Anomalous epoxide formation upon Wittig olefination with 1-iodoethyl triphenylphosphonium ylide

Reaction of iodo Wittig ylide 1 with aldehyde 2 affords moderate yields of cis epoxides 6a and 7a, in addition to the expected olefin 3. A mechanism is proposed to account for both the high Z selectivity and the modest yield of olefin observed with this reagent. Recently we reported the total synthesis of the immuno-suppressive/ antitumor agent (-)-discodermolide (4).¹ A key component of our convergent synthetic strategy was the use of vinyl iodide 3 as the C(9)-C(14) subtarget (Scheme 1). Iodide 3 was conveniently prepared from aldehyde 2 employing the Zhao protocol for 2-iodo-2-alkene construction (Scheme 2).² Accordingly, treatment of aldehyde 2 with ylide 1 afforded 3 as a 6:1 (Z/E) mixture of olefins in 41 % yield.¹.