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Recent Developments in Arene Photocycloadditions

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Recent Developments in Arene Photocycloadditions HOT TOPICS 962 doi:10.2533/chimia.2008.962 CHIMIA 2008, 62, No. 12 Chimia 62 (2008) 962–966 © Schweizerische Chemische Gesellschaft ISSN 0009–4293 Recent Developments in Arene Photocycloadditions Ursula Streit and Christian G. Bochet* Abstract: The photocycloaddition of alkenes with aromatic systems may proceed through three reaction modes, affording different types of products: ortho, meta, and para. These reactions generate a considerable increase in complexity, and yield products arduously attainable by classical synthetic steps. Whereas the meta-photocyclo- addition has gained a lot of attention in the last decades because of its applicability in the total synthesis of numer- ous natural products, the ortho- and para-cycloadditions still have a great potential for further development. In this highlight, we review the latest advances in the field of photocycloaddition of alkenes to aromatic systems. Keywords: Arene photocycloaddition · Lacifodilactone F 1. Introduction [2+2] * [2+4] Aromaticity is a fascinating property, which has been the subject of many studies that ortho para were undertaken from all sorts of perspec- [3+2] meta tives. For the synthetic organic chemist, the extra stabilization of structures which feature cyclic arrays of 4n+2 conjugated π-electrons has been a very useful concept, in terms of a driving force for molecules able to access such a state, or in terms of their Scheme 1. resistance to aggressive reagents. The spe- cific case of benzenoid substrates has been extensively studied in the past century, and For example, benzenoid substrates rarely efficiently by means of classical thermal synthetic transformations of such cores are undergo Diels-Alder [4+2] cycloaddition reactions. We can distinguish three types of now part of the standard synthetic toolbox with olefins or dienes, i.e. as dienes or di- arene-alkene cycloadditions: ortho, meta (e.g. aromatic electrophilic or nucleophilic enophiles. The ‘aromatic resonance energy’ and para (Scheme 1). These descriptors substitutions, metal-halogen exchanges, of the ground state of benzenoid systems only indicate the connection points of the transition-metal catalyzed cross couplings of ca. 20 kcal/mol may significantly raise alkene to the aromatic molecule; attempts etc.). On the other hand, the unusual sta- activation barriers, thus preventing many to draw reaction mechanisms by curly ar- bility of aromatic rings prevents them from processes from occurring thermally at syn- rows are generally futile because, in their engaging in typical reactions of alkenes. thetically relevant temperatures (<150 °C). excited states, molecules often feature an On the other hand, it is well known that electron distribution that cannot be conve- photoexcitation is capable of transmitting a niently represented by simple Lewis struc- substantial amount of energy to a substrate tures. without thermal damage; it may therefore Since the first account of these cycload- be expected that this process can trigger dition by Büchi andAngus and Bryce-Smith normally impossible reactions. This is in- (ortho),[1] Wilzbach and Kaplan, and inde- deed the case, i.e. excited aromatic com- pendently the Bryce-Smith group (meta),[2] pounds are for example capable of under- and Wilzbach and Kaplan (para),[3] there going cycloadditions. The interest of such has been a steady stream of work in this reactions is dual: it may be convenient to area, further boosted by the spectacular ex- exploit the stability of aromatic structures ploitation of such reactions in natural prod- in the initial phases of a synthetic plan, and uct synthesis by Wender and Howbert.[4] then to convert the substrate into a more The field has been reviewed on several oc- [5,6] *Correspondence: Prof. C. G. Bochet complex one by a process that is normally casions (the latest being the excellent Department of Chemistry orthogonal to any other chemical reaction. review by Hoffmann[7]). In this overview, University of Fribourg Secondly, many of the structures that can be we will focus on recent application of these Chemin du Musée 9 CH-1700 Fribourg accessed by arene photocycloadditions are reactions in the synthesis of complex mol- E-Mail: [email protected] so complex that they cannot be assembled ecules. HOT TOPICS 963 CHIMIA 2008, 62, No. 12 2. meta-Cycloaddition O Lacifodilactone F is a triterpenoid OTMS isolated recently from the Chinese herb OTMS Schisandra, bearing a 5,6,7-tricyclic skel- OTMS hυ H eton. Wang and Chen achieved the con- H O struction of its core using a meta-photocy- O (λ =254 nm) H O 89 % O OMe cloaddition as the key step (Scheme 2).[8] OH The first photocycloaddition attempts with linear angular less complex precursors only led to degra- H dation; on the other hand, introduction of a CO2H trimethylsiloxyl group on the tether and an O 9 H O oxygen atom at the 1-position of the arene H 8 proved to be crucial. Surprisingly, the ste- Lacifodilactone F reochemistry at the siloxyl center does not HO H affect the meta-photocycloaddition; howev- HO er silylation helps to orient the olefin group towards the arene moiety thus ensuring a Scheme 2. high yield. The exo products are formed ex- clusively, through a boat-like conformation. The linearly and angularly fused products do not need further purification, and are fur- OMe OMe ther elaborated into the target molecule in MeO hυ H H four additional steps. (λ =254 nm) This flexible approach allows not only 90 % the construction of the 5,6,7-tricyclic skele- H H ton with two adjacent quaternary centers, but OH OH exexo:endoo:endo OH 1:1.2 also to access both configurations at the C(8) OH and C(9) positions of the lacifodilactone F. OH OH The [3+2] meta-photocycloaddition was shown to take place through an exo rather O than an endo exciplex. Additionally it was shown that tethers with four linking units HO H give usually rise to very low quantum yields. H stemodinone HO aphidicolin This observation suggests that the biologi- cally potent aphidicolin might be difficult to access via a meta-photocycloaddition Scheme 3. reaction, since a four-carbon tether and an endo exciplex would be required to build up its core structure.[9] Nevertheless, Russell and coworkers found that a cyclohexane Si n= yield: endo/exo/minor ring on the tether helped to constrain the 1 80 >10:1:<1 Si hυ 2 78 >10:1:<1 tether in a favorable conformation, and the n 3 81 >10:1:<1 photocycloadducts were obtained in good 4 93 4:1:1 n yields (although as a 1:1.2 mixture of exo and endo isomers) (Scheme 3). The core for either the aphidicolin or the stemodinone Scheme 4. could be assembled in such a reaction. The intermolecular arene–olefin photo- cycloaddition is a great tool for rapidly in- OMe 1-Iodo-2-nitrobenzene O Pd(OAc) (5 mol%) creasing molecular complexity, but it suffers OMe hυ 2 dppe(5mol%) sometimes from idiosyncratic behavior and cyclohexane NEt3,DMF,80°C NO substrate-specificity. For example the reac- 19% 2 tion of anisole with cyclopentene furnishes 20% the endo meta-photocycloaddition product in 80–85% yield, whereas the reaction with Scheme 5. cyclohexene is much less stereoselective and efficient (only 20% yield). De Long et al. showed very recently that the yields energy loss due to the broken aromaticity afford bicyclo[3.2.1] compounds. Electro- of arenes with a benzylic silicon atom are during the reaction. This allows the reaction philes are known to add to electron-rich much more reliable (Scheme 4).[10] This of ring systems even larger than cyclopen- olefins and the resulting cationic interme- probably arises from the β-silicon effect, a tene with aromatic rings, in high stereose- diate fragments give bridged bicyclic com- phenomenon well established for ground- lectivities and yields. pounds. Penkett and coworkers described state chemistry. Through hyperconjugation, The reaction products of the meta- recently the first carbocation-mediated re- the silicon atom at the β-position overlaps photocycloaddition are highly strained action on meta-photocycloaddition prod- with the excited-state electron-deficient molecules and can undergo opening of the ucts, coupled with a Heck-type reaction aromatic system, therefore minimizing the cyclopropylic ring with many reagents to (Scheme 5).[11] HOT TOPICS 964 CHIMIA 2008, 62, No. 12 3. Asymmetric meta-Cycloadditions 2R 4S O The reaction of an arene and an alkene O O in a meta-photocycloaddition reaction con- O hυ hυ O verts the flat (thus achiral) reaction sites O into three-dimensional and potentially chi- 2R 4R ral moieties. Four new stereocenters are in- O O O stalledinthis[3+2]cycloaddition.Therefore O it is not surprising that investigations were undertaken to render this process asym- metric. The main strategy towards this goal Scheme 6. was the introduction of a chiral tether for an intramolecular reaction. Sugimura and MeO C coworkers published recently very promis- 2 O 4 ing results with chiral 2,4-pentanediol (PD) O secondary tethers (Scheme 6).[12] In this scenario, the hυ rim 2' vinyl ether adds to both ortho positions of O the phenyl group. The face differentiation O primary MeO2C is excellent (controlled solely by the C(2) CO Me center), but a mixture of regioisomers is 2 rim obtained (the ratio is then dependant on the MeO2C configuration of the position 4, going from 70:1 for the RR to 2.7:1 for the RS). Related Scheme 7. work was reported by Garcia and cowork- ers.[13] Another asymmetric approach was in- vestigated by Van der Eycken and cowork- ers.[14] They studied the intramolecular meta-photocycloaddition in the solid phase in the presence of cyclodextrin (Scheme 7). The meta-substituted arenes formed the [3+2]-photocycloadducts as a 1:1 mixture of two products.
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