Some aspects of the chemistry of 1,1,1-trihalogenoalk-4-en-2-ols, the ene adducts obtained from reaction of chloral and bromal with alkenes

Abstract

The chemistry of the title compounds has been explored and previous studies extended. They are inert to a 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 alkoxide 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 accommodated by the stereoselective electro-cyclic 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 oxygen-ation and then acidification. Reductive dehalogenation of the ene adducts by LiAlH4 or Bun3SnH was partially successful. 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.