We interpret both these results in terms of the general mechanism and the effect of the protecting groups on the covalent triflate/ion pair equilibrium. Thus, the five-membered cyclic 2,3-O protecting groups lead to considerable flattening of the pyra-nose ring and so raise the ground state energy of the triflate. The barrier to formation of the flattened oxacarbenium is therefore reduced and the equilibrium shifted more in its favor. This effect is more pronounced in the carbonate than the acetonide, owing to the sp2 nature of the atoms in the bridge, and so leads to higher «-selectivity in that case.

Zongmin Dai also investigated the use of a 2-O-TBDMS-3-O-benzoyl protected 4,6-O-benzylidene donor (81) but, again, excellent «-selectivity was observed [40]. This result must arise from neighboring group participation by the 3-O-benzoate. Models demonstrate that the twist-boat conformation (82) imposed by such participation on the pyranose ring is not unduly straining even though the system is 4,6-benzylidene protected.

Weiling Cai prepared and investigated the Ley type [42,43] 3,4-bisacetal protected donor 83 in the expectation that the second ring would serve a similar ri-gidifying function as the 4,6-benzylidene group. This was not to be the case, and again excellent «-selectivity was observed in coupling to a number of acceptors (Table 5) [40].

0 0

Post a comment