Rearrangement enolate derivatives

Besides the activation of the olefinic partner by a metal, the unfavorable thermodynamics associated with the addition of an enolate to a carbon—carbon multiple bond could be overwhelmed by using a strained alkene such as a cyclopropene derivative286. Indeed, Nakamura and workers demonstrated that the butylzinc enolate derived from A-methyl-5-valerolactam (447) smoothly reacted with the cyclopropenone ketal 78 and subsequent deuterolysis led to the -substituted cyclopropanone ketal 448, indicating that the carbometallation involved a syn addition process. Moreover, a high level of diastereoselectivity at the newly formed carbon—carbon bond was observed (de = 97%) (equation 191). The butylzinc enolates derived from other amides, lactams, esters and hydrazones also add successfully to the strained cyclopropenone ketal 78. Moreover, the cyclopropylzincs generated are stable and no rearrangements to the more stable zinc enolates occur after the addition. 

In an interesting transformation, reaction of benzoyl trimethylsilane with lithium enolates derived from various methyl ketones gives rise to 1,2-cyclopropanediols, predominantly with the cis configuration, in good yields (Scheme 77). The reaction, which proceeds through addition, Brook rearrangement and cyclization, is also successful with a,/l-unsaturated acyl silanes vide infra, Section IV.D)187. 

The 2-methyl-4/3,5/3-epoxy-3-ketone (244) reacts with methoxide ion to give the three products (245)—(247).208 A Favorskii-type rearrangement is clearly responsible for producing the A-nor-lactone (245), and is considered also to account for the other two products. The 2-methoxy-2-methyl-4-en-3-one (246), however, could possibly result from direct attack of methoxide ion on the A2-enol derivative of the epoxy-ketone (cine substitution), followed by elimination of a 5/3-hydroxy-group to give the a/3-unsaturated ketone. The occurrence of Favorskii rearrangement in the 2-methylated compound (244) contrasts with a simple nucleophilic opening of the unsubstituted 4/3,5/3-epoxy-3-ketone this is one of several recent examples in which an a-substituent favours Favorskii rather than alternative reactions.208... 

Alternatively, the reaction may proceed through formation of a lead enolate derived from the enol followed by intramolecular rearrangement to the desired product (Scheme 2). It appears that the first and last mechanisms are operative, the product distribution reflecting a balance between the two dqiendent on temperature, solvent and substrate. In all three cases however the rate-determining step is the formation of the enol, a factor which heavily influences the choice of reaction conditions. 

Lactams, by Beckmann rearrangement, 343 Lactols, 402 Lactones, 346-349, 397 Lanosterol, 401, 408 Lead tetraacetate, acetoxylation of enol derivatives, 184 —, acetoxylation of ketones, 176-177... 

Asymmetric Ireland-Claisen Rearrangements. Chiral enolates derived from the boron complex (5) and allyl esters rearrange with excellent selectivity upon warming to —20 °C for a period of 1-2 weeks (eqs 9 and 10). As discussed above, the geometry of the intermediate enolate can be controlled by appropriate choice of base and solvent, thus allowing access to either syn or anti configuration in the product The reaction can be completed in 2-4 days with little erosion in selectivity when run at 4 °C. 

The ability of charged substituents to accelerate the 3,3-sigmatropic reiarrangement of allyl vinyl ethers (the Claisen rearrangement) has also been documented. The effect of oxyanion substituents on the rate and course of aliphatic Claisen rearrangements has been the subject of particular attention. " In 1972, Ireland and Mueller reported that the lithium enolate derivatives of allyl esters undergo rapid and effi-... 

The reaction in the shikimic acid pathway is, of course, the [3,3]-sigmatropic shift in which chorismic acid rearranges to prephenic acid on the way to aromatic rings (p. 1403). The simpler reaction given here is one of the family of reactions from Chapter 36 (pp. 944-6) using an allylic alcohol and an enol derivative of a carbonyl compound. In this case we have the enol ether of a ketone. We must combine these to make an allyl vinyl ether for rearrangement. 

A remarkably facile tandem 1,4-conjugate addition Claisen rearrangement is observed by utilizing the copper enolate derived by addition of lithium dimethylcop-per to 2-(allyloxy)-2-cyclohexenone. The resultant hydroxy ketone is obtained as a single stereoisomer in nearly quantitative yield. 

Leost, R, Chantegrel, B., and Deshayes, C., Tandem Wolff rearrangement- fert-amino effect sequence. Synthesis of 2-oxoindolinium enolate derivatives. Tetrahedron, 53, 7557, 1997. 

The enolates (48) and (49) of the transition metals tungsten, rhenium and molybdenum can be successfully prepared by the nucleophilic displacement of a-chloro ketones and a-chloro esters with the appropriate transition metal anion (Scheme 6). They are isolated as C-bound enolate derivatives and, except for the rhenium enolate (49), do not undergo thermal aldol additions to benzaldehyde. However, Bergman and Heathcock et al. have found that an aldol reaction of complex (48) with benzaldehyde can occur on irradiation via the rearranged q -oxaallyl derivative (50), where the metal aldolate (51) can then be... 

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