pHarmacist
(Hive Addict)
05-28-03 18:23
No 436097
      1,5-dibromopentane
(Rated as: excellent)
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It's my opinion that by far the best route to PCP is via via N-benzoylpiperidine. This route is a one-step synthetic beauty with a yield of 75%. Reaction of N-benzoyl amide with the dibromopentane Grignard reagent gives the desired PCP analogue depending on sec. amine used (of course, piperidine itself yields PCP). Details about this route can bee found here (../rhodium/chemistry /pcp/bzp_synth.html).

Nevertheless, here are two routes to the 1,5-dibromopentane - a reagent used to form the cyclohexyl group in the above mentioned procedure.


from pentane-1,5-diol  In a 500-mL 3-necked flask fitted with a stirrer, reflux condenser and dropping funnel, place 154 g (105 mL) of 48% HBr. Cool the flask in an ice bath. Add slowly, with stirring, 130 g (71 mL) of concentrated sulphuric acid. To the resulting ice-cold solution add 35 g (0.33 mol) of redistilled commercial pentane-1,5-diol dropwise. Leave the reaction mixture to stand for 24 hours; heat for 3 hours on a steam bath. The reaction mixture separates into two layers. Separate the lower layer, wash it successively with water, 10% sodium carbonate solution and water, and then dry with magnesium sulphate. The yield of 1,5-dibromopentane, b.p. 9 C/13 mmHg is 39 g (51%).

from tetrahydropyran  Place a mixture of 250 g (170 mL) of 48% hydrobromic acid and 75 g (41 mL) of concentrated sulphuric acid in a 500-mL RB flask, add 21.5 g (24.4 mL, 0.25 mol) of redistilled tetrahydropyran (b.p. 86.5-87.5 C), attach a reflux condenser and reflux gently for 3 hours. Separate the lower layer of dibromide and purify as in the previous preparation. The yield of 1,5-dibromopentane, b.p. 99 C/13 mmHg, is 46 g (80%)

Ref: Vogel's

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