UBC Theses and Dissertations
Studies directed towards the synthesis of (±)-stemodin, a tetracyclic diterpenoid Abeysekera, Brian Frederick
This thesis describes studies directed toward the synthesis of a new class of tetracyclic diterpenoids represented by aphidicolin 7 and stemodin 8. A feasible route to stemodin is established, while the similar route planned for aphidicolin 7 is shown to be non-viable. Well established procedures were utilized to convert the Wleland -Miescher ketone 101 into the bicyclic ketal ketone 57. This was converted into the key intermediate, the tricyclic enone 56, using a new cyclopentenone annulating sequence which compared quite favourably with procedures presently available. This sequence was as follows: the readily available keto phosphonate 75 was converted into its enol ether 79, and allylic bromination with N-bromosuccinimide afforded the bromo compound 81, an excellent alkylating agent. This material was used to alkylate the enolate anions of various ketones, including 57. The sequence is illustrated for cyclohexanone 82, the enolate anion of which on treatment with 81 gave 83. Acid hydrolysis of the enol ether moiety in 83 gave the diketo phosphonate 93 which was cyclized to the enone 97 using the Horner - Emmons reaction. The photoaddition of allene to the enone 56 was previously studied in this laboratory and was reported to yield two photoadducts. While the work described in this thesis supports the structure previously assigned to one of the photoadducts (61), a new structure (109) is proposed for the other. This proposal is based on the conversion of both photoadducts by ozonolysis and subsequent sodium methoxide treatment into the same keto ester 55. The keto ester 55 was subsequently elaborated into the tetracyclic dione 132 in a number of steps, the key reaction in this sequence being a Thorpe-Ziegler condensation. [See thesis for diagrams].
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