Structure, vii, viii

Three possible isomeric forms exist for benzheterocycles containing seven- or eight-membered rings, as depicted by structures (VII), (VIII), and (IX). Most of these heterocycles contain an Si heteroatom, but derivatives of (IX) with several different heteroatoms are known. Only one benzheterocycle with an eight-membered ring has been reported [(IX), M = Si]. 

The effect of substitution on the structure of pyrazolinones is a very important one, because certain of the tautomeric forms then become impossible. For example, substitution at N-l allows only structures (VII), (VIII) and (X). Substitution at N-2 would make... 

A few compounds of platinum(ll) containing four different ligands, [PtABCD], are known. Realizing that either B, C, or D groups may be trans to A, it is apparent that there are three isomeric forms for such a compound. The first complex of this type to be obtained in three forms was the cation [PtNOjNHjCNHjOHjpy] which has structures VII, VIII, and IX. 

Fig. 5. Pyranotropanes (plus chalcostrobamine) and 10,11-dihydropyranotropane structures for Tables VII, VIII and XII.

The molecular structures of compounds (VII), (VIII), and (IX) also have much in common (Fig. If, Table 1) all three compounds consist of [TC6Br6(/t3-Br)5]2 anions and M+ cations where M+ = (C4H9)4N+ or [H30(H20)3]+). For compound (IX) such a structure is established indirectly, because compound (VIII) is easy to obtain by adding [(C4H9)4N] + cations both to the solutions of compound (IX) and to those of compound (VII). Besides, compound (IX) possesses chemical and physico-chemical properties similar to those of compounds (VII) and (VIII). 

Calmodulin Structure (Source) Method Angle (°) A-B (I-II) Angle (°) C-D (III-IV) Angle (°) E-F (V-VI) Angle (°) G-H (VII-VIII) PDB Entry (Reference Number)... 

Structures VII and VIII are identical because rotating either one 180° in the plane of the paper makes is superposable with the other one. VII possesses a symmetry plane and is achiral. Achiral stereoisomers which have chiral centers are called meso. The meso structure is a diastereomer of either of the enantiomers. The meso structure with two chiral sites always has the (RS) configuration. 

Empirically determined chemical shift additivity parameters have been determined- for diene-type polymers. The shift contribution of a quaternary carbon which is fllto the carbon in question was not determined by those authors. However, using their additivity parameters and the shift positions of the ( carbons in Figure 8, a value of +15.4 ppm can be estimated for the contribution of a neighboring (0() quaternary carbon. Using this value, the shift positions of the carbons in structures VII and VIII are calculated as shown. If the first 1,2 unit were on the chain in a 2,1 manner, the methylene carbon resonance would be at a considerably higher field, but it would be difficult to estimate its position with any certainty because the quaternary effect... 

During the racemization of proline (structure VI), the chiral carbon must at some stage become trigonal. In accordance with a trigonal transition state, both structures VII and VIII bind 160 times more tightly than proline.14 15... 

C.K. Ingold, Structure and Mechanism in Organic Chemistry, Bell, London, 1953, Chaps. VII, VIII. 

The stereochemical factors influencing the formation of cyclic carbonates have not been fully elucidated, and the number of derivatives whose structures have been unequivocally proved is quite small. In fact, the structures of the known cyclic carbonate derivatives of pyranosides have all been allocated on the basis of analogies with corresponding 0-isopropylidene acetals. This is particularly unfortunate where the possibility of both five-and six-membered carbonate rings exists, as in the case of 1,2 3,4- and 1,2 4,6-dicarbonate esters of D-galactopyranose (VII VIII). A five-mem-... 

Substituted borohydrides, [BHjX] , are used to form Cu and Ag complexes The anion [HjBCOjR]" reacts with triphenylphosphine complexes of Cu(I) and Ag(I) in chloroform-ethanol, yielding (Ph3P)j MH3BCOjR (where n = 2, 3 and R = H, Me, Et). If 3 mol of phosphine are coordinated to Cu or Ag, structure (VII) results, whereas in complexes with 2 mol of triphenylphosphine, two hydrogen bridges are formed (VIII) ... 

In contrast, in the transition state for C-H deprotonation (Scheme 56, also showing the proton-accepting base H2O), the most important structure is now VIII, representing heterolytic C-H bond cleavage, whereas structure VII (corresponding to III in Scheme 55) should be of minor importance, since it involves the very unfavorable homolytic C-H bond cleavage. 

Chromium hexacarbonyl reacts with Ph2AsCH2AsPh2 yielding a compound of stoichiometry Cr(CO)2(Ph2AsCH2AsPh2) 357) and with triarylphosphines yielding complexes of the type [Cr(CO)2L]2 [L = PPhg, P( i-tolyl)3, and P(p-tolyl)3] (31). The arsine and triphenyl-phosphine compounds have been shown by X-ray analysis to possess the structures (VII) and (VIII), respectively, in which a phenyl group is... 

A few metalloid derivatives are named according to the parent heterocycle, e.g., (VII), M = 2, are metalloindanes and (VII), = 3, are metallochroman derivatives or metallonaphthalene derivatives (hydro prefixes are required to denote saturation). Thus, compounds of structural type (VIII) are metal derivatives of isoindane or isochroman. 

The oxidation of cyclohexene has been the subject of considerable discussion, and it is now apparent that it behaves differently from the straight-chain olefins. Cyclohexene was originally reported to yield both cyclohex-2-en-l-yl acetate, structure (VII), and cyclohex-3-en-l-yl acetate, structure (VIII), in chloride-containing acetic acid (76) and only the allylic isomer with Pd(OAc)a in chloride-free acetic acid (6). However, it has now been demonstrated that if no oxidants are present to regenerate the Pd(0) to Pd(II) in neutral or basic HOAc, the Pd(0) formed will disproportionate the cyclohexene to give benzene (22, 295). In acetic acid containing perchloric acid, cyclohexanone (structure VIII) and cyclohex-1-en-l-yl acetate are formed (22). If Pd(0) is prevented from precipitating by use of oxidants in neutral or basic acetic acid, the allylic and homoallylic acetates are formed. 

The main points of interest of the structures of these polymorphs are (i) the analogies with silica and silicate structures, (ii) the presence of two interpenetrating frameworks in the most dense forms vi and vii (viii), and (iii) the ordering of the protons. Analogies with silica and silicate structures are noted in Table 15.1, namely, ice-iii with a keatite-like structure, ice-vi with two interpenetrating frameworks of the edingtonite type (p. 828), and ice-vii (and viii) with two interpenetrating cristobalite-like frameworks. In these structures, related to those of... 

Figure 2.2 Isomeric structures of C0CI3 (en)2 as predicted by Werner s Coordination Theory (VII,VIII) and Jorgensen s Chain Theory (IX,X).

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