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 B_AL2 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.