Ketones chromophores

The following discussion is divided into three subsections the ketone chromophore (Section 4.4,1.1.), for which configurational assignments are based on the effect of ring dissymmetry and substitution pattern on the rotatory power of the n-rt transition. For conjugated chro-mophores (Section 4.4.1.2.) it is both the helicity of the chromophore and the vicinal substituent effect that determines the rotatory power of the 71-71 transition. Finally, the versatile stereochemical method, exciton chirality method (Section 4.4.2.), is based on the chiral interaction between the electric dipoles of the allowed transitions in two or more chromophores. 

The ketone group is a useful model for other types of chromophores because it can be selectively excited in the presence of other groups in polymer chains such as the phenyl rings in polystyrene and so the locus of excitation is well defined. Furthermore there is a great deal known about the photochemistry of aromatic and aliphatic ketones and one can draw on this information in interpreting the results. A further advantage of the ketone chromophore is that it exhibits at least three photochemical processes from the same excited state and thus one has a probe of the effects of the polymer matrix on these different processes by determination of the quantum yields for the following photophysical or photochemical steps l) fluorescence,... 

Before looking at the effect of the polymeric matrix on quantum yields and efficiencies of photochemical processes it is important to look first at variations which are due to the structure of the ketone chromophore itself which are observable regardless of whether the chromophore is in the solid, liquid, or gaseous state. The first of these is illustrated in Table II which illustrates the quantum yields for esters of dimethyl keto azelate (3). 

FIGURE 13. Extended octant rale applied to enantiomeric dissymmetric orientations of /S,y-unsaturated ketone chromophores. When the absolute of the dihedral angle 0 is >90°, a positive Cotton effect is predicted for the geometry of (a) and a negative for (b). In a transoid arrangement of /S,y-unsaturated ketone (c) the As values are often quite small (References 200 and 201)... 

A variety of studies have been performed in which the chirality induced in ketone chromophores has been characterized by CD spectroscopy. CD has been induced in the n-+ r transition of cyclohexanone upon its dissolution into several alcoholic and ester-functionalized solvent systems [5]. In this work it was established that hydrogen bonding between solute and solvent was not required for the generation of solvent-induced CD. In a more comprehensive work, the CD induced in 20 compounds containing a ketone chromophore was studied in approximately 15 chiral solvent systems [6]. It proved difficult to develop general rules for the observation (or lack thereof) of solvent-induced CD, with the rotational strengths being found to be solvent, temperature, and... 

Screening of L15 was carried out monitoring the appearance of the unsaturated ketone chromophore at 340 nm and calculating the reaction yields in reference to the... 

Despite the ease with which AN and HMAN can rapidly degrade, it is surprising that there have been few reports of the presence of these degradation products, and this may be a consequence of both the lack of commercially available standards and the fact that most studies of anatoxins relied on the use of LC with UV detection for analysis. The degradation products are not detectable using this method as they do not possess the unsaturated ketone chromophore. 

In hydrogen atom abstraction by a ketone the following orbitals are important for the reactant R CO + RH, the orbitals Och and a H of he R—H bond to be broken and the orbitals no, Jico> and jr of the ketone chromophore. For the product RiCOH + -R, the orbitals a and aHn of the newly formed OH bond, plus the orbitals ji(, and of the protonated ketyl radical and the orbital pc of the odd electron on the residue R. Their relative energies can be easily estimated qualitatively. Since both no and Ooh are essentially localized... 

The experimental results fit the Stern—Volmer equation in most cases of energy transfer reactions studied with polymer solutions. Lpkac et al. [61] have used the Stern—Volmer equation to interpret their results concerning the photolysis of polyphenylvinylketone in solution in the presence of quenchers of the excited triplet state of the aromatic ketone chromophore (Fig. 38). The slopes of the straight lines obtained allow the efficiency of various acceptors to be compared. On this basis, the authors have shown that naphthalene is about twenty times more efficient as a triplet quencher when introduced into the polymer chain as vinyl-naphthalene rather than as an additive in the solution. Such a result is not unexpected since the actual concentration of the acceptor inside the polymer coil is much higher than the mean quencher concentration when naphthalene is used as an additive. The process is usually diffusion... 

As shown in Table 11, the photoreduction process is quite efficient in phenyl vinyl ketone copolymers with styrene. Because of the rapid intersystem crossing in the phenyl ketone chromophore, it seems likely that both the reduction and chain scission processes proceed via the intermediacy of the triplet state. 

Through c.d. measurements of the ketones (42), (43), (44), (45), and (46), which were obtained from the corresponding amines, the precise stereochemistry at position 10 has been determined Similarly, c.d. measurements of the 17-ketonic chromophore of the derivatives (35), (37), and (38) allowed a 14/ -H configuration for these products to be postulated. The same diketone (47) has been prepared starting from (35) and from (37). Finally, the configuration has been determined by c.d. measurements of the derived ketone (48), obtained from (35) (Scheme 4). The sign and the amplitude of the Cotton effect of ketone (48) are... 

Alstophylline showed an UV-spectrum consistent with isolated 7-methoxyindole and j8-methoxy-a,)3 unsaturated ketonic chromophores. The NMR-spectrum was informative and identified the methoxyl, the two A-methyls in different environments, the terminal methyl, the... 

Discrepancies are also found in compounds which contain a disturbed P,Y-unsaturated ketone chromophore. Irradiation of 88 leads to the cyclopropyl ketones 89 and 90 in which the methyl groups, originally attached to the methane carbon, are completely scrambled. 40,4i) This result is not in accord with a n2a -b o2a mechanism. On the other hand, 91, which contains the same extended p.y-unsaturated chromophore as 88 gives 92 with the acyl group endo. This is in accord with a n2a -f-... 

C. a, -Unsaturated Ketones Chromophore Type Frequency in CCI4 and CS2 solution, cm " ... 

This small group of antibiotics has received relatively little attention. Dermo-statin has been shown to possess a conjugated hexaene-ketone chromophore and does not contain carbohydrate [26,56]. The recent discovery of candihexin [20], which has certain novel properties, has increased interest in this group. Candihexin has been resolved into eight components [57], designated A—F, of which A, B, E and F constitute 90% of the total. Candihexins A and B were found to have the highest antifungal activity [57]. 

Even very low amounfs of the ketone chromophoric groups in polypropylene can be responsible for the initiation of polymer photo-oxidative degradation which may further occur by mechanisms similar to those described for polyethylene (cf section 3.1.2). 

Estimated value for polymer with isolated ketone chromophore from experimental data on ketone copolymers. (Reprinted with permission from Guillet, J.E., Polymers and Ecological Problems published by Plenum Press,... 

An important observation by Banerjee and Falvey related to the mechanism of phenacyl photochemistry is that, in the presence of H-atom donors, the excited triplet state of the ketone chromophore is quenched to give ketyl radicals 23 clearly observed by LFP. As shown in Scheme 6, carboxylic acids are photoreleased but, again, originally as carboxylate anions, not acyloxy radicals. 

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