Synthesis of furanoeremophilane sesquiterpenoids Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/fb494b57p

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  • Two approaches to the tricyclic core of the furanoeremophilane sesquiterpenoids are described. The first approach entails a projected Diels-Alder/retro Diels-Alder reaction of an acetylenic oxazole 64. Construction of the pivotal aldehyde 67 commenced from ketone 68. The acetyenic moiety was then introduced via a Felkin-Ahn addition of lithiopropyne to aldehyde 67. The final conversion of the cyclohexanone 83 to the acetylenic triflate 65 was unsuccessful. Attempts at addition of lithiated 2-methyloxazole 88 to ketone 83 were also unsuccessful. The second approach exploited a new annulation strategy. The aldehyde 64 was advanced to the 2, 4, 6-triisopropylbenzene sulfonylhydrazone 102 and a Shapiro reaction of 102 then provided alcohol 96. The furyl stananne 114 was readily prepared via a six-step sequence from acetylacetaldehyde dimethyl acetal 106. Unification of allylic bromide 90 and stannane 114 was accomplished through a Stille cross coupling methodology and the resulting product 113 was advanced to the aldehyde 116. However, attempts at further oxidation of this aldehyde to the required acid 89 failed. An alternative furyl stananne 124 with a tert-butyldimethylsilyl substituent at the C2 position was prepared from 3-furoic acid. An analogous sequence to that used with 113 led to aldehyde 131 which was successfully cyclized with the aid of trimethylsilyl trifluromethanesulfonate and 2, 6-lutidine to the tricyclic structure 132. Oxidation of the epimeric mixture of alcohols, followed by stereoselective reduction and removal of the tert-butyldimethylsilyl group from alcohol 134, gave (±)-6β-hydroxyeuroposin (4). Oxidation experiments with 134 were shown to convert the furan in this structure to a butenolide characteristic of the eremophilenolides.
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