Rearrangements, Claisen anion accelerated

The reaction of 2-allyloxyindolin-3-ones with cyanomethylphosphonate has been reported to occur with consecutive olefination, isomerization, deacylation, and anion-accelerated Claisen rearrangement to give the 3,3-disubstituted oxindoles (Scheme 7). ... 

Anionic Claisen rearrangement. Acceleration of the Claisen rearrangement of allyl vinyl ethers was originally observed with potassium hydride in HMPT, but even milder conditions are possible using sodium or lithium dimsylate. The catalyzed rearrangement is as stereoselective as the thermal counterpart. Thus vicinal asymmetric centers are formed selectively on rearrangement of (E)- and (Z)-crotyl ethers (equations I and II). 

A high degree of asymmetric induction has been realized in the carbanion-accelerated Claisen rearrangement of phosphorus-stabilized anions. Treatment of 1,3,2-oxazaphosphorinane (166) with freshly prepared lithium dimsylate led to a 95 5 ratio of a-methyl ketones (167) and (168) (Scheme 33). Li coordination combined with steric interactions provide the necessary control elements for stabilization of the highly organized allyl anion conformation (169). 

The adducts of the reaction of allyl alcohols with allenic sulfones are especially interesting as they allow the carbanion-accelerated Claisen rearrangement to take place. By deprotonation, or directly by addition of an allyl alcoholate anion, a system is generated, which rearranges much more easily than the uncharged system and with high diastereoselectivity (Scheme 62). 

Two other examples of carbanion -accelerated rearrangements have been described in the literature. The first one leads from the anion of the 60-allylic guanine 211 in two consecutive [3.3] sigmatropic shifts (a combined Claisen-Cope rearrangement) via 212 and 213 to the 8-allylic guanine 214 173). 

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. 

Claisen rearrangements introduce the complication of oxygen lone pairs within the rearranging system, rather than as substituents on the perimeter. They may be ignored and the transition structure treated as benzene-like, or they may interrupt the conjugation, and the transition structure is then like a heptatrienyl anion.915 Predictions based on the simple theory above, whichever of these models is taken, match most of the substituent effects, and more elaborate treatments with calculations account for the anomalous accelerating effect of a donor substituent at C-6.916... 

Rate enhancements of Claisen rearrangements can be achieved by the formation of anions a to C-2 (which can formally be considered as strong n-donors in position 2). Ester enolatcs of lithium87 88 91. sodium92-95, magnesium96-98 and zinc" have been widely used to effect chargc-accelerated rearrangements. 

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. 

Table 11.23. Acceleration of the Claisen Rearrangement by Sulfoxide Stabilized Anions...

Other groups have also exploited these simple motifs for the recognition of carboxylates (Fig. 14). Wilcox used ureas such as 18 to complex various anions including carboxylates [55]. Similar receptors have been utilized by Curran to alter the stereochemistry of the alkylation of a-sulfinyl radicals [56] as well as accelerate a Claisen rearrangement [57]. Moran also created a urea-based... 

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. 

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. ... 

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