Photocycloaddition copper -catalyzed

Scheme 15 Copper catalyzed photocycloaddition of diallyl ether.

Norbornene structures can be linked by long chain diesters. The copper catalyzed photocycloaddition of these alkenes gives rise to the cyclobutane shown in Sch. 26 [38]. The intramolecular reaction is stereoselective. 

Grandisol has previously been synthesized using a number of photochemical procedures. Two recent reports demonstrate the utility of the copper catalyzed photocycloaddition (see Sch. 28) in the synthesis of the four member ring observed in the boll weevil pheromone. The first example [41] is a chiral synthesis and the second pathway [42] provides the racemic product. 

Copper-catalyzed [2 + 2] photocycloadditions are related to the latter reactions. These transformations have been extensively studied, frequently in the context of application to organic synthesis [21], When irradiated in the presence of copper(I) triflate, norbomene 12 was effidently transformed into its dimer 13 (Scheme 5.3, reaction 4) [22]. Although complexes such as III are involved in the reaction mechanism [22, 23], it is unclear whether MLCT (metal to ligand charge transfer) or LMCT (ligand to metal charge transfer) excitation induces the transformation. 

Scheme 9 Early example of copper catalyzed intramolecular photocycloaddition.

Salomon has recendy investigated the coi er(I) triflate catalyzed photocycloadditions of allylic alcohols and allylic ethers in an effort to improve the selectivity in photocycloadditions of unactivated al-kenes (equations 103-105,108). ° One particularly interesting case is illustrated in equation (104), where Ae more sterically hindered product is produced as a result of the photocycloaddition of a rigidly held tridentate copper complex. McMurry has also utilized a copper(I) catalyzed [2 + 2] photocycloaddition in the synthesis of -panisene (equation 106)."° Interestingly, the noncatalyzed cycloaddition shown in equation (107), which might also produce a precursor for p-panisene, was unsuccessful. 

Avasthi, K., Raychaudhuri, S. R., Salomon, R. G., Copper Catalysis of Olefin Photoreactions. 13. Synthesis of Bicyclic Vinylcyclobutanes via Copper catalyzed Intramolecular 2ji + 2ji Photocycloadditions of Conjugated Dienes to Alkenes, J. Org. Chem. 1984, 49, 4322 4324. 

Asymmetric [2 + 2] Photocycloadditions. Intramolecular copper-catalyzed [2 + 2] photocycloaddition is a useful methodology for the preparation of bicyclic cyclobutanes and recent studies deal with its asymmetric version albeit with variable success. Diastereoselective reactions are achieved under the control of stereogenic centers incorporated in the dienic precursors. Both CuOTf and the more stable and easy to handle Cu(OTf)2 are suitable catalysts in this context. In the latter case, it is assumed that the copper(I) species is generated from Cu(OTf)2 under the photochemical conditions. A noteworthy example is the application of the CuOTf-catalyzed [2 + 2] photocycloaddition in the stereoselective total synthesis of the tricyclic sesquiterpene kel-soene (eq 128). ... 

The copper(I)-catalyzed photocycloaddition of l,6-heptadien-3-ols 29, in conjunction with oxidation of the resulting bicyclic alcohols to bicyclo[3.2.0]heptan-2-ones, as illustrated by the transformation of 29c to 39 (Scheme 13), provides an alternative route to 2Jt-l- 2Jt intermolecular photocycloaddition of... 

Stereoselective copper(l)-catalyzed [2 -I- 2]-photocycloaddition of 1,6-heptadienols has been used as a key step in the synthesis of several natural products. The total synthesis of the sesquiterpenes a-panasinsene 47 and P-panasinsene 48 has been achieved by CuOTf catalyzed photocycloaddition of the dienol 43 as the key step (Scheme 15). The resulting bicyclo[3.2.0]heptanol 44 was then converted to the natural products through the ketone 46. It is noteworthy that direct access to the requisite intermediate 46 involving photoaddition of isobutylene to the enone 45 failed, illustrating the importance of the copper(l)-catalyzed photocycloaddition of 1,6-heptadienols. 

Copper(I)-catalyzed intramolecular [2 + 2]-photocycloaddition of diallyl ethers and homoallyl vinyl ethers provides a new route to 3-oxabicyclo- and 2-oxabicyclo[3.2.0]heptanes, respectively.Different structural variants of diallyl ethers have been investigated. UV irradiation of diaUyl ethers 77a-f in the presence of CuOTf catalyst produces 3-oxabicyclo[3.2.0]heptanes 78a-f in moderate to excellent yields (Scheme 22). The diallyl ethers 77c and 77d having alkyl substitution at the aUyhc position produce exclusively the 2-exo-alkyl-3-oxabicyclo [3.2.0] heptanes 78c and 78d, respectively. The observed stereoselectivity arises through photocycloaddition of the Cu(l)-diene complex 80, which is stericaUy less crowded than the complex 81 with an axial alkyl group. The bicychc ethers 78 can be oxidized smoothly to the lactones 79 with RuO. Cu(l)-catalyzed photocycloaddition of homoallyl vinyl ethers 82 also proceeds smoothly, producing 2-oxabicyclo[3.2.0]heptanes 83 (Scheme 22). 

The thermo- and photocycloaddition of alkenes will be discussed in Chapter 12, on pericyclic reactions. On the other hand, transition-metals have effectively catalyzed some synthetically useful cycloaddition reactions in water. For example, Lubineau and co-worker reported a [4 + 3] cycloaddition by reacting a,a-dibromo ketones with furan or cyclopen-tadiene mediated by iron or copper, or a-chloro ketones in the presence of triethylamine (Eq. 3.48).185... 

Ghosh, S., Raychaudhuri, S.R., and Salomon, R.G. (1987) Copper(I) catalysis of olefin photoreactions. 15. Synthesis of cyclobutanated butyrolactones via copper(I)-catalyzed intermolecular photocycloadditions of homoallyl vinyl or diallyl ethers. Journal of Organic Chemistry,... 

Galoppini, E., Chebolu, B., Gilardi, R., and Zhang, W. (2001) Copper(I)-catalyzed [2 + 2] photocycloadditions with tethered linkers synthesis of syn-photodimers of dicyclopentadienes. Journal of Organic Chemistry, 66, 162-168. 

Malik, C.K., Vaultier, M., and Ghosh, S. (2007) Copper(I)-catalyzed [2 + 2] photocycloaddition of nonconjugated alkenes in room-temperature ionic liquids. Synthesis, 1247-1250. 

Copper(I) catalysis is very well established to promote intramolecular [2+2] photocycloaddition reactions of l,n-dienes (review [351]). The methodology recently enjoyed a number of applications [352-354], It is assumed that CuOTf, which is commonly applied as the catalyst, coordinates the diene and in this way mediates a preorganization. The Ghosh group recently reported a number of CuOTf-catalyzed photochemical [2+2] cycloaddition reactions, in which an organocopper radical complex was proposed as a cyclization intermediate (which should, however, have a formal Cu(II) oxidation state) (selected references [355-357]). A radical complex must, however, not be invoked, since the process may either proceed by a [2+2] photocycloaddition in the coordination sphere of copper without changing the oxidation state or according to a cycloisomerization/reductive elimination process. 

It is well known that copper salts can catalyze the photocycloaddition of dienes [56]. The reaction can be very diastereoselective and useful as an access to polycyclic molecules, especially 1 - vinyl-2-allylcyclanes [57]. The reaction of vinylglycine, cinnamyl derivatives 76, and analogs is also catalyzed by Cu(I) produced in situ by the reduction of cupric triflate. The coordination of the double bonds to the copper ion during the [2 4- 2]-photocycloaddition process increase ... 

Mattay et al. employed asymmetric copper(I)-catalyzed intramolecular [2 + 2]-photocycloaddition reactions in a synthetic approach to (+)- and (— )-grandisol [56]. Racemic dienol 33 was irradiated in the presence of CuOTf and a chiral ligand to yield mainly cyclobutanes 34 and ent-34 as a mixture of enantiomers. Other 1,6-dienes were also employed. A number of chiral nitrogen-containing bidentate ligands were tested, the most effective of which, (4S,4 S)-4,4 -diisopropyl-2,2 -bisoxazoline (35) and (4R,47 )-4,4 -diethyl-2,2 -bisoxazoline (36), ensured a minor enantiomeric excess of <5% ee (Scheme 12). The coordination of the diene to the chiral Cu(I) complex under formation of a complex of type 37 was proved by CD analysis. The authors suggest a lower reactivity of the chiral complex compared to the copper ion coordinated to solvent molecules as the reason for the low enantioselectivities observed. 

K. Avatschi, S. R. Raychaunduri, and R. G. Salomon, Copper(I) catalysis of olefin photoreactions. 13. Synthesis of bicyclic vinylcyclobutanes via copper(I)-catalyzed intramolecular 2.pi. + 2.pi. photocycloadditions of conjugated dienes to alkenes, J. Org. Chem. 49,4322-4324 (1984). 

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