J. Org.Chem. 1985,50, 941-945
941
Synthesis and Reactivity of Vinyl Quinone Methides Antonio Zanarotti CNR Centro di Studio per le Sostanze Organiche Naturali, Dipartimento di Chimica, Politecnico di Milano, 20133 Milano, Italy Received June 26, 1984
Vinyl quinone methides 4a and 4b were obtained in high yields by Ag20 oxidation of eugenol (3a) and of 2,6-dimethoxy-4-(2-propenyl)phenol (3b). Vinyl quinone methides (VQMs) reacted with alcohols, with phenols, and with acetic acid giving compounds 5 and 6. As the former rearranged to the latter in the reaction medium, the addition of the reported substrates to VQMs turned out to be wholly regioselective toward the formation of the coniferyl and sinapyl derivatives 6. In contrast, addition of carbon nucleophiles (acetylacetone and EtN02) to VQMs gave both compounds 7 and 8. By reaction of the acetates 6a and 6b (R = CH3CO) with LiAIH4 it was possible to perform a novel synthesis of coniferyl and sinapyl alcohols (la and lb). Treatment of the same acetates with an aqueous solution of NaHC03 reproduced VQMs 4, which by subsequent reduction with N&H4 gave propenylphenols 2 and allylphenols 3. Formation of the latter compounds from coniferyl and sinapyl acetates via VQM is here proposed as a possible biosynthetic pathway.
Quinone methides derived from dimerization of phenoxy radicals are recognized as transient intermediates in the biosynthesis of many natural substances.' For instance, the one-electron oxidation of c6c3 phenol units 1-3 leads to dimers (lignans and neolignans)'r2 and to polymers (1ignin)l via quinone methide intermediates.
Scheme I 3 a,b
AgPo
>
R O Y
OH
4
6
1
3
2
4
a X=H b X=OMe
The biosynthetic and synthetic significance of such quinone methides is well r e c o g n i ~ e dwhereas , ~ ~ ~ little attention has been paid to the possible biosynthetic and synthetic role of quinone methides of the type 4,which can derive from a two-electron oxidation of propenylphenols 2 and allylphenols 3 or from dehydration of alcohols 1. Vinyl quinone methides (VQMs) 4 were reported to have been observed only by UV spectroscopy as transient intermediates in very diluted aqueous solution during the enzymatic oxidation of eugenol4 (3a)or in the flash photolysis of coniferyl (la)and sinapyl (lb) alcohol^.^ In the present paper6 VQMs 4 are reported to have been synthetized and characterized for the first time.I These reactive compounds may be phenylpropanoid building bloks (derived in nature from phenylalanine) which are biosynthetic precurs of lignin, lignans, neolignans, stilb(1) 'Oxidative Coupling of Phenols"; Taylor, W. I., Battersby, A. R., Eds.; Marcel Dekker: New York, 1967. (2) (a) Gottlieb, 0. R. Fortschr. Chem. Org. Naturst. 1978,35,1. (b) Ward, R. S. Chem. SOC.Rev. 1982,11,75. (c) Zanarotti, A. J. Chem. Res. Synop. 1983, 306. (3) For reviews on quinone methids, Bee: (a) Wagner, H. U.; Gompper, R. In "The Chemistry of Quinonoid Compounds"; Patai, S., Ed.; J. Wiley and Son: London, New York, 1974; Chapter 18. (b) Gruenanger, P. In "Houben-Weyl Methoden der Organischen Chemie"; Mueller, E., Bayer, O., Eds.; G. Thieme Verlag: Stuttgard, 1979; Vol. VII/3b, p 395. (4) Pew, J. C.; Connors, W. J.; Kunishi, A. In "Chimie et Biochimie de la Lignin, de la Cellulose et des Hemicellulose"; Acte du Symposim International de Grenoble, Juillet 1964;Impimeries Reunies: Grenoble, 1965; p 229. (5) Leary, G. J. Chem. SOC.,Perkin Trans. 2 1972,640. Hemmingson, J. A.; Leary, G. J. Chem. SOC.,Perkin Trans. 2 1975, 1584. (6) Part of this work has been reported in a preliminary communication: Zanarotti, A. Tetrahedron Lett. 1982,23, 3815. (7) Only one synthesis and characterization of a VQM has been reported 80 far: ball,L. K.; Winstein, s.J. Am. Chem. SOC.1972,94,2196.
4 a, b
6
a X=H b X = OMe
Table I. Reactions of 4a and 4b with Hydroxy Compoundso VQM, ROH, productb 5:6 ratio' entry 3 mmol mmol R (% yield)d (total % yield)d 6ae (80) 3.1 (78) 1 4a 250 Me 3.2 (76) 2 4a 170 Et 6a (80) 3 4a 14 PhCHz 6ae (76) 1.5 (70) 4 4a 14 PhCH=CHCHz 1.8 (72) 6a' (71) 5 4b 250 Me 2.2 (86) 6b (89) 6 4b 170 Et 2.3 (88) 6b (90) 7 4b 14 PhCHz 2.4 (75) 6b (81) 2.8 (80) 8 4b 14 PhCH=CHCHZ 6br (79) 9 4a 12 Ph 6a' (45) 10 4a 12 2-OMeC6H, 6ae (38) 11 4a 12 4-OMeC6H4 6ae (54) 12 4b 12 Ph 6b (75) 13 4b 12 2-OMeC6H4 6b' (40) 14 4b 12 4-OMeC6H4 6b (71) 15 4b 12 4-NOzC6H4 6b (20) 16 4a 230 CH3CO 6a (80) 17 4b 230 CHSCO 6b (80) "The reactions of 4a have been performed in CC14 (80 mL), those of 4b in benzene (40 mL). Catalysts: entries 1 and 5 PTSA (2.3 X mmol), entries 2-4 and 6-8 PTSA (1.5 X mmol), entries 9-15 Et3N (0.4 mmol). bReaction time 1-12 h. cReaction time 10-120 s. dYields based on isolated products and calculated on the starting phenols 3a and 3b. e Compound characterized as its acetyl derivative. 'Compound characterized as its methyl derivative.
enes, and flavonoids. The aim of the present research was to explore their reactivity toward selected nucleophiles in order to evaluate their utility as intermediates for synthesis of these natural product classes. Results Synthesis of VQMs 4a and 4b. Oxidation of eugenol (3a)in CC1, (1 g in 160 mL) at 65 "C with a large excess of Ag,O gave VQM 4a in a yield of over 90%.* VQM 4b
0022-3263/85/1950-0941$01.50/0 0 1985 American Chemical Society
942 J . Org. Chem., Vol. 50, No. 7, 1985
was obtained in the same way, at room temperature, in benzene or chloroform solution. A t a concentration of 1 g in 80 mL compound 4b is stable for hours at room temperature. The 'H NMR analysis of the benzene or chloroform solution thus obtained showed that 4b was formed in a quantitative yieldeg Reactivity of VQMs 4a and 4b. VQMs 4 reacted with primary alcohols, in the presence of traces of p-toluenesulfonic acid (PTSA),leading in 10-120 s to the regioisomers 5 and 6 (Scheme I, Table I entries 1-8); the couples of isomers were easily separated by flash chromatography. When the reactions were allowed to proceed for much longer times (6-12 h) only coniferyl and sinapyl ethers 6 were obtained as benzyl ethers 5 slowly rearranged to 6. This was verified by isolating 5a (R = Me) and 5b (R = Et) from short-time reactions of 4a and 4b with MeOH and EtOH and treating the compounds with catalytic amounts of PTSA in the presence of MeOH or EtOH: 6a (R = Me) and 6b (R = Et) were obtained in quantitative yields.'O VQMs 4 reacted with phenols, in the presence of catalytic amounts of Et3N,11leading to coniferyl and sinapyl ethers 6 within 1-4 h (Scheme I, Table I entries 9-15). All reactions showed the same trend during the course of the reaction, the formation of ethers 6 (increasing amounts) and of a second unstable compound (decreasing) was observed by TLC. In one case (entry l l ) , the reaction was stopped before its completion by cooling and treating the mixture with Ac20 and pyridine: the unstable transient compound was thus identified as the benzyl ether 5a (R = 4-OMeC6H,) by isolation of its acetate. A kinetically controlled formation of the two regioisomers occurred in the reaction of 4a and 4b with acetic acid as well; the unstable acetates 5a (R = CH,CO) and 5b (R = CH,CO), initially formed as the major isomers, rearranged to 6a (R = CH,CO) and 6b (R = CH,CO) in the reaction medium;12thus coniferyl and sinapyl acetates were obtained in 80% yield. The latter compounds gave respectively coniferyl (la) and sinapyl (lb) alcohols in quantitative yields by treatment with LiAlH, in tetrahydrofuran. The fast procedure makes the described syntheses of coniferyl and sinapyl alcohols a convenient alternative to the previously reported method based on LiAlH4reduction of the esters of the corresponding acids.13 Attempts to obtain alcohols la and l b by hydrolysis of their acetates or by addition of water to VQMs proved unsuccessful; VQMs 4 appear to be more stable, in neutral or slightly basic aqueous solution, than coniferyl and sinapyl alcohols and their acetates.14 Indeed, VQMs 4 were (8) The synthesis of VQM 4a, which bears the substitution pattern most frequently found in nature in phenylpropanoid units, has demanded a thorough investigation of the reaction conditions in order to avoid the competitive oxygen-o-phenol coupling. Dimerization was minimized by using (1) a nonpolar solvent, (2) a rather diluted solution, (3) high temperature, (4) and a large excess of Ag,O. Compound 4a could not be fully characterized by 'H NMR as the yield of the reaction was not quantitative, moreover the compound is present as a mixture of the E and Z isomers. (9) In ref 6 was reported the 'H NMR spectrum of 4b in CDC13. Due to signals overlap a coupling constant of 2.0 Hz has been uncorrectly assigned to the protons in the 5- and 7-position; it is evident from the quite clear spectrum of 4b in CGD,now obtained that this coupling constant must be assigned to the protons in the 3- and 5-position as described in the Experimental Section. (10) When reacted with PTSA and alcohol also the methyl derivative of 5a (R = Me) rearranged to the methyl derivative of 6a (R = Me) whereas, in the absence of alcohol, polymeric materials were obtained. (11) Addition of VQM 4b on the aromatic ring of phenols occured in the presence of acids: Zanarotti, A. Tetrahedron Lett. 1982,23, 3963. (12) 5a (R = CH,CO) was initially formed in a 68% yield. The compound was isolated as its acetyl derivative. (13) Allen, C. F.H.; Bayers, J. R. J. Am. Chem. SOC.1949, 71,2683. Freudenberg, K.; Huebner, H. H. Chem. Ber. 1952, 85, 1181.
Zanarotti Scheme I1
R 4 a,b
OMe
bH R 1 = M e R2=N02 7
a X=H
R' = R ~ = C H & O
8
b X = OMe
Scheme I11
Nu( 0u-
IH+ HNu -H+
UJ
OH
dH
8
10
stable for hours in neutral or slightly basic aqueous solution, and on the other hand, treatment of coniferyl and sinapyl acetates with an aqueous NaHC03 solution caused formation of VQMs 4a and 4b. This was confirmed by addition of NaBH, to the solution thus obtained: 6a (R = CH3CO)gave a mixture of eugenol (3a) and isoeugenol (2a) via 4a, 6b (R = CH3CO) the corresponding mixture of 3b and 2b via 4b. In fact, it was verified that the reduction of VQMs 4 with NaBH, led to mixtures of 2 and 3. Product ratio was strongly dependent on the reaction medium; for example, the reductions performed in MeOH-water mixtures gave a 2:3 ratio changing from 0.5 to 2.0 for increasing amounts of MeOH in the solution; it is of interest to note that no traces of the 2 isomers of 2a and 2b were detected by gas chromatographic analysis of the reaction mixtures, in agreement with the fact that for all the reactions in Table I no 2 isomers of the compounds 6 were isolated. Nitroethane and acetylacetone, used as substrates in order to test the reactivity of VQMs 4 toward carbon nucleophiles, gave the regioisomers 7 and 8 in a 8 7 ratio 12.7 (Scheme 11). In these cases, in which the regioselectivity of the addition is exclusively under kinetic control as no rearrangements can occur, the attack at the less substituted end of the VQMs appears to be determined by the steric demands of the nucleophiles. Discussion The observed electrophilic reactivity of VQMs and the regioselectivity of the additions (Le., kinetically controlled preferential attack at the benzylic position and selectivity toward l$-addition for thermodynamically controlled re(f4) Only polymeric materiels were obtained by treatingVQMs 4 with acidic or strongly basic aqueous solutions.
J. Org. Chem., Vol. 50, No. 7, 1985 943
Vinyl Quinone Methides
Table 11. Significant 'H NMR Data of Compounds 5 and 6" comDd H-7 H-8 4.60 (d, 6.5) 5.87 (ddd, 6.5, 10.2, 16.2) 5a,b R = Me 5.92 (ddd, 6.6, 10.2, 17.1) 4.71 (d, 6.8) 5a,bR = Et 5.90 (ddd, 6.5, 10.0, 17.2) 4.80 (d, 6.5) 5a,bR = PhCHz 4.81 (d, 6.8) 5.90 (ddd, 6.8, 10.5, 17.0) 5a,bR = PhCH=CHCHz 5.90 (ddd, 6.6, 10.2, 16.2) 4.53 (d, 6.6) 5b, R = Me 4.63 (d, 6.6) 5.90 (ddd, 6.6, 10.2, 17.2) 5b, R = Et 4.74 (d, 6.2) 6.00 (ddd, 6.2, 10.0, 17.2) 5b, R = PhCHz 6.00 (ddd, 6.5, 10.0, 17.0) 4.79 (d, 6.5) 5b, R = PhCH=CHCHZ 6.03 (ddd, 6.0, 10.2, 16.2) 5.43 (d, 6.0) 5a,bR = 4-OMeC6H4 5.97 (ddd, 5.7, 10.0, 16.6) 6.26 (d, 5.7) 5a,bR = CH&O 6.20 (dt, 5.3, 15.7) 6a,bR = Me 6.58 (d, 15.7) 6.10 (dt, 6.0, 15.9) 6.50 (d, 15.8) 6a, R = Et 6a,b R = PhCHz 6.26 (dt, 6.0, 16.0) 6.60 (d, 16.0) 6.20 (m) 6.63 (d, 15.6) 6a,*R = PhCH=CHCHp 6.14 (dt, 6.0, 15.7) 6.53 (d, 15.7) 6b, R = Me 6.17 (dt, 6.0, 16.0) 6b, R = Et 6.50 (d, 16.0) 6.17 (dt, 6.0, 16.0) 6.54 (d, 16.0) 6b, R = PhCHz 6.25 (m) 6b,' R = PhCH-CHCHp 6.51 (d, 16.0) 6.32 (dt, 5.7, 16.5) 6.71 (d, 16.5) 6a,bR = Ph 6.38 (dt, 5.7, 16.5) 6.70 (d, 16.5) 6a,bR = 2-OMeC6H4 6.30 (dt, 5.2, 16.0) 6a,bR = 4-OMeC6H4 6.66 (d, 16.0) 6.25 (dt, 5.7, 16.5) 6.60 (d, 16.5) 6b, R = Ph 6.36 (dt, 4.8, 16.5) 6.66 (d, 16.5) 6b,bR = 2-OMeC6H4 6.20 (dt, 5.2, 16.0) 6.60 (d, 16.0) 6b, R = 4-OMeC6H4 6.26 (dt, 5.4, 16.0) 6.70 (d, 16.0) 6b, R = 4-NOzCsH4 6.10 (dt, 6.5, 16.5) 6.58 (d, 16.5) 6a, R = CH3C0 6.17 (dt, 6.0, 15.5) 6.56 (d, 15.5) 6b, R = CH3CO
H-9 5.22 (m) 5.18 (m) 5.25 (m) 5.25 (m) 5.21 (m) 5.20 (m) 5.25 (m) 5.25 (m) 5.25 (m) 5.22 (m) 4.05 (d, 5.3) 4.08 (d, 6.0) 4.19 (d, 6.0) 4.20 (d, 5.7) 4.06 (d, 6.0) 4.10 (d, 6.0) 4.28 (d, 6.0) 4.10 (d, 5.7) 4.65 (d, 5.7) 4.73 (d, 5.7) 4.60 (d, 5.2) 4.63 (d, 5.7) 4.72 (d, 4.8) 4.58 (d, 5.2) 4.78 (d, 5.4) 4.69 (d, 6.5) 4.70 (d, 6.0)
OSpectra were determined in CDC13 solution at 90 MHz; chemical shifts are expressed in ppm (a) relative to internal Me,Si, coupling constants J in Hz. Compound characterized as its acetyl derivative. Compound characterized as its methyl derivative. Scheme V
Scheme IV
