UV light scheme

In 1960, Anderson and Reese [1] reported that cathecol monoacetate (1) undergoes rearrangement to 2,3- and 3,4-dihydroxyacetophenone (2 and 3, respectively) upon illumination with ultraviolet (UV) light (Scheme 1). This report was the first one of a long list of articles and reviews [2-16] dealing with the photorrerarrangement of aromatic esters, which was extended to amides, carbonates, carbamates, thioesters, sulfonates, and related compounds. 

The same chemistry was used by Maynard and coworkers in a reverse way to immobilize biomolecides on antifouling PEG surfaces [43]. In their approach, acetal-protected aldehydes as chain ends of siuface-bonded poly(3,3 -diethoxypropyl methacrylate) were partially deprotected by irradiation with UV light (Scheme 22). Subsequently, the free aldehydes reacted with V-(aminooxyacetyl)-lV -(D-biotinoyl) hydrazine (ARP). Afterwards, the sample was irradiated again to deprotect further aldehydes. These free aldehydes were then used for the additirMi of aminooxy-terminated PEG, which is well-known for its antifouling properties. In the same step, dye-labeled streptavidin was attached onto ARP. Afterwards, the streptavidin could be used to immobilize any biotinylated proteins. 

A photolabile precursor (170) of the amino acid, glycine (172) containing two phosphate groups attaehed to the indoline moiety at the 4-alkoxy position, has been synthethised by Ogden et al. This precursor released glycine (172) and the nitrosoindole by-product (171) on a sub-ms time scale upon irradiation with near-UV light (Scheme 47)7 ... 

The speciation scheme of Batley and Florence requires eight measurements on four samples. After removing insoluble particulates by filtration, the solution is analyzed for the concentration of anodic stripping voltammetry (ASV) labile metal and the total concentration of metal. A portion of the filtered solution is passed through an ion-exchange column, and the concentrations of ASV metal and total metal are determined. A second portion of the filtered solution is irradiated with UV light, and the concentrations of ASV metal... 

Penicillin sulfoxides can be epimerized by heat to afford thermal equilibrium mixtures of a- and /3-sulfoxides, the position of the equilibrium depending on the C(6) side chain (Scheme 5). Deuterium incorporation studies support a sulfenic acid, e.g. (18), as the intermediate in these transformations. This mechanism is also supported by the finding that when an a-sulfoxide epimerizes to a /3-sulfoxide there is a simultaneous epimerization at C(2) (71JCS(C)3540). With irradiation by UV light it is possible to convert a more thermodynamically stable /3-sulfoxide to the a-sulfoxide (69JA1530). 

In a few cases, azo polymers were synthesized pho-tochemically. Azobenzoin compounds have photo cleavable benzoin groups. Being irradiated with UV light (A = 350 nm) 4,4 -azo-bis(4-cyanopentanoyl)-bis benzoin, ACPB, undergoes a-scission forming two free radicals per initiator molecule (Scheme 24). 

Furthermore, photochemically induced homolytical bond cleavage can also be applied when the prepolymer itself does not contain suitable chromophoric groups [113-115]. Upon thermolysis of ACPA in the presence of styrene, a carboxyl-terminated polystyrene is formed. This styrene-based prepolymer was reacted with lead tetraacetate and irradiated with UV light yielding free radicals capable of initiating the polymerization of a second monomer (Scheme 33) [113]. 

Recently, a photoisomerization reaction of azoferrocene was found to proceed in polar solvents such as benzonitrile and DMSO through both a 7t it transition of the azo-group with a UV light (365 nm) and the MLCT transition with a green light (546 nm) (Fig. 6) (Scheme 1) (153). The quantum yields of the photo-isomerization reaction at 365 nm and 546 nm were estimated to be 0.002 and 0.03, respectively. The transformation into the cis form causes the higher field shift of Cp protons in the 1H-NMR spectrum and an appearance of u(N = N) at 1552 cm-1. The cis form is greatly stabilized in polar media, and dilution of the polar solution of cis-25 with less polar solvents resulted in a prompt recovery of the trans form. 

Photoinduced free radical graft copolymerization onto a polymer surface can be accomplished by several different techniques. The simplest method is to expose the polymer surface (P-RH) to UV light in the presence of a vinyl monomer (M). Alkyl radicals formed, e.g. due to main chain scission or other reactions at the polymer surface can then initiate graft polymerization by addition of monomer (Scheme 1). Homopolymer is also initiated (HRM-). 

The sensitizer in our experiments is benzophenone (BP) which reacts as shown in Scheme 2. UV light of 300 to 400 nm is absorbed and excites the aromatic ketone group to a singlet state which by intersystem crossing (ISC) reverts to a triplet state, abstracts a... 

Electron donation to nucleobases is a fundamental process exploited by nature to achieve the efficient repair of UV induced lesions in DNA [27, 28]. Nature developed to this end two enzymes, CPD photolyases and (6-4) photolyases, which both inject electrons into the UV damaged DNA bases [29, 30]. Both enzymes are, in many species, including plants, essential for the repair of the UV-light induced DNA lesions depicted in Scheme 1 [31]. 

Scheme 1 UV-light induced formation of the two major photo lesions in DNA. T=T cyclobutane pyrimidine dimer. (6-4)-photo product (6-4)-lesion, formed after ring opening of an oxetane intermediate, which is the product of a Paterno-Buchi reaction...

Tandem radical cyclizations of suitably substituted /V-aryl thiocarbamates, thioamides and thioureas have been induced by exposure to 4 equiv. of tnj(trimethylsilyl)silane (TTMSH (TMS)3SiH), 1 equiv. of 2,2 -azo-/h.v-isobu-tyronitrile (AIBN as a radical initiator) and UV light to provide furoquinolines, isofuroquinolines, cyclopentaquinolines, indoloquinolines and related ring systems (Scheme 31).58... 

TPAs bearing cross-linkable oxetane functionalities (52) (Scheme 3.20) has been developed [95,96], This material can be easily photo-patterned by exposure to a very short wavelength UV light source to form a patterned and insoluble HTL. Similar photo-cross-linkable HTMs based on side-chain acrylate or styryl functional groups have been synthesized (53,54) [97]. A series of spiro-linked and photo-cross-linkable HTMs have been patented by the Canon group [98]. 

It is well known that pyrimidine bases convert to photodimers upon irradiation to UV light near the X max( > 270 nm). This photochemical reaction has a lethal effect in biological systems due to the photochemical transformation of pyrimidine bases of nucleic acids. However the photodimerization is a reversible reaction and the photodimers split to afford the original monomers very efficiently upon irradiation at a shorter wavelengths as shown in Scheme 1(1). 

This scheme, set up with reactions in dichloromethane, gave spin adducts from several of the nucleophiles discussed above (F, Cl", AcO, CN, tetramethylsuccinimide anion and triethyl phosphite), provided UV light was employed. With filtered light of A > 435 nm, no spin adducts were detected. This is expected, since PBN cannot then be excited. With water as the nucleophile, only benzoyl nitroxide [9] was seen, indicating that any HO-PBN" disappears too rapidly to be detectable 10 s in acetonitrile) and/or that its rate of formation from PBN +ConW is too low (see above). The complication that nucleophilic addition-oxidation might compete was ruled out experimentally in dichloromethane, but detected for fluoride ion in chloroform, using dioxygen to oxidize the intermediate hydroxylamine anion. 

Figure 26 Scheme of a pixel for generating full color. Different colors are generated by transforming the emitted blue or UV light from the LEDs with the help of colorchanging media. 

The preparation of lactams 68 and 69 from the corresponding dichloramides 66 and 67 in acetonitrile under UV light in 43% and 42% yields, respectively, has been reported. But with dichloramide 67, carried out under similar conditions in aqueous acetonitrile, in addition to the mixture of diastereoisomers of azocinoindole 70 (54%), azepinoindole 71 (25%) was also obtained (92JCS(P1)823, 92JCS(P1)797 Scheme 21). 

---