Chiral carboxylic acids, rearrangement with

The cationic pathway allows the conversion of carboxylic acids into ethers, acetals or amides. From a-aminoacids versatile chiral building blocks are accessible. The eliminative decarboxylation of vicinal diacids or P-silyl carboxylic acids, combined with cycloaddition reactions, allows the efficient construction of cyclobutenes or cyclohexadienes. The induction of cationic rearrangements or fragmentations is a potent way to specifically substituted cyclopentanoids and ring extensions by one-or four carbons. In view of these favorable qualities of Kolbe electrolysis, numerous useful applications of this old reaction can be expected in the future. 

Scheffer and coworkers have intensively studied asymmetric induction for di-Tr-methane rearrangements using the ionic chiral auxiliary approach. Salt crystals of dibenzobarrelene carboxylic acid 65 with optically pure amines underwent enantioselective di-ir-methane photorearrangement to afford chiral dibenzosemi-... 

A semipinacol rearrangement of symmetrically substituted six-membered cyclic P-hydroxy-a-diazo esters with chiral carboxylic acids was reported to give chiral cyclo-heptanones (Scheme 59)7" ... 

Several applications of this methodology are known. For the determination of the relative configuration of the stereocenter and the axial chiral unit of 71, the product of a diastereoselective ester enolate Claisen rearrangement of 70, with AgBF4 a cycli-zation to 72 was initiated. Then the carboxylic acid was reduced to alcohol 73 and the position of the substituents was investigated by NMR and by the use of NMR shift-reagents (Scheme 15.16) [32], Control experiments ensured the stereospecifi-city of the cyclization and the reduction step. There are further examples of this strategy [33]. 

Chiral 2,2-disubstituted cyclobutanones have been obtained by asymmetric rearrangement of chiral sulfinyl- 177,178 and sulfanylcyclopropanes.179 Using readily available cyclopropyl 4-tolyl (/ )-sulfoxide (l),180 the requisite sulfinylcyclopropanes 3 and 3 were obtained by a sequence of lithiation, reaction with carboxylic acid esters and stereoselective addition of Grignard reagents to the ketones 2 thus formed.178 The corresponding sulfanylcyclopropanes 4 and 4 resulted from a sequence of protection, reduction and deprotection.179... 

Generally, Claisen rearrangement of ring-bearing substrates with remote asymmetric centers has led to higher stereoselectivities than in the acyclic series. Kurth and cowoikers used amino acid derived chiral auxiliaries such as (302) for the preparation of enantiomerically pure carboxylic acids. The excellent facial selectivity in these systems was hampered, however, by poor chair vs. boat selectivities. 

Corey and Kania reported an enantioselective Claisen rearrangement reaction of macrocychc lactone for the synthesis of (-l-)-dollabellatrienone (56) [13], Reaction of the lactone 53 with chiral (5,5)-diazaborolidine L2BBr 54 and Barton s base (6) resulted in Claisen rearrangement to give carboxylic acid 55 in 86% yield with >98% ee (diastereos-electivity >98 2) (Scheme 7.10). In this reaction, rapid deprotonation by sterically hindered guanidine base is the key to suppress side reactions. 

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