Ketones, olefinic quinones

Oxetane Formation—The Patemo-Bnchi Reaction. A large number of carbonyl compounds, primarily aldehydes, ketones, and quinones, form oxetanes by photocycloadditions to olefins.61-63 In general, it is observed that (/) carbonyl compounds which have low-lying (77, ) triplet states and which are photoreduced in isopropyl alcohol form oxetanes most readily, and (2) oxetane formation takes place when energy transfer from the carbonyl compound to the olefin is unfavorable because of the relative location of their triplet levels.64,65 Hence, oxetanes are most readily formed from simple olefins and allenes63,66 but are seldom formed from dienes.67 An extensive review by Arnold63 covers the mechanism and scope of this reaction. 

The reactivity of various functional groups toward 9-BBN thus is classified into five broad categories as (1) very rapid-reduction aldehyde and ketone (2) rapid reduction-reaction olefin, quinone, tertiary amide, acid anhydride, acid chloride, and lactone (3) slow-reduction ester, epoxide, and oxime (4) very slow-reduction carboxylic acid, sulfoxide, and azoxy and (5) inert (no reaction)... 

The bismuth ylides, Ph3Bi=CHCOR, do not react with simple ketones and electron-rich olefins probably because of their relatively low electrophilic character. However, Ph3Bi=CHCOR reacts with a-keto esters [46, 67, 68], benzils [46, 67-69], orf/to-quinones [46, 67, 68], and acenaphthenequinone [70] to give epoxides, (9-arovl enolates, 3-hydroxytropones, and 3-hydroxyphenalenones, respectively, accompanied by the formation of Ph3Bi (Scheme 11). In particular, transposition and ring expansion reactions are of interest from a mechanistic point of view, since these reaction modes are unprecedented in ylide chemistry. 

Similar to ketones, quinones have also been known for a long time to undergo PET processes with several donors such as aromatic hydrocarbons, olefins and amines etc. [10b, 11]. It is pertinent here to illustrate one of the synthetically important quinone-olefin [186] reactions which has been utilized for the preparation of Benz (a) anthracene 7,12-dione derivatives (237). In the present example, the excited state of naphthaquinone (234) reacts with ethene (235) to give 236, provided the electron transfer is thermodynamically allowed (ca. AGet < 0). In the follow up processes, (236) is transformed to (237) in 30-50% yield. Another report from the same group has described [187] a novel method of photoallylation of naphthaquinone by allyl stannane. 

The Chemistry of Ring B.—The isolation of taxodione (49), a diterpenoid tumour inhibitor, has stimulated interest in the introduction of oxygen functions at C-6. Taxodione itself has been synthesized from podocarpic acid. The latter was converted to ferruginol benzoate (46). The C-11 hydroxy-group was introduced via the diazo-compound and the product acetoxylated at C-7 to afford (47) this was converted into the A -olefin (48), which was epoxidized and isomerized to the C-6 ketone. The product was then oxidized to give the quinone-methide of taxodione (49). 

As olefinic substrates a,j -unsaturated carbonyl compounds (esters, anhydrides, ketones, quinones), nitriles and nitro compounds can be used. The rate of addition of the diazo compounds is dependent upon the nature of the diazo compound and generally the following order of reactivity can be observed diazomethane > diphenyldiazomethane > methyl diazoacetate > diazoketones. 

These tellurium-containing polymers were checked for their catalytic activity in the ep-oxidation of olefins and as oxidizing agents. The polymeric 4-methoxyphenyl tellurium oxide did not react with amines, amides, alcohols, or phenols, but oxidized hydroquin-ones to quinones, thiols to disulfides, thioketones to ketones, thioesters to esters, and thiobenzamides in organic solvents to cyanobenzenes and in acetic acid to 2,5-diaryl-4,l, 3-thiadiazoles. ... 

Common colored compounds include nitro and nitroso compounds (yellow), a-diketones (yellow), quinones (yellow to red), azo compounds (yellow to red), and polyconjugated olefins and ketones (yellow to red). Phenols and amines are often brown to dark-purple because of traces of air oxidation products. 

Phenols are oxidized by NaBiO3 to polyphenylene oxides, quinones, or cyclohexa-2,4-dienone derivatives, depending on the substituents and the reaction conditions [263]. For example, 2,6-xylenol is oxidized in AcOH to afford a mixture of cyclohexa-dienone and diphenoquinone derivatives (Scheme 14.123) [264] and is oxidatively polymerized in benzene under reflux to give poly(2,6-dimethyl-l,4-phenylene) ether (Scheme 14.124) [265]. Substituted anilines and a poly(phenylene oxide) are oxidatively depolymerized by NaBiO, to afford the corresponding anils [266]. Nal iO, oxidizes olefins to vicinal hydroxy acetates or diacetates in low to moderate yield [267]. Polycyclic aromatic hydrocarbons bearing a benzylic methylene group are converted to aromatic ketones in AcOH under reflux (Scheme 14.125) [268]. 

The types of compounds presently known to donate a hydrogen atom to photoexcited quinones include aldehydes, primary and secondary alcohols, esters and lactones, ethers and thioethers, olefins having allylic hydrogen atoms, alkylbenzenes, benzene, and saturated hydrocarbons. This list is undoubtedly incomplete. Acetone, methyl ethyl ketone, acetic acid, and -butyl alcohol react extremely slowly. 

Other recent examples of CIDNP studies on olefin isomerizations via reverse electron transfer populating the triplet, which all fall into class Ila, concerned all-frans retinal with stilbene as donor or a quinone as an acceptor, and a, P unsaturated ketones with triphenylamine or triphenylphosphine as donors °... 

Furan has been found to form oxetanes with a variety of carbonyl compounds, e.g., ketones,202-205 aldehydes,206 and ethyl cyanofor-mate.207 In most reactions the (2 + 2)-cycloaddition occurred specifically to give a 2,7 dioxabicyclo[3.2.0]hept-3-ene (175) rather than the 2,6-isomer (176). Only the addition of ethyl cyanoformate yielded mixtures of 175 and 176 (R = OEt and R2 = CN), in a ratio of 2 l.207 Two subsequent (2 + 2)-cycloadditions of benzophenone and furans have been reported to give two isomeric products, 177 and 178.205 Substituted furans yielded similar oxetanes.203 Benzo[ >]furans, furo-coumarins, and furochromones also proved to undergo (2 + 2)-cyclo-addition reactions with carbonyl compounds such as ketones, aldehydes, and quinones. Invariably one type of oxetane was formed (179).,37,u3 ,44 200-202 208,20, In the case of 2-methoxycarbonylbenzo[6)-furan, evidence has been provided that the oxetane was produced by addition of the excited triplet state of the olefinic reactant to the ground state of the ketone.208... 

Ketones.-A convenient procedure for the synthesis of 1-formylcyclo-hexenes by one-carbon homologation of the appropriate a-phenylthioketone, e.g. (53), by phosphorus-based olefination and hydrolysis has been reported (Scheme 5).31 Further investigations of the reaction of ethoxycarbonyl-methylenetriphenylphosphorane with <>-quinones have been reported.3 2 Reactions in the presence of triphenylphosphine or alcohols or acetic anhydride led to different products including lactone ylides (54) and furan derivatives (55). 

Nonconjugated olefins, acetylenes, styrenes, a,P-unsaturated aldehydes, a,P-unsaturated ketones, a,P-unsaturated acids and quinones are all arylated under Meerwein conditions. Conjugated dienes and ene-ynes can be also used as substrates. 

Potassium hydroxide in acetonitrile solution with dicyclohexyl-18-crown-6 gives good yields of aj8-unsaturated nitriles from a variety of aldehydes and ketones, including benzophenone. One consequence of the rate enhancement of the [3,3] sigmatropic rearrangement of certain enolates reported last year by Evans is a new route to prenylated quinones (Scheme 14). A development in the synthesis of olefins by the boron mediated cross-coupling reaction is the use of the boronic... 

The photocycloaddition of triplet benzophenone to norbornene was originally reported by Scharf and Korte. The photoproduct 101 that is formed in high exo-selectivity could be thermally cleaved to the 5,e-unsaturated ketone 102, an appHcation of the carbonyl-olefin metathesis (COM) concept. The 1,4-biradical formed in the interaction of norbornene with o-dibenzoyl-benzene was trapped in an intramolecular fashion by the second carbonyl moiety. A highly regioselective reaction of triplet benzophenone was reported with 5-methylenenorborn-2-ene, with preferential attack toward the exo CC double bond. A number of publications have discussed the photocycloaddition reactions of triplet carbonyl compounds to norbornadiene and quadricyclane, as weU as the competition between the Paterno-Biichi reaction and the sensitized norbornadiene/quadricyclane interconversion. Oxetane formation has also been reported for the photoreaction of biacetyl and para-quinones with benzvalene. ... 

---