20 [40,45] 21 [46] 23 [38,43,48]* Yield is not reported. ** Product was not isolated. *** Yield and analytical data are not reported. * Yield is not given, purity of 81%. ** A purity of 4% by in situ 31 P{ 1 H} NMR spectroscopy. *** The crude yield. The pure P,N-acetal have been isolated in a low yield (9-10%) via a reaction with BH3•SMe2 followed by deprotection with HBF4•Et2O.