Photoaddition of allene

Photoaddition of acetylene, 349 Photoaddition of allene, 349 Photoaddition of dichloroethylene, 349 Photoaddition of ethylene, 348 Photoaddition of hexafluoroacetone, 345 Photoaddition of maleic anhydride, 348 Photochemical addition of ethylenes and acetylenes to steroidal enones and dienones, 343... 

The full paper has appeared on a synthesis of steviol. The route was based on the stereocontrolled photoaddition of allene to the cyclopentene-1-aldehyde (178), reduction of the aldehyde, and then solvolysis of the methanesulphonate (179). Papers on the synthesis of the aromatic ether (180) and on the construction of the c/d ring system of the alkaloid chasmanine " have been published. 

Other cycloadditions of interest include the photoaddition [Eq. (52)] of pyrimidine derivatives such as cytosine to the carcinogenic hydrocarbon benzo[a]pyrene,191 and the photoaddition of allene as a vital step [Eq. (53)] in the synthesis of an annotinine derivative.192... 

Some studies directed at the synthesis of the b + c + d rings of gibberellic acid, and in particular the fragment (134), have been reported.The Diels-Alder addition of butadiene to the cyclopentenone (135) afforded (136), which was converted via its iodo-lactone (137) into the tricyclic compound (138). However, the synthesis broke down at the removal of the ring D substituents. l-Hydroxy-7-methylenebicyclo[3,2,l]octane (139) provides a model for the gibbane-steviol c/d ring system. A synthetic route involves the photoaddition of allene to 1-cyclopentene-l-aldehyde to give l-formyl-7-methylenebicyclo-... 

The photoaddition of allenic esters (31) to the enone (32) has been studied. The major products from these reactions are the dienes (33) and the cyclobutanes (34) and (35). It seems likely that the addition proceeds by the traditional biradical path to an intermediate such as (36). Dis-proportionation affords (33) while C-C bond formation yields the two cyclobutanes.The cycloaddition of isobutylene to the enone (37a) in a variety of solvents affords the adduct (38, 20-40X) and the alkylated... 

Earlier work by Wiesner proposed a set of empirical rules to account for the outcome of cycloaddition reactions to enones. Wiesner et al. have now examined the photoaddition of allene to the sterically hindered system (61). This work has... 

Fig. (14). The diol (137) was converted to the ketone (157) by the standard organic reactions. Photoaddition of allene to (157) affords the methylene cyclobutene (158) which is converted to the aldol (162) by ketalization, ozonolysis, reduction and hydrolysis respectively. The derivative (164) prepared from (162) undergoes a chugaev reaction affording the alcohol (165) whose mesyl derivative (166) suffered rearrangement furnishing the allylic alcohol (167) which is converted to the ketone (168). Itis subjected tl lithium ammonia reduction to obtain (169) wich can be converted to aphidicolin (148) by reaction already desccribed.

Stereocontrolled photoaddition of allene to cyclopent-l-ene-l-carboxaldehyde 29 has been utilized in the introduction of exocyclic methylene group in steviol 30 [32]. 

Two separate studies on photocyclization of allenes include examples of how Lewis acids and the polarity of the solvent affect the regioselectivity of the addition of activated olefins and 1,1-dicyclopropylallene, and provide details of the intramolecular photoaddition of allenes to a,p-cyclohexenones. Copper(i) trifluoromethanesulphonate has been found to catalyse the intramolecular photocyclization of several monocyclic (3- and Y"(pent-4-enyl)allyl alcohols for instance, the allyl alcohol (19) forms the tricyclic hydrocarbon (20), which is an... 

Intramolecular photoaddition of (terminal) allenes or ketenes to a cyclohex-2-enone C —C double bond have been investigated. The former reaction occurs regiospecifically in high yield,2 the latter, wherein the ketene moiety is photochemically generated, proceeds in moderate yield.3... 

The use of silica gel surfaces to modify photoprocesses continues to be of interest. The present report deals with photoaddition of alkenes to the steroidal enone (44).It is presumed that, in the adsorbed state on silica gel, the steroid will present the normally hindered side, i.e., the g-face is attacked by the alkene and the a-face is adsorbed on to the gel. The photoadditions were compared with those carried out in methanol. Thus with allene the two photoproducts (45) and (46) were obtained in 90 and 10% yields, respectively, at -78 °C in methanol, while on silica gel, both at -78 °C and at ambient temperature, the same two products were obtained in 46 and 53% yield. Similar results were obtained using ethene and cyclopentene. The photocycloaddition of ethyne and propyne to progesterone and testosterone in solution yields the cycloadducts (47) and (48), with a preference for the former mode of addition being observed. These results are in conflict with those of de Mayo and co-workers. ... 

The cycloaddition of the allenes (63) to cyclopentenone results in the formation of the two adducts (64) and (65) in a ratio of 4 1. The addition reaction occurs primarily across the less substituted double bond of the aliens. Cycloaddition of acetylene and but-l-yne to the enones (66) can be brought about using left-circularly polarised light. The resultant cyclobutane derivatives are readily converted to the optically active enones (67). The photoaddition of hex-1-yne to the lactone (68) affords the two isomeric adducts... 

A study of the influence of silica gel surfaces on the photochemical addition of allene to a series of cyclic enones has been carried out. Using this technique cycloaddition occurs to the more hindered face of the enone. A typical example of allene addition, where addition takes place to the less hindered face, is reported by Piers et al. in the photoaddition to the enone (65) which affords the four isomeric products (66) in ratios of 40 51 6 3. The two major products (66a) and (66b) were used as starting materials for total syntheses of naturally occurring compounds of the stemodane type. ... 

Neutral carbon of organic compounds is inherently a very weak base due to the lack of nonbonding electrons. However, the 7r-system of alkenes, alkynes, and allenes are potential basic sites due to the availability of 7t-electrons. Wooldridge and Roberts [61] first reported the facile photohydration of arylacetylenes to the corresponding ketones (in an overall Markownikoff sense) in dilute aqueous acid solutions. The facile protonation of the excited state was attributed to the enhanced basicity of the alkyne moiety in the excited state, more specifically as a result of extensive charge migration from the aryl group to the acetylene moiety. The photoaddition of methanol [62] and acetic acid to 7c-electron systems such as alkenes, alkynes, and allenes may be rationalized in the same way. 

The intramolecular photoaddition of vinylogous esters with allenes has also been explored. Thus photocycloadition of 82 leads to the formation of the bicyclic furan 84 in moderate yield. This furan derivative is prepared from the cyclobutane intermediate 83, which results from the parallel addition to the terminal olefin of the allene. However, this approach does not work for the acyclic vinylogous ester 85. Irradiation of this ester provides none of the expected furan product 87 and results only in isomerization of the vinylogous ester. 

The factors governing the stereochemistry of the photoaddition of aP-unsaturated ketones to olefins have been fully discussed by Wiesner, with examples from his own work. The stereochemistry of the product may be predicted by assuming that the p-carbon atom of the excited system is pyramidal, and can select a most stable conformation as judged by the principles of conformational analysis. The product is formed by reaction of the most stable conformer with the olefin. For example, the most stable conformation of the excited state of the yP-unsaturated ketone (125) may be considered to be (126), rather than (127), and it is the most stable form (126) which gives rise to the product (128), by 2 + 2 addition to allene. 

Novel photoadditions to benzene are reported in Scheme 67.261.412—414 In the low-yield addition of allenes the 1,4-mode of addition is favoured, which contrasts with the addition of simple mono-olefins where the 1,3-mode is favoured. Further study shows that the 1,3-mode of addition is facilitated by strained olefins, i.e. olefins of lower ionization potential. In the case of the olefin adducts the stereochemistry has not been rigorously defined. On treatment with base the allene adducts fail to isomerize to give semi-bullvalenes, but it has not been established whether this is due to kinetic or thermodynamic factors. 

Some of the earliest work on the [2 + 2] photocycloaddition chemistry of vinylogous imides was reported by Wiesner and coworkers, who described the isolation of the [2 + 2] photoaddition products resulting from the irradiation of cyclic vinylogous imides with ethyl acrylate and allene, respectively (Scheme 12)29. 

In a more extended study Dauben et al. (119) found that a similar photoaddition involving the allene (167), instead of the ene component, yielded the all-cis product in 60% yield (Scheme 43). This approach has the added merit of generating an exocyclic ene which can be used to introduce the... 

Benzene and other aromatic hydrocarbons undergo photoaddition reactions with furan, dienes, and allenes to give para-like adducts as major products by allowed (jt" s + Ti s) cycloaddition process. For examples, benzene reacts with furan in molar ratio 1 1 gives para-like adduct as major product [19] and with allene gives para-and meta-adducts in a ratio of 2 1 [20]. 

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