- Some Aspects of the Chemistry of 1,1,1-Trihalgenoalk-4-en-2-ols, the Ene Adducts Obtained from Reaction of Chloral and Bromal with Alkenes
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The chemistry of the title compounds has been explored and previous studies extended.They are inert to wide range of oxidising agents, and conversion into trihalogenomethyl ketones was effected (CrO3-H2SO4-AcOH) only after hydrogenation of the double bonds.Simple dehydration also failed, but the tosylate esters reacted with NaOEt-EtOH to afford ethyl alka-2,4-dienoates.Reaction of the ene adducts with alcoholic KOH gave α-alkoxy acids in high yield, as noted previously by Weizmann.A similar result was obtained with NaOR-ROH unless excess base was neutralized before work-up when α-alkoxy ester was isolated.It appears that the ester to acid conversion by the BAL2 mechanism is relatively fast at high alcoxide ion concentrations.The trichloromethyl alcohol to α-alkoxy acid or ester conversion has been shown to occur with net retention of stereochemistry, and a mechanism based on a dichloroepoxide intermediate is proposed.The stereochemical results are accomodated by the stereoselective electrocyclic ring opening rearrangement of the dichloroepoxide.Similar considerations apply to the hydrolysis using aqueous base, when α-hydroxy acids with retention of stereochemistry were obtained; in one instance competing chloroform elimination (to give aldehyde) was observed.The dianions from the α-methoxy acids are readily converted into the corresponding β,γ-unsaturated methyl esters by oxygenation and then acidification.Reductive dehalogenation of the ene adducts by LiAlH4 or Bun3SnH was partially succesful.The exocyclic chiral centres of the (-)-β-pinene ene adducts were detached, without loss of chirality, by oxidative degradation of the pinenyl ring; enantiometrically pure methyl (S)-3-acetoxy-4,4,4-trichlorobutanoate was obtained by this route.
- Benner, Jill P.,Gill, G. Bryon,Parrott, Stephen J.,Wallace, Brian
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p. 331 - 342
(2007/10/02)
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