Photochemical rearrangement of α-hydroxy-ketones to lactones

Abstract

U.v. irradiation of 3β-acetoxy-5α-hydroxycholestan-6-one (I; R = Ac) and of the 5β-epimer (VI; R = Ac) through Pyrex gives 3β-acetoxy-5α-hydroxy-5,6-secocholestan-6-oic lactone (II; R = Ac) and its 5β-epimer respectively, in almost quantitative yield. The derived 3-oxo-lactones are equilibrated heavily in favour of the trans-isomer (VIII). On photoisomerisation of the O-deuterio-compounds (I and VI; R = Ac) in O-deuterio-ethanol one atom of deuterium is incorporated at C-7 only in one configuration. The whole rearrangement is thus stereospecific.

Some confusion in the literature is removed by showing that in the formation of 3β,5β-dihydroxycholestan-6-one (VI; R = H) by reaction of 3β-acetoxy-5α-bromocholestan-6-one (V) with sodium hydroxide in methanol the 6,6-dimethoxy-compound (IX) is an intermediate. The product of oxidation of 3β-acetoxycholestan-6-one (XV) with perbenzoic acid, which had been described as the 5,6-lactone (VII; R = Ac), is actually a mixture containing mostly the isomeric 6,7-lactone (XVI; R = Ac).

Irradiation of trans-1-acetyl-4-t-butylcyclohexanol (XXII) yields almost no 4-t-butylcyclohexyl acetate, but mainly 4-t-butyl-6-methylbicyclo[3,1,1]heptane-1,6-diol (XXIII) formed through the usual six-atom transition state. 4-t-Butylcyclohexanone, which is also formed (31%), becomes the principal product (78%) from cis-1-acetyl-4-t-butylcyclohexanol (XXV).