The [1,4]oxazino[4,3-a]azepine ring system: Construction from morpholinoallenes and a structural study

Morpholinoallenes 4 react with dimethyl and di(tert-butyl) acetylenedicarbo xylate to form 4-alkylidene-3-morpholino-1-cyclobutenes 7 which undergo the rmal rearrangement to [1,4]oxazino[4,3-a]azepine derivatives 8 and 9. The s tructures of 7a, 8aA, and 8c have been determined by single-crystal X-ray d iffraction. A cis-fusion of the chair-like morpholine ring and the dihydroa zepine ring is found in both 8aA and 8c, but the angular hydrogen atom is e quatorial in 8aA and axial in sc. NMR studies show that the respective chai r conformation of the morpholine ring which is found in the solid state is also favored in solution, but temperature-variable spectra indicate that ri ng inversion is taking place. (C) 1999 Elsevier Science Ltd. All rights res erved.