Rearrangements unsaturated carbonyl compounds

Steric factors are also important in hydrodimerizations carried out in acidic media. Excessive steric hindrance about the 0-carbon in an a, 0-unsaturated carbonyl compound can retard tail-to-tail coupling, e.g., 2 130 - 131, and lead to products of head-to-head (and occasionally head-to-tail) coupling. Thus in the reduction of mesityl oxide at pH lg.4 there is also formed a small amount of ketone 140, apparently formed via head-to-head coupling of 130 and subsequent pinacol rearrangement of 139 134) ... 

A simple two-step protocol for the generation of a terminal diene is to add allyl magnesium bromide to an aldehyde or a ketone and subsequent acid or base catalysed dehydration (equation 34)72. Cheng and coworkers used this sequence for the synthesis of some indole natural products (equation 35)72a. Regiospecific dienones can be prepared by 1,2-addition of vinyllithium to a,/l-unsaturated carbonyl compounds and oxidative rearrangement of the resulting dienols with pyridinium dichromate (equation 36)73. 

The product is a y,8-unsaturated carbonyl compound (a 1,5-diene), hinting that the last step is a Claisen rearrangement. 

In the Lewis acid catalysed reactions of a,/J-unsaturated carbonyl compounds with dienes, sometimes the products of a [2 + 4]-cycloaddition, where the carbonyl compounds function as heterodienes, were isolated. It was proposed that the intermediate of the [2 + 4]-cycloaddition is formed first in this case, followed by a Cope rearrangement which leads to the normal Diels-Alder product (Scheme 7). 

With respect to the above-mentioned unsaturated carbonyl compounds with a double bond and a carbonyl group separated by three carbon atoms (14), it can be stated here that they may be disconnected to an alkyl vinyl ketone and an allylic anion (Scheme 7.5), through an oxy-Cope rearrangement (C/. Scheme 5.22). 

Hydroxymethylenecyclopropanols (340) have been shown" to be intermediates in the photochemical rearrangement of a, -unsaturated carbonyl compounds (339) to 1,4-dicarbonyl compounds (341). The products are eventually obtained by double tautomerization of the enol and cyclopropanol portions of (340). 

As far as we are aware, these observations are the first that show that the well-known Norrish Type I reactions of p,7-unsaturated carbonyl compounds can take place by excitation of the alkene moiety rather than the carbonyl group. This unusual reactivity may be due to the fact that the TiC-ir, -ir ) excited states of 53 and 55 possess sufficient energy to promote the homolytic allylic bond fission to form the stabilized pentadienyl radical 57. As a result, photodecarbony-lation competes favorably with the ODPM rearrangement. 

Table 12. Addition of l-(Methoxymethoxy)cyclopropyllithium and 1-Ethoxycyclopropyllithium to Saturated and Unsaturated Carbonyl Compounds. Rearrangement to Cyclobutanones and Vinylcyclobu-tanones...

Oxidation of the allylic carbon of alkenes may lead to allylic alcohols and derivatives or a, 3-unsaturated carbonyl compounds. Selenium dioxide is the reagent of choice to carry out the former transformation. In the latter process, which is more difficult to accomplish, Cr(VI) compounds are usually applied. In certain cases, mixture of products of both types of oxidation, as well as isomeric compounds resulting from allylic rearrangement, may be formed. Oxidation of 2-alkenes to the corresponding cc,p-unsaturated carboxylic acids, particularly the oxidation of propylene to acrolein and acrylic acid, as well as ammoxidation to acrylonitrile, has commercial importance (see Sections 9.5.2 and 9.5.3). 

Furan-2-methanols are cleaved to derivatives of levulinic ester by methanolic hydrogen chloride a mechanism involving the carbonium ion (375) has been proposed. Under similar conditions, a,(3- unsaturated carbonyl compounds of type (384) undergo a similar rearrangement, a reaction known as the Marckwald rearrangement, and afford keto esters of type (385), as shown in Scheme 105 in an example drawn from a synthesis of equilenin (70AJC547). 

A wide range of alkenes may be used as substrates. The reaction is most commonly performed with alkenes of normal electronic nature, but electron deficient alkenes, such as a, 3-unsaturated carbonyl compounds, and very electron rich alkenes, such as enol ethers and enamines, have also been used successfully. Not surprisingly, the cyclopropylcarbinyl ethers and amines that are formed in the latter reactions (see Table S) are subject to facile rearrangements. 

The aliphatic Claisen Rearrangement is a [3,3]-sigmatropic rearrangement in which an allyl vinyl ether is converted thermally to an unsaturated carbonyl compound. 

Gallium(III) triflate catalysed the rearrangement of 2-substituted vinyl epoxides into /3,y-unsaturated carbonyl compounds with high regio- and chemo-selectivity with low catalyst loading. Alkyl-substituted trimethylsilylvinyl epoxides gave /i,y-unsaluralcd ketones, but aryl-substituted vinyl epoxides gave the aldehydes.80... 

The aliphatic Claisen rearrangement is simpler in that there is no rearomatisation at the end but there is an ionic step first as the vinyl ether 67 has to be made and the easiest way to do that is from the allylic alcohol 65 by acetal exchange with another vinyl ether to give 66 and elimination to give 67. All these steps, including the rearrangement occur under the same conditions16 and the product is a y,S-unsaturated carbonyl compound 68. 

The reaction of ketene with a,(1-unsaturated carbonyl compounds in the presence of a cationic palladium(ll) complex leads to the formation of 4-vinyloxetan-2-one intermediates 863, which rearrange under the reaction conditions to give 3,6-dihydropyran-2-ones 864. ot,(3-Unsaturated aldehydes provide higher yields of the desired 3,6-dihydropyran-2-ones than their corresponding ketones (Scheme 239, Table 37) <2000CC73, 2002T5215>. 

The next three chapters by Fleming, Armesto, and Rao deal with different aspects of alkene photochemistry alkene [2+2]-photocycloaddi-tions to other alkenes, di-7i-methane (DPM) rearrangements of 1,4-dienes and oxa-di-7i-methane (ODPM) rearrangements of 3,y-unsaturated carbonyl compounds. Photocycloaddition of an ether-tethered 1,6-diene by Cu(I)-catalysis leads to the exo-selective formation of the bicyclic tetrahydrofuran derivative 4 [4]. By direct electronic excitation of a... 

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. 

The 4-vinyloxetan-2-ones generated in a Pd-catalysed [2+2] cycloaddition between ketene and a,P-unsaturated carbonyl compounds undergo a spontaneous allylic rearrangment. A zwitterionic intermediate is proposed that cyclises to a 3,6-dihydro-2//-pyran-2-one, but which may alternatively decarboxylate to a diene <02T5215>. 

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