1,4-Dimethyl-2,5-dioxabicyclo[2.2.1]heptane-3,6-dione: optical resolution, absolute configuration and circular dichroism

X-ray diffraction study of DL-1,4-dimethyl-2,5-dioxabicyclo[2.2.1]heptane-3,6-dione: the sense of twist and folding

“…The main characteristic feature of the molecular structure of dilactone 1 with C 2 symmetry in a crystal † is the sense of its skeleton twist as synchro-(+,+) (t 0 = 1.8°) and synchro-(-,-) (t 0 = -1.8°) for (R,R)-(Figure 1) and (S,S)-enantiomeric forms, respectively, which is in accordance with the ab initio model (MP2/6-31G*, t 0 = 0.9°) 6 of enantiomer (R,R)-1.…”

“…The analogous twist character for a bicyclo[2.2.2]skeleton was observed for dilactone (±)-3 in a crystal without the retention of C 2 symmetry (t 0 = |4.4| and |4.6|°) 8 and confirmed by DFT (B3LYP/6-31G*) models of homochiral dilactones (R,R)-3 and (R,R)-4 [a single synchro-(+,+,+)-twist form, t 0 = 2.9° (3), 3.1° (4)]. 12 As expected, the planarity of the lactone group [C-C(O)-O-C, t 0 ] and the folding of the dilactone ring [the fold angle between mean planes of lactone groups is f = 109.0° (XRD), † 110.2°( MP2) 6 ] in 1 increased as compared with 3 [f = 124.9° (XRD) 8 ].…”

“…Zelinsky's dilactone 1 has a long history of interest in the synthesis, [1][2][3][4][5] optical resolution, 6 spectroscopy (CD, VCD) 6,7 and theoretical (MP2/6-31G*) 6 studies. However, its structure was not determined by X-ray diffraction (XRD), MW or GED in contrast to its bridged homologue 3 (XRD).…”

“…This interesting effect is displayed in chiroptical properties, namely in the opposite sign of the n-π* Cotton effect (CE) of (R,R)-1 6 and γ-lactones 17 with the same enantiomeric conformation ring (see 7). Previous correlations 17,18 between the sign of the n-π* CE and the γ-lactone ring enantiomeric conformation did not take into account the sense of twisting of the lactone group.…”