Triphenyl-carbinol 1 preparation

The Grignard reagent—magnesium phenyl bromide—is prepared as described in Preparation 21 from 1-2 gms. (1 mol.) of dry magnesium and 8 gms. (1 mol.) of dry bromobenzene. 6-8 gms. (1 mol.) of dry methyl benzoate dissolved in 25 gms. of sodium-dried ether are added to the cold solution, slowly and with constant shaking. The liquid is then heated on a water bath until no further change takes place. Ice and dilute sulphuric acid are added to the cold reaction mixture, which, when the precipitate has dissolved, is steam distilled. The triphenyl carbinol which remains is recrystallised from benzene. 

Ring cleavage. Tetraphenyl-l,3-cy-clobutanedione added portionwise to an ethereal soln. of phenyllithium prepared from bromobenzene and Li, refluxed 1.5 hrs. after the reaction has subsided, and allowed to stand overnight si/m-tetraphenyl-acetone (Y 96%) and triphenyl-carbinol (Y 95%).— The startg. m. failed to give a noticeable reaction with phenylmagnesium bromide even when heated at 66° for 6 hrs. 

The phosphonium salt 25 was prepared from this key intermediate. Vinylation of 24 gave a mixture of C(9)-epimeric vinyl carbinols 26. These were smoothly transformed via an acetoxycarbinol mixture into the acetoxybromide which, on treatment with triphenyl phosphine and aqueous NaCl, afforded the phosphonium chloride 25. This was identified by m.p., [a]"and H-NMR [24]. 

For the synthesis of the C4o-carbon skeleton the Ci3 + Ci4+Ci3 = C4o and the Ci5 + Cio + Ci5 = C4o strategies have been used, and the latter has been developed on a technical scale. The Ci5-phosphonium salt 72 was prepared by oxidation of 68 to (Rj-3-hydroxy-p-ionone (73), followed by vinylation to the epimeric vinyl carbinols 74 and treatment with triphenyl-phosphine hydrochloride (Scheme 16). 

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