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|Title:||The Stereochemistry and Mechanism of The Biosynthesis of Ent-Sandaracopimaradiene and Related Diterpenes|
|Author(s):||Drengler, Keith Allan|
|Department / Program:||Chemistry|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||The stereochemistry and mechanism of the cyclization of copalyl pyrophosphate to (+)-sandaracopimaradiene by enzyme extracts from R. cummunis was investigated. (S)-Geranyl-geraniol-1-d was prepared by enzymatic reduction of geranyl-geranial-1-d with horse liver alcohol dehydrogenase and NAD('+) with ethanol as a proton source. Inspection of the vinyl region of the ('1)H NMR spectrum of sandaracopimaradiene biosynthesized from (S)-geranylgeranyl pyrophosphate-1-d indicated that the deuteriovinyl group had the E configuration. The S(,N') cyclization of copalyl pyrophosphate to sandaracopimaradiene therefore proceeds with anti stereochemistry.
The question as to whether the pro-E or pro-Z hydrogen at C-17 of copalyl pyrophosphate is eventually eliminated during the last step of sandaracopimaradiene was ascertained using (17E)-copalyl pyrophosphate-17-d. The exocyclic double bond of copalol was labelled by halogen-metal exchange on (17E)-bromo copalyl tetrahydropyranyl ether and hydrolysis with deuterium oxide. The E stereochemistry at C-17 was established by a stereospecific intramolecular cyclopropanation reaction of a side chain diazo ketone onto the exocyclic methylene of a partially degraded sample obtained from copalol-17-d. Inspection of the coupling constant between the two residual hydrogens on the cyclopropane ring confirmed the assigned stereochemistry.
Integration of the NMR spectrum of (+)-sandaracopimaradiene biosynthesized from (17E)-copalyl pyrophosphate-17-d established that the C-17 pro-Z hydrogen was stereoselectively removed. The results are also consistent with cyclization between C-17 and C-13 of copalyl pyrophosphate occurring from the si face of C-17.
The mechanism of the S(,N') reaction was probed by studying the incorporation of structural analogues of copalyl pyrophosphate. A mixture of ent-manool and ent-13-epimanool was prepared by selective epoxidation of the allylic double bond of copalol, conversion to the mesylate, and reductive opening of the epoxide ring coupled with cleavage of the mesylate group. 16-Nor-copalol was prepared by a Wittig condensation between trimethyl phosphonoacetate and the aldehyde derived from partial degradation of the side chain of copalol. cis-Copalol was prepared by reduction of cis-copalal obtained as a side product from the activated manganese dioxide oxidation of trans-copalol.
The pyrophosphate esters of the analogues were incubated with cell free enzyme extracts from R. communis and M. macrocarpus. All the analogues except 16-norcopalyl pyrophosphate were efficiently incorporated into the diterpene hydrocarbons. The results indicate either the presence of an isomerase activity capable of interconverting all the analogues or that the pyrophosphate binding pocket of the diterpene cyclases is capable of ionizing the pyrophosphate group from several positions on the allylic system. The mechanism of the S(,N') reaction appears to proceed by ionization of the pyrophosphate group to an ion pair followed by condensation to close the C-ring.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1980.
|Date Available in IDEALS:||2014-12-13|