Bromide intermediates, epoxide formation

One example of neighboring-group participation without the formation of cationic intermediates is the aminolysis of 2-bromoethanols (last example, Scheme4.49). In this instance epoxide formation and opening must be faster or as fast as direct bimolecular substitution of bromide by the amine otherwise no rearranged product would be observed. 

The photocatalytic ability of POM to induce bromide-assisted functionalization process was also studied by Molinari et al. [27] in the bromination of some aromatics and alkenes. They reported the possibility to convert phenol and anisole to the corresponding monobrominated derivates and a wide range of cydoalkenes to dibromides and bromohydrins, the last ones as intermediates for the formation of epoxides. 

In the second approach, 16-dehydropregnenolone was employed, the double bond of which at the 16,17- position was epoxidised, the epoxide cleaved with hydrogen bromide to the bromohydrin which was reduced and following formation of the 3-formoxy derivative, acetylation of the 17-hydroxy compound gave an intermediate which was oxidised by the Oppenauer reaction to give cortexolone (56) as the diacetate to which the microbiological oxidation could be applied. The process is outlined. 

In the reaction of 2-hydroxyalkyl halides with cyanide ions the intermediate formation of oxiranes can give rise to rearrangement products. Epoxides may also be encountered if CN ions are reacted with a-halo ketones (Scheme On heating with quaternary ammonium halides in acetonitrile or DMSO at 80-130 C these cyanooxiranes (3) are converted into 2-cyano ketones (4). Phenacyl bromide, in contrast, gives rise to 3-oxo-3-phenylpropionitrile (5) on treatment with cyanide ions. Not much is known about the reaction of the corresponding a-halo sulfides with cyanide ethylthioacetonitrile was obtained from the chloro sulfide by treatment with Hg(CN)2. ... 

Reactions with —OH Groups and Epoxides.—The formation of A -l,2-oxaphos-pholen derivatives from propargylic alcohols and phosphorus trichloride has been studied in detail. Intermediate phosphites (24) and allenic phosphonates (25) are described, and the A -l,2-oxaphosphoIen is produced in the final stage, as shown. Improved conditions have been outlined for the preparation of allylic bromides (26) from allylic alcohols and phosphorus tribromide. Related reactions of primary alcohols with the complex of phosphorus trichloride and DMF lead to the chloride (27) 22 addition of zinc bromide to the reaction results in the formation of alkyl bromides, but an attempt to extend this exchange to the preparation of cyanides was not successful. ... 

The possible mechanism for present investigation involves the nucleophilic attack of a bromide ion at the less substituted C-3 position of the spiro epoxide leading to the formation of an intermediate (II and 12). Further, as sulfur is abetter nucleophile than nitrogen, in the next step cyclization leads to formation of an oxathiolane ring (Scheme 8.57). 

There has been a fuller account given of the formation of the 2,2 -anhydro-system 18 ftom the 2, 3 -ene by treatment with N-bromoacetamide (see Vol. 26, p. 227). Treatment of 18 with NaOH in acetone formed the D-Zyro-epoxide 19. When the xylose-derived nucleoside 20 was treated with diphenyl carbonate and NaHCOs, the 2,2 -anhydronucleoside 21 was obtained, and it was proposed that the mechanism of this process proceeded through the 3 ,5 -cyclic carbonate and the 2, 3 -ribo-epoxide as intermediates. Treatment of 21 with pyridinium bromide followed by hydrogenolysis gave thymidine. The oxazoline 22 can be used as a precursor of various 5-substituted 2,2 -anhydrouridine derivatives, as in the case shown in Scheme 2. The... 

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