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UBC Theses and Dissertations
Carbocycle construction in terpenoid synthesis : subtitle the total synthesis of (±)-sarcodonin G and (±)-1-epi-9-norpresilphiperfolan-9-one Gilbert, Michael W.
Abstract
Carbocycle construction represents a significant challenge in synthetic studies geared towards terpenoid natural products. A step-wise annulation approach to the construction of the diterpenoid framework of the cyathanes (54) was investigated, ultimately leading to the first reported total synthesis of (±)-sarcodonin G (36). The sesquiterpenoid carbon skeleton of the presilphiperfolanes (270) was assembled via a tandem cyclization strategy, affording the known ketone, (±)-1-epi-9- norpresilphiperfolan-9-one (306). The construction of the cyathane 5-6-7 fused tricyclic framework commenced with 3-methyl-2-cyclohexen-1-one 38, representing the B-ring. The annulation sequences for A and C-rings utilized the bifunctional reagents 39 and 44, respectively. With the help of these reagents, the 5-6-6 fused tricycle 155 was quickly assembled from 38. Intermediate 155 was further elaborated, including a key SmI₂-mediated ring expansion, to afford the 5-6-7 fused tricycle 159, which displays the complete cyathane framework. The target structure, (±)-sarcodonin G (36), was obtained in several steps from 159. In addition, an advanced intermediate in the synthesis of 36 was transformed into substance 269, a likely synthetic precursor of the corresponding keto triol natural product, (±)-cyathin A₄ (37). Assembly of the presilphiperfolane 5-5-6 fused tricyclic carbon skeleton 270 was approached via a tandem cyclization strategy involving free-radical reactions. The key intermediates of this synthetic study, xanthates 367, were readily prepared from enone 49 in several steps. Bu₃SnH-mediated free-radical reaction of 367 yielded a mixture of cyclization products, alkenes 368, whose frameworks include the carbon skeleton of the natural product presilphiperfolan-9-ol (53). Alkenes 368 were subjected to oxidative cleavage conditions, affording the known ketone (±)-1-epi-9-norpresilphiperfolan-9-one (306). [diagram]
Item Metadata
Title |
Carbocycle construction in terpenoid synthesis : subtitle the total synthesis of (±)-sarcodonin G and (±)-1-epi-9-norpresilphiperfolan-9-one
|
Creator | |
Publisher |
University of British Columbia
|
Date Issued |
2002
|
Description |
Carbocycle construction represents a significant challenge in synthetic studies
geared towards terpenoid natural products. A step-wise annulation approach to the
construction of the diterpenoid framework of the cyathanes (54) was investigated,
ultimately leading to the first reported total synthesis of (±)-sarcodonin G (36). The
sesquiterpenoid carbon skeleton of the presilphiperfolanes (270) was assembled via a
tandem cyclization strategy, affording the known ketone, (±)-1-epi-9-
norpresilphiperfolan-9-one (306).
The construction of the cyathane 5-6-7 fused tricyclic framework commenced
with 3-methyl-2-cyclohexen-1-one 38, representing the B-ring. The annulation sequences
for A and C-rings utilized the bifunctional reagents 39 and 44, respectively. With the help
of these reagents, the 5-6-6 fused tricycle 155 was quickly assembled from 38.
Intermediate 155 was further elaborated, including a key SmI₂-mediated ring expansion,
to afford the 5-6-7 fused tricycle 159, which displays the complete cyathane framework.
The target structure, (±)-sarcodonin G (36), was obtained in several steps from 159. In
addition, an advanced intermediate in the synthesis of 36 was transformed into substance
269, a likely synthetic precursor of the corresponding keto triol natural product, (±)-cyathin A₄ (37).
Assembly of the presilphiperfolane 5-5-6 fused tricyclic carbon skeleton 270 was
approached via a tandem cyclization strategy involving free-radical reactions. The key
intermediates of this synthetic study, xanthates 367, were readily prepared from enone 49
in several steps. Bu₃SnH-mediated free-radical reaction of 367 yielded a mixture of cyclization products, alkenes 368, whose frameworks include the carbon skeleton of the
natural product presilphiperfolan-9-ol (53). Alkenes 368 were subjected to oxidative
cleavage conditions, affording the known ketone (±)-1-epi-9-norpresilphiperfolan-9-one
(306).
[diagram]
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Extent |
7691361 bytes
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Genre | |
Type | |
File Format |
application/pdf
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Language |
eng
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Date Available |
2009-10-01
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Provider |
Vancouver : University of British Columbia Library
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Rights |
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.
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DOI |
10.14288/1.0061283
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URI | |
Degree | |
Program | |
Affiliation | |
Degree Grantor |
University of British Columbia
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Graduation Date |
2002-05
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Campus | |
Scholarly Level |
Graduate
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Aggregated Source Repository |
DSpace
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Rights
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.