Unsaturated ketones rearrangement

Most p.-y-unsaturated ketones rearrange upon triplet sensitization to cyclopropyl ketones via the so-called oxa-di-TT-methane (ODPM) rearrangement, the result of which is an overall 1,2-acyl shift as schematically shown for (1) -+ (2). p,y-Enone arrangements in which the alkene moiety is further conjugated to either phenyl or C—O sites also undergo this transformation upon direct excitation, i.e. via the singlet excited state. 

Cyclic a,) -unsaturated ketones present a rich array of photochemical reactions, some of which are of considerable synthetic value (see Section 6.4 of Part B). For cyclohex-enones, two prominent reactions are the di-rr-methane rearrangement (path A) and the lumiketone rearrangement (path B). 

Dibromo-3-ketones may also be used as substrates for the preparation of A -3-ketones by Joly s method. " Hexamethylphosphoramide has been recommended as a medium suitable for the dehydrobromination of a-bromo ketones to give a, -unsaturated ketones in high yield without rearrange-... 

A-ring unsaturated ketones and the sensitive 11 j5-hydroxyl group will survive conditions of the Mattox rearrangement. 

A related reaction is the oxa-di-n-methane rearrangement, where one of the C=C double bonds is replaced by a C=0 double bond. The substrates are thus /3,y-unsaturated ketones. The rearrangement proceeds from the triplet state. This oxa-variant gives access to highly strained molecules containing small rings, as has been demonstrated by irradiation of norborn-5-ene-2-one 10 ... 

Recently, a study of this rearrangement has been repeated and extended in order to determine the influence of a- and y-substitution on the position of the 68-69 equilibrium in the presence of silica, and the utility of this reaction for a novel and convenient synthesis of highly substituted a, / -unsaturated ketones, by subsequent treatment with CuCl2 in methanol-water136. An ion-pair mechanism can also be suggested for the facile rearrangement of sulfone 70 to 71, a key intermediate in the Hoffmann-La Roche Sulfone Route to Vitamin A137. 

In a, 3-unsaturated ketones, nitriles, and esters (e.g., 125), the y hydrogen assumes the acidity normally held by the position a to the carbonyl group, especially when R is not hydrogen and so cannot compete. This principle, called vinylology, operates because the resonance effect is transmitted through the double bond. However, because of the resonance, alkylation at the a position (with allylic rearrangement) competes with alkylation at the y position and usually predominates. 

It is possible to treat ketones with allyl alcohol and an acid catalyst to give y,5-unsaturated ketones directly, presumably by initial formation of the vinylic ethers, and then Claisen rearrangement.In an analogous procedure, the enolates (126) of allylic esters [formed by treatment of the esters with lithium isopropylcyclohex-... 

Cyclic 0,y-unsaturated ketones undergo a photochemical rearrangement thought to involve a-cleavage and radical rearrangement. The products of these cyclic ketones are j8,y-unsaturated ketones possessing exocyclic olefinic linkages,<41,4a)... 

S,y-Unsaturated ketones undergo a rearrangement that is formally like the di-77-methane photorearrangement. An example of this rearrangement is provided by the photolysis of l,2,4,4-tetraphenyl-3-butenone ... 

This rearrangement of j8,y-unsaturated ketones was utilized to achieve a photochemical synthesis of homocubanone, 475... 

Dihydro-2H-pyran-2-ones (e. g., 4-195) are valuable intermediates in the synthesis of several natural products [67]. Hattori, Miyano and coworkers [68] have recently shown that these compounds can be easily obtained in high yield by a Pd2+-catalyzed [2+2] cycloaddition of ct, 3-unsaturated aldehydes 4-192 with ketene 4-193, followed by an allylic rearrangement of the intermediate 4-194 (Scheme 4.42). In this reaction the Pd2+-compound acts as a mild Lewis acid. a,(3-unsaturated ketones can also be used, but the yields are below 20%. 

Oxidative rearrangement of the tertiary-cyclopropylcarbinol 262 using PCC leads to the / ,y-unsaturated ketone 263 and halide 264 [149], (Scheme 104)... 

For allyl ethers 790 with R1 =Ph, treatment with LDA generates anions 791 which undergo [2,3]-Wittig rearrangement to more stable alkoxides 792 (Scheme 126). Spontaneous expulsion of benzotriazole anion from 792 generates (3,y-unsaturated ketones 793 that are isolated in high yields (86-92%) <1996JOC4035>. In the case of... 

Direct irradiation of (3,y-unsaturated ketones affords rearranged 3,y-unsaturated ketones via a 1,3 shift of the acyl group 214a> b). This is exemplified in the preparation of a bicyclo[3,3,l]nonanone (2.10)215),... 

Enones undergo a variety of photorearrangements 333). In contrast to a,P-unsaturated ketones which react in many different ways, p,y-unsaturated ketones undergo the oxadi-7c-methane rearrangement in sensitized irradiations in complete analogy the products formed are cyclopropyl ketones... 

Reaction of saturated acylzirconocene chlorides with (CH3)2Cu(CN)Li2 gives the secondary alcohol (73%), and D20 work-up of the reaction mixture gives the a-deuterio alcohol. This observation suggests the formation of a ketone—zirconocene complex (Scheme 5.40 see also Section 5.3.2.1). Thus, for the reaction of a,p-unsaturated acylzirconocene chlorides with R2Cu(CN)Li2, initial formation of an unsaturated ketone—zirconocene complex followed by 1,3-rearrangement of the zirconocene moiety to an oxazirconacyclopentene, which is a ketone carbanion equivalent, has been proposed (Scheme 5.41). 

The alkoxide generated by KH in THF was believed to be effective in accelerating the vinylcyclobutane ring expansion in the synthesis of 6-membered ring compounds 142). As an example, the cyclobutanol (416) reacted with KH and rearranged to (417), which upon subsequent oxidation, provided (418) 142) in 64.5% yield. The a,P-unsaturated ketone (418) was converted to (—)- 3-selinene (419)142). Similarly, furancyclohexanol (421) could be obtained from the cyclobutanol (420) 142). 

Stereocontrolled oxy-Cope rearrangement. 1,2-Addition of a chiral vinyl-lithium reagent to a chiral (3,-y-unsaturated ketone could give rise to at least eight... 

By changing the groups the process has increased the versatility of Claisen rearrangements and has made it a powerful synthetic tool. Thus long chain unsaturated ketones can be prepared in good yields by the following reaction. 

The extension of the di-jr-methane rearrangement from 1,4-dienes and /3,/-unsaturated ketones to the use of 1-aza-1,4-dienes has been achieved129 (equation 73). 

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