Cyclic, photoreactions

Photoisomerization of retinal from the all-trans to the 13-ct.v form leads to a cyclic photoreaction with intermediates, bR, K, L, M, N, and O, which are subsequently formed before recovery of the initial state, as schematically illustrated, with the absorption wavelength in the suffix (Fig. 5), together with their individual lifetimes.50 51 The first proton transfer in the photocycle of the all-trans, 15-anti isomer is from the retinal Schiff base to Asp 85 at the central part of the protein. Then, a proton is released from the proton release complex (PRC) consisting of Glu 204, 194 and a bound water molecule at the extracellular surface, as illustrated by arrow A. This is followed by reprotonation of... 

Chromium aminocarbenes [39] are readily available from the reaction of K2Cr(CO)5 with iminium chlorides [40] or amides and trimethylsilyl chloride [41]. Those from formamides (H on carbene carbon) readily underwent photoreaction with a variety of imines to produce /J-lactams, while those having R-groups (e.g.,Me) on the carbene carbon produced little or no /J-lactam products [13]. The dibenzylaminocarbene complex underwent reaction with high diastereoselectivity (Table 4). As previously observed, cyclic, optically active imines produced /J-lactams with high enantioselectivity, while acyclic, optically active imines induced little asymmetry. An intramolecular version produced an unusual anti-Bredt lactam rather than the expected /J-lactam (Eq. 8) [44]. 

FIG. 13 Cyclic voltammogram (a) and potential dependence of the photoresponses (b)-(c) to chopped illumination and lock-in detection associated with the photoreaction in Eq. (40). The CV shows that the polarizable window extended to less than 100 mV. The photocurrent measurements carried out were done in the presence (trace 3) and absence (trace 2) of the redox quencher in the organic phase. (Reprinted with permission from Ref 48. Copyright 1989 American Chemical Society.)... 

A number of examples of this type of photorearrangement have been observed with cyclic and bicyclic molecules. Some photoreactions involving cyclohexenones are as follows ... 

Systematic studies of the photoreactions of cyclic imides and in particular of phthalimide derivatives have been the subject of a review.278 One of the... 

Prolonged irradiation of the mixture leads to the formation of a huge variety of secondary products due to competing photoreactions of the triene members of the series P, T and D. Many of these are formed as a result of the more common triene photoprocesses which have already been discussed. Many more are formed via less generic processes which involve the cyclic moieties at the two ends of the triene or intervention of the solvent in either catalytic or direct fashion. The so-called toxisterols arise from irradiation... 

Steric encumbrance in the attacking reactant blocks the Sjjj l reaction by a standard manner (Look and Norris 1999). If the attacking reactant protrudes in the ion-radical form, the reaction results depend on the manner of spin-charge distribution within this form. Thns, Af-phenylpyrrolidine cation-radical forms a nitrogen-containing heterocycle as a result of cycloaddition to menthoxyfuranone, whereas the A-mesitylpyrrolidine cation-radical is unable to form the cyclic product compare courses of the two photoreactions shown in Scheme 6.4 (Griesbeck et al. 2007). 

There are two principal modes for introducing the photophore into a peptide (1) Coupling a photoreactive N-protected amino acid during the stepwise assembly of the peptide e.g. in Scheme 3, the amino acid 4-benzoylphenylalanine (3) is incorporated into position 8 of substance P (9). (2) Post-synthetic modification of a fully assembled peptide, at either a free N-terminal a-amino group or a side-chain functionality, by the photophore in a site-specific or nonspecific manner e.g. in Scheme 3, treatment of cyclic R-G-D-containing peptide 10 with 4-benzoylbenzoic anhydride (11) gives the modified peptide 12. 

One may well ask why the isomerization of alkenes discussed in the preceding section requires a sensitizer. Why cannot the same result be achieved by direct irradiation One reason is that a tt — tt singlet excited state (5,) produced by direct irradiation of an alkene or arene crosses over to the triplet state (Ij) inefficiently (compared to n —> it excitation of ketones). Also, the Si state leads to other reactions beside isomerization which, in the case of 1,2-diphenyl-ethene and other conjugated hydrocarbons, produce cyclic products. For example, cw-l,2-diphenylethene irradiated in the presence of oxygen gives phenanthrene by the sequence of Equation 28-8. The primary photoreaction is cyclization to a dihydrophenanthrene intermediate, 6, which, in the presence of oxygen, is converted to phenanthrene ... 

One of the many photoreactions of conjugated dienes is the formation of cyclobutenes,99 and this has been reported both in cyclic and acyclic dienes. The stereochemistry of such photocyclizations has been discussed in terms of orbital symmetry by Woodward and Hoffmann.100... 

Relatively few examples of photoreactions of seven-membered and larger heterocycles have been reported. The major products of irradiation of liquid oxepin are hex-5-en-l-ol and hexanal234 photodecomposition of cyclic ethers has, in general been shown to be largely dependent on ring size. Irradiation of the valence bond isomer (287) of hexakis(trifluoromethyl) oxepin gave the unexpectedly stable oxet (288) via a mixture of (Z)- and... 

Fig. 18 Photoreaction of the photoproduct of [Os3(CO)io( Ji-PPh2)] with Hg2+ to yield a cyclic heptametallic complex [102]...

The high stereoselectivity observed in this cyclization reaction was attributed to the location of the carbonyl moiety in a rigid cyclic ring framework, allowing H-abstraction from the same face of the initially formed five-membered ring. The same group has further utilized the constraint imparted by the benzylidene acetal moiety on the excited state photoreactivity of the carbonyl moiety of the D-xylose derivative 52. Thus, 8-H-abstraction results in the formation of a diastereomeric mixture of cyclized product 53, which is further transformed into D-glucopyranose 54 and D-idopyranose 55 derivatives, as shown in Scheme 8.16 [17]. 

Lewis and his coworkers reported the intramolecular photoaddition of aliphatic secondary amines to arylalkenes to give 5- and 6- membered cyclic amines (Scheme 17) [66-67]. Ohashi reported that the other type of photoaddition reaction takes place when 1,4-dicyanobenzene and triethylamine are used as substrates. This photoreaction can be explained in terms of the deamination of the initially produced diethylaminoethylated compound (Scheme 18) [68]. 

Yonemitsu and his coworkers found that the intramolecular photoaddition of chloroacetoamide derivatives with an electron donating aromatic ring in molecules gives the cyclic amide compounds in water [69], However, the photoreaction in or nic solvents proceeds via the exciplex to give the different cyclized products. 

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