Photodimerization of Enones

However, in an inclusion crystal of chalcone 70a with 1,1,6,6-tetraphenylhexa-2,4-diyne-l,6-diol host compound 91, the 70a molecules aggregate in close posi- 

Photoirradiations of both neat and benzene solution of 2-cyclohexenone (95) give a complex mixture of the syn-trans-96 and anti-trans-dlmer 97, and two other dimers of unknown structure [57], When a l 2 inclusion complex of the chiral host compound (-)-l,4-bis[3-(o-chlorophenyl)-3-hydroxy-3-phenylprop-l- 

Photoreactions of coumarin (99) in EtOH both in the absence and presence of benzophenone as a sensitizer give, respectively, a mixture of, vyn-head-to-head dimer 100 and 5vn-head-to-tail dimer 101 [59], and an//-head-to-head dimer 102 together with a small amount of nnh-head-to-tail dimer 103 [60]. On the other hand, photoirradiation of a 1 2 inclusion complex of 10 with 99 and of a 1 2 inclusion complex of (5,5)-(-)-l,6-(2,4-dimethylphenyl)-l,6-diphenylhexa-2,4-diyne-1,6-diol (104) with 99 in the solid state gave 100 (74.5%) and rac-103 (94%), respectively, in the yields indicated [61]. 

Interestingly, however, irradiation of a 1 1 inclusion complex of 2a with 99 prepared by recrystallization of the two components from ethyl acetate-hexane gave (-)-102 of 96% ee, although the same irradiation of a 1 1 inclusion complex of 2a with 99 prepared by recrystallization from toluene-hexane gave 100 [61]. Photoconversion of 99 into 100 by the irradiation of the powdered 1 2 inclusion complex of 10 with 99 in the solid state has also been carried out recently by the Venkatesan research group [62], 

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The cycloaddition of enones to olefins is a reaction of considerable synthetic interest 14°). Oxetane formation and cyclobutane formation are sometimes competitive 141>, but the latter reaction is the more common. The photodimerization of enones 142> is a special case of such cycloaddition. It has been shown that triplets are involved in these cycloadditions, since intersystem crossing quantum yields are unity 143> and cycloaddition is totally quenchable by triplet quenchers. Careful kinetic analysis indicates an intermediate which can partially revert to ground state reactants, since quantum yields are lower than unity even when extrapolated to infinite substrate olefin concentration. That a diradical is... 

Scheme 18 Enantioselective [2 + 2] photodimerization of enones in their host-guest crystals.

Scheme 3.13 Schematic representation of the photodimerization of enones in cavities of 33b-tube...

More recent investigations of the photodimerizations of cyclopentenone and cyclohexenone<134) have revealed that both reactions result from the lowest triplet states of the enones (Et > 70 kcal/mole), which is probably 77- 77 in character/1331 Both photodimerizations occur via reversibly... 

Tanaka K, Mochizuki E, Yasui N, Kai Y, Miyahara I, Hirotsu K, Toda F (2000) Single-crystal-to-single-crystal enantioselective [2-1-2] photodimerization of coumarin, thiocoumarin and cyclohex-2-enone in the inclusion complexes with chiral host compounds. Tetrahedron 56 6853-6865... 

Cyclic enones participate in several other photochemical reactions. Irradiation of neat cyclopentenone417 or cyclohexenone418 leads to formation of dimers. Eaton has shown that the reaction is quenched by piperylene and that there is a strong polar solvent effect on the ratio of head-to-head and head-to-tail dimers formed.511 Similar solvent effects have been noted for the triplet-state photodimerization of isophorone.512 Head-to-head dimers are favored in polar solvents and in neat ketone, while head-to-tail dimers are favored in nonpolar media. 

Example of head-to-tail and head-to-head photodimerization of an enone in solution and at the interface of a micelle... 

The unusual increase in regioselectivity with a decrease in pressure is not present for the photodimerization of a similar enone, isophorone, in SCF CO2 (31). The difference for the two enones is a result of differences in their phase diagrams and differences in the phase region in which the experiments were performed. As shown in Figure 8, the critical point for the cyclohexenone-ethane mixture is close to that of pure ethane. Hence the compressibility is large. In contrast, the... 

Additions to Cyclohexenones and Related Systems - A re-investigation of the photodimerization of isophorone (37) has been reported. The study examined the influence of solvent and of the concentration of the enone. Some of the results and the yields of dimers obtained are shown in Scheme 4. From this detailed study the authors suggest that supramolecular structures are involved in the dimerization. These apparently take part even at low concentrations of enone. The photocycloaddition of enones such as (39) to buckminsterfullerene (Ceo) has been studied. The outcome of the addition is the formation of low yields of furanylfullerenes. This addition occurs to the exclusion of de Mayo type of addition. Photocycloaddition of the cyanocyclohexenone derivative (40) to alkenes has been reported. ... 

Preorientation (alignment) effects on photoreactions were first reported in photodimerizations and photoadditions of enones (107,136-140). Substrates like... 

Incorporation of a triplet sensitizer benzophenone into the macrocycle 33c has little influence on the self-assembly nor the dimension of the resulting nanotube, but the reaction environment of the cavity is totally changed. Photodimerization of the above enones could not happen within 33c-tube. Instead, the nanotube was able to act as a triplet sensitizer for the cis-trans photoisomerization of trans- >-mQ hy -styrene and selective oxidation of 2-methyl-2-butene to a primary allylic alcohol (Scheme 3.14) [70, 109]. As well, an enlarged nanotube 33d-tube was shown to facilitate the selective photodimerization of coumarin to the awr/-head-to-head product [71]. 

The intramolecular photodimerization and [2 + 2]-photocycloaddition in DNA involves thymine or cytosine as the chromophore. This chemistry has been intensively investigated with regards to DNA damage and repair [131]. Despite the fact that the area is of continuous interest [132], the synthetic applications are limited and are not covered here in detail. However, some preparative aspects of 4-pyrimidinone photocycloaddition chemistry will be addressed. Aitken et al. have prepared a plethora of constrained cyclobutane P-amino acids by intra- or intermolecular [2 + 2]-photocycloaddition to uracil and its derivatives [133, 134]. In a chiral adaptation of this method, the uracil-derived enone 140 was employed to prepare the diastereomeric cyclobutanes 141 in very good yield (Scheme 6.49). The compounds are easily separated and were - despite the relatively low auxiliary-induced diastereoselectivity - well suited to prepare the as-2-aminocyclobutanecarboxylic acids 142 in enantiomerically pure form. Enantioselective access to the corresponding trans-products was feasible by epimerization in a-position to the carboxyl group [135],... 

The first such reaction published in 1908 by Ciamician and Silber was the light induced carvone —> carvonecamphor isomerization, corresponding to type b [1]. Between 1930 and 1960 some examples of photodimerizations (type c) of steroidal cyclohexenones and 3-alkylcyclohexenones were reported [2-5]. In 1964, Eaton and Cole accomplished the synthesis of cubane, wherein the key step is again a type b) photocycloisomerization [6]. The first examples of type a) reactions were the cyclopent-2-enone + cyclopentene photocycloaddition (Eaton, 1962) and then the photoaddition of cyclohex-2-enone to a variety of alkenes (Corey, 1964) [7,8]. Very soon thereafter the first reviews on photocycloaddition of a,(3-unsaturated ketones to alkenes appeared [9,10]. Finally, one early example of a type d) isomerization was communicated in 1981 [11]. This chapter will focus mainly on intermolecular enone + alkene cycloadditions, i.e., type a), reactions and also comprise some recent developments in the intramolecular, i.e., type b) cycloisomerizations. 

Irradiation of the 2 1 host-guest crystals of cyclohex-2-enone 81 with the axle-wheel-type host compound (— )-5 as an aqueous suspension caused regio- and enantioselective [2 + 2] photodimerization to afford the (— )-anti-head-to-head dimer 82 of 48% ee in 75% chemical yield (Scheme 18) [86]. Similarly, solid-state photolysis of the 3 2 complex of cycloocta-2,4-dien-l-one 83 with (R,R)-( — )-4 gave the (-)-anti-head-to-head dimer 84 in moderate optical yield [87]. 

Photodimerization and cross-cycloaddition of coumarins are improved by Lewis acids [74]. Similarly, photochemical [2-1-2] cycloadditions [75] of 1- and 2-naphthols [76] with ethylene are promoted by aluminum halides yielding the [2-1-2] adduct from the (complexed) enone form. According to the structure, substitution (e. g., methyl) vicinal to the OH group in 1-naphthol gives rise to ring-contracted indanone products. The formation of (ring-contracted) benzo-bicyclo[3.1.0]bicyclohexenone was already observed by irradiation (AICI3) of 1-naphthol without ethylene (Scheme 6) [77],... 

The [2-1-2]-cycloaddition reactions in inclusion crystals, or host-guest crystals, have been thoroughly reviewed. Coumarin 61a, thiocoumarin 61b, and cyclohex-2-enone 62 efficiently undergo single-crystal-to-single-crystal enantioselective photodimerization in inclusion complexes with chiral host compounds (R,R)-(-)-fra s 63a, (R,R)- -)-trans 63b, and (-)-64, respectively. The products are (-)-anri-HH 65a (100% EE), +)-anti-HH 65b (100% EE), and -)-syn-trans 66 (48% EE) dimers. An example for enantioselective intramolecular [2+2]-photocycloadditions in inclusion crystals is the reaction of guest 67 in chiral host 68. Irradiation of the powdered inclusion crystals in water suspension affords the optically active photocyclization product 69 with 100% EE in 90% yield. 

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