J. zyxwvut Org. Chem. zyxwvut 1983,48, zyxwvu 5111-5112 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONML 5111 excess of 10% Pd/C in EtOAc;" (PPh3),RhC1 in benzene12) were unsuccessful. 2-Norbornanoneethylene dithioketal was obtained in 90% yield from 2-norbornanone following the procedure used for preparation of 1: 13C NMR (CDCl3) 6 72.2 zyxwvutsr (5, 1 C), 51.8 (d, 1 C), C), 26.6 (t, 1 C); 'H NMR (CDCl,) 6 3.4-3.0 (m, 4 H), 2.4-2.1 (m, 3 H), 2.0-1.1 (m, 7 H); IR zyxwvutsr (film) 2960 (s), 2872 (m), 1452 (m), 1307 (w), 1275 (w), 970 (w) cm-'; MS, mlz (relative intensity) 186 (M', 43), 158 (loo), 125 (19),93 (la), 67 (31), 66 (26). Anal. Calcd for C9H14S2: C, 58.06; H, 7.53; S, 34.41. Found: C, 58.22; H, 7.74; S, 34.69. Treatment of 2-norbornanone ethylene dithioketal with bro- mine under the same conditions as those used for 1 yielded - 100% of a 8:l mixture of two products. The products were separated by column chromatography on silica gel with methylene chloride-pentane (1:4) as the eluent. The major product was isolated in 75% yield (2.0 g) as a bright yellow oil and was shown to be 3-bromo-2-norbornanone ethylene dithioketal(198% pure by GC, DEGS 170 "C): 13C NMR (CDC13) 6 78.8 (s), 69.2 (d), 53.1 (d), 49.2 (d), 40.1 (m), 39.5 (m), 38.0 (t), 28.3 (t), 26.9 (t); 'H NMR (CDCl,) 6 4.33 (d, J = 2.4 Hz, 1 H), 3.4-3.0 (m, 4 H), 2.6-2.4 (m, 2 H), 2.2-1.2 (m, 6 H); IR (film) 2965 (s), 2920 (s), 2875 (m), 1475 (w), 1450 (m), 1420 (w), 1308 (w), 1210 (w), 780 (w), 660 (w) cm-'; MS, m/z (relative intensity) 266 (M', 6), 264 (M', 6), 238 (7), 236 (6), 187 (9), 186 (ll), 185 (loo), 157 (20), 156 (13), 131 (12), 105 (9), 81 (16). Anal. Calcd for CSH13S2Br:C 40.75; H, 4.91; S, 24.15; Br, 30.19. Found: C, 41.03; H, 4.87; S, 24.07; Br, 29.91. The minor product (isolated as white crystals in -10% yield &r recrystallization from pentane) exhibited the following characteristics: mp 37-39 "C; 13CNMR (CDCl,) 6 213.7 (s), 71.9 (s), 48.6 (d), 48.4 (d), 39.4 (m), 39.0 (m), 37.4 (t), 27.0 (t), 25.5 (t); 'H NMR (CDCl,) 6 3.7-3.2 (m, 4 H), 2.8 (br s, 2 H), 2.3-1.5 (m, 6 H); IR (KBr) 2960 (s), 2870 (m), 1742 (s), 1450 (m), 1282 (m), 1180 (m), 1078 (m), 755 (m) cm-I; MS, m/z (relative intensity) 200 (M+,6), 172 (ll), 144 (22), 131 (loo), 71 (39). Anal. Calcd for CsH12S20: C, 54.05; H, 6.05; S, 32.01. Found: C, 54.25; H, 6.27; S, 31.80. Acknowledgment. This work was supported by a grant from the Research Council of the Republic of Croatia (SIZ 11). Registry No. 1, 50511-34-7; 2, 87450-45-1;BIZ,7726-95-6; 4-homoadamantanone, 24669-56-5; 1,2-ethanedithiol, 540-63-6; 2-norbornanone ethylene dithioketal, 172-69-0; 3-bromo-2-nor- bomanone ethylene dithioketal, 87450-46-2; 2,3-norbomanedione monoethylene dithioketal, 87450-47-3; 2-norbornanone,497-38-1. 49.4 (t, 1 C), 39.8 (t, 1 C), 39.7 (m, 2 C), 36.8 (d, 1 C), 28.3 (t, 1 (11) (a) Eelinton. G.: Lardv. I. A.: Rauhael. R. A.: Sim. A. G. J. Chem. Soc. 1964;1f54. (bj Parham;$. W.; Rider, zyxwvutsrq M. T.; Hasek, R. W. zyxwvutsrq J. Am. Chem. SOC. 1963, 75, 1647. (12) Birch, J. A.; Walker K. A. M. Tetrahedron Lett. 1967, 1935. Simple and Efficient Synthesis of 3-Oxo-2,6,6-trimethylcyclohex-l-ene-l-carboxylic Acid, a Key Synthon for (*)-Strig01 Manuel Gonzklez Sierra,* Roland0 A. Spanevello, and Edmundo A. Rdveda* Instituto de Quimica Orgbnica de Sintesis (CONICET- UNR), Facultad de Ciencias Bbsicas, Suipacha 531, 2000 Rosario, Argentina Received April 21, 1983 Methyl 3-oxo-2,6,6-trimethylcyclohex-l-ene-l- carboxylate (lb) is an important A-ring intermediate for the synthesis of (f)-strigol (3), a well-known potent wit- chweed seed germination stimulant. due to the possible role of 3 as a control agent in witchweed-infested fields, much effort has recently been devoted to improve the preparation of la and/or lb. 0022-326318311948-5111$01.50/0 la, R = H 2a, R = H b, R = Me b,R=Me 3 Sih et a1.l in their total synthesis of 3, described two sequences for their preparations. One of them startes from cy-cyclocitral, requires three steps, and produces la in 36% overall yield. The second one, from P-cyclocitral, gives lb after five steps in 40% overall yield but requires chro- matographic purification. Later, Pepperman2 carried out a careful analysis of the fist sequence showing lower yields than those reported, but by introduction of a two-step oxidation procedure the final yield was significantly in- creased. Recently, Brooks et al.3 reported a five-step synthesis of lb from a-ionone with a 48% overall yield. We describe herein a simple, efficient one-step preparation of la from a mixture of a- and P-cyclocitrals. Treatment of the mixture of cy- and P-cyclocitrals* by a modification of the procedure for the oxidation of allylic methylenes previously described5 produced a ca. 9:l mix- ture of two carboxylic acids. In a first run they were chromatographically separated as methyl esters and spectroscopically characterized as lb, the major product, and 2b. Subsequent experiments showed that from the crude reaction product pure crystalline la could be ob- tained and Jones oxidation of the mother liquors furnished and additional amount of la, increasing the total yield to better than 70%. Experimental Section Melting points were determined on an Ernst Leitz hot-stage microscope and are uncorrected. IR spectra were measured with a Beckman Acculab 8 spectrophotometer. 'H and 13C NMR spectra were recorded at 80.13 and 20.15 MHz, respectively, on a Bruker WP zyxwv 80 SY spectrometer in CDCl, solutions. GC analyses were conducted by using a Perkin-Elmer 820 gas chromatograph equipped with a 0.3 X 300 cm 15% Carbowax 20M column. Silica gel GFw4 (type 60) was utilized for TLC, and spots were visualized by staining with anisaldehyde-sulfuric acid.6 Methyl 3-0xo-2,6,6-trimethylcyclohex-l-ene-l-carboxylate (lb) and Methyl 3-Hydroxy-2,6,6-trimethylcyclohex- l-ene- 1-carboxylate(2b). The crude product obtained from the NBS oxidation as described below (500 mg) was dissolved in EGO (20 mL) and treated with an excess of diazomethane. Chromatog- raphy of the resulting product over silica gel (50 g) with hexane and increasing amounts of EtOAc resulted in the isolation of lb (405 mg) and 2b (46 mg) as colorless oils. lb: IR (film) 1727, 1678, 1620, 1240, 1053, 1020 cm-'; 'H NMR 6 1.24 (s,6 H, C-6, 2 X Me), 1.72 (s, 3 H, C-2, Me), 1.90 (m, 2 H, H-5), 2.55 (m, 2 H, H-4), 3.83 (s, 3 H, OMe); 13CNMR 6 12.8 (q, C-2, Me), 26.6 (q, (1) Heather, J. B.; Mittal, R. S. D.; Sih, C. J. J. Am. Chem. SOC. 1976, (2) Pepperman, A. B. J. Org. Chem. 1981,46, 5039. (3) Brooks, D. W.; Kennedy, E. J. Org. Chem. 1983,48, 277. (4) Gedye, R. N.; Arora, P. C.; Deck, K. Can. J. Chem. 1971,49,1764. (5) Finucane, B. W.; Thomson, J. B. J. Chem. SOC. D 1969, 1220. (6) Stahl, E. "Thin-Layer Chromatography, A Laboratory Handbook", 2nd. ed.; Springer-Verlag: West Berlin, Heidelberg, New York, 1969; p 857. 98, 3661. 0 1983 American Chemical Society