Monocyclic rings

For methanol clusters [36], it was found that the dimer is linear, while clusters of 3 and 4 molecules exist as monocyclic ring structures. There also is evidence that there are two cyclic ring trimer confomiers in the molecular beam. 

All of the naturally-occurring monobactams discovered as of this writing have exhibited poor antibacterial activity. However, as in the case of the penicillins and cephalosporins, alteration of the C-3 amide side chain led to many potent new compounds (12). Furthermore, the monobactam nucleus provides a unique opportunity to study the effect of stmctural modifications at the N-1 and C-4 positions of the a2etidinone ring on biological activity. In contrast to the bicycHc P-lactams, these positions on the monocyclic ring system are readily accessible by synthesis. 

Sulfimides have been utilized extensively for the synthesis of monocyclic ring systems as well as ring annulations. 

Our survey of heteroaromatic tautomerism will be completed in three further chapters, two of which will appear in Volume 77 of our series covering the tautomerism of (i) rings with three, four, and seven or more members and (ii) two or more fused five- or six-membered heterocyclic rings. The final chapter dealing with the tautomerism of six-membered monocyclic ring systems will appear in a subsequent volume. 

Tlie first two chapters in this volume continue the survey of heteroaromatic tautomerism that was the topic of Volume 76 of Advances in Heterocyclic Chemistry. Tliis whole subject was first dealt with comprehensively in Volumes 1 and 2 of our series, which date back to 1963 and 1964.Tlie area was updated in a special supplementary volume of the series that appeared in 1976 but is now seriously out of date. Tire chapters in Volume 76 deal with a general introduction and the tautomerism of tive-membered monocyclic rings systems. 

Based on 68 isomers of sulfur clusters S ( i=3-ll), Au et al. have shown that two-fold coordination is generally favoured in sulfur clusters [55]. Structures with atom(s) in one-fold and three-fold coordinations are higher in energy. As a consequence, many large sulfur clusters exist as monocyclic rings while the open-chain forms are significantly less stable. 

Large monocyclic rings tend to transform into polycylic structures by forming stable a bonds between unstable n bonds. 

Figure 2. The new species added to our chemical models of interstellar clouds. The species range in complexity from 10-64 carbon atoms and comprise the following groups of molecules linear carbon chains, monocyclic rings, tricyclic rings, and fullerenes. The synthetic pathways are also indicated. See ref. 83. Reproduced from the International Journal of Mass Spectrometry and Ion Processes, vol. 149/150, R.P.A. Bettens, Eric Herbst "The interstellar gas phase production of highly complex hydrocarbons construction of a model", pp 321-343 (1995) with kind permission from Elsevier Science-NL, Sara Burgerhartstraat 25,1055 KV, Amsterdam, The Netherlands.

As background to the material described in the chapters which follow, it is appropriate at this point to introduce a few further examples of particular (monocyclic) ring types. When taken together with the macrocycles described so far, these examples enable some perspective to be gained of the structural diversity for simple rings which now exists. Nevertheless, it is emphasized that collectively these systems represent only a very minor selection from the large number of such rings now reported. 

Monocyclic ring systems are usually classified according to the number of carbon atoms in the ring and thus we have ... 

The six possible five-membered monocyclic rings containing three oxygen or sulfur atoms in 1,2, and 4 positions are shown in (1) to (6). Examples of each ring system are known, although 1,2,4-trioxolane (1) and 1,2,4-trithiolane (2) are the only parent rings that have been synthesized and characterized. The other systems are found as derivatives of the parent ring, with 1,4,2-dioxa-thiolanes and 1,2,4-oxadithiolanes only reported in the form of S-oxides. 

The definition of aromaticity conceived by Hiickel strictly applies to monocyclic ring systems, but indole, constructed from the fusion of benzene and pyrrole, behaves as an aromatic compound, like quinoline and isoquinoline. The ring fusion, however, affects the properties of both components. This is reflected in the valence bond description of indole, shown in Scheme 7.1, where one canonical representation shows electron density shared between N-1 and C-3 in the pyrrole unit (implying enamine character). Note that although other canonical forms can be drawn, where the lone-pair electrons are delocalized into the benzenoid ring, their energy content is relatively high and they are of limited importance. 

Both single and double cyclizations of the Schiff bases 65A and 66A occur in the multicomponent tautomeric mixtures derived from as- and /ra t-cyclohexane aminodiols 69 and 70, to give monocyclized ring forms of C-2-epimeric perhydro-l,3-benzoxazines (B and G) and oxazolidines (D and E), besides 7,ll-dioxa-9-azatricyclo[7.2.1.0 °]-dodecane diastereomers 67 and 68 (Scheme 10). The tautomeric ratios are found to satisfy by Equation (1) (Table 1) and are influenced by the cis- or rra t-geometry of the cyclohexane substituents in the Schiff base (A). For the equilibria... 

The formation of a weak a-bond by overlap of two singly occupied 7i -orbitals is a characteristic feature of certain 7i-electron-rich inorganic systems. This structural feature can occur via an intramolecular process resulting in a transannular interaction that converts a monocyclic ring into a bicyclic system. Alternatively, it may involve an intermolecular contact in which two cyclic monomers are weakly associated to give a dimeric species. 

For the heavier group 13 elements, there is an extensive cluster chemistry of neutral oligomers of the type (MX) (X = halide), (MR) (R = alkyl) or (MNR2) . For example, the series of tetramers [MC(SiMe3)3]4 (M = A1, Ga, In, Tl) all form tetrahedral structures,cf. (B Bu)4. In the formal + 1 oxidation state, these elements do not form monocyclic ring systems even with the stabilising influence of dialkylamino substituents. 

In contrast to the extensive homologous series known for cyclosiloxanes, monocyclic ring systems of the type (R2SiS) are limited to four- and six-membered rings n = 2, 3). The most common synthetic method involves the reaction of a diorganodichlorosilane with H2S in the presence of a base such as triethylamine or pyridine [eqn (10.34)]. ... 

Synthesis of Monocyclic Rings with Two or More Heteroatoms 551... 

---