Enantioselective dimerization coumarins

The efficient enantioselective dimerization reaction of 97 in the inclusion complex (100) with 12a suggests that two coumarin molecules are arranged in chiral positions in 100 and their [2 + 2] photodimerization occurs by keeping the chirality in 100. The chiral arrangement of 97 was detected by CD spectral measurement of the inclusion complex in Nujol mulls. The 1 1 complex of 97 with 12a and 13a showed CD spectra with a mirror-imaged relation (Fig. 17). After photoirradiation, the CD absorptions of the complex (100) at 225, 275, 300, and 330 nm disappeared, and the new CD absorption due to the inclusion complex (101) of 98 at 240 nm appeared. The photodimerization of 97 was also followed by measurement of infrared (IR) spectra as Nujol mulls. Upon photoirradiation, the nCO absorption of 97 in 100 at 1700 cm 1 decreased gradually and finally disappeared after 4 hr, and new vCO absorption due to 98 in 101 appeared at 1740 cm-1. 

The steric course of the photodimerization reactions of coumarin (37a) and thiocoumarin (37b) succeeded in being controlled almost perfectly by carrying out the reaction in inclusion complexes using various host compounds. Furthermore, enantioselective dimerization reactions of 37a and 37b were found to proceed through a single-crystal to single-crystal process. 

The solid-state chemistry of coumarin has also received considerable attention, mainly due to the biological importance of coumarin derivatives, and has been summarized in several reports. " In a comprehensive study of nearly 30 coumarin derivatives in the solid state, Venkatesan, Ramamurthy, and co-workers discuss the criteria for topochemical reactivity in the solid set forth 20 years earlier by Schmidt. Their results, though not invalidating previous guidelines for solid-state reactivity, indicate that caution, rather than rigor, is recommended in their application. The irradiation of coumarin within chiral inclusion complexes has also been used as a means to achieve enantioselective dimer synthesis. " ... 

Toda et al. reported that the topotactic and enantioselective photodimerization of coumarin and thiocoumarin takes place in single crystals without significant molecular rearrangements [49]. Molecular motion needs to be called upon to explain the photochemically activated cycloaddition reaction of 2-benzyl-5-benzylidenecyclopentanone. The dimer molecules, once formed, move smoothly in the reactant crystal to form the product crystal [50]. Harris et al. investigated the reactivity of 10-hydroxy-10,9-boroxophenanthrene in the solid state and the mechanism of the solid-state reaction was characterized by both X-ray diffraction and thermal analysis [51]. It was demonstrated that the solution chemistry of 10-hydroxy-10,9-boroxophenanthrene is different from that in the solid state, where it undergoes dimerization and dehydration to form a monohydride derivative. 

Enantioselective photodimerization of coumarin (97) was accomplished in the inclusion complex (100) with 12a. For example, irradiation of the 1 1 inclusion compound of 97 with 12a gave the anti-head-to-head dimer (—)-98 of 96% ee in 96% yield [52], This photochemical reaction of 100 to the complex (101) of 98 was also found to proceed by a single-crystal to single-crystal manner. However, 1 2 inclusion compound of 11 and 97 gave the. vw-head-to-head dimer (99) upon irradiation in 75% yield [53],... 

The [2+2] photodimerization of coumarin has been extensively studied and previously reviewed in this series <1984CHEC>. An enantioselective variant of this transformation has recently been described <20050L1501>. Irradiation of coumarin or 6-methylcoumarin in the presence of an optically active host compound provided anti-head-to-head dimers with up to 98% ee (Equation 15). 

Similarly, irradiation in the solid state of a 1 1 inclusion complex of coumarin with 241a led to the cis-anti-cis head-to-head dimer 244 as an almost pure enantiomer [187]. The selectivity observed was very sensitive to the nature of the solvent used for crystallization. Replacing a mixture of ethylacetate-hexane with toluene-hexane led to a different 1 1 complex and different photodimers. Furthermore, among the numerous inclusion complexes tested, only the preceding example using 241a as the host led to an enantioselective photodimerization of coumarin. 

Pyrrolo[2,l-a]isoquinoline derivatives (27) were synthesized using Cgo-Bodipy dyads (28) as excellent photosensitizers (Scheme 13)/ Direct irradiation of glycine methyl ester to Ceo afforded [3 - - 2] cycloadduct/ Akasaka and his coworkers found the photocycloaddition of 2-ada-mantane-2,3-[3ff]diazirine and disilyliranes to Cso-metallofullerenes/ Intra- and inter-molecular photocycloaddition of alkenes to coumarin, quinolone, and isoquinolone derivatives have been reported by several groups. Griesbeck et al. found that the intramolecular photocycloaddition of cyclohexene moiety to coumarin (29) was catalysed by molecular ojygen. [2 + 2] Photocross dimer (33) of coumarin derivative (31) and 5-fluorouracil derivative (32) was obtained by laser irradiation/ Bach reported enantioselective intramolecular photocycloaddition of coumarins (34) to alkenes catalysed by a chiral Lewis acid (36) (Scheme 14)/ ... 

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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