Transformation of Carboxylic Acid Derivatives

Microwave irradiation of a mixture of an acid anhydride, an amine adsorbed on silica gel, and TaCl5/Si02 is a solvent-free method for the synthesis of A-alkyl and A-aryl-imides [47]. Ni(II) promotes the conversion of an acrylamide to ethyl acrylate via a Diels-Alder adduct with (2-pyridyl)anthracene [48], Aromatic carboxylic acids [49] and mandelic acid [50] are efficiently esterified with Fc2(S04)3 XH2O as catalyst. Co(II) perchlorate in MeOH catalyzes the methanolysis of acetyl imidazole and acetyl pyrazole [51]. Hiyama et al. used FeCb as a catalyst for the acylation of a silylated cyanohydrin. The resulting ester was then cyclized to 4-amino-2(5H)-furanones (Sch. 5) [52]. 

One means of stereoselective cleavage of biaryl lactones [53] is activation of the carbonyl group with a Lewis acid and subsequent attack with a chiral nucleophile. Conversely, activation can be effected with a chiral Lewis acid followed by attack of an achiral nucleophile. Complexation of a biaryl lactone to the chiral fragment [CpRe (NO)(PPh3)j then reduction with K(s-Bu)3BH (K-selectride) and ring opening of the intermediate rhenium lactolate gives the metalated aldehyde (dr = 75 25) which is converted to the alcohol without essential loss of optical purity (Sch. 6) [54]. 

Better results are obtained with ruthenium complexes of the corresponding thiono-lactones. Reductive ring cleavage with LiAlH4, then methylation and decomplexation, leads to the thioether (76 % ee). The chiral ruthenium complex recovered in this step can be converted back into the starting complex in two steps (Sch. 7) [55]. 

Achiral ruthenium fragments can also be used with a chiral reductant. The reduction of a [CpRu(DPPE)(thionolactone)] complex with BINAL-H and decomplexa-tion as above gave the thioether in 84 % ee [53], 

1 Formation of Enantiomer-Enriched Epoxides via Lewis Acid-Promoted Asymmetric Cyclization 

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The reactions of this type play an important role in many organic and bioorganic processes since all enzymatic transformations of carboxylic acid derivatives belong to this category [84, 85]. 

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