Role of structure and pH in cyclization of allene oxide fatty acids: implications for the reaction mechanism.
Incubations of allene oxide synthases of flax or maize with the E,E-isomers of the 13- and 9-hydroperoxides of linoleic acid (E,E-13- and E,E-9-HPOD, respectively) at pH 7.5 afforded substantial yields of trans-disubstituted cyclopentenones. Under the conditions used, (Z,E)-HPODs were converted mainly into alpha-ketols and afforded only trace amount of cyclopentenones. These findings indicated that changing the double bond geometry from Z to E dramatically increased the rate of formation of the pericyclic pentadienyl cation intermediate necessary for electrocyclization of 18:2-allene oxides and thus the yield of cyclopentenones. The well-known cyclization of the homoallylic allene oxide (12,13-EOT) derived from alpha-linolenic acid 13-hydroperoxide (E,Z-13-HPOT) into cis-12-oxo-10,15-phytodienoic acid was suppressed at pH below neutral and was not observable at pH 4.5. In contrast, cyclization of the allene oxide ((9E)-12,13-EOD) derived from (E,E)-13-HPOD was slightly favoured at low pH. The finding that the cyclizations of 12,13-EOT and (9E)-12,13-EOD were differently affected by changes in pH suggested that the mechanisms of cyclization of these allene oxides are distinct.
Grechkin
Alexander N
AN
Kazan Institute of Biochemistry and Biophysics, Russian Academy of Sciences, P.O. Box 30, 420503, Kazan, Russia. grechkin@mail.knc.ru
Chechetkin
Ivan R
IR
Mukhtarova
Lucia S
LS
Hamberg
Mats
M
D003500:Cyclization; D003517:Cyclopentanes / Q000737:chemistry / Q000378:metabolism; D005227:Fatty Acids / Q000378:metabolism*; D019597:Flax / Q000201:enzymology; D006863:Hydrogen-Ion Concentration; D019746:Intramolecular Oxidoreductases / Q000378:metabolism*; D019787:Linoleic Acid / Q000378:metabolism; D015394:Molecular Structure; D010545:Peroxides; D013237:Stereoisomerism; D003313:Zea mays / Q000201:enzymology
D003517:Cyclopentanes; D005227:Fatty Acids; D010545:Peroxides; D019787:Linoleic Acid; D019746:Intramolecular Oxidoreductases; C021377:hydroperoxide isomerase; C013905:cyclopentenone