The course of the dehydration of α-hydroxy-γ-oxotrimethylsilanes is highly dependent on the conditions to produce stereoselectively (Z)- and (E)-γ-oxovinyltrirnethylsilanes, which are potential precursors for preparation of stereodefined exocyclic tetrasubstituted alkenes.
Syntheses and Theoretical Studies of Exocyclic .gamma.-Oxoalkenyltrimethylsilanes. An Approach to the Stereodefined Exocyclic Tetrasubstituted Alkenes
作者:Kazuhiko Nakatani、Tomoko Izawa、Sachihiko Isoe
DOI:10.1021/jo00099a027
日期:1994.10
The Z-selectivity in the dehydration of alpha-hydroxy-gamma-oxoalkyltrimethylsilanes under acidic conditions was studied from experimental and theoretical points of view. Experimental results showed that (Z)-gamma-oxoalkenyltrimethylsilanes were thermodynamic products under these conditions. The dehydration studies of the compound involving the tert-butyl group instead of the TMS group pointed out that not only steric but also electronic effects of Si could contribute to the distribution of the products as well as their stereochemistries. Theoretical studies using ab initio calculation at the 6-31G* level indicated that the (Z)-isomer was thermodynamically more stable than the corresponding (E)-isomer. Detail examinations of the optimized structures showed that the configuration of Si in the (Z)-isomer was slightly distorted from tetrahedral. Interpretation of the geometrical change of Si to rationalization of the thermodynamic preference was discussed fi om the viewpoint of possible coordination of the carbonyl oxygen to Si. Those (Z)-gamma-oxoalkenyltrimethylsilanes would have potential to be the novel type of alkenylmetal compounds in organic synthesis as we demonstrated in the construction of stereodefined exocyclic tetrasubstituted alkene.
Removable Silyl Group as a “Masked Proton” in Oxy-2-oxonia(azonia)-Cope Rearrangement: Applications in Stereoselective Total Synthesis of Natural Macrolides
In the presence of a Lewis acid, trimethylsilyl-substituted β,γ-unsaturated ketones and aldehydes (imines) undergo nucleophilic addition to produce zwitterionic intermediates, followed by oxy-2-oxonia(azonia)-Cope rearrangements to give homoallylic esters (amides). In the case of TMS-containing 2-vinylcycloalkanones, the process results in ring-enlargement, providing 10- to 16-membered lactones. This