1,2-Thio-Wittig rearrangement

The lithium enolate formed from methyl S-trityl mercaptoacetate (8) has been C-alkylated in high yield at or below —40 °C (Scheme 4).37 At higher temperatures, the [ 1,2]-thio-Wittig rearrangement of the enolate was the predominant process. ESR evidence has indicated that the rearrangement occurred by a radical mechanism. 

The [l,2]-thio-Wittig rearrangement has been examined by several groups, and is known for both simple thioethers and a-thiocarbonyl derivatives. In a representative transformation, deprotonation of ester 69 followed by warming to -40 °C led to [l,2]-thio-Wittig rearrangement. A 75% yield of 70 was obtained after a CH3I quench. ... 

The pioneering work on thia-[2,3]-Wittig rearrangement was reported by Rautenstrauch in 1971 °. The reaction of allyl sulfides 137 with n-BuLi at —30°C for 1.5-4 h gave homoallyl sulfide or thiol 138 ([2,3]-product) exclusively (equation 81). In contrast, a similar reaction of vinyl sulfide 139 gave no [1,2]-rearrangement product 141 albeit a-thio-carbanion was generated as shown by its methylation to 140 (equation 82) °. 

For the Thio-[2,3]-Wittig Rearrangement, Nakai reported the following relative reaction rates R = Ph > C02Li > CN > C02Et > COMe, and for R = Ph > H > CH3. Reactions in this series were conducted at temperatures of from -80 °C to +60 °C. 

Dienes, as the simplest polyene systems, are widely encountered in sex pheromones like (4E, 7Z)-4,7-tridecadienyl acetate (100), a component of the sex pheromone of the potato tuberworm moth (Phthorimaea operculella). This compund was synthesized by Kim and Park [54] starting from a protected 5-hexen-l-ol and utilizing the chemistry of thio-substituted phosphonates (Scheme 28). Thus, the Pummerer rearrangement of a-phosphoryl sulfoxide 101 with 5-hexenyl acetate 102 afforded 7-acetoxy-l-methylthiohept-3-enyl phos-phonate 103 as an E/Z=85 15 mixture, being unreactive in the Horner-Wittig reaction, even after OAc deprotection/THP reprotection. However, the reaction of the more reactive a-phosphoryl sulfone 104 with w-hexanal successfully led to the required 1,4-diene 105, as an ElZ = 70 30 mixture, in 86 % yield. The final pheromone 100 was obtained after the desulfonylation of 105 with sodium hydrosulfite and the OTHP OH OAc deprotection/reprotection procedures. 

---