Carboazidation of chiral allylsilanes: Experimental and theoretical investigations

The carboazidation of chiral allylsilanes has been investigated by varying the nature of the substituents at the silicon center and on the carbon framework. The influence of temperature and the nature of the sulfonyl azide, as well as the stereochemistry of the remote stereogenic center, on the 1,2-diastereocontrol of the process were considered. Good to excellent levels of diastereocontrol were generally observed, with the syn-beta-azidosilane always being formed as the major isomer. An illustration of the value of this methodology has been provided with a short and efficient synthesis of an analogue of castanospermine. EPR spectroscopy was carried out on various P-silyl radicals providing useful information about their conformations in the ground state. Based on this experimental evidence and DFT calculations, reactant-like transition state models were finally proposed that rationalize the observed 1,2-stereoinduction.