Cyclopentenones, photocycloaddition

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. 

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 addition of the 2-substituted 2-cyclopentenone in (4.57) to ethylene affords an intermediate which is converted to tricyclo[4.2.0.01,4]octane 469a). A similar broken window compound was also obtained by an intramolecular photocycloaddition of a 2-cyclopentenone (4.58)469b). 

A slight modification of this method was reported by Vandewalle291 The photocycloaddition between the cyclopentenones (86) and p,p-diethoxyacrylate (87) at —40 °C afforded stereoselectively the head to tail adduct (88) (Table 4) 29). Compound (88) was reduced with NaBH4 in EtOH at —40 °C, and was followed... 

Cyclopentenones (448a) and (448b) underwent photocycloaddition with 1,2-6w[trimethylsiloxy]cyclopentene to give the epimeric (449a) and (449b) in 65 % and... 

In cycloadditions of enones to alkenes novel strategies have been adopted for ring expansion of the cycloadducts, either by the choice of appropriate alkenes, e.g. 2-(trimethylsiloxy)buta-1,3-diene,70 vmv-2-trimethylsiloxybuten-2-oales71 or 3,3-dimethylcyclopropene,72 or by using 3-oxo-l-cyeloalkene-l-carboxylates as enones.73 Asymmetric [2 + 2] photocycloaddition of cyclopent-2-enone to a (+ )-dihydrofuran acetonide constitutes the cornerstone of the synthetic strategy in the first total synthesis of the novel antitumor metabolite ( )-echinosporin.74 The cycloaddition product 25 from treatment of 2-(2-carbomethoxyethyl)-2-cyclopentenone (24) with ethene has been used as a precursor for the preparation of tricyclo[4.2.0.01,4]octane.75... 

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. 

Cis,anti,cis products are favored in the intermolecular photocycloaddition of cyclopentenones with cyclic alkenes87 (Scheme 34). However, intermolecular photoaddition of some cyclohexenones provided preferred /raw. -fused product as the major product with cyclic alkenes60. From the large number of examples presented in Schemes 37, 42, 43, 45, 46, 48 and other examples, it could be concluded that cis -fused products are usually preferred. 

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. 

Cyclopentenones undergo a clean [2 + 2]-photocycloaddition to a variety of alkenes, resulting in products with a bicyclo[3.2.0]heptanone skeleton. The two annelated... 

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]. 

The single stereogenic center of y-silylsubstituted cyclopentenone 12 allowed for an excellent stereocontrol in the [2 + 2]-photocycloaddition to the strained cyclobutene 13, which in turn had been obtained by a [2 + 2]-photocycloaddition-ring contraction sequence (Scheme 6.6) [26]. Alkene 13 dictates the exo-approach of the cyclopentenone, but as a meso-compound is of course not capable of controlling the absolute configuration. Further elaboration of product 14 led to (+)-pentacyclo-anammoxic acid (15). 

In a pioneering study, Zandomeneghi and Cavazza showed that the [2 + 2]-photocycloaddition of racemic 4-acetyloxy-2-cydopentenone can lead to a small, but detectable, enantiomeric excess (ee) of the product if circularly polarized light (CPL) is used as an irradiation source. By preferential excitation of the (—(-antipode of the cyclopentenone with 1-CPL (351-363 nm laser) in the presence of acetylene, an... 

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]. 

Cyclohexenones require essentially identical irradiation conditions as cyclopente-nones (vide supra). The outcome of the intermolecular [2 + 2]-photocycloaddition to alkenes is somewhat more complex as compared to cyclopentenones, because the... 

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... 

H)-Furanones (4-oxa-2-cyclopentenones) have been extensively explored as [2 + 2]-photocycloaddition substrates [59, 60]. A recent intermolecular example (Scheme 6.19) illustrates the application of their photocycloaddition chemistry to... 

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. 

All these results indicate that enantioselective photocycloadditions of s thetic interest should be possible with the help of a removable chiral auxilii as soon as the right chiral auxiliaries could be defined. In order to test the limit of this strategy, functionalized cyclohexenones and cyclopentenones were si lected to look for new chiral inductors. When co-alkenyl substituents were attach to the cyclic enone through an enamide, a carboxamide, or an ester group (Schei... 

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 . 

The sequence of intramolecular photocycloaddition-cyclobutane fragmentation has been used by Crimmins in an excellent manner for the synthesis of pentalenenes and the even more sophisticated lauren-l-ene (27). The latter synthesis was accomplished in 27 steps from cyclopentenone U66). One of the key steps is the reductive cleavage of the tetracyclic cyclobutane (167), followed by hydrogenation of the resulting P,7-unsaturated ester to give keto ester (168 Scheme 57). 

The [2+2]photocycloaddition of 5-arylfuran-2,3-diones (496) to trimethylsilyl-oxyethylenes (495) occurs with high regio- and stereoselectivity to efficiently yield polyfunctionalised cyclobutanes (497). ° A convenient four step stereoselective synthesis of ( + )-norasteriscanolide (500) has used the [2+2]photoadduct (499) from 2-cyclopentenone and the trimethylsilyl enol ether (498) to assemble the 5/8 ring system present in many terpenoids. 2-Naphthaldehyde undergoes regio-and stereoselective [2+2]photoadditon to the cyclic ketene silyl acetals (501) to... 

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