Ethers, crotyl propargyl

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. 

It is noteworthy that the rearrangement of crotyl propargyl ether 86 with f-BuLi/f, )-27 at —95 °C carried out in pentane provided [2,3]-shift product 87 in high enantiopurity (89% ee, 47% yield) with high (>95%) anf/-selectivity (equation 48). 

Reaction (e) is also useful for preparing mixtures of ether adducts of allyl- ", crotyl- , propargyl- and benzylaluminum bromides. The organo bromides are used to insure enhanced reactivity (as compared with the chlorides) and less coupling (as compared with the iodides). 

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. 

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. 

Liang T, Zhang W, Chen T-Y, Nguyen KD, Krische MJ (2015) Ruthenium-catalyzed diastereo- and enantioselective coupling of propargyl ethers with alcohols siloxy-crotylation via hydride shift enabled conversion of alkynes to ji-allyls. J Am Chem Soc 137 13066-13071... 

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