According to the isoprene rule proposed by Ruzicka, terpenes can be considered to arise from head-to-tail joining of simple five-carbon Isoprene unIts. CH3 CH2-C-CH-CH Head Isoprene Tail 2-Methyl-1, 3-butadiene Tail Head rene Head Pinene◼ According to the isoprene rule proposed by Ruzicka, terpenes can be considered to arise from head-to-tail joining of simple five-carbon isoprene units. CH2 C CH CH3 CH2 1 2 3 4 Isoprene 2-Methyl-1,3-butadiene Head Tail Tail Head Myrcene Tail Head -Pinene