Photoaddition of alcohol

Two types of addition to pyrimidine bases appear to exist. The first, the formation of pyrimidine photohydrates, has been the subject of a detailed review.251 Results suggest that two reactive species may be involved in the photohydration of 1,3-dimethyluracil.252 A recent example of this type of addition is to be found in 6-azacytosine (308) which forms a photohydration product (309) analogous to that found in cytosine.253 The second type of addition proceeds via radical intermediates and is illustrated by the addition of propan-2-ol to the trimethylcytosine 310 to give the alcohol 311 and the dihydro derivative 312.254 The same adduct is formed by a di-tert-butyl peroxide-initiated free radical reaction. Numerous other photoreactions involving the formation by hydrogen abstraction of hydroxyalkyl radicals and their subsequent addition to heterocycles have been reported. Systems studied include 3-aminopyrido[4,3-c]us-triazine,255 02,2 -anhydrouri-dine,256 and sym-triazolo[4,3-fe]pyridazine.257 The photoaddition of alcohols to purines is also a well-documented transformation. The stereospecific addition of methanol to the purine 313, for example, is an important step in the synthesis of coformycin.258 These reactions are frequently more... 

Although the excited states of alkynes are known to be attacked very rapidly by proton acids, the products obtained by photoaddition of alcohols (2.9B) often arise by a radical mechanism that results in hydrogen abstraction from the position adjacent to the alcohol hydroxyl group, rather than by an ionic mechanism, which would result in abstraction of the —OH proton. 

The photoaddition of alcohols and phenols to alkenes has been observed.404 The equivalent intramolecular process results in cyclization and the formation of oxygen heterocycles. Irradiation of 2-allyl-4-(-butylphenol (388) affords 2,3-dihydro-2-methyl-5-<-butyl-benzofuran (389), whereas o-3-methylbut-2-enylphenol (390) gives 2,2-dimethylchroman (391). In both cases, therefore, the addition can be said to occur in a Markovnikov direction. 

The intramolecular version of photoaddition of alcohols to alkenes was also reported. Mizuno and Otsuji reported the DCA-sensitized photocyclization of co-hydroxy-l,l-diphenylalkenes (n = 3-5) via exciplex in benzene [38]. The long chain cu-hydroxyalkenes (n = 6,10) do not give the corresponding cycHzed ethers. Gassman reported the photocyclization of unsaturated carboxylic acids by the use of a sensitizer system consisting of sterically hindered electron acceptors and biphenyl to give y-lactones (Scheme 10) [39]. 

The photoaddition of alcohols to 126a and 126b depends on the nature of alcohols and the basicity of solutions. The photoaddition of pure ROH to 126a and 126b is only... 

Enantiodifferentiating anti-Markovnikov polar photoadditions of alcohols to 1,1-diphenyl-l-alkenes 107 and 108 sensitized by optically active naphthalene(di)car-boxylates 41-43, 71, 72, and 91 were investigated in detail (Scheme 19) [70], Since this photoaddition involves the attack of alcohol to a radical cationic species of the substrate alkene [71], the use of polar solvents is desirable for obtaining the adduct in a high yield. However, in polar solvents, the radical ionic sensitizer-substrate pair produced upon photoexcitation is immediately dissociated by solvation, and no chirality transfer is expected to occur. Thus the optical and chemical yields are often conflicting issues, and therefore the critical control of solvent polarity is essential for obtaining the optically active product with an appreciable ee in reasonable chemical yield. In fact, the initial attempts on 107, employing naphthalenecarboxylate sensitizers with chiral terpenoid auxiliaries (a-c and f) and a pentane solvent afforded a best ee of 27% for adduct 110 (R = Me), but in < 2% yield [70a]. 

Figure 8 Microenvironmental polarity control upon enantiodifferentiating polar photoaddition of alcohol (ROH) to aromatic olefin (D) sensitized by naphthalenedicarboxylate with saccharide auxiliaries (A ) the local polarity is enhanced around the saccharide moieties, facilitating electron transfer from exited sensitizer (A ) to substrate olefin (D) to produce a radical cation (D -h). The radical cation produced cannot escape from the high polarity region around the saccharide to the low-polarity bulk solution and is accordingly attacked by ROH in the chiral environment of saccharide to produce the adduct in high ee.

Photoaddition of alcohols and ethers to chiral conjugated lactones (207 and 210) chemically sensitized by benzophenone proceeds with high regio-and stereoselectivity [165-167]. The stereoselectivity is considerably lower... 

The photoaddition of alcohols to nitroalkenes has been described (Scheme 2). Three of the same authors have reported similar work on the photolysis of 3-nitro-2-enopyranosides in 1,3-dioxolane to give the corresponding 2-C-(dioxolan-2-yl)-3-nitro derivatives. ... 

The photoaddition of alcohols to epoxides which is catalyzed by Fe203 (Fe3+ and other metal ions) [39], Scheme 14(c) functions similarly. The metal catalysis of this reaction was detected when the solvent and reagent methanol had been distilled from EDTA to remove metal ion traces and the reaction found to proceed orders of magnitude slower in the purified solvent. Photo-electron transfer from the epoxide to Fe(III) as acceptor generates an oxonium cation that is highly susceptible to nucleophilic attack by the alcohol. Since the catalysis by metal ions in this case is quite general, their functioning simply as Lewis acids cannot be excluded. 

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