Cyclic compounds polycyclic

The intramolecular version for synthesizing cyclic and polycyclic compounds offers a powerful synthetic method for naturally occurring macrocyclic and polycyclic compounds, and novel total syntheses of many naturally occurring complex molecules have been achieved by synthetic designs based on this methodology. Cyclization by the coupling of an enone and alkenyl iodide has been applied to the synthesis of a model compound of l6-membered car-bomycin B 162 in 55% yield. A stoichiometric amount of the catalyst was used because the reaction was carried out under high dilution conditions[132]. 

Shaw, R. Golden, D. M., and Benson, S. W., Thermochemistry of Some Six-Membered Cyclic and Polycyclic Compounds Related to Coal. J. Phys. Chem, 1977. 81 pp. 1716-1729. 

Very recently, von Hanisch et al. reported on the syntheses and crystal structures of cyclic and polycyclic GaSb compounds featuring low coordinated Sb centers.146 Reaction of Ph(w-Pr)2P -> GaCl3 with i-Pr3SiSb(SiMe3)2 yielded [Ph(n-Pr)2P -> Ga(Cl)SbSi(/-Pr)3]2 84 and [(GaCl)4(P( -Pr)2Ph)2(SbSi(/-Pr)3)4] 85, respectively147 (Scheme 20). 

A concise collection of some modem methods of radical formation via rupture of C-E, C-G and C-A bonds, as well as from alkenes and cyclopropanes, by metals, organometallic hydrides, and photochemical and electrochemical means, are given in the last Chapter of Giese s book [28]. For some examples of radical generation and reactions leading to cyclic and polycyclic compounds see next Chapter (Heading 6.1.3). 

As in the case of acyclic compounds, the level of difficulty of the synthesis of a cyclic compound depends upon whether the molecule is a consonant or a dissonant system. However, some additional difficulties may be encountered in molecules with medium-sized rings as well as in polycyclic bridged compounds, which are treated in the next Chapter. On the other hand, as we have seen in Heading 4.3, even simple monofunctionalised cyclic molecules may require a FGA operation before bond disconnection of the cyclic network at the ipso-, a- or 13-positions can be effected. 

Cycloisomerization represents another approach for the construction of cyclic compounds from acyclic substrates, with iridium complexes functioning as efficient catalysts. The reaction of enynes has been widely studied for example, Chatani et al. reported the transformation of 1,6-enynes into 1-vinylcyclopentenes using [lrCl(CO)3]n (Scheme 11.26) [39]. In contrast, when 1,6-enynes were submitted in the presence of [lrCl(cod)]2 and AcOH, cyclopentanes with two exo-olefin moieties were obtained (Scheme 11.27) [39]. Interestingly, however, when the Ir-DPPF complex was used, the geometry of olefinic moiety in the product was opposite (Scheme 11.28) [17]. The Ir-catalyzed cycloisomerization was efficiently utilized in a tandem reaction along with a Cu(l)-catalyzed three-component coupling, Diels-Alder reaction, and dehydrogenation for the synthesis of polycyclic pyrroles [40]. 

Hydrogen abstraction can occur from a position within the ketone molecule, and this generates a biradical that may cyclize by combination of the radical centres. The overall photocydization process is observed for a wide variety of compound types, and it has been used extensively to make cyclic or polycyclic systems, in an unconstrained system a ketone (n,n ) excited state shows a preference tor abstraction from the y-position (4.42), which can be understood on... 

A hybrid HF-DFT approach has been used to determine gas-phase acidities (AH values) for a range of aliphatic, cyclic, and polycyclic carbon acids and reference compounds. The results obtained have been discussed in terms of strain in three- and four-membered rings, carbon hybridization aromaticity and topological analysis of electron density.13... 

The conjugated system is often part of a cyclic or polycyclic compound, e.g. 

When a mineral or Lewis acid replaces the carboxylic component in the Passerini reaction, the final products are usually a-hydroxyamides. Also in this case, when chiral carbonyl compounds or isocyanides are employed, the asymmetric induction is, with very few exceptions, scarce [18, 19]. For example, the pyridinium trifluoroacetate-mediated reaction of racemic cyclic ketone 14 with t-butyl isocyanide is reported to afford a single isomer [19] (Scheme 1.7). This example, together with those reported in Schemes 1.3 and 1.4, suggests that high induction may be obtained only by using rigid cyclic or polycyclic substrates. 

Several cyclic and polycyclic oligostannanes with a rich structural diversity have been reported in the literature16,21 83 and it was concluded by Sita21 that the concepts and strategies established in organic chemistry can successfully be applied for the construction of polystannane frameworks as well. The polycyclic oligostannanes 16-19 are representative examples for this class of compounds. 

The synthesis of enantiomerically pure compounds is the challenging problem for organic chemists. The synthesis becomes obsolete if the intermediates produce racemic mixtures. The problem is particularly acute when the asymmetric centers do not reside in a rigid cyclic or polycyclic framework. To be able to carry out efficient syntheses of complex molecules, chemists have to control the sense of chirality at each chiral center as it is introduced in the course of synthesis. Monoalkyl- or dialkyl-boranes exhibit a remarkable chemo-, stereo-, and regioselectivity for the hydroboration of unsaturated compounds. This property, coupled with the capability for asymmetric creation of chiral centers with chiral hydroboration agents, makes the reaction most valuable for asymmetric organic synthesis. In some of the cases, however, this has been achieved by diborane itself as shown in the synthesis of monensin by Kishi et al. A stereospecific synthesis of its seven carbon. component has been accomplished by two hydroboration reactions (Eq. 129) 209. ... 

Radical [6 + 0] cyclization is of some importance for the preparation of polycyclic azines, but is of no significant synthetic utility for the preparation of mono- or bi-cyclic compounds. Photochemical oxidative cyclizations of aromatic Schiff bases (equation 49) and azo compounds (equation 50) constitute important procedures for the preparation of phenan-thridines and benzocinnolines. These reactions proceed by initial formation of the dihydro compounds and aromatization is effected with either oxygen or, preferably, iodine (present in the reaction mixture). 

In addition to such divalent species, the elements form compounds in formal oxidation states IV and VI with four, five, or six bonds tellurium may give an 8-coordinate ion TeFg". Some examples of compounds of Group 16 elements and their stereochemistries are listed in Table 12-2. They also form cyclic and polycyclic cations (e.g., S2+, Te +) under special conditions. 

SxNy Compounds. A number of such compounds are known, for example, S4N2, SnN2, (S7N)2SJt, and S5N6, and have been shown to have cyclic or polycyclic structures, as shown in 12-XI to 12-XIII ... 

Thio analogs of these compounds, for example, PhP(S)(NHR)2 can also be prepared similarly, from the corresponding thiophosphonic halides and amines or by oxidative addition of sulfur to diaminophosphines, RP(NR92 Phenylphosphonic acid diamides, PhP(Q)(NHBu )2 (Q = S, Se) were prepared from PhP(NHBfr)2 by oxidative addition of sulfur or selenium and the 0x0 derivative PhP(0)(NHBu )2 by oxidation with tert-butyl peroxide. Their lithiation produces unusual cyclic or polycyclic dUithium salts of the phosphonate dianions [PhP(Q)(NBfr)2] (Q = O, S, Se). The dimethylaluminum derivatives PhP(Q)(NBu )2AlMe2 (Q = S, Se) obtained from PhP(Q)(NHBu )2 with AlMes are fom-membered chelate rings. ... 

The Hiickel rule is strictly applicable only to monocyclic compounds, but the general concept of aromaticity can be extended beyond simple mono-cyclic compounds to include polycyclic aromatic compounds. Naphthalene, with two benzene-like rings fused together, anthracene, 1,2-benzpyrene, and coronene are all well-known compounds. Benzolalpyrene is particularly interesting because it is one of the cancer-causing substances that has been isolated from tobacco smoke,... 

Aromatic compounds are defined as cyclic or polycyclic systems which sustain a diamagnetic ring current and consequently exhibit a total diatropic lowfield 1H NMR chemical shift relative to that of vinylic protons. Paramagnetic ring current is expected and was shown to be induced in antiaromatic species and to result in a total highfield H NMR band displacement, while non aromatic compounds give rise to characteristic vinylic XH patterns. 

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