SHORT COMMUNICATIONSHydrogenation of the double C=C bond in the B ring of ecdysteroids leads to their 7,8-dihydro analogs which are structurally related to brassinosteroids [1]. Attempts to obtain 7,8-dihydro analogs from 20-hydroxyecdysone and its derivatives by reaction with alkali metals in liquid ammonia [2], which is widely used for selective reduction of double bonds in conjugated ketones of the steroid series were unsuccessful [3,4]. Better results were obtained with the use of lithium tetrahydridoaluminate, but the reduction of the C 7 =C 8 double bond was accompanied by reduction of the 6-oxo group [5].Catalytic hydrogenation of ecydisteroids over 10% Pd/C in ethanol was not selective, and the reaction mixtures contained a number of products, including unreacted initial compounds, instead of the desired 7,8-dihydro derivatives [6]. Suksamrarn et al. [7] recently reported on stereoselective catalytic hydrogenation of ∆ 7 -6-oxo steroids in the presence of sodium nitrite [7]; however, 7,8-dihydro analogs of 20-hydroxyecdysone and its derivatives could not be obtained according to this procedure [5].We found that addition of excess metallic sodium to a solution of 20-hydroxyecdysone (I) in methanol and subsequent hydrogenation over 10% Pd/C at room temperature under atmospheric pressure (reaction time ~3 h) gives 20-hydroxy-7,8-dihydroecdysone (II) as the only product which was isolated in ~90% yield by column chromatography on silica gel. Likewise, 2,3:20,22-di-O-isopropylidene-20-hydroxyecdysone (III) and 2,3 : 20,22-di-O-isopropylideneponasterone A (IV) were converted under the same conditions into the corresponding dihydro analogs V and VI (Scheme 1).It is necessary to follow the above operation sequence to obtain 7,8-dihydro analogs of ecdysteroids. When ecdysteroid was added to a preliminarily prepared solution of sodium methoxide in methanol and the catalyst was then added, no hydrogenation of the C 7 =C 8 bond occurred. It was also necessary to bubble hydrogen through the reaction mixture contain-I, II, R 1 = R 2 = R 3 = R 4 = H; III-VI, R 1 R 2 = R 3 R 4 = Me 2 C; I, II, III, V, R 5 = OH; IV, VI, R 5 = H.
