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This paper reports an easy characterization of carboxylic-trifluoroacetic mixed anhydrides by 1 H and 13 C NMR, and gives evidence that mixed anhydrides are produced instantaneously, at room temperature, from a carboxylic acid and trifluoroacetic anhydride and even from carboxylic anhydrides and trifluoroacetic acid. The esterification of phenol with the carboxylictrifluoroacetic anhydrides is highly chemoselective, and affords pure products, in quantitative yields, and in the case of cholesterol, some cholesteryl trifluoroacetate is also produced.

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Uma rota conveniente para a síntese de 3β-hidroxi-6-oxo-5α-esteroides a partir de ∆ 5 -esteroides é descrita tendo sido aplicada para a síntese da laxogenina, substância que apresenta atividade como hormônio de crescimento vegetal. O método é uma alternativa para instalar este grupo funcional importante encontrado em esteróides naturais. O processo descrito utiliza reagentes baratos e pode ser executado em quatro etapas. As etapas de oxidação e de tratamento ácido não afetam a cadeia lateral espirocetálica presente na diosgenina (16).A convenient pathway to obtain 3β-hydroxy-6-oxo-5α-steroids from 3β-acetoxy ∆ 5 -steroids is reported; the methodology was applied to the synthesis of laxogenin (7), substance that behaves as a plant growth hormone. This is an alternative way to produce an important functionality found in many examples of naturally occurring steroids. The developed procedure uses inexpensive reagents and can be carried out in four steps. The oxidizing and acidic steps used in this methodology did not affect the labile spiroketal side chain present in diosgenin (16).Keywords: laxogenin, diastereoselective epoxidation, 3β-hydroxy-6-oxo-5α-steroids IntroductionThe 3β-hydroxy-6-oxo moiety is present in different naturally occurring steroids.1 The brassinosteroid teasterone (1), 2 the steroid glycoside osladin (2), 3 lactones as chiogralactone (3) 4 and dendrosterone (4), 5 and steroidal alkaloids like petisidine (5) 6 and petisine (6) 7 are some examples of naturally occurring steroids bearing such functionality (Figure 1).Recently, we have synthesized laxogenin (7), a steroidal sapogenin isolated from Smilax sieboldi 8 and its C-23 substituted derivatives 8 and 9 and reported that they have shown plant growth promoting activity similar of that of brassinosteroids ( Figure 2). 8 In that report, a protocol based on Brown´s hydroboration for the introduction of the 3β-hydroxy-6-oxo moiety was developed.The 5β,6β-epoxy moiety has been previously used for the preparation of 6-oxosteroids. In particular, Henbest and Wrigley reported 9 that treatment of 5,6β-epoxy-5β-cholestan-3β-ol acetate with BF 3 ·Et 2 O led to the corresponding 3β-acetoxy-5-fluoro-6β-hydroxy-5α-steroid which was oxidized to 3β-acetoxy-5-fluoro-5α-cholestan-6-one using Jones reagent. More recently, an extension of this procedure to stigmasterol has been used to obtain brassinosteroid analogues bearing the 5α-fluoro-6-oxo moiety. 9Owing our interest on steroids bearing such functionality, and after some reports on the highly stereoselective β-epoxidation of ∆ 5 -steroids using biphasic systems involving potassium permanganate and metal salts, 10 we envisaged the β-epoxidation of the C5-C6 double bond followed by the regioselective oxirane ring opening as the key steps for the introduction of an oxygen atom at position C-6. This led us to an alternative protocol for the synthesis of 3β-hydroxy-6-oxo steroids. Fe 2 (SO 4 ) 3 resulted in the highly diasteroselective β-epoxidation of the C5-C6 double bond; only traces of the α-epoxide could be...

The main 1 H and 13 C NMR characteristics of synthetic (25S)-5β-spirostantes are described. Changes on chemical shifts due to substitution at C-23 are briefly analyzed.

(1)H and (13)C NMR spectroscopic data for 5alpha-androstanes and halo-5alpha-androstanes with different substituents at positions C-3, C-9, C-11 and C-17 were examined and assigned by a combination of 1D and 2D NMR experiments. The substituent effects on the (13)C chemical shifts were compared with those of epi-androsterone, used as a reference compound. The coupling constants (n)J((19)F,(13)C) were measured for compounds 6, 8, 11 and 14.

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