Photocycloadditions with ethylene

Pentafluoropyridine is known to undergo photocycloaddition with ethylene [89,90] and cycloalkenes [91] at the 3,4-position of the pyridine ring. Recently, Sakamoto et al. reported that metacrylonitrile adds to the 2,3-position of 2-alkoxy-3-cyano-4,6-dimethylpyridines (24a,b) to give 7-azabicyclo[4.2.0]octa-2,7-di-enes (25a,b) [92] (Scheme 10). This work was developed to the photocycloaddition of furan to pyridine ring as a first example (Scheme 10) [93],... 

As previously discussed, 2 -i- 2 photocycloaddition can be stereoselective when carried out with rigid starting materials. When unsaturated lactam 178, prepared in a few steps from levulinic acid and (S)-( -f )-valinol, was subjected to photocycloaddition with ethylene, a very diastereoselective addition (de = 85%) takes place [152] (Scheme 29). Acidic methanolysis produces the corresponding esters and recovered valinol. Preferential formation of 179 involves an attack of ethylene from the convex side of the molecule. 

Photo-induced conversions are also important synthetically. The photocycloaddition of 5-substituted uracils 69 with ethylene has been used for the synthesis of 2-aminocyclobutanecarboxylic acids. The addition reaction worked well with carbon and fluorine substituents and also with a Cbz-protected amino. However, uracils with other 5-nitrogen substituents (NH2, NHBn and NO2) failed, only starting material being recovered. The sequence also worked for the 6-isomers but somewhat less consistently <06SL1394>. 

Ring enlargement.1 A new route to seven-membered ring systems from a cyclohexenone (1) involves a photocycloaddition of ethylene to provide the bicy-clooctanone 2. Addition of lithio-1,3-dithiane to 2 provides the adduct 3, which on reaction with HgO and HBF4 forms an unstable rearranged hydroxy aldehyde... 

Wiesner and coworkers have emphasized that while the equilibrium constants between the two diastereomeric photoexcited states and anionic intermediates respectively should be similar, there is no reason to expect that they must be numerically identical. Small differences in equilibrium constants could in some cases reverse the stereoselectivity of photocycloaddition with respect to metal reduction. The group of Cargill94 examined the validity of Wiesner s models by the photoaddition of tert-butylcyclohexenone 178 with ethylene. Irradiation at low temperature (—78 °C) afforded a mixture of three isomers 179-181, in which the photoproduct 179 is the major product while isomer 180, expected to be the major one based on the first model, was obtained as the minor isomer. This result seems to rule out the first model (it does not take into consideration the reversibility of the first bond formation in the intermolecular photoadditions), however, it is consistent with the second model (Scheme 39). 

From a preparative point of view the heteroatom in P-position has an influence because important latent cyclobutane cleavage pathways exist in the product. The prototypical reaction of this type is the [2 + 2]-photocycloaddition/retro-aldol reaction sequence (de Mayo reaction) [55-57], the course of which is illustrated for substrate A3 (Q = O, PG = protecting group) in the reaction with ethylene as a generic olefin. 

The biradical (42) is widely considered to be the intermediate in the mela photocycloaddition of ethylenes to benzenoid compounds. Evidence for this rests largely on the observation that with substituted benzenes the 1-substituted isomer of (36) is formed preferentially. Sheridan has attempted to generate (42) independently and hence explore its possible intermediacy in the... 

Intramolecular Cyclization Reactions This Section is concerned with photo-induced cyclization reactions of aromatic compounds in which the final product is also an arene. Examples of intramolecular cyclization in which the product is a stable aliphatic system (e.g. intramolecular meta photocycloaddition of ethylenes to benzenes) are considered in Section 3 of this Chapter. The present type of process occurs for a wide variety of systems and may involve arene-arene and arene-ethylene photo-oxidative cyclization or reactions proceeding by loss of halogen acid. For several years now one of the major interests in these processes has been in their use as synthetic procedures and again within the review period there have appeared a number of accounts which illustrate this feature. Stilbene-... 

The reaction of an olefin with a 1,3-diketone enol, known as the de Mayo reaction [116], is an important member of the [23-2] photocycloaddition reaction family. This and related processes were discussed by Sato et al. [117]. 1,3-Dioxi-nones (62) react with ethylene to give cyclobutane products. (Kaneko et al. [118] and Demuth et al. [119] have written reviews on this subject.) The intermolecular reactions of olefins with enones, carried out by Organ et al. [120], are complementary to the work on spirodioxinone derivatives (e.g., 63) by Sato et al. [121]. Reaction of enone 64 with cyclohexene led to a mixture of seven products, with the all-cis isomer formed in 32% yield. However, higher selectivity was seen for the reaction with a protected cyclohexenone (65), which afforded the all-cis isomer (66) in 54% yield, and reaction with cyclopentene (67), which gave 68 as a single product in 90% yield, as shown in Scheme 17 (also see Fig. 8). 

Photocycloadditions of ethylene with chiral lactam 1.92 (R1 = R" = Me, R = /-Pr) take place selectively on the exo (convex) face of the a,(3-unsaturated system. After acid treatment and epimerization of the liberated methyl ketone, Meyers and coworkers [327] obtained ketone 9.19 with an excellent enantiomeric excess (Figure 9.8). This compound is a precursor of optically active grandisol. Photocycloaddition of chiral fiimarate 9.20 with frans-stilbene is also highly stereoselective [1533] (Figure 9.8). 

Diastereoselective [2+2] photocycloaddition of a polymer-supported cyclic chiral enone with ethylene has been reported (Scheme 12.33) [43]. The auxiliary was derived from (-)-8-(p-methoxyphenyl) menthol (87). Protection of the secondary alcohol and demethylation were carried out to give (-)-8-(p-hydroxyphenyl)menthyl acetate (88). An alkyl linker was introduced and finally loaded to poly (ethylene glycol) grafted Wang resin. Deprotection of the alcohol functionality was followed by esterification with cyclohexen-3-one-l-carboxylic acid to provide the chiral enone 89. The photochemical reaction with ethylene was performed by irradiating with light (k > 280 nm). Trifluoroacetic acid (TFA) or aqueous hydrolysis with... 

Scheme 29. Enantioselective [2 -1- 2] photocycloadditions by using a chiral unsaturated lactam or lactone with ethylene.

Similarly photocycloaddition of ethylene with chiral 5-alkoxyfuranones 181 proceeds with facial diastereoselection [153], Although selectivity remains low, the reaction has been used for the enantiospecihc synthesis of grandisol because it is easy to separate the initial diastereoisomers [154]. Selectivity is greatly influenced by temperature and by substituents on the ethylenic bond of the furanone ring. It was concluded that steric hindrance of the alkoxy group and stereoelectronic effects of substituents on the conjugated system are responsible for the selectivity observed. It was also proposed that the diastereoselection of the photocycloaddition of ethylene with 181 was dominated by the approach of ethylene to the vibrationally relaxed (71,71 ) excited furanone rather than by the selectivity of the reactions of biradicals. 

In photocycloadditions of 1,2-dioxine-4-ones A derived from menthone with unsymmetrically substituted alkenes moderate regioselectivity was observed. However, the cycloaddition reaction with cyclopentene proceeded highly stereoselective. " Chiral bicyclic lactams diastereoselectively react with ethylene furnishing optically active cyclobutanes. ... 

The diastereoselective 2 -i- 2-photocycloadditions of cyclohexenone derivatives with ethylene and cyclopentene in supercritical carbon dioxide have been investigated. High diastereoselectivity was observed with both ethylene and cyclopentene. ... 

The chemical selectivity in the photocycloaddition of ethylene (78) to benzene (77) was discussed. Diels-Alder cycloadducts of 1,2-benzo-quinones (82) with benzynes generated from (83) were converted to naphthalene derivatives (85) by photoinduced double decarbonylation of (84) (Scheme 22). ... 

Gauzy C, Saby B, Pereira E, Faure S, Aitken DJ (2006) The [2+2] photocycloaddition of uradl derivatives with ethylene as a general route to cis-cyclobutane p-amino acids. Synlett 9 1394-1398... 

A survey of the literature reveals some characteristic photochemical features of homoquinones (1) [2+2]-photocycloaddition with alkenes and alkynes, (2) hydrogen abstraction and/or ring opening of incorporated cyclopropane, and (3) photoinduced electron transfer reactions, as described below. The photocycloaddition with alkenes and alkynes took place exclusively at the ethylene linkage of homobenzoquinones to afford cyclobutanes (or cyclobutene derivatives) regio- and exo/endo stereoselectively, depending on the identity of both the homobenzoquinones as well as the alkenes. 

Photochemical [2 + 2] cycloaddition is a powerful way to produce cyclobutanes, which, in turn, are reactive synthesis intermediates. N-Methylpyrrole adds aldehydes via [2 -I- 2] photocycloaddition to give transient oxetanes with high regioselectivity Ring-opening produces 3-(oi-hydroxyalkyl)pyrroles which are oxidized easily to 3-arylpyrroles, such as 3-BUTYROYL-l-METHYL-PYRROLE. With a special apparatus, ethylene is conveniently added to 3-methyl-... 

In contrast to the preceding syntheses of kelsoene. Piers and Orellana used ethylene 8 in the [2-1-2]-photocycloaddition key step to furnish tricyclic ketone rac-lO diastereoselectively (Scheme 2) [11]. In accordance with the report by Srinivas and Mehta, attempts to homologate the sterically encumbered ketone rac-lO using the Wittig, Magnus [18, 19] or Taguchi [20] pro-... 

The importance of the ethylene ketal described above with respect to stereocontrol of the de-Mayo reaction is emphasized by later published works of two other teams. Fetizon and co-workers obviously followed a similar concept and carried out photocycloaddition reactions with 96a in model studies (Scheme 23) (Si). As can be seen from retroaldol product 98, exclusive a attack of the cyclohexene has taken place. Thus, the relative stereochemistry of the BC ring connection is opposite to that of taxane. Totally comparable results were obtained by Berkowitz et al. in the course of cycloaddition reactions of cyclopentene, cyclohexene, or those of a cyclohexenone ketal to the camphor derivative 96b (.84). 

As a suitable reaction we selected 2n + 2n) photocycloaddition as demonstrated by Schmidt, disubstituted ethylenes, appropriately oriented in the crystal and with the double bonds at a distance 4 A, form on u.v. irradiation cyclobutane dimers with a stereochemistry that directly reflects the symmetry relating the monomers in the mother phase (topochemical dimerization). Subsequently, Hasegawa and Naka-nishi have demonstrated that, when the double bonds are appropriately oriented and spaced, symmetrical disubstituted / -distyryl derivatives undergo, by the same mechanism, topochemical solid-state photopolymerization. 

Addition of simple ethylenes to polynuclear aromatic compounds has been reported for a variety of systems in recent years.10 The light-induced 1,2-addition of acrylonitrile to naphthalene has been intensively studied by McCullough and co-workers.ei It is, however, now interesting to read that polymerization of the ethylene can occur under such conditions.82 and his group have carried out several studies in this area and they have now described the stereospecific 1,2-photocycloaddition of 1-naphthonitrile with cis- and //wts-l-phenoxypropenes.88... 

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