Cyclopentenones, alkene photocycloaddition

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... 

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). 

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... 

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 . 

Additions to Cyclopentenones and Related Systems. (2 + 2)-Cycloadditions are reported following the irradiation of mixtures of alkyl and aryl 2-thioxo-3/f-benzoxazole-3-carboxylates with alkenes. Cycloaddition also occurs to the CS double bond. The photochemical additions of arylalkenes to 3-phenylcyclo-pentenone and 3-phenyl cyclohexenone have been studied. The regio- and stereochemistry observed in the additions has been rationalized in terms of the stability of the intermediate biradicals. Photocycloaddition of allene to the cyclopentenone derivative (6) in methylene chloride solution at — 78°C affords... 

A preliminary examination of asymmetric induction in photochemical [2 + 2] cycloaddition reaction of an enone to an alkene employed a chiral auxiliary attached to the alkene component74. The photocycloaddition of 2-cyclopentenone to the optically active ketene acetal 1 led both to oxetanes and cyclobutanes in the ratio 6.5 3.5 with a total yield of 60%. The m-[2C + 2C] addition of the chiral alkene I to 2-cyclopentenone was completely regioselective and gave four diastereomeric head-to-tail cycloadducts 2-4 in the ratio 6 29 33 32. 

Investigation of [2 -l- 2] photocycloaddition of enones to monosubstituted alkynes indicates that the reaction proceeds with an opposite regioselectivity compared to that of alkenes [71]. Generally, head-to-head adducts are major products except when the group substituted on the alkyne is -CO2R or -OR (see Scheme 17). Thus, methyl propynoate (60, R = CO2CH3) photoadds to cyclopentenone 59 to give 1 1 HH and HT products. Cycloaddition of cyclohexenone to 1-hexyne has also been studied and the HH HT ratio varies from 2 1 to 7 1 as a function of substituents at the enone 3-position. It... 

The approach of orbitals prior to bonding is also an important factor in stereoselectivity of [2 + 2] photocycloadditions. For example, the alkene-teth-ered cyclopentenones 97 and 98 both gave 99 and 100 in a 1 2 ratio in low-conversion experiments (Scheme 32) [112]. 

For the photoadducts derived from cyclic enones and alkenes, stereochemistry at the ring junction is influenced by the structure and especially by the ring size of the starting reagents. For steric reasons, only cis-fused cycloadducts can be formed on photocycloaddition of cyclopentenones (n = 0). From cyclohexenone derivatives (n = 1), cis- and transfused adducts can be isolated, even if the cis-fused structure is thermodynamically more stable. This indicates that trans-fused cycloadducts result from a kinetic rather than a thermodynamic control. Fortunately, trans-fused cycloadducts can be epimerized easily to the more stable cis stereoisomers (Scheme 11). 

The importance of steric hindrance on facial selectivity is illustrated first for asymmetric cycloalkenes. High facial selectivity is observed with cycloalkenes having one stereogenic center in an allylic position. Photocycloaddition of cyclopentenone 25 with cycloalkene 26 led to a mixture of regio-isomers 27 and 28, resulting in a selective cycloaddition of the excited enone from the diastereoface opposite the large isopropyl group [69], Similar results are observed with alkenes 32, 35, and 38 [70-73] (Scheme 13). 

Ketals 201 from cyclopentenone and dialkyl tartrates have a rigid cyclopentene ring. Submitted to 2 -i- 2 photocycloaddition with a cyc-lohexenone and a cyclopentenone carboxylic ester, diastereoselectivities of up to 84% were obtained. The selectivity was very sensitive to the steric hindrance of the chiral auxiliary and tartrate derivatives gave better results than the corresponding threitoldibenzylethers [162]. When the chiral auxiliary was introduced into an acylic enone system (204), the diastereoselec-tivity of the cycloaddition was low. This indicates that the chiral alkene in the ground state exists as a mixture of conformers leading to opposite facial diastereoselection [163]. 

As previously established, the mechanism of the [2-1-2]-photocycloaddition of cyclopentenones with alkenes can be summarized as shown in Scheme 1. The interaction of the alkene with the enone, excited... 

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. 

Following Eaton s original discovery of the photocyclodimerization of 2-cyclopentenone (CP) and of photocycloaddition of this enone to cyclopentene (Eq. (72.1)),- Corey and co-workers reported [2+2]-photocycloadditions of 2-cyclohexenone (CH) to a variety of alkenes, and established many of the characteristic features of this reaction. 

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