Denmark rearrangement

Denmark, S.E. and Miller, PC., Asymmetric [2,3]-Wittig rearrangements with chiral, phosphorus anion-stabilizing groups, Tetrahedron Lett., 37, 6631, 1995. 

Denmark and Harmata studied the reaction of 1,2-allenyl sulfones and 2-prope-nols under the catalysis of 5 mol% of sodium alkoxide affording 2-alloxy-2-propenyl sulfone 217, which can be converted to 2-oxo-5-alkenyl sulfone 218 by treatment with 1.5equiv. of KH in HMPA, a carbanion-accelerated Claisen rearrangement [116, 117]. 

A new perspective was opened up recently when Denmark demonstrated diat with chirally modified phosphoryl-activated allenes an asymmetric induction could be effected. From easily generated allenyl phosphoramidates containing an optically active amino alcohol, the diastereomeric adducts (37) and (39) could be obtained by addition of dlyl alcohol. When the separated adducts were employed in the carb-anionic Claisen rearrangement, a remarkable asymmetric induction (90 10) could be achieved with preferential formation of the diastereomers (38) or (40) respectively, whereas in a thermal reaction no stereoselection was observed (Scheme 63). Another example of an asymmetric induction in Claisen rearrangements is reported by Welch. ... 

In the laboratory of F.G. West, the stereoselective silyl-directed [1,2]-Stevens rearrangement of ammonium ylides was investigated as a potential key step toward the enantioselective synthesis of various hydroxylated quinolizidines. The dimethylphenylsilyl group served as a surrogate for one of the hydroxyl groups in the product. The Fleming-Tamao oxidation was performed under Denmark s conditions to avoid oxidation of the tertiary amine to the corresponding A/-oxide, and the desired quinolizidine did was obtained in 81% yield. 

Denmark, S. E., Harmata, M. A. Carbanlon-accelerated Clalsen rearrangements. 2. Studies on Internal asymmetric Induction. J. Org. 

Based on this observation Denmark and coworkers developed a carbanion -accelerated Claisen rearrangement with X = CHS02Aryl K+ 164). While the C6H5S02CH2-substituted vinyl-allyl-ether 182 does not rearrange to give 185 when kept at 50 °C in HMPA 167) for 3.5 h, rearrangement occurs under the same conditions in the presence of potassium hydride. Undoubtedly, with potassium hydride the... 

Denmark, S.E., and Marlin, J.E., Carbanion-accelerated Claisen rearrangements. Part 7. Phosphine oxide and phosphonate anion stabilizing groups, J. Org. Chem., 56, 1003, 1991. 

It is possible to enhance the rate of a Claisen rearrangement, especially in the enolate Claisen reaction. Denmark et al. showed that other carbanionic centers accelerate the Claisen rearrangement (as in Table 11.23).467 Generation of the anion of sulfone 631 (sec. 8.6.A) with various bases led to acceleration of the reaction relative to the thermal reaction of 631 and also influenced the syn/anti ratio (632/633). In general, a donor group at the allyl position accelerates the rate and the presence of an amino stabilizing group increases the rate even more. 

Denmark has developed the carbanion accelerated Claisen rearrangement [15, 72-78]. It is known that the nature of the transition state of Claisen rearrangement is dependent on the substituents present [79]. The effects of substituents have been studied in detail by Carpenter [80]. At positions Cl, C2, and C4 both donor and acceptor substituents accelerate the reaction with respect to hydrogen (Eq. 3.1.58). Furthermore, the synthetic versatility of the Claisen rearrangement primarily resides in the variety of the substituents X that determine the carbonyl derivative... 

Denmark has also investigated the carbanion accelerated Claisen rearrangement of allyl vinyl ethers with cydic phosphonamides as carbanion-stabihzing groups [78], because phosphonamide allyl anion is well established and has potential for chiral modification. 

Denmark and co-workers have published extensively on the use of (3-silyl substituted divinyl ketones (see 82) in the Nazarov cyclization. Such silyl groups control the collapse of the intermediate cyclopentenylic cations 84, and thus aid the regioselectivity of elimination, as well as the minimization of side reactions (secondary cationic rearrangements). Such stabilization derives from the known P-cation stabilizing effect of silicon, which through stabilization of 84, ensures maximum efficiency of the cyclization, with controlled formation of the final double bond. An important consequence of the final elimination step is that the double bond is placed in the thermodynamically less stable position (see 85). The most common Lewis acid used in the silicon-directed Nazarov cyclization is anhydrous iron(III) chloride, at temperatures below ambient. Alternatively, in cases where the... 

In 1912, Claisen first observed the thermal rearrangement of O-allyl acetoacetate 20 to yield the j, 5-unsaturated ketone 21. The reaction later proved to be general for a broad series of compounds which presented the allyl vinyl ether motif. Carpenter demonstrated that substituents have an important role in the reaction and the presence of anionic 7t-donor as illustrated in (22) greatly accelerate the reaction as documented by Denmark. ... 

Denmark SE, Harmata MA, White KS. Studies on the addition of allylc oxides to sulfonylations. Preparation of highly substituted allyl vinyl ethers for carhanionic Claisen rearrangements. 7. Org. Chem. 1987 52 4031-4042. 

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