Carbonyl compounds, p.y-unsaturated

Bis-acceptor-substituted diazomethanes are most conveniently prepared by diazo group transfer to CH acidic compounds either with sulfonyl azides under basic conditions [949,950] or with l-alkyl-2-azidopyridinium salts [951] under neutral or acidic conditions [952-954]. Diazo group transfer with both types of reagents usually proceeds in high yield with malonic acid derivatives, 3-keto esters and amides, 1,3-diketones, or p, y-unsaturated carbonyl compounds [955,956]. Cyano-, sulfonyl, or nitrodiazomethanes, which can be unstable or sensitive to bases, can often only be prepared with 2-azidopyridinium salts, which accomplish diazo group transfer under neutral or slightly acidic reaction conditions. Other problematic substrates include amides of the type Z-CHj-CONHR and P-imino esters or the tautomeric 3-amino-2-propenoic esters, which upon diazo group transfer cyclize to 1,2,3-triazoles [957-959]. 

Later Julia and coworkers developed another important mode of ring breaking addition of Grignard compounds or reduction converts the ester 22 to the corresponding alcohols which are startingpoints for a cyclopropylcarbinyl/homoallyl cation rearrangement. After acid treatment p/y-unsaturated carbonyl compounds (e.g. 23) can be isolated, which sometimes isomerize to the oc,(3-unsaturated systems 10). 

The reduction of methyl 2-siloxycyclopropanecarboxylates can also be started at the ester function when lithium aluminum hydride in ether is the reagent. The resulting alcohols undergo the wellknown cyclopropylcarbinyl/homoallyl rearrangement upon treatment with acid to provide P/y-unsaturated carbonyl compounds 117. These are synthesized isomerically pure and in good yields in a number of cases, if the two-phase-system 2N hydrochloric acid/pentane is employed 78). Otherwise the very easy isomerization to the conjugated a,p-unsaturated compounds 118 occurs to some extend, which can intentionally be completed by base catalysis. 

It is difficult to keep a p,y-unsaturated carbonyl compound because the double bond tends to move into conjugation with the carbonyl group in the presence of traces of acid or base. The intermediate is, of course, an enol in acid solution but an enolate ion in base. 

In addition to the normal photochemical reactions of saturated ketones, p,y-unsaturated carbonyl compounds undergo carbonyl migration via a [1,3] shift. Compound 139 in Scheme 45 represents a typical example. Compounds with an alkyl substituent in the P position such as 140 may also undergo a Norrish type II reaction (Kiefer and Carlson, I%7), while for ketones with electron-rich double bonds such as 141, oxetane formation is also observed (Schexnayder and Engel, 1975). 

The cyclopropylimine-pyiToline rearrangement (equation 68) has been exploited by Stevens in alkaloid syntheses. - Wenkert s cyclopropylcarbinyl rearrangement (equation 69) served extremely well in the design of 1,4-dicarbonyl synthons or p,y-unsaturated carbonyl compounds which then were expressed in numerous syntheses of terpenoid and alkaloid natural products. Donor-accrotor concepts continue to be expressed in the applicability of these rearrangements to organic synthesis. 

Dehydration Dissolution of 2-(a-hydroxyalkyl)cyclopropanols in CF3COOH causes ionization and ring opening, p,y-unsaturated carbonyl compounds are formed. Since the substrates are usually synthesized from conjugated lower homologues, the overall result is a methylene group insertion between the carbonyl group and the a-carbon atom. 

Removal of a proton from carbon atom 4 of aldol was considered not easy (point) In the carbocation, however, this proton should be much more acidic as again the carbanion can be stabilised by the adjacent positive charge. Thus, this bond becomes reactive and the point representing this heterolysis is again shifted more to the right of the reactivity space. However, the product is a p,y-unsaturated carbonyl compound, which is not as stabilised as the a,p-isomer. This is reflected in the fact that point 12 is not as far to the right as point 11. 

The ODPM rearrangement reaction is almost invariably a triplet-mediated process. Nevertheless, the reaction can proceed under conditions of direct irradiation because intersystem crossing is possible and which thus allows for the necessary singlet-to-triplet conversion. The crucial role of the sensitizer in the more conventional triplet-mediated variant of the reaction has been emphasized. Most particularly, the triplet energies of the sensitizers used to effect the reaction must be close to those of the triplet states of the alkene moiety associated with the p,y-unsaturated carbonyl compound serving as the substrate. So, for... 

J.1.2 Transformation of allyl alcohob to rearrangedhomoaifytakohob or p,y-unsaturated carbonyl compounds... 

The P, y-unsaturated carbonyl compounds are non-conjugated systems and show remarkable properties in ultra-violet region. Bicylo [2.2.1]-hept-5-en- 2-one is an example and shows absorption at 308 nm. 

Beside [1, 2] shift and [1, 3] shift p, y-unsaturated carbonyl compounds undergo characteristic reactions of chromophores, i.e., dimerisation, oxetane formation, reduction, Norrish t3T)e I and Norrish type II reactions, are also reported. 

The oxa-di-Jt-methane rearrangement of p, y-unsaturated carbonyl compounds is most commonly observed in rigid systems where isomerisation of the alkene is inhibited. It occurs from the lowest tc -> n triplet state and can be sensitised by triplet sensitiser. 

Write down the products formed after the photolysis of following P, Y Unsaturated carbonyl compounds q... 

As far as we are aware, these observations show for the first time that the well-known Norrish Type 1 reactions of P,y-unsaturated carbonyl compounds can take place by excitation of the alkene moiety rather than the carbonyl group. The reason for this unusual reactivity may be that the Tj (itit ) excited states of 47 and 49 have sufficient energy to promote the homolytic bond fission between the carbonyl and the a-carbon due to the stabihty of the resulting pentadienyl radical 51. As a consequence, the photodecar-bonylation path competes favorably with the ODPM rearrangement. 

New Results on the Triplet Reactivity of p,y-Unsaturated Carbonyl Compounds... 

Pratt, A.C., Photochemistry of P,y-unsaturated carbonyl compounds. 3,3-Dimethyl-5,5-diphenyl-pent-4-en-2-one and 2,2-dimethyl-4,4-diphenylbut-3-enal, J. Chem. Soc., Perkin Trans. 1, 2496-2499, 1973. 

The studies carried out on di-Jt-methane for many years involved the direct or triplet-sensitized irradiation of 1,4-unsaturated systems. The results obtained show that, in general terms, acyclic 1,4-dienes undergo the DPM reaction in the singlet excited state while cyclic and polycyclic dienes are reactive as triplets. > > f " >" The ODPM, 1-ADPM, and 2-ADPM counterparts only occur in the triplet excited state of P,y-unsaturated carbonyl compounds, 8 1-aza-1,4-dienes,and 2-aza-l,4-dienes, > > respectively. An attempt to observe di-7i-methane reactions promoted by SET sensitization was unsuccessful. Thus, Zimmerman and Hoffacker carried out a study on the photoreactivity of 1,4-dienes 21 using 9,10-dicyanoanthrathene (DCA) or 1,4-dicyanonaphthalene (DCN) as electron-acceptor sensitizers. The results obtained show that these compounds undergo intramolecular cychzation to yield dihydronaphthalenes 22 (Scheme 5). The corresponding DPM products are not produced in these photoreactions. 

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