Intramolecular photoaddition

Various reactions discussed so far have been applied to cleave complex vinylcyclopropanes which are available by a novel method of extreme convergency. In 1981 Wender and Howbert reported their first example for an intramolecular 1,3-photoaddition of an olefin unit to an adjacent arene moiety This ingenious strategy generates functionalized polycyclic vinylcyclopropanes in a regio- and stereocontrolled fashion which is perfect to synthesize certain terpenes. The key steps of a short route to a-cedrene are displayed in equation 182. ... 

Intramolecular photoaddition of tertiary amine and styrene moieties has been extensively studied by Aoyama29 and Lewis group28,30,31 (equations 4-8). Equations 4 and 5 show that if the intramolecular additions result in the formation of a five- or six-membered ring, the product yields are excellent. Highly regioselective intramolecular proton transfer is proposed to occur via least motion pathways from the lowest energy... 

The solvent-controlled differential reactivity was also applied to the intramolecular photoaddition of 36 and 37 (equations 11 and 12)45,46. 

Irradiation rraws-2-[3-(7V-methylamino)propyl] stilbene 89 results in the formation of 7V-methyl-l-benzyltetrahydro-2-benzazepine 90 as the only significant primary photoproduct (equation 26), which in turn undergoes secondary photochemical N-demethylation. The final mixture contains 90 (38%) and 91 (25%) at high (>95%) conversion. Intramolecular photoadditions of these (equations 24-26) secondary (aminoalkyl)-stilbenes are highly regioselective processes24. 

Intramolecular photoaddition of (terminal) allenes or ketenes to a cyclohex-2-enone C —C double bond have been investigated. The former reaction occurs regiospecifically in high yield,2 the latter, wherein the ketene moiety is photochemically generated, proceeds in moderate yield.3... 

A series of intramolecular photoadditions of one anthracene nucleus to another resulting in the formation of heterocycles have been reported in certain dianthryl derivatives.273 9-Anthryl-9-anthramide (262), on irradiation in benzene yields the /3-lactam (263) the corresponding carbonate and azo compound undergo analogous cyclizations. 

Intramolecular photoaddition in cis-5,5,6-trimethylhepta-3,6-dien-2-one (284) takes a different course,296 yielding not the oxetane but the two dihydropyrans (285 and 286). This is in contrast to the inter-molecular cycloaddition of a, jS-unsaturated aldehydes to alkenes which affords only oxetanes, and has been accounted for in terms of diradical intermediates (287 and 288) formed from the s-cis conformation (284) of the dienone. The intermolecular equivalent is thought to occur by addition to the s-trans conformation. 

Intramolecular photoaddition has been observed in nonconjugated olefins. Thus irradiation of Formula 410 gives the cage structure of Formula 411 (181). Two isomers of the hexachlorocyclopentadiene-cyclooctatetraene adduct give analogous cage structures on irradiation... 

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. 

Another example is Crimmins and Gould s114 total synthesis of ( )-laurenene 243 via the intramolecular photoaddition of enone 241, affording an epimeric mixture with very high facial selectivity, followed by subsequent transformations (Scheme 52). 

Very high diastereofacial selectivity was obtained in the intramolecular photoadditions of the chiral unsaturated lactones115 244, 246 and 248 (Scheme 53). 

Scheme 24 Quantum yields of intramolecular photoaddition products from various 5 -(4-trifluoro acety lpheny 1) -1-pentenes.

From 1987 onward, many examples of rearrangements of ortho photocycloadducts to derivatives of cyclooctatriene and subsequent reactions of these compounds have been reported. The same year marks the start of a series of investigations on intramolecular ortho photocycloadditions. This may not be a coincidence, because ortho adducts formed by intramolecular photoaddition seem especially prone to undergo rearrangement, although it has also frequently been observed in adducts arising from intermolecular addition. 

Photoinduced electron-transfer reactions generate the radical ion species from the electron-donating molecule to the electron-accepting molecules. The radical cations of aromatic compounds are favorably attacked by nucleophiles [Eq. (5)]. On the contrary, the radical anions of aromatic compounds react with electrophiles [Eq. (6)] or carbon radical species generated from the radical cations [Eq. (7)]. In some cases, the coupling reactions between the radical cations and the radical anions directly take place [Eq. (8)] or the proton transfer from the radical cation to the radical anion followed by the radical coupling occurs as a major pathway. In this section, we will mainly deal with the intermolecular and intramolecular photoaddition to the aromatic rings via photoinduced electron transfer. 

Photoinduced electron transfer between amines and aromatic hydrocarbons occurs to generate radical cations of amines and radical anions of aromatic hydrocarbons. Pac and Sakurai reported the photoaddition of N,N-dimethylaniline to anthracene via photoinduced electron transfer [60]. In benzene, the 4n + 4n) photocyclodimer of anthracene is produced as a sole isolable product, although an emission due to the exciplex formed from anthracene and JV,N-dimethylaniline is observed. In acetonitrile, the addition of dimethylaniline to anthracene occurs via their radical ions to give 9,10- dihydro-9-(4 -dimethylaminophenyl)anthracene as the major product. However, the photoamination on anthracene takes place even in benzene when iV-methylani-line is used as an electron donor. Sugimoto and his coworkers reported the intramolecular photoaddition of anilines to aromatic hydrocarbons to give cyclic amino compounds (Scheme 16) [61-63]. 

Lewis and his coworkers reported the intramolecular photoaddition of aliphatic secondary amines to arylalkenes to give 5- and 6- membered cyclic amines (Scheme 17) [66-67]. Ohashi reported that the other type of photoaddition reaction takes place when 1,4-dicyanobenzene and triethylamine are used as substrates. This photoreaction can be explained in terms of the deamination of the initially produced diethylaminoethylated compound (Scheme 18) [68]. 

Yonemitsu and his coworkers found that the intramolecular photoaddition of chloroacetoamide derivatives with an electron donating aromatic ring in molecules gives the cyclic amide compounds in water [69], However, the photoreaction in or nic solvents proceeds via the exciplex to give the different cyclized products. 

Intramolecular photoadditions are also possible34,35. The method can give better results than the cyclization of JV-chloro amines35 (see Section 7.2.5.4), but the utmost precautions are required for the handling of the carcinogenic nitroso amines. 

Intramolecular photoaddition reactions of aminoarenes which are of interest from the synthetic point of view, was first reported by Bryce-Smith et al., for dimethylaminoalkyl)benzenes (Scheme 51) [281]. 

Aoyama and coworkers have synthesized several pyrrolidine and piperidine derivatives via intramolecular photoaddition reactions of tertiary (aminoalkyl) styrene (Scheme 52) [282, 283]. 

Similar intramolecular photoaddition reactions of styrylamides have also been reported by Aoyama and coworkers [282-286]. 

Lewis and coworkers have also observed efficient intramolecular photoaddition for several (A,A-dimethylaminoalkyl)styrenes, leading to the formation of nitrogen heterocycles or aminocycloalkanes (Scheme 53) [287]. 

Intramolecular photoaddition reactions of secondary amines linked to styrenes can result in the formation of C-N bonds at the a- or / -carbon (Scheme 54) [288-290], The product yields and the ratio of the two products are shown in Table 11. 

Intramolecular photoaddition of secondary amines linked to various other arenes and stilbenes also provide an efficient method for the synthesis of A -heterocycles (Scheme 55) [291-293]. 

Intramolecular photoaddition reactions of amines for the construction of A -heterocycles has also been successfully explored by Sugimoto and coworkers (Scheme 57) [296-302]. 

The intramolecular photoaddition of the enone (32) in chlorobenzene using wavelengths > 350 nm affords the adducts... 

Intramolecular photoadditions can also occur as so-called x[2 -P 2] cycloadditions, as demonstrated for 1,5-hexadiene (55) the terminal carbon atom of each double bond adds to the internal carbon atom of the other double bond in such a way the resulting a bonds cross each other. 

A fragmentation of a similar tricyclus has been used by Oppolzer for the synthesis of the hydroazulene ketone (74 Scheme 25), precursor of the bulnesenes (75). The tricyclic ketone (71), obtained by intramolecular photoaddition, is converted to the diol (72). Fragmentation is accomplished in one... 

Contrary to earlier work, Caldwell et al. conclude from a detailed study of the 4,4-dimethylcyclohex-2-enone-l,l-diphenylethene system, that the (2n + 2n) photocycloaddition arises from a triplet exciplex, and Crimmins et al. report that the enone (15) undergoes highly diastereoselective intramolecular photoaddition to give (16) which is a key intermediate in a new route to spiro[5.4]decanes. Similar intramolecular processes with other enones, followed by treatment with tributyltin hydride and AIBN, provide a convenient route to bicyclic compounds such as (17) in good yield (Crimmins et al.), and irradiation of the optically active silylallene (18) gives (19) with an ee in excess of 99% (Shepard and Carreira). 

The ( )-isomers of N-acyl-a-dehydro(l-naphthyl)alanines (125 n = 2,3 R = Me, Ph R = Me, Et, Pr) produce l,2-dihydrobenzo[f]quinolines (126) by an electron-transfer reaction, and the )- and (Z)-isomers produce minor amounts of the benzo[f]isoquinoline (127) and 1-azetidine derivatives respectively by an intramolecular photoaddition. Evidence suggests that the bulky diisopropylamino donor and the N-benzoyl group enhance the relative yield of (127) to (126). 

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