Ring systems, table

The new macrolides are semisynthetic molecules that differ from the original compounds in the substitution pattern of the lactone ring system (Table 5.3, Figs 5.12 and 5.13). 

The placement of the tether at the C(3) position of the nitronate provides access to the spiro mode of dipolar cycloadditions. This class has been evaluated for the preparation of five- and six-membered spiro ring systems (Table 2.54) (254). The dipolar cycloaddition proceeds in good yields with mild heating, however, the three-atom tether proceeds faster than the four-atom tether. 

Six-membered ring systems with four heteroatoms are much less well known only 18 out of more than 80 possible systems are described, and their chemistry remains essentially unexplored. Thus only one of each of the several possible oxadithiazines and dioxathiazines (Table 6) have been reported. Two oxathiadiazine ring systems (Table 6) are known,-with the symmetrical 1,4,3,5-isomer being fairly well described. Isolated examples of... 

This rearrangement offers a general synthesis of a- and (3-2-deoxy sugars with pyranoid and furanoid ring systems (Table).5... 

The formation of [5,5]- and [6,5]-ring systems under the same conditions is quite favorable, in contrast to that of [7,5]-ring systems (Table 36). 

This precaution stated, a simultaneous theoretical investigation of the essential spectroscopic features of the lactam and lactim forms of the three fundamental monohydroxypurines yields some interesting results. These are summarized in Table XII, together with the corresponding experimental data, which in the case of lactim forms refer to the methoxy derivatives. As in all these compounds there is besides the lactam-lactim tautomerism, the possibility of an oscillation of the imidazole proton between the different N atoms of the ring system, Table XII indicates the exact tautomeric form or forms to which the calculations refer. In case of the lactam forms these are the most probable tautomers of such forms obtained in the study described in the next section. For the lactim forms they are the a priori most probable ones. 

In order to achieve high selectivities with thermostable zeolite-based membranes, zeolites can be choosen with pore apertures matching the kinetic diameters of the molecules to be separated. Moreover, the hydrophobicity of all-silica zeolites provides continuous separation, independently of traces of water in the gas streams applied. In the total spectrum of tectosilica(te)s there is only one all-silica 8-ring system Deca-dodecasil 3R (DD3R) but several all-silica 6-ring systems Table 6). 

As might be expected, ease of desulfurization depends on the type of carbon-sulfur bonds involved. Alkyl sulfides and polysulfides possess weak carbon-sulfur bonds and react rapidly and completely under hydroprocessing conditions, as their uses in catalyst sulfiding agents testify. Sulfur incorporated in thiophene systems have both stronger C(sp2)-S bonds than alkyl C(sp3)-S bonds and the sulfur as well is in the aromatic five-membered thiophene ring system. Table 8.12... 

The BZAs have been developed as broad-spectrum anthelmintic agents. The most useful have modifications at the 2 and/or 5 positions of the benzimidazole ring system (Table 41-1). Thiabendazole, mebendazole, and albendazole have been used extensively for the treatment of human helminth infections. 

In the cyclopentyl series, 5y/i-elimination occurs from a planar transition state, but a/in -elimination is slightly distorted from a dihedral angle of 180°. Consequently, the preference for n/m -elimination is less marked in the five-than in the six-membered ring systems (Table 12). With the small neutral base trimethylamine, electrostatic forces of attraction between the sulphonate ester and the partially neutralised base balance the normal preference for n/iti-elimination and the reaction is almost non-stereospecific. None of the reactions follows the carbanion mechanism, as general base catalysis is observed for the 2-p-tolylsulphonylcyclopentyl tosylate elimination and the studies on 2-phenylcyclopentyl tosylates revealed large isotope effects (A h/A d) and p values smaller than in the 2-phenylethyl series (see Table 9, p. 209) " . 

Phenazines compounds based on the phenazine ring system (Table). All known naturally occurring P. are produced only by bacteria, which excrete them into the growth medium. Both six-membered carbon rings of P. are biosynthesized in the shikimate pathway of aromatic biosynthesis, via chorismic acid (not from anthranilate, as reported earlier). The earliest identified biosynthetic intermediate after chorismate is phenazine 1,6-dicarboxylate, which has been isolated from Pseudomonas phenazinium and from non-... 

A number of tables of these bending modes have been developed to aid in making the assignment of substitution patterns in a large number of aromatic ring systems. Table 5.3 is one such table for the benzene ring system. 

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