Applications of Crown Ethers in Organic Synthesis

Applications of Crown Ethers in Organic Synthesis. - The first selective CH-insertion of aryl-halocarbenes has been reported. The free carbene, generated in situ from PhCHCl2, Bu O K, and 18-crown-6, reacts with (76) to give (77). Aryl diazo-cyanides are readily formed from the diazonium tetra-fluoroborates with KCN and 18-crown-6. The cyanides undergo [2 + 4] 

Anions from the Schiffs base (78) can be C- or A -alkylated with ethyl iodide or diethyl sulphate. The ratio of the products depends both on the solvent and on the presence of 18-crown-6. In non-polar solvents, the crown ether increases the solubility of the base, and C-alkylation is the major pathway. In dipolar aprotic solvents, the 18-crown-6 breaks up ion pairs by solvation of the Na cation, and favours A -alkylation. A nerylsulphonamide, formed from (79), undergoes regiospecific reductive desulphonylation to give nerol (80), which implies that (79) is an effective synthon for cisoid iso-prenoids. Chiral complexes of crown ethers, e.g. (81), catalyse the Michael addition reaction of j3-keto-esters and methyl vinyl ketone to give adducts in high optical yields.  

Ohsawa, T. Takagaki, A. Haneda, and T. Oishi, Tetrahedron Lett., 1981, 22, 2583. 

The formation of macrolides from co-alkoxy-thioester derivatives of crown ethers (82) and Bu O has been studied. The role of the crown ether is to provide stabilization of the transition state for the attack of the alkoxide ion on the carbonyl group of the thioester by placing the carbonyl oxygen adjacent to the complexed ion.  

Immobilized derivatives of 18-crown-6 are efficient catalysts for the preparation of cyanoalkanes from the bromo-derivatives.  

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