The reactions of bicyclo[2.1.1]hexene (2) and its tricyclic derivative 3 with 2,5-diphenyl-6H-1,3,4-oxadiazin-6-one (1a) gave the 3,4-dihydro-alpha-pyrones 6 and 7, respectively. In contrast, the methyl phenyloxadiazinonecarboxylate 1b and 2 afforded a mixture of the nine-membered, bridged, alpha,beta-unsaturated enol lactone 11 and the beta-lactone 12 in a ratio of about 10:1. Olefin 3 reacted with 1b to furnish small amounts of the bridged derivative 16 of 11 as well as the 2:1 product 14, a saturated delta-lactone. Unlike 16, enol lactone 11 reveals dynamic phenomena, which have been investigated by means of line shape analyses of the temperature dependent C-13-NMR spectra. The interconvertion of the cis- and trans-lactone conformers (11a reversible 11b), being present in a 1:2 ratio, proceeds with DELTA-H(c,t) not-equal = 48.8 kJ mol-1, whereas the racemization of the trans-lactone enantiomers (11b --> 11b') requires less activation (DELTA-H(t,t) not-equal = 42.3 kJ mol-1). The structure of cis-lactone 11a has been established by an X-ray analysis.
|Number of pages||10|
|Publication status||Published - Jun 1991|