ester of the suitable 2,3-di-O-isopropylidenetartaric acid. Oxidation of the hydroxy group to aldheyde and subsequent acid-catalyzed trans-acetalization with the two hydroxy groups of the tartaric acid moiety provided 3-aza-2-oxo-6,8-dioxabicyclo[3.2.1]octane-7-carboxylic acid methyl esters [named BTAa(O)] in good yield and, in most cases, as single enantiopure diastereoisomers. This strategy has been applied to the preparation of BTAa(O) starting from (R,R)-, (S,S)-, or meso-tartaric acid and glycine, l- and d-phenylalanine, l- and d-alanine, and (±)-phenylglycine. In the cases of glycine, l- and d-phenylalanine, and l- and d-alanine, the selective reduction by BH3·DMS of the amide group succeeding to the cyclization step, or the reduction of both amide and ester functions followed by reoxidation of the hydroxy to carboxylic group, provided in good yield the 3-aza-3-benzyl-6,8-dioxabicyclo[3.2.1]octane-7-carboxylic acids (or their methyl ester) BTAa, having the side chain of the amino acid precursors at position 4. The stability and rigidity of the bicyclic skeleton, the complete control of all the stereocenters, the possibility of introducing the side chains of l- or d-amino acids, and the demonstrated compatibility with the conditions required for solid-phase peptide synthesis make the BTAa compounds potential dipeptide isosteres useful for the synthesis of modified peptides.

An important aspect in drug discovery is the early structural identification of the metabolites of potential new drugs. This gives information on the metabolically labile points in the molecules under investigation, suggesting structural modifications to improve their metabolic stability, and allowing an early safety assessment via the identification of metabolic activation products. From an analytical point of view, metabolite identification still remains a challenging task, especially for in vivo samples, in which they occur at trace levels together with high amounts of endogenous compounds. Here we describe a method, based on LC-ion trap tandem MS, for the rapid in vivo metabolite identification. It is based on the automatic, data-dependent acquisition of multiple product ion MS/MS scans, followed by a postacquisition search, within the entire MS/MS data set obtained, for specific neutral losses or marker ions in the tandem mass spectra of parent molecule and putative metabolites. One advantage of the method is speed, since it requires minimum sample preparation and all the necessary data can be obtained in one chromatographic run. In addition, it is highly sensitive and selective, allowing detection of trace metabolites even in the presence of a complex matrix. As an example of application, we present the studies of the in vivo metabolism of the compound MEN 15916 (1). The method allowed identification of monohydroxy ([M + H](+) = m/z 655), dihydroxy ([M + H](+) = m/z 671), and trihydroxy ([M + H](+) = m/z 687) metabolites, as well as some unexpected biotransformation products such as a carboxylic acid ([M + H](+) = m/z 669), a N-dealkylated metabolite ([M + H](+) = m/z 541), and its hydroxy-analog ([M + H](+) = m/z 557).

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A new series of monocyclic pseudopeptide tachykinin NK-2 receptor antagonists has been derived from the lead compound MEN11558. A synthesis for these molecules sharing the same intermediate was designed and performed. The replacement of the succinic moiety with an aspartic acid and the functionalization of its amino group with a wide variety of substituents led to very potent and selective NK-2 antagonists. Best results were obtained through the insertion in position 12 of an amino group with R configuration, linked by a short spacer to a saturated nitrogen heterocycle (morpholine, piperidine, or piperazine). The study led to compounds 54 and 57, endowed with high in vivo potency at very low doses and long duration of action in animal models of bronchoconstriction. In particular 54 and 57 completely inhibited NK-2 agonist induced bronchoconstriction in guinea pig after intratracheal administration at subnanomolar doses (ED(50) = 0.27 nmol/kg and 0.15 nmol/kg, respectively).