Graduate Thesis Or Dissertation
 

Absolute stereochemistry and total synthesis of botryococcene

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  • Studies were carried out to determine the absolute stereochemistry of botryococcene (1). The natural product was degraded to two sets of 'y- and 8- valerolactones. All enantiomeric variations of these lactones were synthesized in optically pure form starting from (S)- and (R)-methyloxiranes. 1H NMR correlation of natural and synthetic materials in the presence of chiral shift reagent, Eu(hfc)3, led to the assignment of absolute configuration to 1 as 3S, 7S, 10S, 13R, 16S, 20S. An enantiospecific, total synthesis of botryococcene was accomplished. The key steps are formation of (2R,6S)- 3- methyleno- 2,6,7- trimethyl -7- octenol (66), formation of (2S,5S) and (2R,5S) diastereomers of methyl 2,5-dimethy1-2-hydroxymethy1- 6-[(methoxymethypoxy]-3-E-hexenoate (126 and 127), and an organocopper coupling reaction. Methyl (2S)-3-hydroxy-2-methylpropionate was converted to (2R)-2,3- dimethy1-3-butenol, the dianion of which reacted with its tosylate in a head-to-tail manner to give the octenol. An alkylcopper species was prepared from the octenol via its iodide. Methyl (2R)-3-hydroxy-2-methylpropionate was transformed into methoxymethyl ( 5S)- 2, 5- dimethyl- 6- [(methoxymethyl)oxy] -2 -E- hexenoate which underwent an unusual fragmentation-recombination reaction to give a mixture of (2S,5S) and (2R,5S) diastereomers 126 and 127. The (2S,5S) diastereomer (126) was converted to (2R,5S)-2,5-dimethy1-6-[(methoxymethypoxy]-2-[(methoxymethypoxy]methyl-3- E-hexen-1-ol; the latter compound was also obtained from the (2R,5S) diastereomer (127) through an effective inversion at C-2, thus bringing both diastereomers into stereoconvergence. The hexenol was elaborated into (3S,6S)-3,6-dimethy1-7-iodo-3- iodo-methy1-4-E-hepta-1,4-diene which reacted with two equivalents of the octenylcopper species to furnish botryococcene.
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