Mechanism of oxidative cyclization of squalene. Evidence for cyclization of squalene from either end of the squalene molecule in the in vivo biosynthesis of fusidic acid by Fusidium coccineum

Abstract

phenyl phosphide was prepared by the reaction of diphenylphosphine with n-butyllithium in THF. Lithium diphenylamide was prepared by the reaction of diphenylamine with methyllithium in THF. Sodium dimsylate was prepared as described by Greenwald, Chaykovsky, and Corey.80 All of the above were reacted with carbon monoxide at 200 psi; only sodium dimsylate took up an appreciable quantity of carbon monoxide (0.5 equiv of carbon monoxide per equivalent of dimsylate over a 24-hr period). Dilithium Benzophenone Dianion. Butyllithium in hexane (6.0 mmol) was added slowly to 1,Zdibromoethane (0.957 g, 5.1 mmol) in a 40-ml centrifuge tube capped with a No-Air stopper. The resulting precipitate of anhydrous lithium bromide was washed four times with 20-ml portions of pentane, dissolved in 7 ml of hot DME, and added to 2 mmol of dipotassium benzophenone dianion in 30 ml of D M E containing 0.1534 g of n-nonadecane as internal standard in a stoppered 40-ml centrifuge tube. The solution was thoroughly mixed by shaking and the precipitate of potassium bromide separated by centrifugation. The resulting solution of dilithium benzophenone79 was added to a solution of a,a-diphenyla-hydroxyacetophenone (0.0967 g, 0.336 mmol) in 10 ml of DME. The red color of dilithium benzophenone persisted after the addition. After stirring at room temperature for 2 hr, the reaction mixture was hydrolyzed with aqueous potassium hydroxide solution; the aqueous layer was separated and extracted with one 50-ml portion of ethyl ether, and the organic layers were combined and dried (Na2SOa). Analysis by glpc indicated the presence of a,adiphenylacetophenone (72 %) and ap-diphenyl-a-hydroxyacetophenone (9 %).Preparation of Other Anions. Lithium phenyl acetylide and lithium butyl acetylide were prepared by reaction of the corresponding acetylene with methyllithium in ether solution. Lithium di-(79) Solutions of 18 prepared using this procedure were analyzed for potassium by hydrolysis and treatment with lithium tetraphenylborate: cf. D. N. Bhattacharyya, C. L. Lee, J. Smid, and M. Szwarc, J. Phys. Chem., 69, 608 (1965). Less than 1 % of the potassium originally present as dipotassium benzophenone remained in solution after addition of the lithium bromide.Abstract: Fusidic acid biosynthesized by F. coccineum from (3RS,5S)-[2-' 4C,5-3H]me~alonic acid was shown to contain 0.5 atom of tritium at the C-llp and C-12 positions. From the known mechanism of squalene formation the 0.5 atom of tritium at C-12 must have the LY configuration. Our results indicate that either one of the terminal double bonds of squalene is epoxidized to an equal degree, and that the ensuing cyclization to prosterol occurs from either end of the squalene molecule. This shows that the geometrical asymmetry imparted to the squalene on the squalene synthethase is not retained during the conversion to oxidosqualene. These observations are consistent with the hypothesis of the release of squalene into a free squalene pool prior to epoxidation. he biosynthetic steps involved in the formation...

Stereochemistry of trans-phytoene synthesis. Lycopersene as a carotene precursor and a mechanism for the synthesis of cis- and trans-phytoene

“…By examining the distribution of in lanosterol synthesized from farnesyl pyrophosphate in the presence of NADP3H, Etemadi et al (1969) concluded that the biosynthesized squalene had randomized. Ebersole et al (1973) incubated (3i? )-[2-14C]-( 55)-[5-3H]mevalonate with cultures of Fusidium coccineum.…”

New Steroid Acids from Antrodia cinnamomea, a Fungal Parasite of Cinnamomum micranthum

“…Zhankuic acid B (2, 20 mg) was obtained from fraction 4 by prep, tic (Si gel, toluene-EtOAc-HOAc, 10:1:0.5). Zhankuic acid C (3, 300 mg) was obtained directly from the recrystallization of fraction 1 and 2; hreims mlz 468.2872 (C29H40O3, caled 468.2876); eims mlz 468 (M+, 56), 450 (9), 424 (78), 354 (64), 339 (33), 326 (22), 313 (18), 311 (63), 297 (11), 286 (42), 271 (22), 260 (30), 246 (10), 231 (8), 220 (23), 215 (7), 201 (6), 189 (8), 175 (9), 161 (9), 149 (9), 135 (14), 123 (20), 121 (17) 109 (38), 95 (100), 82 (13), 68 (30). Zhankuic acid A methyl ester [4],-A solution of 1(100 mg) was treated overnight with an excess amount of CH2N2 in Et20.…”

Biosynthesis of tetrahymanol by Tetrahymena pyriformis: mechanistic and evolutionary implications

“…Elimination of a proton would result in dehydrogammacerane (17b). Alternatively, 15 could rearrange to the hopanoidyl cation 13b which in turn could be stabilized in a number of ways. For example, loss of a proton would result in hop-22-ene (18), while acquisition of a hydroxyl would yield diplopterol (26).…”