COMMUNICATIONS 17c tissues, it follows that the ratio 3:5 obtained for ergosterol (Table 1) indicates the presence of 3H on C-24.This same carbon atom carries an extra methyl group in ergosterol, and Lederer (1964) has considered a number of schemes for the alkylation step.These involve intermediates with the following arrangements in the side chain:which are then converted into ergosterol carrying the methyl group on C-24 thus: CH3 I-ICH3 ==CH-CH-CH 24 -CH3Our results deny any scheme that involves inter- mediates containing the groups (i) and (ii) in the side chain, for these would require the absence of 3H on C-24 in ergosterol derived from 4R-3H-labelled mevalonate.However, intermediates of types (iii) and (iv) are allowed, as schemes involving these do not necessarily require loss of the 3H on C-24.This argument holds whether the compound that is alkylated is desmosterol (Lederer, 1964) or, as seems more likely, some compound closely related to lanosterol (Goad & Goodwin, 1965).
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Whittle et al. (1965) studied this question.