With 1 N-atom

In the four-membered rings, the P-lactam moiety is part of the classical penicillin and cephalosporin antibiotics. The most prominent example of a drug with a five-membered ring with one nitrogen atom is Lipitor (see Table 1.1). In the five-membered rings with 2 N atoms, imidazoles are found both in modern agrochemicals, especially the imidazolinones (e.g., Imazapyr), and pharmaceuticals, such as antimycotics (e.g., isoconazole, ketoconazole, and... 

Benzodiazepine derivates, seven-membered rings with 2 N atoms in 1,4 position are the pivotal structures of the benzodiazepine class of breakthrough CNS (central nervous system) drugs such as Librium and Valium. The most recent drug in this class, Sepracor s Lunesta (eszopiclone), is composed almost entirely of A-heterocyclics. Like most modern drugs, esczopi-clone also has a chiral center ... 

ESR measurements on irradiation of 3,3-diethoxycarbonyldiaziridine in r-butyl peroxide showed signals of the diaziridinyl radical (43). The structure of the radical followed from coupling with two N atoms with Qn = 14.2 and Qn = 11.7 g and one proton with Oh = 45.7 g (76TL4205). 

Some polymer is also formed but this can be converted into the bicyclic S11N2 by refluxing in CS2. The X-ray crystal structure (Fig. 15.37b) shows that the 2 N atoms are planar.This has been interpreted in terms of sp hybridization at N, with some delocalization of the p. lone-pair of electrons into S-based orbitals, thus explaining the considerably diminished donor power of the molecule. S11N2 is stable at room temperature but begins to decompose when heated above 145°. 

Results from an array of methods, including X-ray absorption, EXAFS, esr and magnetic circular dichroism, suggest that in all ureases the active sites are a pair of Ni" atoms. In at least one urease,these are 350 pm apart and are bridged by a carboxylate group. One nickel is attached to 2 N atoms with a fourth site probably used for binding to urea. The second nickel has a trigonal bipyramidal coordination sphere. 

Merck s thienamycin synthesis commences with mono (V-silylation of dibenzyl aspartate (13, Scheme 2), the bis(benzyl) ester of aspartic acid (12). Thus, treatment of a cooled (0°C) solution of 13 in ether with trimethylsilyl chloride and triethylamine, followed by filtration to remove the triethylamine hydrochloride by-product, provides 11. When 11 is exposed to the action of one equivalent of tm-butylmagnesium chloride, the active hydrogen attached to nitrogen is removed, and the resultant anion spontaneously condenses with the electrophilic ester carbonyl four atoms away. After hydrolysis of the reaction mixture with 2 n HC1 saturated with ammonium chloride, enantiomerically pure azetidinone ester 10 is formed in 65-70% yield from 13. Although it is conceivable that... 

Figure 5. Left panel lowest energy equilibrium structures of AunTM+ clusters with 2

FORMATION OF A FIVE-MEMBERED RING WITH ONE N ATOM AT A RING JUNCTION 668... 

The nitrogen rule helps us propose compositions for molecular ions If a compound has an odd number of nitrogen atoms—in addition to any number of C, H, halogens, O, S, Si, and P—then M+ has an odd nominal mass. For a compound with an even number of nitrogen atoms (0, 2, 4, and so on), M+ has an even nominal mass. A molecular ion at mlz 128 can have 0 or 2 N atoms, but it cannot have 1 N atom. 

We use different hybridization schemes to describe different arrangements of electron pairs. For example, to explain a trigonal planar electron arrangement, we mix one s-orbital with two p-orbitals and so produce three sp2 hybrid orbitals. They point toward the corners of an equilateral triangle (Fig. 3.19a). A linear arrangement of pairs requires two hybrid orbitals, so we mix an s-orbital with a p-orbital to obtain two sp hybrid orbitals (Fig. 3.19b). Table 3.2 summarizes the relationship between electron arrangement and hybridization type. No matter how many atomic orbitals we mix together, the number of hybrid orbitals is always the same as the number of atomic orbitals we started with, so N atomic orbitals produce N hybrid orbitals. 

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