Carbanion-accelerated rearrangements

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). 

The results obtained in the carbanion-accelerated rearrangement of phosphane oxides and phosphonates are similar to those of the arylsulfonates518. Substrate 15 gives the sjn-product 21 as the major diastereomer in the thermal as well as in the anionic process, and the anti-product 22 is favored in the case of the Z-configurated substrate 16. The rearrangement of 16 is slower because of eclipsed interactions in the transition state 19. 

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]. 

The Claisen rearrangement also benefits from a geminal donor to the framework oxygen atom [229]. This forms the basis of many synthetic applications. A newer version of the Claisen rearrangement is the carbanion-accelerated process [230]. It is noted that in all these cases the facilitation is always achieved by alternating polarity accentuation. 

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 arylsulfonyl carbanion accelerated Claisen rearrangement is completely regioselective and has also been found to be highly diastereoselective (Scheme 2). The stereochemical course of the reaction conforms to the familiar chair-like transition state model usually invoked for the classic thermal process. Recently, high degrees of asymmetric induction have been observed in tlie rearrangements of chiral cyclic phosphoramidate stabilized carbanion derivatives. ... 

Blechert has developed an interesting synthesis of 2-substituted indoles which involves the conjugate addition of V-phenylhydroxylamine salts (or V-phenylnitrones) to electron-deficient allenes, followed by carbanion-accelerated hetero-Cc rearrangement of the Michael adduct. For exanple, addition of the hydroxylamine salt (46) to the allenyl sulfone (47) produces the anion (48), which undergoes rapid 3.3-sigmatropic rearrangement to afford the -keto sulfone (49). Cyclization to the indole proceeds smoothly upon exposure to formic acid (Scheme 3). 

Finally, no examples of carbanion-accelerated 1,3-sigmatropic rearrangements appear to have been observed to date, although Okamura and coworkers have reported a transformation which may involve a sulfinyl carbanion accelerated 1,5-hydrogen shift. ... 

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). 

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... 

The carbanion -accelerated Claisen rearrangement is also a viable reaction for the creation of vicinal quaternary centers, as shown, e.g., by the rearrangement of 193 to 194 164 c). A similar carbanion -accelerated Claisen rearrangement of 197 could... 

A carbanion -accelerated hetero-Cope rearrangement was reported by Blechert 170). Reaction of the sodium salt of the hydroxamic acid (202) with the allene sulfone 203 led at 0 °C within 10 min to 206. Undoubtedly, the carbanions 204 and 205 are the important intermediates in this transformation. A carbanion -accelerated... 

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. 

Carbanion-accelerated Vinylcyclopropane Rearrangements 8.3.2.4 Carbocation-accelerated Vinylcyclopropane Rearrangements... 

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