Dienes tritium-labelled

Chlorella ellipsoidea was found to be able to convert tritium-labelled 5a-ergosta-8,14-dien-3/3-ol and 4 -methyl-5a-ergost-8-en-3/3-ol into ergost-5-en-3)8-ol, but brassicasterol (ergosta-5,22-dien-3/3-ol) was not labelled,180 and thus it is probably not derived from a saturated side-chain precursor. 

The inverse procedure, double bond tritiation followed by re-oxidation, allows for the tritium labeling of a,/8-unsaturated carbonyl compounds (Figure 10.7). Applied to the complex steroid 22 it provided [ H]25, in which 95% of the tritium was located at C2. If most tritium in the intermediate 34 had been on the a-face and equally distributed between Cl and C2, in analogy with the preparation of [ C]24 under very similar conditions, then the benzeneselenic anhydride-mediated dehydrogenation must have constituted a net la,2)3 elimination. This selectivity is consistent with the DDQ dehydrogenation of [la,2a- H]androst-4-ene-3,17-dione (36) to give [2- H]androsta-l,4-dien-3,17-dione (2Z) with little tritium at Cl, and DDQ dehydrogenation of [l)3,2)3- H]testosterone 1381 to provide mainly 17)3-hydroxy[l- H]androsta-l,4-dien-3-one (39) . 

Syntheses and uses of isotopically labelled dienes and polyenes 821 2. Synthesis of simple seven-membered ring compounds labelled with tritium... 

Addition of deuterium to ergosterol acetate was shown to form the 5a,6a-dideuterio reduction product under mild conditions (Equation 15.4). This result illustrates a number of features of the reactivity of Wilkinson s catalyst. First, the conjugated diene is reduced in preference to the C21-C22 double bond. Second, the diene is reduced from the less-crowded a-face of the steroid. Neither the remaining trisubstituted double bond nor the trans disubstituted A double bond are affected. Finally, no isotopic exchange takes place during this reaction. This selectivity has been used to specifically label prostaglandins with tritium (Equation 15.5). The sensitive 3-keto alcohol is unperturbed. 

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