Sterol Biosynthesis: Strong Inhibition of Maize Δ5,7-Sterol Δ7-Reductase by Novel 6-Aza-B-homosteroids and Other Analogs of a Presumptive Carbocationic Intermediate of the Reduction Reaction

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Abstract
A series of mono- and diazasteroids have been synthesized as analogs of a predicted carbocationic intermediate of Δ5,7-sterol Δ7-reductase (Δ7-SR). 6-Aza-B-homo-5α-cholest-7-en-3β-ol (4), a novel compound whose synthesis is described for the first time, and 6,7-diaza-5α-cholest-8(14)-en-3β-ol (6) were shown to be very powerful inhibitors of Δ7-SR in a preparation isolated from maize (Zea mays) (Ki,app = 50−70 nM, Ki,app/Km,app = 1.0 × 10-4 to 1.3 × 10-4). The data are consistent with a carbonium ion mechanism for the reduction; compounds 4 and 6 probably act as reaction intermediate analogs. Compound 4, in contrast to compound 6, displayed in the same microsomal preparation more than 50-fold selectivity for inhibition of the Δ7-SR versus Δ8-Δ7-sterol isomerase, cycloeucalenol isomerase, and Δ8,14-sterol Δ14-reductase, the mechanism of these four enzymes involving presumptive cationic intermediates centered respectively at C7, C8, C9, and C14. These observations highlight the paramount importance of the location of the positively charged nitrogen atom(s) in the B-ring structure for selectivity among these enzymes involving structurally close cationic reaction intermediates. Efficient in vivo inhibition of sterol biosynthesis in bramble cell suspension cultures by a low concentration of compound 4 was demonstrated and confirmed the in vitro properties of this derivative.