Journal
JOURNAL OF ORGANIC CHEMISTRY
Volume 68, Issue 22, Pages 8453-8458Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jo035003j
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Funding
- NIGMS NIH HHS [GM 62160, GM 57335] Funding Source: Medline
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It is demonstrated that the beta-selectivity observed in the insoluble silver salt mediated couplings of 2,3-O-carbonate-protected rhamnosyl bromides is uniquely due to the heterogeneous nature of the reaction. In homogeneous solution these same donors are alpha-selective, a fact that is attributed to the half-chair conformation of these substances which reduces the energy barrier to oxacarbenium ion formation. It is suggested that the 2,3-O-carbonate group be dubbed torsionally arming in the manno- and rhamnopyranose series. When the carbonate group is removed to the 3,4-O-position a beta-selective system is formed, in both homogeneous and heterogeneous conditions, and it is demonstrated that this selectivity arises from the combination of the electron-withdrawing nature of the carbonate and its inability to take part in neighboring participation.
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