Novel tocopherol derivatives. Part 32: On the bromination of pyrano32-fchromenes related to γ-tocopherol

摘要

While bromination of γ-tocopherol (>2) with elemental bromine affords 5-bromo-γ-tocopherol quantitatively (>3), the analogous reaction of its truncated model compound, 2,2,7,8-tetramethylchromanol (>2a) is known to be accompanied by side reactions and to produce hitherto unknown byproducts. These compounds originate from pyrano[3,2-f]chromene (>6), a byproduct in the synthesis of model compound >2a, which affords bromochromene >7 as the major product. The reaction mechanism was shown to proceed via chromene >8 and its 1,2-dibromo addition compound >9, which eliminates HBr in an E1 process to finally afford >7. Analytical data including crystal structures of both >6 and >7 are reported.