Nitro compounds are important precursors in organic synthesis,^1-3 the reduction of conjugated nitroalkenes to ketones being one important transformation, and various reagents have been used.^4-9 Expensive reagents and stringent reaction conditions are the major drawbacks of most of these reported procedures.

The reaction of SnCl₂?2H₂O with 6-nitro-Δ5-steroids has been reported to give 5α-hydroxy-6-keto steroids¹⁰ and the Nernst potential difference of Cd/Cd²⁺, Mg/Mg²⁺ is higher than that of Sn/Sn⁴⁺,¹¹ it was therefore expected that the combination of CdCl₂?Mg in a protic solvent would be a useful reducing system for the transformation of conjugated nitroalkenes to other functionalities. Our surmise indeed proved to be correct when we treated 3β-acetoxy-6-nitro-cholest-5-ene in tetrahydrofuran (THF) with CdCl₂?Mg?H₂O system at room temp., within 15 min 3β-acetoxy-5α-cholestan-6-one was obtained in 85% yield. In this reaction 15% of 3β-acetoxy-5β-cholestan-6-one was also obtained.

Table 1
Reduction of nitroalkenes to ketocompounds^a

Entry	Substrate	Productb	Yieldc	Time
1	3β-Acetoxy-6-nitrocholest-5-ene	3β-Acetoxycholestan-6-one	100%	15 min
2	3β-Acetoxy-6-nitrostigmast-5-ene	3β-Acetoxystigmastan-6-one	90%	15 min
3	6-Nitrocholest-5-ene	Cholestan-6-one	95%	15 min
4	3β-Chloro-6-nitrocholest-5-ene	3β-Chlorocholestan-6-one	90%	15 min
5			90%	15 min
6	3-Nitrocholest-2-ene	Cholestan-3-one	95%	15 min
7	1-Nitrocyclohex-1-ene	Cyclohexanone	85%	15 min
8	1-Phenyl-2-Nitropropene	1-Phenyl-2-Propanone	80%	15 min
9			60%d	15 min

Notes

1. ln a typical reaction, 1 mmol of the nitroalkenes in dry THF (3 cm³) was added with stirring to a mixture of anhydrous CdCl₂ (8 mmol). Mg powder (15 mmol). Then water (100 mmol) was added dropwise to this mixture over a period of 5 min. The exothermic reaction took place instantly with the liberation of hydrogen. After 15 min, the reaction mixture was thoroughly washed with CH₂Cl₂ (200 cm³). The CH₂Cl₂ solution was dried (Na₂SO₄). evaporated under reduced pressure to give a residue which on purification by TLC gave the product as a gum or solid. All products were characterized by spectral analysis and direct comparison with authentic compounds.
2. For entries 1-5, both 5α-ketosteroid and 5β-ketosteroids are formed in a ratio of 85:15, respectively.
3. The reaction was conducted with other protic solvents such as MeOH. EtOH. glycols, diglyme and in each case the yield was found to be same.
4. ln this reaction 37% of the unreacted starting material was also recovered from the reaction mixture.

In order to assess the scope of the reaction, 1-nitro-1-cyclohexene, β-methyl-β-nitro-styrene and the compound obtained from natural products¹² (Entry 9) were treated with the CdCl₂?Mg-H₂O system and in each case corresponding ketocompounds were obtained in 60?85% yield (Table 1). Although the mechanism of this reduction process is not clearly defined at this stage, the following observations were noted. Anhydrous CdCl₂ and Mg in anhydrous THF does not react even after long exposure. But addition of a few drops of water to this mixture initiates vigorous exothermic reaction with evolution of hydrogen (deuterium when D₂O is added) and the formation of metallic cadmium particles. Since CdCl₂ is not hydrolysed readily and the system has pH ~7, hydrogen is released from the water present and is possibly replaced by magnesium.