There are many theories regarding the origin of the minus sign. According to one theory, it originates from a handwriting form of the letter m (for minus) that came to resemble a horizontal bar,[1] Another theory traces it back to a hieratic sign.
Decomposition of 4 in the presence of cis-piperylene at room temperature results in the stereocontrolled formation of trans-cycloheptadiene 27 in 90% ee (96% ee at -78 °C).