新键的生成在反应体系的同面叫同面环加成 (suprafacial cycloaddition H H HC CH CH CH H 丌S+ HC CH H H HC CH H3C、 CH IT2sT2sI CH3 CH C=C H H 般环加成为同面环加成新键的生成在反应体系的同面叫同面环加成 （suprafacial cycloaddition) C C H CH3 H H3C C C H CH3 H H3C hν π s 2 2 π s CH3 CH3 CH CH3 3 C C H3C H CH3 H C C H CH3 H H3C CH3 CH3 CH3 CH3 π s 2 2 π s hν 一般环加成为同面环加成