Cyclopentenones photocycloaddition reactions

In a careful study of cyclopentenone photocycloaddition reaction, DeMayo and coworkers215 have noted that ketone sensitizers of triplet energy less than 71.2 kcal did not sensitize cycloaddition to cyclohexene. Triphenylene (Et = 66.6 kcal) and acenaphthene (ET = 59.3 kcal) were exceptional since they resulted in quantum yields of 0.10 and 0.21, respectively. This behavior, as well as the fact that 0.1 M cyclopentenone quenched acenaphthene fluorescence by 90% in an EPA (ether-isopentane-alcohol) glass at 77°K strongly implicate singlet energy transfer. 

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. 

We examined initially the photochemical reactivity of 2-ethoxypyrrolin-5-one (12) with olefins because of its obvious similarity to 2-cyclopentenone. Irradiation of 12 in the presence of 1,1-dimethoxyethene or cyclohexene in ferr-butyl alcohol solvent did not give a (2 + 2)-cycloadduct but the rearrangement product ferr-butyl N-(ethoxycyclopropyl)carbamate (I3)l4 The novelty and synthetic potential of the rearrangement of 12 to 13 lured us temporarily away from our original pursuit. However, as will become evident later, our studies of the rearrangement reaction were important in our eventual discovery of the (2 + 2)-photocycloaddition reaction. 

The photocycloaddition reaction seems to be equally successful in systems where the alkene absorbs the radiation initially, and this must happen when simple alkyl-substituted alkynes react with enones (equation 47) or with enediones (equation 48) . The reactions of cyclopentenone probably occur through a triplet state of the enone and perhaps through a complex between this excited state and the alkyne . 

A series of (2+2) photocycloaddition reactions have been carried out using (5R)-5-menthyloxy-2(5H)-furanone (32) as the substrate. Photoaddition of cyclopentenone to this substrate gives the four products (33)-(36) with some level of regioselectivity but no facial selectivity. Interestingly, cyclohexenone, cyclohepten-one and cyclooctenone fail to undergo the mixed addition. High facial selectivity is observed when more complex enones such as the 3,5,5-trimethylcyclohexenone and isophorone, (37) are used. The reaction affords adducts of the type illustrated... 

In contrast to cyclopentenones and cyclohexenones, medium-ring vinylogous esters are not suitable for photocycloaddition reaction unless the olfin coupling partner is substituted. For example, when ( -3-(but-3-enyloxy)cyclooct-2-enone 78a is irradiated under a variety of conditions, no intramolecular cycloadduction occurs. However, upon substitution of a vinyl or phenyl on the olefin, the cycloaddition proceeds efficiently to give diastereomeric mixtures 79b/c, and 80b/c, respectively. The dramatically enhanced yields and rates of the photoaddition reactions upon olefin substitution result from the stabilization of the 1,4-biradical by a vinyl or phenyl. A mixture of diastereomers is formed presumable because the rotational relaxation of the intermediate 1,4-diradical is faster than [2 + 2] ring closure. 

All of these photoadducts obtained are polycyclic, and even polyfunctional molecules that have an a-cyclobutyl keto group. They can be used as very versatile starting materials for the synthesis of complex molecules. The [2-i-2]-photocycloaddition reaction constitutes a method of choice for creating a four-membered ring in polycycKc molecules. The photoreaction of a cyclopentenone unit with an alkene has recently been used in the synthesis of two natural products containing a cyclobutane unit — namely, sulcatine G and ( )-kelsoene (Scheme 14). ... 

Furanones, maleic anhydride, maleimides, and cyclopentenones behave similarly when irradiated in the presence of alkenes. [2-1-2]-Photocycloaddition reactions of furanones, especially y-substituted furanones with alkenes, can be highly diastereoselective. A detailed analysis of the cycloaddition of ethylene with substituted furanones gave a deep insight into the origin of the diastereoselectivity. Based on product ratio dependence on temperature and the substituents, it was proposed that a pyramidalization of the P-carbon of furanones in their relaxed excited state and a homo-anomeric effect determined the... 

Weedon, A. C, Photocycloaddition reactions of cyclopentenones with alkenes, in CRC Handbook of Organic Photochemistry and Photobiology, Horspool, W. M. and Song P-S., Eds., CRC Press, Boca Raton, FL, 1995, 634-651. 

Photochemical Cycloadditions. Isobutene has been widely used in intermolecular [2 + 2] photocycloaddition reactions with enones. The weakly polarized isobutene is often used to study the regioselectivity of the photocycloaddition. Cyclohexenones (eq 9), cyclopentenones (eq 10), and functionalized enones (eq 11 ) undergo cycloaddition with isobutene. Patemo-Biichi... 

Most investigations of the photocycloaddition of simple a,0-unsaturated carbonyls have involved the reactions of the ring compounds cyclopentenone and cyclohexenone. The photoaddition of 2-cyelohexenone to a number of different olefins has been reported by Corey and co-workers(94) ... 

The fadal diastereoselectivity of intermolecular cyclopentenone [2 + 2]-photocy-cloaddition reactions is predictable if the cyclopentenone or a cyclic alkene reaction partner is chiral. Addition occurs from the more accessible side, and good stereocontrol can be expected if the stereogenic center is located at the a-position to the double bond. In their total synthesis of ( )-kelsoene (11), Piers et al. [22] utilized cyclopentenone 9 in the [2 + 2]-photocycloaddition to ethylene (Scheme 6.5). The cyclobutane 10 was obtained as a single diastereoisomer. In a similar fashion, Mehta et al. have frequently employed the fact that an approach to diquinane-type cis-bicydo [3.3.0]octenones occurs from the more accessible convex face. Applications can be found in the syntheses of (+)-kelsoene [23], (—)-sulcatine G [24], and ( )-merri-lactone A [25]. 

In general, the stereoelectronic influence of substituents in [2 + 2]-photocycload-dition reactions is minor, and the preferred ground-state conformation often accounts for the formation of the major diastereoisomer. Inspection of molecular models and force field calculations provide a good picture of possible transition states leading via 1,4-biradicals to cyclobutane products. The total synthesis of (+)-guanacastepene represents another recent example for the use of stereoselective intramolecular cyclopentenone-olefin photocycloadditions in natural products synthesis [32]. 

Intramolecular reactions of cyclohexenones follow pathways similar to those of cyclopentenones, both with regard to regio- and stereocontrol. The initially mentioned intramolecular [2 + 2]-photocycloaddition of carvone (1) is a typical example for five-membered ring formation with high diastereofacial control (Scheme 6.1). In this case, the rule of five requires the terminal carbon atom of the intermolecular... 

A very useful extension of the de Mayo reaction has been recently introduced by Blechert et al. (Scheme 6.26) [78]. The retro-aldol fragmentation was combined with an intramolecular enantioselective allylation (asymmetric ring-expanding allylation) catalyzed by a chiral Pd complex. Bicycloheptane 68, for example, was accessible by intermolecular [2 + 2]-photocycloaddition of cyclopentenone 67 with allene. Further transformation in the presence of Pd2(dba)3 (dba = dibenzylideneacetone) and the chiral oxazoline ligand 69 (tBu-phox) resulted in the enantioselective formation of cycloheptadione 70. 

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