Two general 5-endo-trigreversals of 7-oxabicyclo[2.2.1]heptanes and heptenes are described and discussed. The reverse-Michael reaction occurs with the aldehyde, ketone, ester, and nitrile derivatives while the reverse aldol reaction catalysed by acid is confined to the aldehydes and ketones. These properties of the title compounds are rationalized in terms of the geometric alignments of the bonding and antibonding orbitals of the bridging oxygen atom and its neighboring carbons. New isobenzofuran and cyclohexadiene syntheses form a part of the report.