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  4. The Study of Radical Cyclization Reactions with Acylsilanes Derived from Carbohydrate Templates
 
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The Study of Radical Cyclization Reactions with Acylsilanes Derived from Carbohydrate Templates

Date Issued
2010
Date
2010
Author(s)
Zheng, Wei-Lin
URI
http://ntur.lib.ntu.edu.tw//handle/246246/257415
Abstract
In the first part of this thesis, we wanted to understand why the radical cyclization was not efficient using acylsilanes derived from hexoses. We used 2-deoxy-D-glucose as a chiral template to synthesize a related acylsilane as a radical cyclization precursor. Our method conserved the orginal stereochemistry of 2-deoxy-D-glucose. The reducing end was converted to an acylsilane functionality whereas the non-reducing end was transformed into a halide. All the secondary hydroxyl groups were protected as methyl ethers. We expected captodative stabilization of a radical on C(2) of acylsilane could be avoided when using this acylsilane. By using tributyltin deuteride as reagent, we found the presence of severe 1,5-hydrogen shift in this system thus disproved the importance of captodative effect. We identified cyclization products, and discussed the stereoselectivity. Combining previous research in D-glucose and D-mannose systems, we concluded that the crucial factor controlling the ratio of radical cyclization and 1,5-hydrogen transfer is the conformation in transition state. In the second part of this thesis, we used D-ribose as a chiral template to synthesize a related acylsilane as a radical cyclization precursor. Our method conserved the orginal stereochemistry of D-ribose. We protected hydroxyl groups on C(2) and C(3) as acetonide, and that on C(4) with methyl ether. Due to the acetonide, the molecular structure is locked in a puckerd form, and highly stereoselective origin of radical cyclization was realized. We identified cyclization product, and discussed the stereoselectivity. Furthermore, the cyclization product with three different types of protecting groups could enable us to futher functionalize the structure.
Subjects
radical
acylsilane
captodative effect
acetonide
Type
thesis
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