Ethers, crotyl Wittig rearrangement

The bis(oxazoline) S, 5)-(115) has been used as an external chiral ligand to induce asymmetric diastereoselective lithiation by r-BuLi during [2,3]-Wittig rearrangement of achiral substrates, (fj-crotyl propargylic ethers.It is believed that the enantios-electivity is determined predominantly at the lithiation step. 

Chiral bis(oxazoline) 27 is an effective chiral coordinating agent for enantiocontrol in the [2,3]-Wittig rearrangement. The rearrangement of (Z)-crotyl benzyl ether 84 with f-BuLi/(5, 5)-27 (1.5 equivalents each) in hexane provided [2,3]-shift product (l/ ,25 )-85 in 40% ee (equation 46The feasibility of the asymmetric catalytic version was also examined. In this case, the rearrangement with 20 mol% of 27 in ether was found to provide the same level of enantioselectivity (34% ee). 

Hauser noted that diallyl ether (8) also undergoes Wittig rearrangement upon base treatment and suggested that product formation could involve either a 1,2-shift or a cyclic mechanism (equation 3). Later studies by Schdllkopf and Makisumi with substituted allylic ethers (10,11 and 14-16 equations 4 and 5) pointed to a cyclic (SnO mechanism a process allowed by the Woodward-Hoffmann rules. The diastereoselectivity of the reaction was not determined in these cases, but Schollkopf subsequently found that benzyl rrans-crotyl ether (20 equation 6) affords mainly the anti products upon rearrangement of ether (20) with BuLi in THF. Rautenstrauch observed a 1 1 mixture of syn and anti products upon rearrangement of ether (20) in the presence of TMEDA, whereas the cis isomer (23) gave only the syn product (22 equation 7). ... 

Wittig rearrangements of alkynyl ethers follow the stereochemical trends demonstrated by other w-donor-stabilized substrates, and simple diastereoselection is typically excellent for both E- and Z-substrates 37 " 39. Propargyl-stabilized anions of E- and Z-crotyl ethers 27 exhibit high levels of simple diastereoselection. 

Wittig rearrangement. 9-Crotyloxy-9-fluorenyllithium (367a) and the (1-methylallyl)-isomeric 367b were found to isomerize to lithium 9-(l-methylallyl)-9-fluorenolate and lithium 9-crotyl-9-fluorenolate, respectively (Scheme 1-285). Analogous reactions of crotyl and 1-methylallyl 2-quinolylmethyl ethers and of dicinnamyl ether were reported shortly afterward. 

Mikami K, Azuma K-I, Nakai T. [2,3]-Wittig sigmatropic rearrangement of crotyl propargyl ether system. An emerging tool for control of acyclic stereochemistry. Tetrahedron 1984 40(12) 2303-2308. 

---