Macrocyclic ethers Wittig ring contractions

Macrocyclic ethers with remote sp stereocenters can undergo diastereoselective 2,3-Wittig ring contractions. Thus, the alcohol (266a equation 58) and several of its ether derivatives (266b) and (266c) upon treatment with Bu"Li in pentane-THF-TMEDA afforded the four diastereomeric rearrangement... 

For the rearrangement of conformationally constrained [2,3] Wittig substrates, chiral bases can exert a more pronounced influence on the stereochemical outcome of the reaction, as evidenced by the macrocyclic ring contraction of cyclic ether 47 86. 

Marshall has reported an innovative application of the Wittig rearrangement in the synthesis of the 14-membered cembrenoid carbocycles (Scheme 16.21) [104], The stereoselectivity of this process is coupled with the conformational preferences of the macrocyclic ether. The rearrangement reaction of the 17-membered macrocycle 187 provided a 90% yield of the 14-membered alcohols, with 188 as the major diastereomer (dr=81 7 6 7). This represents a remarkable example of macrocyclic stereocontrol (see also Chapter 1) to effect a stereoselective ring-contraction by use of a Wittig rearrangement. 

---