Indian Journal of Chemistry Vol. 51B, July 2012, pp. 1027-1031 Note Studies on the reaction of N-bromo- succinimide in dimethylsulphoxide: Part X — Action on cholest-4-en-3,6-dione B P Pradhan, Utpal Debnath, Nagendra Narayan Pradhan & Pranab Ghosh* Natural Product and Polymer Chemistry Laboratory, Department of Chemistry, University of North Bengal, Darjeeling, West Bengal 734 013, India E-mail: pizy12@yahoo.com Received 7 December 2010; accepted (revised) 18 April 2012 The reaction of cholest-4-en-3,6-dione, 1 with N-bromo- succinimide in dimethyl sulphoxide furnishes three compounds which have been identified as 2,7α-dibromocholest-1,4-dien-3,6- dione 2, 2,2,7α-tribromocholest-4-en-3,6-dione, 3 and 2α,7α- dibromocholest-4-en-3,6-dione 4 by various spectroscopic methods. In our previous communications we have reported the reaction products of N-bromosuccinimide in dimethyl sulphoxide on various reactive sites viz. (a) olefinic double bonds (i) with suitably placed carbo- xyl/methoxycarbonyl/hydroxyl group 1 (ii) without any functional group at its vicinity as in the case of cholesteryl acetate 2 , (iii) isopropenyl double bond where ring enlargement is possible as in the case of lupenyl acetate 3 , (iv) isopropenyl double bond where ring enlargement is restricted as in the case of moretenyl acetate 4 ; (b) keto functional group of 3- keto-triterpenoids with active hydrogen at C-4 as in the case of friedelin 5 , cerin and cerin acetate 6 or the C-4 is blocked by gem dimethyl group as in the case of lupanone and methyl betulonate 7 and (c) keto oximes of 3-ketotriterpenoids 8 . As the products of the reaction in each case are of mechanistic value, the investigation of the reaction of N-bromosuccinimide (NBS) in dimethyl sulphoxide has been extended to the present titled compound to unveil the nature of the products formed in the reaction. Results and Discussion A solution of cholest-4-en-3,6-dione 1 dissolved in minimum amount of chloroform was mixed with dimethyl sulphoxide. NBS was added in excess in small proportions at a time so that the temperature did not rise. The solution was kept under darkness for thirty days and worked up in the usual way. The product obtained was chromatographed over neutral silica gel column. Elution with petroleum ether-ethyl acetate(2%), petroleum ether-ethyl acetate(4%) and petroleum ether-ethyl acetate(5%) furnished compounds 2, 3 and 4 respectively. The compound 2 purified by repeated crystal- lizations from CHCl 3 -MeOH that was analyzed for C 27 H 38 Br 2 O 2 had the m.p. 200-02°C. IR spectrum showed absorption at 1660 cm -1 and UV showed absorption at 259 nm indicating the presence of α,β- unsaturated carbonyl function. The higher wavelength in the UV spectrum (1 shows absorption at 250 nm) showed that one of the bromine atoms is attached to the olefinic double bond of the α,β-unsaturated carbonyl group 9,10 . MS showed molecular ion peaks at m/z 556, 554, 552 in the ratio of 1:2:1(M + ) indicating the presence of two bromine atoms in compound 2; the other fragments appeared at m/z 476, 475, 474, 473 [M + -HBr], 447, 445, 395, 361,247,233, 230, 215(base), 213, 133, 57(96%), and 55(98%). 1 H NMR spectrum of compound 2 showed five methyl groups of which two appeared as singlets at δ 0.77 and 1.24 and three appeared as doublets at δ 0.86, 0.87, and 0.93 (J = 6.5Hz each). The doublet centered at  4.41 integrating for one proton with a small coupling constant of 3.5Hz indicated that one of the bromine atoms is axially attached to the C-7 carbon atom, the geminal proton (β-equatorial) which couples with C- 8(β-axial) proton; the singlet at  6.57 integrating for one proton is obviously due to C-4 proton and the downfield singlet at  7.45 integrated for one proton is definitely due to the vinyl proton at C-1 position. 13 C NMR accounted for all 27 carbons. The compound 2 exhibited singlets downfield at  177.5 and 194.0 due to C-3 and C-6 carbonyl groups respectively. The singlet at  124.6 and doublet at 152.8 were assigned to carbons involved in ,-unsaturated double bond with respect to carbonyl group. C-4 carbon appeared at  127.1 as a doublet, C-5 appeared at  157.9 respectively. C-7 carbon appeared at  41.9. Hence from the above spectral data the compound 2 has been assigned as 2,7α-dibromocholest-1,4-dien-3,6-dione. It is noteworthy that the substitution at the C-2 by a bromine atom causes an upfield shielding effect on the 13 C NMR of the carbonyl carbon at C-3 position