The stereoselectivity of the SCOOPY reaction, in which allyl alc. derivs. are prepd. by reaction of betaine ylides with carbonyl compds., was investigated by the synthesis of 1-hydroxy-2-octene (I), and 1-hydroxymethyl-2-octene (II) from Ph3P+-CH2 and Ph3P+-CHMe, resp., using HCHO and octanal. Treatment of III with HCHO in the presence of LiBr followed by treatment with n-C5H11CHO gave 10% I having a 97:3 cis-trans ratio. A 36% yield of II with a 30:70 cis-trans ratio was obtained by treating III with n-C5H11CHO and then with HCHO. Treatment of Ph3P+-CHMe with n-C5H11CHO and then with HCHO gave 71% II with a 99.3:0.7 cis-trans ratio, while treatment with HCHO and then with n-C5H11CHO gave 48% II with a 36:64 ratio. [on SciFinder (R)]