Cylindrospermopsins (CYNs) are guanidine alkaloid hepatotoxins, produced by a number of cyanobacterial genera (see later). Currently, five analogues of CYN are known, namely CYN, 7‐epi‐CYN, and 7‐deoxy‐CYN (Fig. 12.1) and the two recently characterized congeners, 7‐deoxydesulfo‐CYN and 7‐deoxydesulfo‐12‐acetyl‐CYN . The structures of CYN and 7‐epi‐CYN have repeatedly gone through name changes, with each at some point being called CYN. At the present, 7‐(S)‐CYN is considered to be the principal toxin produced by Cylindrospermopsis raciborskii. Based on reevaluation of absolute configuration at C‐7 of the isolated CYN, revised synthesis [3, 4], and asymmetric synthesis , the absolute configuration of all of the asymmetric centres of CYN, 7‐epi‐CYN, and 7‐deoxy‐CYN were established.