Phenethyl tosylate

Phenethyl tosylate solvolyzes in CF3C02H orders of magnitude faster than ethyl tosylate (Figure 2.25). Because the neighboring phenyl ring can make a -electron pair available, a phe-nonium ion intermediate is formed. Phenonium ions are derivatives of the spirooctadienyl... 

Fig. 2.25. Acceleration of the trifluoroacetolysis of phenethyl tosylate by neighboring group participation.

Another approach to cephalotaxine intermediate 43 was reported by Weinstein and Craig (65) in 1976 (Scheme 33). The reaction of 3,4-(methy-lenedioxy)-iS-phenethyl tosylate (191), a derivative of the previously used nosylate 117, and sodium 2-carboethoxypyrrole (192), followed by hydrolysis, produced the carboxylic add 193. Intramolecular Friedel-Crafts acylation of 193 with stannic chloride and trifluoroacetic anhydride yielded the benzazepine 194, which was reduced, hydrogenated, and finally oxidized to produce the tricyclic Dolby-Weinreb enamine 43. 

Mandelic acid derivatives are useful resolving agents. While (/ )-( +)-phenethyl alcohol (77) is commercially available, it is relatively expensive. As shown in Scheme 16, (5)-l can be converted readily into multigram quantities of 77. Reduction of 1 with borane-dimethyl sulfide provides the diol 76, which is selectively tosylated at the primary hydroxy position and then detosylated with lithium aluminum hydride to provide 77 in 48% overall isolated yield (Scheme 16). The low yield is a result of the problematic tosylation step, in which ditosylation is unavoidable. 

---