This article reviews the current research interest in the selective preparation of hydroperoxides in photosensitized oxygenation.This involves two sections; (i) selective formation of allylic hydroperoxides by ene type reaction of olefins with singlet oxygen and (ii) stereospecific preparation of α-hydroperoxysulfide with singlet oxygen oxidation of thiazolidine derivatives.
This review describes the recent studies on organic photoreactions via charge-transfer complexes of small-ring compounds with various electron-acceptors, such as valence isomerizations, rearrangements, cycloadditions and oxygenations. In especial, mechanisms of oxygenations of three-membered compounds through the charge-transfer complexes with TCNE were discussed in detail, comparing with those reported for photoelectron transfer induced oxygenations.
Contrary to the hitherto accepted concept that every olefins undergo mutual isomerization between their cis-and trans-isomers, olefins substituted by anthracenyl and other aromatic polycyclic groups undergo only one-way isomerization from their cis-to trans-isomers, and the reverse isomerization from their trans-tocis-isomers never occurs. The mechanism for the one-way isomerization and the features to distinguish one-way or two-way isomerization is discussed to get whole view of the isomerization of olefins.
The reactivity features of the following photoreactions which proceed via exciplexes or radical ion species are described: (1) Regio-and stereoselective photocycloadditions between electron-donative and electron-attractive compounds via exciplexes. (2) inter- and intramolecular photocycloaddition of vinyl ethers through termolecular interactions. (3) Photocross- addition between two different electron- rich compounds via photoinduced electron transfer. (4) Photochemistry of 9, 10- dicyanoanthracene (DCA)- 1, 2- diarylcyclopropane system; (4π + 2 σ) photocycloaddition and cis-trans photoisomerization of cyclopropane derivatives via radical cation species. (5) DCA- sensitized photooxygenation of cyclopropane derivatives and polycyclic aromatics via photoinduced electron transfer. (6) DCA- Cu (II) sensitized two-electron oxidation of cyclopropane derivatives and alkenes. (7) Photoallylation of dicyanoaromatics and α, β-unsaturated nitriles by allylic silanes. (8) Double vicinal C-C bond formation of benzylidene malononitriles by use of allylic stannanes and alkyl iodides.
The author's results on the organic photochemistry of nucleic acid derivatives have been reviewed. These include, i) a novel photoreaction between thymidine and primary amines, ii) its use in determining T residues of DNA fragments in DNA sequencing, iii) new photochemical methods for C-C bond formation at C- 5 and C- 6 positions of pyrimidine nucleosides, and iv) a selective deoxygenation method of ribonucleosides to 2'- deoxy-and 2', 3'-dideoxyribonucleosides by utilizing photosensitized electoron-transfer reaction.
The metal-catalyzed photoreaction was carried out on some sugar molecules. Under the irradiation in the presence of titanium (IV) chloride in methanol, D- glucose, D- mannose, or D- galactose underwent a selective bond cleavage at the C5-C6 position, producing the corresponding pentodialdose derivatives, while these aldohexoses underwent the selective bond cleavage at the C1-C2 position under the irradiation in the presence of iron (III) chloride in pyridine, producing aldopentoses. Both reactions are considered to proceed through an electron transfer within a metal chelate. Synthetic utilities of the products were discussed.
Novel Synthetic approaches to β-lactams and Corey lactone analogues via four-membered ring intermediates formed by photochemical pericyclic reaction, followed by subsequent retro-aldol-type C-C bond fission are described. The same approach to carbocyclic C-nucleosides which involves thermal 4 + 2 addition reaction in the first step is also reported. These examples demonstrate that pericyclic reactions, if combined with C-C bond fission of the carbocyclic rings thus formed, provide productive synthetic methods of biologically active substances which allow strict regio-and stereochemical control of the required functionality of the target molecules.
A new combination of functional groups leads to a new reactive system with excited pathways. Photoreactions of chromophoric systems consisting of a nitrogen and one or two thiocarbonyls are systematically studied, and general photochemical behavior of cyclic thioimides and aza-aromatic thiones is reviewed and discussed.
Photocycloaddition reactions as synthetic methods for carbon frameworks of terpenoids were reviewed with particular emphasis of the enolized β - diketone photocycloaddition, de Mayo reaction, of methyl 2, 4- dioxoalkanoates developed in the authors' laboratory. In contrast to simple β- diketone, e.g., acetylacetone, methyl-2, 4-dioxoalkanoates, thoroughly existing in linearly conjugated structure in various media, cycloadd regiospecifically to various olefinic compounds upon UV irradiation. Thus, conjugated olefins having other functional groups in a vicinity smoothly produced [2 + 2] π cycloadducts from which further transformations could be conveniently carried out. In addition to the well- documented cyclization to cyclohexenones, the cyclopentene formation via reductive cyclization extended the usefulness of the method. Using this procedure, we have synthesized several terpenoids, from a monoterpenoid, dehydroiridodial, to a diterpene, cycloaraneosene, some of which have been discussed in detail. Although a few side reactions are found in this photocycloaddition, generality of this new version of de Mayo reaction is satisfactory.
In the context of the importance of photoregulation of functions of organic substances, recent developement of photochromic molecules is described, focusing attention on the following topics. a) New photochromic compounds appearring in recent literatures. b) Molecular and environment design for getting the kinetic stability of photochrome isomers. c) What kind of property of materials, into which a photochromic molecule is incorporated, can be regulated by the photoisomerization of the chromophore ? d) Hitherto unknown phenomenon accompanying photochromism. Successful example for b) is heteroaromatic homologues of fulgide system which reveales a remarkable kinetic stability of the colored form and is immune to fatigue. A variety of properties of polymers, which are photochemically regulated, are related closely to the fundamental character of the photochromism of chromophoric unit in the polymer. Photo-voltaic phenomenon of the thin film of triphenylformazan is a unique two photon process consisting of photoisomerization of the red-form to the yellow-form and the successive photo-ionization of the latter.