Intramolecular reaction photocycloaddition

Inter- and intra-molecular [3 + 2] photocycloadditions of 1,3-dipolar species generated by elimination of nitrogen from tetrazoles (103) with alkenes (104) have been reported giving pyrazolines (105). Polycyclic azaheterocycles (108) (110) were also synthesized by [3 + 2] tandem photocycloaddition reactions. Intramolecular-proton-transfer mediated photocycloaddition of 3-hydroxyquinolones (111) with alkenes (113) to give [3 + 2] cycloadducts (114). ... 

In this synthesis, we have witnessed the dramatic productivity of the intramolecular enone-olefin [2+2] photocycloaddition reaction. This single reaction creates three contiguous and fully substituted stereocenters and a strained four-membered ring that eventually provides the driving force for a skeletal rearrangement to give isocomene. 

Dihydropyran-4-ones are a source of phenols via an intramolecular [2+2] photocycloaddition reaction and a Lewis-acid catalysed cleavage of the cyclobutane moiety <96TL1663>. 

The synthetic applications 440) and mechanistic aspects 4411 of intermolecular photocycloaddition reactions of arenes to olefins have been reviewed recently. Intramolecular cycloadditions442a,b) have been studied in the context of the photochemical behaviour of bichromophoric molecules, as to investigate interchromophoric interactions in polyfunctional molecules. Three types of addition products can be formed in the photocycloaddition of benzene to an alkene (4.37)441. ... 

The literature of diene and polyene photochemistry provides many cases of synthetically useful reactions. As a result, certain arbitrary decisions have been made regarding what is covered in this chapter. For example, intramolecular [2 + 2]-photocycloaddition reactions of a, >-dienes can be formally included under the general rubric of diene photochemistry. However, we have chosen to restrict our discussion to dienes and polyenes which constitute a self-contained chromophore, viz. conjugated, cross-conjugated and 1,4-diene systems. Likewise, arene-olefin photocycloadditions will not be considered. These two broad classes of photoreactions have been applied extensively in synthesis, and have been the subject of recent reviews3,4. 

The skeleton of 47 is a heterocyclic tricyclo[6.2.0.0 ]decane and the similarity to the tricyclic kelsoene is obvious. In the course of the above-mentioned studies we had become curious whether the high facial diastereocontrol in the photocycloaddition reaction could be extended to other bridged 1,6-hexadienes. Kelsoene was an ideal test case. The retrosynthetic strategy for kelsoene along an intramolecular [2+2]-photocycloaddition pathway appeared straightforward. To avoid chemoselectivity problems the precursor to kelsoene should not contain additional double bonds. Alcohol 48, the hydroxy group of which was possibly to be protected, seemed to be a suitable substrate for the photocycloaddition (Scheme 14). Access to the 1,2,3-substi-... 

An example where this abstraction from the carbonyl compound competes with photocycloaddition was discussed in the previous section (p. 312). In that case, 2-methylbenzophenone underwent intramolecular hydrogen abstraction, yielding the tautomeric enol,46 a process which completely inhibited the photocycloaddition reaction.37... 

The N=N double bon d does take part in a few photocycloaddition reactions to give cyclic compounds with two adjacent nitrogen atoms in the ring. Intermolecular (2 + 2 cycloadditions are not known, but some intramolecular examples of this reaction are reported for quite complex compounds 15.27 in which the reacting groups are held fairly rigidly in an orientation suitable for reaction. A (4 + 2) cycloaddition takes place when naphthalene is irradiated with an electron-deficient cvdrc azo-compound (5.28). 

Photocycloaddition proceeds between allylsilane and A-methylphthalimide to yield a mixture of [2 + 2] and [4 + 2] adducts along with the allylated product291. Intramolecular cycloadditions of the vinylsilanes with the cyclopentenone moieties in 200 furnish good yields of cyclic products stereoselectively (equation 164)292. In the presence of 1,4-dicyanonaphthalene, diallylsilane 201 undergoes an intramolecular photocycloaddition reaction in an aromatic solvent to give a four-membered ring product (equation 165)293. 

Demonstration of the unique synthetic utility of the [2 + 2] photocycloaddition reaction of enones to alkenes and the success in controlling the stereoselectivity, to some extent, in the intermolecular additions (discussed above) prompted further studies and development of new synthetic applications in the intramolecular photoadditions during the last decade. In most cases that have been studied, the alkene was tethered to the cyclic enone by three carbon units or two carbons and one heteroatom. 

Photocycloaddition reactions have been used as key transformations in many organic syntheses to construct organic compounds having unique structures that are hardly accessible by other methods. However, their stereoselectivity is not necessarily high, and many efforts have been done to attain the highly regioselective and stereoselective photocycloadditions. They are discussed in terms of the electronic nature of substrates, the steric repulsions between substituents, and the conformational restrictions of intramolecular reactions. 

Intermolecular and intramolecular photocycloaddition and photoaddition to aromatic rings in the electron-donor and electron-acceptor systems were discussed in this chapter. The highly stereoselective and regioselective photocycloaddition is a synthetically useful method for the construction of polycyclic carbon-skeleton compounds, including natural products. New aspects for the stereoselective intermolecular and intramolecular photocycloaddition reactions via exciplexes in less... 

The dimerization of thioformylketene was investigated by B3LYP and G3MP2B3 methods. The 4 + 4-pathway has the lowest energy barrier and calculations suggest that the reaction is pseudopericyclic.231 The stereospecific intramolecular 4 + 4-cycloaddition reaction between cyclohexadiene iron tricarbonyl complex and appended dienes (198) generates cyclooctadiene tricyclic adducts (199) (Scheme 56).232 The first example of an asymmetric intermolecular 4 + 4-photocycloaddition reaction in solution between 9-cyanoanthracene and chiral 2-methoxy-l-naphthamides has been reported. The frozen chirality is effectively transferred to the optically active product.233... 

CONTENTS Preface, Mark Lautens. Photocyclization and Photocycloaddition Reactions of 4- and 2-Pyrones, Frederick G. West. Intramolecular [4+3] Cycloaddition Reactions, Michael Harmata. Lewis Acid Catalyzed [2+2] Cycloaddition Reactions of Vinyl Sulfides and Their Analogues Catalytic Asymmetric [2+2] Cycloaddition Reactions, Koichi Narasaka and Yujiro Hayashi. Vinylboranes as Diels-Alder Dienophiles, Daniel A. Singleton. Preparation and Exo-Selective [4+2] Cycloaddition Reactions of Cobaloxime-Substituted 1,3-Dienes, P... 

Bach, T. (2002) Stereoselective total synthesis of the tricyclic sesquiterpene (i)-kelsoene by an intramolecular Cu(I)-catalyzed [2 + 2]-photocycloaddition reaction. Synlett, 1305-1307. 

The photochemical cycloaddition of two different alkenes leads to multiply substituted cyclobutanes, and allows for a general access to this class of compounds. More specifically, ever since Ciamician observed the light-induced isomerization of carvone (1 —> 2) (Scheme 6.1) in 1908 [1], the inter- and intramolecular reaction between an a,(3-unsaturated carbonyl compound and an alkene has become the most intensively studied and most widely used class of [2 + 2]-photocycloaddition reactions [2-9]. 

Control over the absolute configuration in cyclohexenone photocycloadditions has been achieved by auxiliary-induced diastereoselectivity. In particular, esters related to compound 26, which are derived from a chiral alcohol but not from methanol, lend themselves as potential precursors, from which the chiral auxiliary can be effectively cleaved [42, 43]. In a recent study, the use of additives was advertised to increase the diastereomeric excess in these reactions [44], An intriguing auxiliary-induced approach was presented by Piva et al., who employed chiral 13-hydroxy-carboxylic adds as tethers to control both the regioselectivity and the diastereoselectivity of intramolecular [2 + 2]-photocycloaddition reactions [45]. In Scheme 6.14 the reaction of the (S)-mandelic acid derived substrate 38 is depicted, which led with very good stereocontrol almost exclusively to product 39a, with the other diastereoisomer 39b being formed only in minor quantities (39a/39b = 96/4). Other acids, such as (S)-lactic acid, performed equally well. The chiral tether could be cleaved under basic conditions to afford enantiomerically pure cydobutane lactones in good yields. 

Substrates A3 (Q = O) have been employed not only as starting materials for fragmentation reactions but also to probe novel stereoselectivity concepts. The photochemical transformation of axial chirality into central chirality was achieved by Carreira et al., who employed chiral, enantiomerically pure allenes in intramolecular [2 + 2]-photocycloaddition reactions (Scheme 6.27) [79]. The reaction of enantiomerically pure (99% ee) cyclohexenone 71, for example, yielded the two diastereomeric products 72a and 72b, which differed only in the double bond configuration. Apparently, the chiral control element directs the attack at the allene to its re face. The double bond isomerization is due to the known configurational liability of the vinyl radical formed as intermediate after the first C—Cbond formation step (see Scheme 6.2, intermediate C). 

Recently, pyrone [2 + 2]-photocycloaddition reactions were used to construct macrocyclic compounds. A dipyrone was irradiated in the presences of a,(0-diolefins yielding 18- to 25-membered rings by a sequential intermolecular and intramolecular cycloaddition [146]. 

In enantioselective photocycloaddition reactions, 4-alkoxyquinolones perform in superior fashion to l,5-dihydropyrrol-2-ones and 5,6-dihydro-lff-pyridin-2-ones. Both, intermolecular and intramolecular reactions were performed with excellent enantioselectivity in the presence of the chiral template 115, or of its enantiomer ent-115 [147, 148], The well-established photocycloaddition reactions [149, 150] enabled access to a variety of chiral dihydroquinolones. 4-Methoxyquinolone (157) produced, upon direct irradiation in the presence of allyl acetate, the formal HT product 158 in 80% yield and with 92% ee (Scheme 6.56) [151]. 

Bach, T., Bergmann, H., and Harms, K. (2000) Enantioselective intramolecular [2 + 2]-photocycloaddition reactions in solution. Angewandte Chemie, International Edition, 39, 2302-2304. 

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. 

H. G. Heller and M. Jottaway, Intramolecular [4 + 4] photocycloaddition reactions of the photo-chromic compounds, 3,4-bis(9-anthryl methylene)-tetrahydrofuran-2,5-dione and its 5-dicyano-methylene derivative, J. Chem. Soc., Chem. Commun., 1995, 479-480. 

The utility of /f-enaminones (vinylogous amides) and / -enamidones (vinylogous imides) has been successfully demonstrated in both inter- and intramolecular [2 + 2] photocycloaddition reactions. The vinylogous amide, which reacts as a /Mieteroatom substituted enone, serves as the nitrogen analogue of the enol form of a / -diketone in the de Mayo reaction24. 

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. 

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