Cyclopropanes chromophore

A. The Cyclopropane Chromophore and the Circular Dichroism of Cyclopropane Hydrocarbons... 

The relevant electronic features of the cyclopropane chromophore, in particular, the unusual conjugation properties of cyclopropane (112) can be discussed referring to those... 

All the above considerations reveal that the absorption spectrum of 112 will be a complexity of overlapping bands and, therefore, the assignment of electronic features of the absorption spectrum of 112 is still far from being clear. This also complicates the discussion of the CD of the cyclopropane chromophore. 

CD effects of the cyclopropane chromophore have also been detected for gem-dimethylcyclopropanes which are attached cis to another ring, such as 116-120 . ... 

The phenyl substitution provides both the chromophore necessary for photoactivity and the stabilization of the initially formed radical. The reported photochemical extrusion of SO from 2,2,4,4-tetraacetylthietane263b to give the corresponding cyclopropane appears to be a unique case associated with the particular features of the irradiated molecule. 

The ultraviolet spectroscopy of epoxides has received relatively little attention in the literature, since their ability to delocalize ir-ciectrons in a chromophore, although detectable, is even smaller than that of the cyclopropane ring.1"1 A representative selection of Amu values is shown in Table 3, For additional references from the held of steroid chemistry an excellent review by Dorfman468 should be consulted. 

Studies on the ability of a cyclopropane to conjugate have involved a wide variety of approaches including spectroscopic, thermochemical, structural and theoretical examinations. Two overall thrusts are apparent in the reported work. One approach has been to investigate the impact of a cyclopropyl substituent on the properties of an attached functional group or molecule. Typically, techniques used include measurement of the substituent parameters of a cyclopropyl group, determination of the acidity or basicity of functional groups attached to a cyclopropane, or measurement of the impact of a cyclopropyl substituent on the absorption spectrum of an attached chromophore ... 

Photolysis of thietane sulfones (e.g., 169 ) also yields cyclopropanes. The need for a chromophore and a 2-substituent to stabilize an intermediate diradical has been emphasized. The sulfone 170 undergoes an unusual transformation. ... 

The present chapter is largely restricted to cyclopropane derivatives as data on other compounds with a three-membered ring are lacking in the literature. Generally, chiroptical properties intrinsic to the cyclopropane system as well as substituent effects of the cyclopropyl group on chiroptical properties of other types of molecules or chromophores, respectively, will be dealt with. 

Determinations of absolute configurations of trans-cyclopropane-l,2-dicarboxylic acids on the basis of a definite Cotton effect seem to be possible for carboxylic acid thioamides. In these molecules, such as 123a and 123b, there is a Cotton effect near 330 nm which results from the (n, n ) excitation of the thioamide chromophore . In the... 

The vinylcyclopropane chromophore takes an exceptional position among the compounds in which a double bonded group is linked to the cyclopropane ring. In the parent molecule, vinylcyclopropane (127), there is a s-trans-gauche conformational equilibrium whereas in all other related double bonded compounds (acids, aldehydes, etc.) s-trans and s-cis conformers are preferred... 

In chiral vinylcyclopropanes the chromophore can be viewed as inherently chiral, i.e. all the electronic transitions have simultaneously an electric (/i) and a magnetic (m) transition moment. For transoid or cisoid (indicated in broken lines below) arrangements the magnetic moments are generated by a circular displacement of electric charge density around the bond between the cyclopropane and the ethylenic moieties. The helical movement of electron density involves essentially p-atomic orbitals twisted relative to each... 

As (-H )-carvone (143) has the same absolute arrangement of the carbonyl and cyclopropyl moieties as 141, but opposite (n, tc ) CE, XXVIII has been ruled out as a conformational representative of 143 . For 143 with the gem-dimethyl moiety steric interactions will be severe in the conformations XXIX and (especially) XXX. XXVII is assumed to be the most favorable representation of 143. In XXVII the carbonyl n and cyclopropane p orbitals are almost orthogonal. Hence, there will be no contributions from an inherently chiral chromophore and the observed (n, n ) CE can be rationalized by the sum of the normal octant-like contributions of the C(6)methyl and gem-dimethyl group In XXVII all components of the gem-dimethyl group lie nominally in a (+) back... 

Referring to the very similar rotational strengths of a-fenchocampherone (159) and spiro(cyclopropane-l,7 -norbornanone) (155) (Figure 13 Table 9) it was suggested that 155 has an inherently symmetric chromophore in which the normal octant perturbers dominate the contributions to the chirality of the (n, k ) transition. 

Photochemical studies of cyclopropane derivatives in solution have concerned mainly molecules in which a chromophore capable of absorbing at wavelength above 200 nm is attached to the ring. The principal reaction is the cleavage of one bond to give a diradical. Cyclopropane and alkylcyclopropanes have been studied in the gas phase at 147 nm or... 

If viewed from the oxygen, most optically active carbonyl compounds have their substituents only in the rear octants. The appearance of the plane that separates the rear octants from the front octants is not determined by the symmetry of the isolated chromophore. Calculations have shown that it has approximately the shape depicted in Figure 3.4b. Many examples have verified the validity of the octant rule, but there are also cases where it is not applicable, at least not in its original, simple form. This is true for ketones with a cyclopropane ring in the a, jS-position and for fluorosubstituted ketones, for which the experimentally observed sign can be reproduced only if the perturbation due to the fluorine atom is assumed to be smaller than that due to the hydrogen atom. More recent detailed calculations solved some of these problems. (Cf. Charney, 1979.)... 

Comparison of the NMR data and the circular dichroism of toxlsterol C2 with those of the now known Cl suggests that C2 should have formula II. The circular dichroism curve of C2 is opposite in sign (Ae227 -7.09) to that of Cl (Ae233 +4.55), The sign of the Cotton effect, associated with the absorption band at 227 to 229 nm, reflects the configuration in the chromophore concerned—that Is, the relative orientation of the cyclopropane ring and the double bond. This rules out formula... 

Another fairly general class of photochemical isomerizations are the di-Ti-methane photoisomerizations. These reactions involve the photochemical conversion of a divinyl methane unit to a corresponding vinyl cyclopropane through intermediate biradical states. The example shown below is interesting in that the naphthalene chromophore in (66) absorbs the light, undergoes intersystem crossing, and sensitizes the benzo-bicyclo-[2.22]-octatriene for reaction. 

A variety of compounds containing a P,y-enone chromophore in a complex molecular framework have been synthesized and their photochemical reactions have been examined. In the majority of the chromophoric systems, the P,y-enone group is part of a bicyclo[2.2.2] octane framework in which the oxa-di-ji-methane reaction is most general and efficient. Mehta and Srikrishna reported an oxa-di-Jt-methane reaction in the tricychc compound 131 and its halo analogue that, upon sensitized irradiation, gave 132. It is interesting to note that [jt -l-Jt ]-cycloaddition was not observed. Similarly, the P,y-enone 133 underwent the ODPM reaction to give the compound 134 with a cyclopropane annulated bicyclo [3.3.0] octane framework, in excellent yield (Scheme 25). ... 

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