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Total synthesis of stemodane-type diterpenoids : (±)-maritimol, (±)-stemodin, (±)-stemodinone and (±)-2-desoxystemodinone

University of British Columbia

This thesis describes work leading to the completion of a total synthesis of (±)-stemodin 3 and (±)-maritimol 5, as well as a formal total synthesis of (±)-stemodinone 4 and (±)-2-desoxystemodinone 6. The tricyclic enone 66 was prepared, in a series of steps, from the Weiland-Miescher ketone 68. Photoaddition of allene to this enone provided equal amounts of two photoadducts, 79 and 80. Ozonolysis, followed by treatment of the resulting 1,3-dione with sodium methoxide in methanol, generated the same keto ester 65 from both photoadducts. Two possible mechanisms for the unexpected conversion of the oc-photoadduct 80 into the keto ester 65 are presented. The keto ester 65 was elaborated, in a series of steps, into the tetracyclic dlone 63. The key step in this reaction sequence was a Thorpe-Ziegler condensation of the dinitriles 77. The gemlnal methyl groups required at C-4 in the target natural products 3 - 6 were introduced using a 5-step sequence. Treatment of the resulting alkylated dione 62 with methyltriisopropoxytitaniura afforded the keto alcohol 61, admixed with its C-13 epimer, in the ratio of 5:1. This keto alcohol 61 was subsequently converted into (±)-maritimol 5, and also into (±)-stemodin 3, via compound 55. Since the keto alcohol 61 has previously been converted into (±)-2-desoxystemodinone 6, and 55 has been converted into (±)-stemodinone 4, the work described here also constitutes a formal total synthesis of compounds 4 and 6.

Text

2010-05-02

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http://hdl.handle.net/2429/24368

Chemistry

University of British Columbia

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