[1] In the presence of lithium or aluminum amide bases, epoxides may open to give the corresponding allylic alcohols.
Removal of a proton adjacent to the epoxide, elimination, and neutralization of the resulting alkoxide lead to synthetically useful allylic alcohol products.
(1)Isomerization of epoxides to allylic alcohols under strongly basic conditions proceeds by a β-elimination process.
[5] Concerted C–O bond cleavage and deprotonation proceeds via a syn transition state to give an allylic alkoxide, which is protonated upon workup.
1,2-hydrogen migration leads to ketones,[2] while intramolecular C–H insertion affords cyclic alcohols with the formation of a new carbon-carbon bond.
[3](3)In many cases when hexamethylphosphoramide (HMPA) is used as an additive with lithium amide bases, selectivity for the formation of allylic alcohols increases.
The use of bulky aluminum amide bases facilitates elimination at the substituent cis to hydrogen, which occurs from the less sterically hindered epoxide-base complex.
(8)Suitably substituted unsaturated epoxides may undergo vinylogous elimination, which leads to conjugated allylic alcohols.
The residue was purified by preparative TLC (Rf 0.22 in 1:2 ether–hexane) to give 99% of (E)-2-cyclododecenol: IR (neat) 3330–3370, 1465, 1450, 970 cm−1; NMR (CCl4) δ 3.73–4.20 (1, m), 4.97–5.82 (2, m); mass spectrum (m/z) 182 (16), 164 (13), 139 (32), 125 (46), and 98 (100).