1718
J. Org. Chem. 1991,56, 1718-1725
isolated yield by method C from 42.93 g (0.271 mol) of l a and 31.33 g (0.359 mol) of tert-amylamine in 50 mL of toluene. 'H NMR 6 0.76 (t,J = 7.45 Hz,3 H), 1.21 (s,6 H), 1.62 (q, J = 7.45 Hz, 2 H), 2.18 (8, 3 H), 3.26 (s, 2 H), 6.69 (br s, 1 H, NH). 13C NMR 6 7.96 (CHJ, 25.99 (2 C, CHS), 30.72 (CHS), 32.57 (CHZ), 50.72 (CH3,53.96 (C), 164.70,205.49. IR: 3340,3080,2980,2940, 2890,1730,1665,1560,1468,1420,1370,1340,1208,1169 cm-'. EI-MS: m l r (re1 int) 171 (l),156 (3), 142 (35), 72 (43), 58 (1001, 43 (43). HRMS: 171.1260 (calcd for C9HI7NO2171.1259).
Acknowledgment. We would like t o thank Drs. John Hyatt, Bob Clemens, John Hubbs, Kevin Edgar, and Joe
Zoeller for helpful discussions on various aspects of this work and the expert technical assistance of Mr. James Little, Mr. Robert Hale, and Ms. Donna Kilgore in obtaining mass spectral data. J.S.W. would like to express a special word of thanks to Mr. R. D. Burpitt for his enthusiastic support of this work. Supplementary Material Available: A listing of additional spectral data for compounds 6,8-12,14, and 15 as well as 'H and 13C spectra for compounds 5, 16,23,24,and 25 (12 pages). Ordering information is given on any current masthead page.
Construction of Trifluoromethylated Quarternary Carbons via Diels-Alder Reactions of 2-(Trifluoromethy1)propenoic Acid Derivatives: Application to the Synthesis of 16,16,16-Trifluororetinal' Yuji Hanzawa, Makoto Suzuki, Yoshiro Kobayashi, and Takeo Taguchi* Tokyo College of Pharmacy, 1432-1 Horinouchi, Hachioji, Tokyo 192-03, Japan
Yoichi Iitaka Faculty of Medicine, Teikyo University, 359 Ohtsuka, Hachioji, Tokyo 192-03, Japan Received March 5, 1990
Diels-Alder reactions of 2-(trifluoromethy1)propenoicacid (1) and its 2,2,2-trifluoroethylester 2 with various dienes gave adducts in good yields. In Lewis acid catalyzed Diels-Alder reactions of 2, the combination of the Lewis acid and solvent proved to be crucial. For example, polymerization occurred in the case of TiCl4-CH2Cl2 and adduct formation was observed with TiClrtoluene and TiC12(O-i-Pr)z-CH2C12.In the TiCl,-catalyzed Diels-Alder reaction of the 2-(trifluoromethy1)propenoate ester 3 of Dpantolactone with butadiene, the formation of the R configurational quaternary carbon bearing the trifluoromethyl group was confirmed by X-ray crystallographic analysis of the adduct 20. No polymerization of ester 3 could be detected in the presence of TiC14 in CH2Clz. The reactivity difference between 2 and 3 in TiC1,-catalyzed Diels-Alder reactions may possibly be attributable to the stabilization of the 3-Tic4 complex or weakening of Lewis acidity by coordination of the (4), which is considered to be an bidentate ester group of 3. The synthesis of all-trans-16,16,16-trifluororetinal important analogue for the study of retinal-bindingprotein, was conducted on the basis of these results. Comparison of the absorption maximum (362 nm) of 4 with other trifluororetinals 34 (362 nm) and 35 (382 nm) reported previously suggests the possibility of a large torsion of the conjugated system between the ring and the polyenal side chain in 4.
Introduction The introduction of fluoro substituents into biochemically important molecules has attracted much attention in organic, biological, and medicinal chemistry for studying the biochemical process of the parent molecule a n d / o r enhanced or altered Activity of the fluoro analogue.* For this purpose, new a n d / o r more effective methods for t h e selective introduction of fluoro substituents into organic molecules should be developed. The growing number of fluorinated compounds, commercially available, should serve as starting materials for preparing functionalized fluoro molecules. It is also interesting t o compare t h e reactivity of t h e fluorinated molecule with the related hydrocarbon during reactions. In recent examinations of the reactivity of fluorinated compounds, the electronic effect of the fluoroalkyl group had been found t o considerably affect the stereochemical results in the asymmetric reduction of fluoroalkyl ketones with binaphthol-mediated (1) Preliminary communication: Hanzawa, Y.; Suzuki, M.; Kobayashi, Y. Tetrahedron Lett. 1989, 30, 571. (2) (a) Filler, R.; Kobayaahi, Y. Biomedicinal Aspects of Fluorine Chemistry; Kodansha, Ltd.,and Elsevier Biomedical Press: Amsterdam, 1982. (b)Welch, J. T. Tetrahedron 1987,43,3123.
aluminum hydride reagent,3 in nucleophilic attack on trifluoromethylated cyclohexanone derivative^,^ in the a-hydroxylation of the enolate prepared from T,T,ytrifluoroester d e r i ~ a t i v e ,and ~ in Pd(0)-mediated intramolecular lactonization.6 We describe herein the results of Diels-Alder reactions of 2-(trifluoromethyl)propenoic acid (1) and esters 2 and 3 for construction of quaternary carbons bearing a trifluoromethyl group and the preparation of 16,16,16-trifluororetinal (4).'
Results and D i s c u s s i o n 2-(Trifluoromethyl)propenoicacid (1)and ita derivatives are strong acceptors in the Michael reaction and have been utilized in the synthesis of the fluoro analogues of nucleoside bases and amino acid
derivative^.^ T h e cyclo-
(3) Hanzawa, Y.; Kawagoe, K.; Kobayaahi, Y. Chem. Pharm. Bull. 1987,35, 2609. (4) Cieplak, A. S.; Tait, B. D.; Johnson, C. R. J. Am. Chem. SOC.1989, 111,8447.(5) Morizawa, Y.; Yasuda, A.; Uchida, K. Tetrahedron Lett. 1986,27, 1833.
(6) Hanzawa, Y.; Ishizawa, S.; Ito,H.; Kobayaahi, Y.; Taguchi, T. J. Chem. SOC.,Chem. Commun. 1990,394. (7) Numbering of the retinoids system waa used.
0022-3263/91f 1956-1718$02.50/0 0 1991 American Chemical Society
J . Org. Chem., Vol. 56, No. 5, 1991 1719
Trifluoromethylated Quaternary Carbons addition of 1with diazomethane has been reported to give a 1,8dipolar cycloadd~ct.~ Although 1 and ita esters are expected to be good dienophiles owing to the electronegative trifluoromethyl and carbonyl functions,1°td the best of our knowledge, there are no r e p o on ~ their Diels-Alder reactivity. Dim
Table 1. Diels-Alder Reactions of 2 with Isoprene with and without Lewis Acid Catalysts reactn ratio yield temp of 9 ("C) (f/isoprene) Lewis acid (equiv) solvent 1/1.1 none CH& 92 120 1/1.1 EtAlClz (0.8) CHZC1, 51 -20 TiC1, (0.10) CH2Cl, polymrzn -20 'I3 TiC1, (0.13) toluene 38 0 1/3 0-25 1/30 TiClZ(O-i-Pr),(1.0) CHzCIP 54 1/3.6 BFs.OEt, (0.8) CHzClz polymrzn 0
-
"The ratio of the regioisomer (para isomerlmeta isomer); 295%.
0
~
4
Initially, we examined the Diels-Alder reactivity of 1 with typical dienes. Reaction with cyclopentadiene proceeded instantaneously at room temperature to give a mixture of stereoisomers 5a and 5b (2:l ratio) (Scheme I). The stereochemistry of the adduct 5 was determined by iodo lactonization of the crude reaction mixture (12-K1, NaHC03), in which only endo carboxylic acid 5a was cyclized to give iodo lactone 7. It should be noted that the ratio (endo-COOH/exo-COOH = 2) of the cyclopentadiene adduct 5 was the inverse of that obtained in reactions of a-alkylated propenoic acids with cyclopentadiene," which give the exo-COOH product preferentially. Considering the steric size of the trifluoromethyl group being similar to that of an isopropyl group,12the pseudo n-character or the electronic effect of the trifluoromethyl group may possibly affect the approach of the dien0phi1e.l~ Similar results have also have been observed in Diels-Alder reactions of 3-(trifluoromethy1)propenoic acid and 3,3,3trifluoro- l-(phenylsulfonyl)propene.14 Adducts were also obtained in good yields with acyclic dienes at higher temperature (110-120 "C, sealed tube). Regiochemistry of the isoprene adduct 8 favored the para isomer 8a (para/meta = l O / l ) . The structure of the major isomer of 8a was confirmed by IR and 13C NMR spectra of the lactone 13 obtained by treating a mixture of 8a and 8b with iodine in basic media followed by the reduction (nBu3SnH, AIBN) of iodide 11. In the IR and 13C NMR spectra of 13,a typical &lactone carbonyl absorption ( u 1750 cm-') and seven 13CNMR signals at 23.5, 25, 30,46, 82,125, and 169 ppm indicated the lactone 13 to have a symmetry plane, which would not be present in the ~~~
~~
(8) (a) Fuchikami, T.; Ojima, I. Tetrahedron Lett. 1982,23,4099. (b) Fuchikami, T.; Yamanouchi, A.; Ojima, I. Synthesis 1984, 766. (c) Ki-
tazume, T.; Ohnogi,T. Ibid. 1988,614. (d) Ojima, I.; Kato, K.; Nakahaehi, K.J . Org. Chem. 1989,54,4511. (e) Ymazaki, T.; Ohnogi, T.; Kitazume, T. Tetrahedron: Asymmetry 1990, I , 215. (9)Fuchikami,T.; Shibata, Y.;Suzuki,Y.Tetrahedron Lett. 1986,27, 3173. (10) Diels-Alder reaction of 3,3,34rifluoropropene: (a) Ojima, I.; Yatab, M.; Funcikami, T. J . Org. Chem. 1986, 47, 2051. (b) Gaede, B.; Balthazor, T. M. Ibid. 1983,48,276. (11)(a) Meek, J. 5.; Trapp, W. B. J. Am. Chem. SOC.1957, 79,3909. (b) Martin, J. 0.; Hill, R. K. Chem. Reu. 1961, 61,537. (12) Bott, G. J . Am. Chem. SOC.1980,102,618. (13) Bamgardner, C. L.; Bunch, J. E.; Whangbo, M.-H. Tetrahedron Lett. 1986,27, 1883 and references cited therein. (14) (a) McBee, E.T.; Keogh, M. J.; Levek, R. P.; Weseler, E. P. J . Org. Chem. 1983,38,632. (b) Taguchi, T.; Hoeoda, A.; Tomizawa, G.; Kawara, A.; Mmuo, T.; Suda, Y.; Nakajima, M.; Kobayashi, Y . Chem. Pharm. Bull. 1987,36,909.
Table 11. Lewis Acids in Diels-Alder Reactions of 3 with IsoDrene" 'Ondtns yield (%) diastereomeric Lewis ratioC temp ("C) time (h) acidb of 10 TiCl, 80 2/98 -23 94 6/92 -23 ZrCl, 67 40/55 -23 2 EtAlClz 65 22/32 -23 2 Et,AlCl -23 BFS.OEt, polymrzn -23-rt 24 SnC1, recvry of 3
"The reaction was carried out in CH2Clz/hexane7/1 (3, 1 mmol/20 mL). No detectable amount of the meta isomer of 10 waa observed. bAnequimolar amount of Lewis acid to 3 was used. CDeterminedby GLC and NMR ('H and I9F). Table 111. Diels-Alder Lewis diene acid" cyclopentadiene TiCl, butadiene TiCL isoprene TiCl; 16 TiCl, 16 ZrC1;
Reactions of 3 with Dienes at -23 OC yield diastereomeric (%) excess product 95* 98c 6a (endo-COOR) 86 98( 20 80 96c 10 81 45* 19 70 57
"0.13 equiv of Lewis acid to 3 (1 mmol/l mL of CH2C12)was used. The ratio of endo/exo = 8. Determined by GLC and NMR ('H and I9F). dSee ref 23.
product from the minor iodo lactone 12.15 The reaction of 2,4-pentadien-l-oll6 with 1 gave volatile lactone 14 (48%). An attempted intramolecular Diels-Alder reaction of 15 prepared by reaction of 2-(trifluoromethy1)propenoyl chloride" with 2,4-pentadien-l-o1 failed to produce the cycloadduct. It is thus evident that lactone 14 is formed through an intermolecular Diels-Alder reaction of 2,4pentadien-l-oP with 1 and subsequent lactonization. Reaction of ester 218 with trans-l-methoxy-3-[(trimethylsilyl)oxy] 1,3-butadiene (Danishefsky's diene)lS proceeded exothermically at 0 OC to give a crude primary product, which was converted directly to enone 17 with acid (TsOH/MeOH) in 78% yield. Attempts to effect the Diels-Alder reaction of 2 with isoprene by Lewis acid catalysis resulted in extensive polymerization in the TiC14/CH2C12system (Table I). Although polymerization was prevented by TiC1, in toluene or TiC12(0-i-Pr)220in CH2C12,only moderate yields of 9 were obtained. The relation between the acidity of the Lewis acid and basicity of the solvent is important for successful Diels-Alder reactions of 2. It should be noted that the regiochemical
-
(15) In the iodo lactonization of the mixture of the isoprene adduct
8, &lactoneand y-lactone were obtained from the mejor 80 (para homer)
and minor 8b (meta isomer) adducts, respectively. (16) White, J. D.; Sheldon, B. G. J. Org. Chem. 1981,46, 2273. (17) Prepared by the reaction of 1 with phthaloyl dichloride in a good yield. (18) Jpn Kokai Tokkyo Koho, J P 60 42,252; Chem. Abstr. 1986,103, 14147211. (19) Danishefsky, S.;Kitahara, T. J. Am. Chem. SOC.1974,96,7807. (20) Mukaiyama, T.; Ishida, A. Chem. Lett. 1976, 319.
1720 J. Org. Chem., Vol. 56,No.5, 1991
Hanzawa et al. Scheme I
_.
6OoR
5b (exu-Co
