Ethers, allyl lithiomethyl

Table 4 2,3-Wittig Rearrangement of Lithiomethyl Allyl Ethers...

Scheme 4 Transition states for lithiomethyl allyl ether rearrangements...

The TMS-propargyl ether (104) rearranges to an 80 20 mixture of the ( )- and (Z)-isomers (106) and (108) in an apparent violation of this transition state proposal (equation 26). (Note that because of the stereochemical descriptor rules the sense of ( ) and (Z) is inverted by the TMS substituent.) However, replacement of the vinylic TMS grouping by hydrogen led almost exclusively to the ( )-product (109). Evidently, the normally favored transition state (E) is destabilized by steric interactions between the vinylic TMS substituent and the n-pentyl grouping (R ). The alternative conformation (F) lacks this interaction. The situation is analogous to that noted by Still in the rearrangement of lithiomethyl allyl ethers (Scheme 4). 

---