Effect of Substituents at

Stereochemistry of glycosylations with partially benzylated glucosyl donors is also sensitive to the presence of an acyl group at 0-6. The results of glycosylation of acceptor 113 with a series of glucosyl donors 112 showed that the presence of an 

Glucosyl donors with acyl groups at either 0-6 (112g) or at both 0-6 and 0-3 (118) were successively applied for the synthesis of glycoforms of the outer core 

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Studies of the alkylation of indazoles (67HC(22)1) have been updated by Nunn (73JCS(PD2371) and Palmer (75JCS(P1)1695). The ratio of methylation at positions 1 and 2 is relatively sensitive to the steric effect of substituents at positions 3 and 7 as shown by the results obtained in basic medium for unsubstituted indazole (55 45) and its 3-phenyl (74 26) and7-nitro derivatives (29 71). 

Eckstein, J. W., Hastings, J. W., and Ghisla, S. (1993). Mechanism of bacterial bioluminescence. 4a,5-Dihydroflavin analogs as models for luciferase hydroperoxide intermediates and the effect of substituents at the 8-position of flavin on luciferase kinetics. Biochemistry 32 404 111. 

Inner-outer-ring dienes are very useful in the synthesis of polycyclic molecules. Their reactivity in the Diels-Alder reaction depends on the type of ring (carbo-cyclic, heterocyclic, aromatic) that bears the ethenyl group or on the electronic effects of substituents at the diene moiety [30]. 

Fig. 3 Structure-photophysical property relationship of coumarin derivatives, (a) Schematic representation of the correlation between electronic effect of substituents at the C-3 and C-7 position and photophysical properties (b) Structure and their emission maxima of various coumarins...

The same Suzuki methodology was used to synthesize a similar copolymer 446 [548], The polymer showed a solvent-dependent green-yellow emission (from 545 nm in THF to 565 nm in chloroform) as often observed for polar chromophores. The PL QE also varied with the solvent (from 11% in THF to 21% in decalin) but, in contrast to copolymer 445, no strong decrease in emission efficiency was observed in the solid state (4>p1 n= 13%) that could be attributed to the effects of substituents at the thiophene ring. LEDs based on 446 showed, for an ITO/PEDOT/446/Ca/Al architecture, a turn-on voltage of ca. 10 V with a maximum brightness of 340cd/m2 at 22 V and appreciable el = 0.14%. 

The first are relative rates for [14] - [17] obtained by Schleyer and coworkers and given below on the effects of substituents at the 6-position in augmenting solvolysis of 2-exo-norbornyl tosylates (Schleyer, 1972 Schleyer et al., 1965 Strang and Schleyer, 1968). 

A measure of the susceptibility to any substitution effect of reaction series for families of organic compounds as modified by para-positions of certain reactions of aromatic compounds, log(kx/ ku) = pcTy, where constant characteristic of substituent X and its position. 

Steric effects of substituents at the ring ortho position are most evident when comparing the formation of 1 and 12. The quantum yield for formation of 1,... 

The mechanism of the aminolysis and the electronic effects of substituents at C-2 or C-4 on the kinetics of amide bond formation have been studied. In some cases, ring opening with amines occurs with partial isomerization of the exocyclic double bond. However, with more hindered compounds, such as unsaturated oxazolones derived from ketones, ring opening is stereospecific.Ring opening using diamines has also been described. Selected examples of dehydroamino acid amides prepared by aminolysis of unsaturated 5(4//)-oxazolones are shown in Table 7.40 (Fig. 7.51). 

The mechanism of decarboxylation of /3-lactones has attracted much attention. The gas-phase decomposition of 2-oxetanone is a unimolecular first-order process. It has a considerably lower energy of activation than the pyrolysis of oxetane and a much higher entropy of activation, indicating a loose activated complex (69JA7743). The ease of the reaction is greatly affected by the electronic effect of substituents at position-4, but not at position-3. The Hammett treatment of a series of rrans-4-aryl-3-methyl-2-oxetanones gave a good correlation with [Pg.374]

In contrast, the need to evaluate the relative rates of competing radical reactions pervades synthetic planning of radical additions and cyclizations. Further, absolute rate constants are now accurately known for many prototypical radical reactions over wide temperature ranges.19,33 3S These absolute rate constants serve to calibrate a much larger body of known relative rates of radical reactions.33 Because rates of radical reactions show small solvent dependence, rate constants that are measured in one solvent can often be applied to reactions in another, especially if the two solvents are similar in polarity. Finally, because the effects of substituents near a radical center are often predictable, and because the effects of substituents at remote centers are often negligible, rate constants measured on simple compounds can often provide useful models for the reactions of complex substrates with similar substitution patterns. 

The course of reaction and the character of the final products are determined by electronic effects of substituents at the aryl ring (Ar) of N-protonated azomethine 130. The most interesting is the reaction leading to 2-styrylbenzo[crf]indolium salts 132 the formation of which can be represented as the result of a rearrangement of intermediate 134. The latter is afforded by C-acylation of azomethine perchlorate 130. 

Formation of the sterically unfavourable (Z)-alkadienes on dehydrosulfonylation of a, a-dialkylated (/i)-allylic sulfones promoted by Bu OK/Bu OH has been attributed to a bvn-cffect which, by definition, stabilizes the. vy -conformation required on approach to the transition state.13 The. syn-effect of substituents at the -position of the (C)-allylic sulfones decreases in the order RO— CH3— > RS— > — CFb— > (CH3)2CH—> (CH3)3C—> C6H5— and in accord with previous observations of isomerization of (/i)-vinylie to allylic sulfones. 

The effect of substituents at the a-carbon of the enone on the stereoselectivity was examined on compounds 270, with the stereogenic center located at the alkenyl side chain. Crimmins and DeLoach122 found that the stereoselectivity encountered upon irradiation of 270 depends on the degree of steric hindrance associated with the ester group linked to the double bond. The ratio of the two epimeric centers at the C-9 position varied from 13 1 (R = Me) to 17 1 (R = Et), then 20 1 (R = i-Pr). These results demonstrate that steric effects play an important role in controlling the stereofacial selectivity in these and related systems. Fragmentation of the photoproduced four-membered ring and simple transformations afforded synthesis of ( )-pentalene 274, (i)-pentalenic acid 275 and (i)-deoxypentalenic acid 276 (Scheme 59). 

Reaction constants p in the Hammet equation describing the influence of the substituents in the phenyl ring of 2-arylbenzoxazoles, 2-arylbenzoth-iazoles (68MI2), and 2-arylbenzotellurazoles are approximately the same value, indicating a similar mechanism of expressing the electronic effects of substituents at the nitrogen atom in different benzazoles. 

In addition, most compounds usually have several different types of CH bonds, double bonds, or aromatic rings. For each type of reaction,the effect of substituents at positions a and P to the reaction center must also be taken into account. The detailed calculation of kHO (air) from these SARs has been simplified considerably by availability of a computerized estimation procedure that uses the molecular structure of the compound as the starting point for the calculation (Howard, 1993). 

The effect of substituents at the position ortho to the tether on the regio-chemistry of the addition in para-alkenyloxyacetophenones has been examined... 

Acid-catalysed solvolysis of both classes of amides (1) and (2) proceeds according to the A-2 mechanism. In consequence, variation in the polar effects of substituents at the nitrogen has two opposite effects. Electron donation increases the concentration of substrate s conjugate acid but at the same time slows... 

CoMFA Quinolones, naphthyridinones Analysis of the effects of substituents at different positions of the quinolone/naphthyridi-none scaffold [27]... 

It follows from the data in [101, 102] that the Pfitzinger reaction is sensitive to steric factors arising not only from the effect of substituents at the ortho positions of the benzene ring but to some degree also from the size of the substituent at the para position. 

UV-visible spectral measurements show that substituted indolo[ 3,2-ib] carbazoles (5) absorb significantly below 450 nm (Fig. 4.12). Their band gaps in the solid state, estimated from the on-set UV-visible absorption, were >2.55 eV, which are significantly larger than those of most p-channel organic semiconductors for OTFTs (Table 4.1). In addition, whereas chlorine substitution at the 3,9 positions of 5,ll-dialkylindolo[3,2-jb]carbazole (i.e. 5d) did not cause noticeable changes in the spectral properties, substitution at the 2,8 positions (i.e. 5c) results in red-shifts in both solution and thin film absorption - a phenomenon that clearly indicates the pronounced electronic effects of substituents at positions para to the radical cation sites (5-N and 11-N positions). 

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