Cyclic esters and cyclodepsipeptides derived from lactide and 2,5-morpholinediones.

The reaction between Bu(n)Li in benzene and the solid polystyrene support PS-C6H4CH2NH2 leads to a lithiated species that can be represented as PS-C6H4CH2NHLi(LiBu)x, where x approximately 4, which is active in the ring-opening of the cyclic esters L-lactide, rac-lactide, and 2,5-morpholinediones. With approximately 10 eq of these monomeric six-membered rings and with heating, cyclic esters (MeCHC(O)O)n and [MeCHC(O)OCHRC(NH)O]n are reversibly released to the solution. These have been characterized by electrospray ionization MS, and some small rings have been separated by gel-permeation chromatography. Addition of NaBPh4 to a heated benzene solution containing these rings preferentially removes the 18-membered rings from solution. For lactide this is shown to form the basis for chemical amplification from a dynamic combinatorial library and lactide can be converted to (MeCHC(O)O)6 in >80% yield. Metallated supports derived from Me2Mg and Et2Zn are less reactive but do show some ability for lactide ring-enlarging. The 18-membered ring (R,R,R,S,S,S)- and meso-(R,S,R,S,R,S)-(MeCHC(O)O)6 and the 24-membered ring (MeCHC(O)OCHPr(i)C(NH)O)4 have been characterized by single-crystal x-ray diffraction studies, together with the complex Na[eta3-S,S,S,S,S,S-(MeCHC(O)O)6]2BPh4.