18-Membered cyclic esters derived from glycolide and lactide: preparations, structures and coordination to sodium ions

Abstract

From reactions between glycolide or lactide (4 equiv.) with 4-dimethylaminopyridine, DMAP (1 equiv.) and NaBPh₄ (1 equiv.) in benzene at 70 °C the cyclic ester adducts (CH₂C(O)O)₆NaBPh₄ and (CHMeC(O)O)₆NaBPh₄ are formed respectively. The structures of the salts Na[(S,R,S,R,S,R)-(CH₃CHC(O)O)₆]₂BPh₄·CH₃CN and (CH₂C(O)O)₆NaBPh₄·(CH₃CN)₂ are reported. The cyclic esters were separated by chromatography and the structures of (CH₂C(O)O)₆, (S,R,R,R,R,R)-(CHMeC(O)O)₆ and (S,S,R,R,R,R)-(CHMeC(O)O)₆ were determined. The ¹H and ¹³C NMR data are reported for one of each of the six enantiomers of (CHMeC(O)O)₆ and the two meso isomers. The mechanism for the formation of these 18-membered rings is discussed in terms of an initial reaction between DMAP and NaBPh₄ in hot benzene that produces NaPh and DMAP : BPh₃ in the presence of the monomer lactide. The cyclic esters (CHMeC(O)O)₆ can also be obtained from the reaction between polylactide, PLA, in the presence of DMAP and NaBPh₄. The cyclic esters 3-methyl-1,4-dioxane-2,5-dione and 3,6,6-trimethyl-1,4-dioxane-2,5-dione undergo similar ring enlarging reactions to give cyclic 18-membered ring esters as determined by ESI-MS.