Ketones, unsaturated photochemical cycloaddition reactions

It is also worthwhile comparing the intramolecular photochemical cycloaddition reactions of ethylenic aldehydes and ketones with free radical intramolecular additions. For instance, irradiation of 5-hexen-2-one (470) (Scheme 161) in the gas phase gives the oxetane 471 as only cyclized product, as expected from the known photochemical intermolecular reaction between olefins and ketones. If the irradiation is conducted in solution 470 gives 471 (26%) and 472 (18%). With other y,< -unsaturated ketones, the bicyclic compound analogous to 472 may become the major product. With 2-allylcyclanones such as 473 (Scheme 161) bicyclic compounds are obtained (80% yield) as a mixture of 474 and 475, with 475 being the major product, but such compounds are difficult to isolate. " In the same manner, selective irradiation of the carbonyl group of 2-acyl-2,3-dihydro-4/f-pyrans (476) leads exclusively (23% yield) to exo-brevicomin (477) (a sex attractant), neither oxetane formation nor Norrish type II reaction being observed. The formation of the compounds 472, 475, and 477 which was considered as unexpected... 

Photocycloaddition Reactions ofEnones. Cyclic a,(3-unsaturated ketones are another class of molecules that undergo photochemical cycloadditions.188 The reactive... 

Another important class of cycloaddition reactions is the formation of oxetane rings between a photoexcited carbonyl compound and an unsaturated molecule. These reactions also occur probably through an exciplex although these exciplexes are non-fluorescent as they are formed from the triplet state of the ketone or aldehyde. The formation of the four-membered oxetane ring is an interesting example of a typical photochemical reaction... 

The photochemistry of o< -unsaturated ketones has attracted much attention and is still a field fiettd of current interest. /Numerous examples of such photochemical transformations are well-documented for cyclic enones and dienones, including both cycloaddition reactions and rearrangements. For example, cyclopentenones 1 and 2 readily rearrange to... 

Another class of molecules that are quite prone to undergo photochemical cycloadditions are a,/3-unsaturated carbonyl compounds. The reactive state in photochemical cycloadditions of or,-unsaturated ketones is believed to be the n-Tt triplet." Conservation of spin then implies that the initial intermediate is a triplet diradical, and the reaction need not be stereospecific with respect to the alkene component." The reaction has been most thoroughly studied in the case of cyclopen-... 

The photochemical addition of simple olefins to a,j -unsaturated ketones is a reaction of great current interest. The steroidal A -20-ketone system is especially prone to cycloaddition under mild conditions. Sunder-Plassmann et irradiated 3j5-acetoxypregna-5,16-dien-20-one (67) in the presence... 

The enone system itself is usually part of a five- or six membered ring, although acyclic a,(3-unsaturated ketones and enols of P-diketones are also found to undergo cycloadditions under certain conditions. For seven- and higher membered rings the primary photochemical event is Z—E isomerization around the C—C double bond, the E-isomer then eventually undergoing further thermal reactions. 

Four-membered rings can be synthesised by [2 + 2] cycloadditions. However, thermal [2 + 2] cycloadditions occur only with difficulty they are not concerted but involve diradicals. Photochemical [2 + 2] reactions are common and although some of these may occur by a stepwise mechanism many are concerted. An example of a [2 + 2] reaction is the photodimerisation of cyclopent-2-enone. This compound, as the neat liquid, or in a variety of solvents, on irradiation with light of wavelength greater than 300 nm (the n - n excited state is involved) is converted to a mixture of the head-to-head (48) and head-to-tail (49) dimers, both having the cis,anti,cis stereochemistry as shown. It is believed that the reaction proceeds by attack of an n - n triplet excited species on a ground state molecule of the unsaturated ketone (Section 2.17.5, p. 106). In the reaction described (Expt 7.24) the cyclopent-2-enone is irradiated in methanol and the head-to-tail dimer further reacts with the solvent to form the di-acetal which conveniently crystallises from the reaction medium as the irradiation proceeds the other dimer (the minor product under these conditions) remains in solution. The di-acetal is converted to the diketone by treatment with the two-phase dilute hydrochloric acid-dichloromethane system. 

Cyclobutane formation, occasionally in fairly low yield due to ring-opening reactions, also occurred by photochemical [2 - - 2] cycloaddition of vinylcyclopropane derivatives to an a, -unsaturated ketone moiety, " a cyclobuta-1,3-diene, a 1,4-benzoquinone, 2-acylthiophenes, and on irradiation of e t/o,enr/o-2,4-bis[( )-2-(methoxycarbonyl)vinyl]bi-cyclo[1.1.0]butane. A formal [2-1-2] cycloaddition also occurred on reaction of tricyclo[3.1.0.0 ]hex-3-ene with methyl 6-oxo-5-phenyl-l,3,4-oxadiazine-2-carboxylate to give methyl 9-oxo-10-phenyl-8-oxapentacyclo[4.4.0.0 ". 0 .0 °]decane-7-carboxylate, albeit in very low yield, after nitrogen extrusion. ... 

In 1984, Schreiber published a synthesis of the pheromone in which the majority of steps involve pericyclic reactions. Make sure you understand each one as it appears—re-read the appropriate part of Chapter 34 or this chapter if you have any problems. The first step is a photochemical [2 + 2] cycloaddition. You could not have predicted the regiochemistry, but it is typical of the cycloaddition of allenes with unsaturated ketones. 

Most of a, P unsaturated compounds undergo photochemical change like migration of the alkenic jt-bond to give a p, y-unsaturated ketones, dimerisation by photochemically allowed [x s+tc s] cycloaddition, rearrangement and cis-trans isomerisation. These are among the commonest photochemical reactions of a, P-unsaturated ketones. 

The photochemical dimerisation of a, P-unsalurated ketones provides a further class of [ji +jt ] cycloaddition and is demonstrated for 3-methylcyclopent-2-enone and 3-methylcyclohex-2-enone, with the formation of head to head dimers and head to tail dimers. The a, (5-unsaturated enone absorbs light of wavelength 300 nm and get n,n excitation. The reaction is believed to occur from triplet state because ISC is efficient. While it,n triplet excited enone also cause dimerisation. 

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