Allenes photocycloaddition reactions

The [2 + 2] photocycloaddition reaction of enones with allenes was first reported in 1966. A diradical intermediate is formed from a triplet enone via an exciplex. The triplet diradical cyclizes to the product after spin inversion to the singlet state [31,32]. 

Although most of the photocycloaddition reactions have involved olefins, there are several examples of additions to other types of un-saturated systems (e.g., allenes, acetylenes, ketenimines, etc.) which extend their synthetic utility. It is impossible, at this time, to define the limitations of the reaction as applied to other systems however, this will unquestionably be an active and fruitful area of research in the future. 

Substrates A3 (Q = O) have been employed not only as starting materials for fragmentation reactions but also to probe novel stereoselectivity concepts. The photochemical transformation of axial chirality into central chirality was achieved by Carreira et al., who employed chiral, enantiomerically pure allenes in intramolecular [2 + 2]-photocycloaddition reactions (Scheme 6.27) [79]. The reaction of enantiomerically pure (99% ee) cyclohexenone 71, for example, yielded the two diastereomeric products 72a and 72b, which differed only in the double bond configuration. Apparently, the chiral control element directs the attack at the allene to its re face. The double bond isomerization is due to the known configurational liability of the vinyl radical formed as intermediate after the first C—Cbond formation step (see Scheme 6.2, intermediate C). 

A new method for the construction of the AB-ring core of paclitaxel (Taxol), an anticancer drug, was developed utilizing the cyclopentanone (143) allene (144) photocycloaddition reaction to give the bicyclic product 145, which was subsequently transformed to a bicyclic diketone 146 in several steps (Scheme 6.66) in 42% overall chemical yield.763... 

An intramolecular [2-1-2] photocycloaddition reaction was reported as the key step in constructing the tricyclic core 218 of solanoeclepin A, which includes an intricate bicycle[ 2.1.1 jhexanone moiety. Allene butenolide 217 as... 

Four-membered heterocycles are easily formed via [2-I-2] cycloaddition reac tions [65] These cycloaddmon reactions normally represent multistep processes with dipolar or biradical intermediates The fact that heterocumulenes, like isocyanates, react with electron-deficient C=X systems is well-known [116] Via this route, (1 lactones are formed on addition of ketene derivatives to hexafluoroacetone [117, 118] The presence of a trifluoromethyl group adjacent to the C=N bond in quinoxalines, 1,4-benzoxazin-2-ones, l,2,4-triazm-5-ones, and l,2,4-tnazin-3,5-diones accelerates [2-I-2] photocycloaddition processes with ketenes and allenes [106] to yield the corresponding azetidine derivatives Starting from olefins, fluonnaied oxetanes are formed thermally and photochemically [119, 120] The reaction of 5//-l,2-azaphospholes with fluonnated ketones leads to [2-i-2j cycloadducts [121] (equation 27)... 

A novel intramolecular photocycloaddition involving vinylogous amides and allenes led to an interesting type lb entry to functionalized pyrroles <060L4031>. For example, photolysis of allene 11 provided fused pyrrole 12 via a [2+2] cycloaddition and retro-Mannich reaction. 

With conjugated dienes, photocycloaddition of carbonyl compounds occurs at one of the double bonds to give vinyloxetanes. An interesting example is the reaction of acetone with 2-methyl-l,3-butadiene, which gave the two oxetanes (60) and (61) in a ratio of 3 1 and a total yield of about 20% (72JA8761). Other alkenes which have been used for photosynthesis of oxetanes include enol ethers, ketene acetals, enamines, allenes and diketene, with the reaction of the last compound with benzaldehyde illustrated in equation (105) (75CPB365). 

Photochemical reactions of quinones with allenes have also been studied and in some cases cyclobutane formation occurs, although in competition with products derived from attack of the allene on the carbonyl oxygen. Thus, photocycloaddition of tetramethyl-l,4-benzoquinone with 1,1-dimethylallene affords the four-membered carbocycle 6 in good yield.12... 

The formation of trans-products is observed to a lesser extent in the reaction of 3-alkoxycarbonyl-substituted cyclohexenones, in the reaction with electron-deficient alkenes and in the reaction with olefinic reaction partners, such as alkynes and allenes, in which the four-membered ring is highly strained (Scheme 6.11). The ester 26 reacted with cyclopentene upon irradiation in toluene to only two diastereomeric products 27 [36]. The exo-product 27a (cis-anti-cis) prevailed over the endo-product 27b (cis-syn-cis) the formation of trans-products was not observed. The well-known [2 + 2]-photocycloaddition of cyclohexenone (24) to acrylonitrile was recently reinvestigated in connection with a comprehensive study [37]. The product distribution, with the two major products 28a and 28b being isolated in 90% purity, nicely illustrates the preferential formation of HH (head-to-head) cyclobutanes with electron-acceptor substituted olefins. The low simple diastereoselectivity can be interpreted by the fact that the cyano group is relatively small and does not exhibit a significant preference for being positioned in an exo-fashion. 

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. 

Cycloaddition of but-l-yne to the quinolone (58a) gave the head-to-tail [2 + 2] adduct (59). This approach was coupled with a ring-opening reaction to provide a synthesis of quinolones bearing a substituent at C-3. For example, the cycloadduct (60), obtained from the quinolone (58b) and 2-methylbut-3-yn-2-ol, was transformed into edulinine (61). The photocycloaddition of allene to the quinolone (58b) affords the two [2 + 2] adducts (62, 59.6%) and (63, 9.7%). Diketene has also been used in cycloadditions to quinolones (58c) and (58d). The addition process is selective in that cycloaddition to (58c) yields the adduct (64) whereas (58d) affords (65). In the latter case, the cycloadduct is accompanied by the rearranged product (66). These adducts were used in further chemical transformations. 

Reaction Wave function Alkene, photocycloaddition, 366. 420-23 addition to benzene. 420 substituted, 432 Alkyl amines, tertiary. 466 Alkyl aryl ketones, 399, 402 Alkylethylenes, 420 Alkylidenecyclopropene. 57-58 N-Alkylimine, 375 Alkyl iodide, 471 Alkyl methyl ketones. 383-84 Alkyl radical, 380 Allene. 416 Ally radical. 102, 460 Allyl resonance, 461... 

Alkenylenones have not been as widely utilized as the corresponding 2- and 3-alkenylenones. The photocycloaddition of various allenes attached to cyclohexenones and cvclopentenones at the 4-position has been reported and two examples are shown. Irradiation of the cyclopentenone 1 resulted in the formation of the straight adduct 2 and product 3 in a 1 1 ratio140. Compound 3 is formed by reaction across the 1,2-double bond of the allene function. Photocycloaddition of the cyclohexenone derivative 4141, as a 3 2 mixture of anti- and, vv -diastereomers at — 70 C, resulted in quantitative cycloaddition of the anti-diastereomer to yield the cis decalin system 5. The syn-diastereomer gave three minor photoproducts which were not characterized. 

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