Ketones 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... 

A-Substituted pyrroles, furans and dialkylthiophenes undergo photosensitized [2 + 2] cycloaddition reactions with carbonyl compounds to give oxetanes. This is illustrated by the addition of furan and benzophenone to give the oxetane (138). The photochemical reaction of pyrroles with aliphatic aldehydes and ketones results in the regiospecific formation of 3-(l-hydroxyalkyl)pyrroles (e.g. 139). The intermediate oxetane undergoes rearrangement under the reaction conditions (79JOC2949). 

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... 

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

Trimethyloxazole 257 undergoes photochemically induced [2 + 2] cycloaddition with aromatic and aliphatic aldehydes to provide bicyclic oxazolines 258 with excellent regiochemical and stereochemical control. Diastereoselec-tivities from 75-99% can be achieved, which is the first reported example of a Paterno-Biichi reaction involving an oxazole. The oxetane cycloadducts can be hydrolyzed to a-amino-(3-hydroxy ketones. Other oxazoles have not been evaluated to determine if they undergo the photochemical cycloaddition (Scheme 8.71). 

Cyano-substituted ethylenes react in a different way with aliphatic ketones. The orientation of photochemical cycloaddition (4.661 is the opposite of that found for electron-rich alkenes, and the reaction is highly stereoselective (4.69) in the early stages. These processes involve the formation and subsequent decay of an excited complex (exciplex) from the (n,n ) singlet state of the ketone and the alkene. Aryl ketones undergo intersystem crossing so efficiently that such a singlet-state reaction is rarely observed, but the reaction of a benzoate ester with an electron-rich alkene 14.70 rnay well be of this type, with the ester acting as electron-acceptor rather than electron-donor. 

Four-membered heterocycles are easily formed via [2+2] cycloaddition reac tions [63] These cycloaddition 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, p 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, l,4-benzoxazm-2-ones, l,2,4-tnazin-5-ones, and l,2,4-tnazm-3,5-diones accelerates [2+2] photocycloaddition processes with ketenes and allenes [106] to yield the corresponding azetidme derivatives Starting from olefins, fluonnated oxetanes are formed thermally and photochemically [119, 120] The reaction of 5//-1,2-azaphospholes with fluonnated ketones leads to [2+2] cycloadducts [121] (equation 27)... 

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... 

Cycloaddition reactions of acyl silanes appear to be rare, but Brook has shown that a-silyloxy bis(trimethylsilyl)silenes (52), generated photochemically from acyl tris(trimethylsilyl)silanes (vide infra, Section IV.A.4), undergo [2 + 2] and [4 + 2] cycloaddition reactions with ketones, and [4 + 2] cycloaddition reactions with less bulky acyl silanes, as illustrated in Scheme 8717,24 26 72 73,201. They do not, however, react with their parent acyl tris(trimethylsilyl)silanes. 

Fused aziridines are interesting compounds owing to the fact that the strained three-membered ring can easily open and cause dipolar cycloaddition reactions as well as their photochromic properties. Therefore, most of this chapter covers the chemical and photochemical properties of bi- and tricyclic aziridines. Some properties of aziridinyl ketones are also reviewed, in particular, reactions leading to aziridinyl anils. 

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... 

Fluorine-containing aldehydes, ketones and acid fluorides undergo photoinitiated cycloaddition reactions with fluorinated alkenes to give oxetanes.90 91 The addition of hexafluoroacetone (11) to fluoroalkenes can also be performed in the presence of the Lewis acid, aluminum chloride fluoride.92-1 22-1 23 Unlike the photochemical cycloaddition of hexafluoroacetone with trifluoroethene, the Lewis acid catalyzed addition is regioselective.92 Fluorooxetanes (c. g., 14) have also been synthesized by the addition of formaldehyde to fluoroalkenes in hydrogen fluoride.94 Examples of the formation of fluorooxetanes by [2 + 2]-cycloaddition reactions arc-given in Table 8. 

The photochemical cycloaddition of a carbonyl, generally from an aldehyde or ketone, and an alkene is called the Patemd-Buchi reaction This [2 + 2]-cycloaddition gives an oxetane (213) and the reaction is believed to proceed via a diradical intermediate. Silyl enol ethers react with aldehydes under nonphoto-chemical conditions using ZnCl2 at 25°C or SnCl4 at —78°C. ... 

Copper-catalysed Michael addition followed by an aldol reaction with formaldehyde, gives a 1 1 mixture of diastereoisomers of the aldol 86 that can be eliminated to the enone 83. The resulting efficient photochemical cycloaddition gives ketone 82 with total regioselectivity probably because it is intramolecular. 

The mechanisms of two particular classes of photochemical (2 + 2)-cycloaddition reactions have been studied extensively, viz. (i) of reactions in which either the heterocyclic or the substrate contains an enone moiety,1231,24 which are believed to occur via the excited triplet state of the enone, and (ii) of oxetane formation, which is thought to proceed via n — 7T excitation of the carbonyl group, followed by intersystem crossing to the triplet state and addition to the carbon-carbon double bond in its ground state6 in some cases, however, an oxetane has been reported to be formed from a ground-state ketone and an excited-state olefin (Section III,C,2). 

Ketones and aldehydes can undergo photochemical [2-1-2] cycloaddition reactions with alkenes to give oxetanes. This is called the Paterno-Buchi reaction. For alkyl carbonyl compounds both singlet and triplet excited states seem to be involved, but for aromatic compounds the reaction occurs through the triplet state.The regiochemistry can usually be accounted for on the basis of formation of the most stable 2-oxa-1,4-diradical. For example, styrene and benzaldehyde give 2,3- not 2,4-diphenyloxetane. ... 

Dioxinones made from achiral disymmetric ketones having the ketalic carbon as the only asymmetric center can be prepared in optically pure form from 3-hydroxybutyric acid [158]. Replacement of menthone by other carbonyl residues has a large effect on the complexity of the reaction mixture. Up to seven stereoisomers can be characterized from the photochemical cycloaddition of 191 with cyclohexene [157b,c] (Scheme 31). 

A successful photochemical cycloaddition of various fluoroalkyl ketones, aldehydes, and acyl fluorides to fluoroolefins was reported by Harris and Coffman. Originally it was reported that among four possible stereoisomers, only the regioi-somers of type A with CF2 unit next to oxygen were formed (Scheme 2.3). It was rationalized that the reaction proceeds through the most stable diradical intermediate. The yields of oxetanes in this reaction varied mostly from 30% to 70% when X = CF3 and were about 15% for X = Cl. Presumably the mechanism of this reaction involves... 

Alkylsilanes react with acid chlorides under catalysis with AICI3 to give unsym-metrical ketones.Compound (45) is obtained by reaction of 5-tri-methylsilylcyclopentadiene with dichloroacetyl chloride in the presence of tri-ethylamine, and it may be converted into prostaglandins or into loganin aglycone acetate (46), by subsequent modifications. Compounds (47) and (48) are obtained from photochemical cycloadditions of alkynylsilanes, RC=CSiMe3, with cyclopent-2-enone. ... 

---