Photocycloadditions principle

Further studies, on the same principle, were carried out by Lange and coworkers on the intermolecular photocycloadditions of cyclohexenones 167 to alkene 168, possessing different chiral auxiliaries at the enone91 or alkene92. Diastereomeric mixtures of cis,anti,cis 169 and cis,syn,cis isomers were obtained in low to moderate diastereomeric excess (Scheme 37). 

Direct UV irradiation of DNA leads to a variety of lesions at the nucleo-base. The principle products are pyrimidine cyclobutane dimers ToT, ToC, and CoC, formed in that order of preference, by [2+2] photocycloaddition of two adjacent pyrimidine nucleosides in the same oligonucleotide strand (Scheme B.21.4). With approximately 50% of the frequency of cyclobutane dimers, [2+2] dimerization involves a C=0 double bond leading to an intermediate oxetane 29, which later decomposes to the [6-4] photoproduct 30. 

The desired cyclization leading to the six-membered ring was achieved by intramolecular photocycloaddition of dioxolenone 107 as shown in Scheme 26. Consecutive cleavage of the ketal followed by retroaldol reaction augmented the initial ring system by a C2 unit to trans-bicycle 108 (90). Attempts to apply this reaction principle to a trans-decalin system in order to reach a taxane skeleton failed (97). 

Photochemical 67T-6tt cycloaddition of two benzene rings, in principle, produces benzene dimers having two 1,3-dienes units. However, as expected, the dimers are unstable and revert back to benzene rings easily. Prinzbach and coworkers found that two benzene rings, locked in face-to-face relationship, undergo 67r-67r photocycloaddition on irradiation... 

Early studies of solid-state polymerization of vinyl derivatives have been reviewed (5). The initial discussions of the topochemicrd principle 0 were replete with examples from the photocycloaddition of cinnamic acid derivatives. 

Dioxinones (194) can be prepared as enantiomerically pure compounds. The intramolecular 2 -I- 2 photocycloaddition of 194 usually leads to a mixture of two regioisomers, 195 and 196, and even to only isomer when m = 2 and w = 0. The principle merit of this approach resides in the ability of the ester and ketalic groups in the photoadducts to be cleaved easily into adducts similar to those provided by intermolecular photocycloaddition of dioxinones with the corresponding alkenes [159d]. Only one stereoisomer... 

The photocycloaddition of a,p-unsaturated carbonyls (enones) with an olefin to form cyclobutanes is an especially efficient reaction (Eqs. 16.33 and 16.34). This reaction has proven to be of considerable synthetic utility. In principle these types of reactions can proceed from 7t,TT or Tr,ir states of the carbonyl, and for enones these two states can often be quite close in energy, making detailed analysis complicated. 

In their communication, however, the authors only reported overall yields of photoproducts, and no kinetic or binding data are available to characterize the intervention of the supramolecular assembly during the photocycloaddition step. In this regard, the choice of ethyl cinnamate as a model compound is also unfortunate, as it cannot be ruled out that the enhancement in yield results from an increase in the single state hfetime of the cinnamate chromophore due to hindered ,Z-isomerization. Although the observed products are the P- and 5-truxinate dimers, which are usually the major products from solution photodimerization, the presence of methylene tethers of different lengths could, in principle, be used to provide evidence for the proposed hydrogen-bonded assembly. 

In photocycloaddition reactions, the degree of enantioselectivity depends on the effectiveness with which the enantiotopic faces of a prochiral olefin can be distinguished (enantioface-differentiation). The conventional way to achieve this goal is to covalently attach a chiral auxiliary (see Section 61.2) to the prochiral substrate. The consecutive reaction is diastereoselective, and the auxiliary has to be removed afterwards. The degree of diastereoselectivity in the selectivity-determining step is measured by the diastereomeric excess (DE), which is defined by analogy to the enantiomeric excess, and the ee corresponds to the DE after removal of the auxiliary. The auxiliary-based process consequently consists of three steps, two of which are, in principle, superfluous as they are not concerned with the reaction under scrutiny. 

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