Page 180 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry
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               336                                                                                Heterocyclic Chemistry


                 The oxidative condensation of certain pyrroles can lead  ligand. In this biologically active and important molecule,
               directly to the porphyrin nucleus, as in the case of oc-  it is unsurprising to find a further type of heterocycle: a
               taethylporphyrin.                                 ribose ring, one of the sugars based on a hydrogenated
                                                                 furan ring.
                                                                   Other examples of naturally occurring furans are vita-
                                                                 min C (ascorbic acid), first isolated from orange juice in
                          COOEt                NH  N
                               1. KOH                            1928, and furfural, the artificial oil of ants, from which the
                        NH     2. AcOH, ∆, air                   solvent tetrahydrofuran is made.
                                               N  HN
                                                                            HO
                        NEt 2
                                                                                 O               O
                                                                        HO           O               CHO
                                             octaethylporphyrin               HO   OH
                                                                              ascorbic acid     furfural
                 Heme, the coloring portion of hemoglobin, and the
               chlorophylls, the green pigments of plants, are both
                                                                   Manyimportantindolesarefoundinnature:tryptamine,
               porphyrins.
                                                                 the parent of the essential amino acid tryptophan; the
                                                                 powerful vasoconstrictor serotonin; and the powerful hal-
                                                                 lucinogen lysergic acid diethylamide (LSD) are a few
                       N   N                     N   N           examples.
                         Fe                       Mg
                       N   N                     N   N                             NH 2  HO          NH 2

                                                                              N                  N
                                                                              H                  H
                                  COOH                  O
                HOOC                  C 20 H 39 OOC  MeOOC                tryptamine         serotonin
                        heme                       chlorophyll a
                                                                                   O
                          H 2 NOC
                                           CONH 2                              Et 2 N
                                                                                        NMe
                                                                                          H
                                               CONH 2
                         H 2 NOC
                                   N CN  N
                                    Co +
                               H                                                          N
                                   N   N                                                  H
                         H 2 NOC                                                 lysergic acid diethylamide
                                                                   Indoles have been used since ancient times; woad, the
                                             CONH 2
                            O
                               NH                                coloring matter worn by Boudicca’s warriors, was an im-
                                                                 pure form of indigotin (indigo) extracted from the plant
                                                                 Indigofera tinctoria. Tyrian purple, the highly prized dye
                              _  O       N
                              O  P                               worn on the garments of emperors, is 6,6 -dibromoindigo,
                                  O    OH
                               O          N                      and was extracted from the mollusc murex.
                                H
                                                                                      O
                                    O                                                     H
                                                                                          N       R
                              HO
                                  cyanocobalamin
                                                                              R      N H
                 Vitamin B 12 (cyanocobalamin) is used to combat per-                    O
                                                                                R = H   indigo
               nicious anemia; it was synthesized by Woodward and Es-           R = Br Tyrian purple
               chenmoser in brilliant and unparalleled sequences of over
               30 reactions. The skeleton of vitamin B 12 is the corrin  Thiophene itself is found in coal tar. Biotin, the human
               macrocycle, differing from a porphyrin ring by having one  growth factor vitamin H, contains a tetrahydrothiophene
               less carbon atom in the linking of the four pyrrole rings.  ring alpha-Lipoic acid, when enzyme-bound, is converted
               Here, the nitrogen atoms of the pyrrole rings bind (act as  into acetyl lipoate, which acetylates coenzyme A giving
               ligands to) a central carbon atom. Moreover, a nitrogen  acetyl coenzyme A, the biological building block that sup-
               atom of a benzimidazole (heterocyclic) ring also acts as a  plies two carbon atoms.
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