18. Using a Fischer representation, draw a representative meso compound. 19. Which of the following is optically active? 20. Cephalotaxine, shown below, is a lehmizidine alkaloid that has garnered interest from the organic synthesis community, including from your very own Beng. Identify the different chirality centers using asterisks and designate the chiral centers as either R or S. Cephalotaxine (1R,3aR,14bS)2-methoxy-1,5,6,8,9,14b-hexahydro-4H-[1,3]dioxolo[4,5':4,5]benzo[1,2- d ]cyclopenta[b]pyrrolo[ [1,2a] urepin-1-ol