L-bromopropane

A series of benzimidazole and benzimidazolone derivatives from the Janssen laboratories has provided an unusually large number of biologically active compounds, particularly in the area of the central nervous system. Reaction of imidazolone itself with isopropenyl acetate leads to the singly protected imidazolone derivative 51. Alkylation of this with 3-chloro-l-bromopropane affords the functionalized derivative Use of this... 

An early synthesis of A5-palmitoy]-.S -[2,3-bis(palmitoyloxy)propyl]cysteine employed cysteine methyl ester, however, this leads to difficulties in the saponification step of the tri-palmitoylated residue. 96 The optimized procedure, in which the cystine di-fert-butyl ester is used, 90 is outlined in Scheme 6 after N-acylation with palmitoyl chloride, the ester is reduced to the cysteine derivative for S-alkylation with l-bromopropane-2,3-diol to yield chirally defined isomers if optically pure bromo derivatives are used. Esterification of the hydroxy groups is best carried out with a 1.25-fold excess of palmitic acid, DCC, and DMAP. The use of a larger excess of palmitoyl chloride is not recommended due to purification problems. The diastereomeric mixture can be separated by silica gel chromatography using CH2Cl2/EtOAc (20 1) as eluent and the configuration was assigned by comparison with an optically pure sample obtained with 2R)- -bromopropane-2,3-diol. 

To a mixture of anhyd ethylene glycol (250 mL) and anhyd KF (168 g, 3.0 mol) in a mechanically stirred 1-L flask fitted with a distillation head and a pressure-equalizing addition funnel was added dropwise l-bromopropan-2-one (1 102 g, 0.75 mol) at 160 C. The crude product 2 distilled from the mixture over a 70— 120 C boiling point range. The crude distillate was dried (K2C03) and then fractionally distilled yield 9.00 g (20%) bp 75-77 "C/760 Torr. 

A = 4880 C3H6O3 C3H7Br Methyl Carbonate l-Bromopropane 90.35 71.0 Nonazeotrope 227... 

Propyleneiodonium ion 162-1 and propylenebromonium ion 162-Br were also obtained by the ionization of 2-fluoro-1-iodopropane and 2-fluoro-l-bromopropane in SbF5 S02ClF solution at 78°C386 [Eq. (4.116)]. The propylenechloronium ion is also known. 

A similar reaction takes place when 1-chloropropane is photolysed with light of wavelength less than 2200 A. The corresponding radiolysis reaction occurs less readily with l-bromopropane - . Apart from the major product of 2-chloropropane, Benson et found that the °Co y-radiolysis of 1-chloro-... 

The energy of the E2 transition state for I-bromopropane is similar to that for 2-bromopropane, because the products are the same (propene). So the E2 rate constants for these two compounds will be similar. However. 1-bromopropane, being primary, will have a faster competing SN2 reaction than will the secondary 2-bromopropane. Overall, it is reasonable to estimate that the actual E2 rate for l-bromopropane will be intermediate between the rates for bromoethane and 2-bromopropane. 

These compounds have been prepared from 1-bromopropane and 2-methyl-l-bromopropane via the corresponding Grignard reagent (equation 11). The appearance of the C—D stretching band at 2180cm" in the IR spectrum of 1 has been observed. 

Entry 5 is an example of a stereospecific anti elimination reaction of an alkyl halide in which the transition state requires the proton being removed and the bromide lost to be in an anti orientation with respect to each other. The dias-tereomeric threo- and eryfhro-l,2-diphenyl-l-bromopropanes undergo base-catalyzed jS-elimination to produce stereoisomeric products. Entry 6 is an example of pyrolytic elimination requiring a syn orientation of the proton being removed and the amine oxide nitrogen atom. This elimination reaction is a concerted one in which the proton is abstracted by the oxygen of the amine oxide group. 

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