1082
Alkylation of Dianions of 6-Keto Esters Stuart N. Huckin and Larry Weiler” Contribution from the Department of Chemistry, University of British Columbia, Vancouver 8, British Columbia, Canada. Received August 13,1973 Abstract: The dianions from a variety of P-keto esters have been generated using 1 equiv of sodium hydride and 1 equiv of n-butyllithium or methyllithium or 2 equiv of lithium diisopropylamide. The dianions react with a range
of alkylating agents to produce y-alkylated products exclusively in good yield. Reaction of these dianions with a , w dihaloalkanes yields cyclic 1 : 1 adducts, cyclic 1 : 2 adducts, or acyclic 1 : 2 adducts, depending on the alkylating agent and the reaction conditions. This method offers ready access to a wide range of compounds which were previously difficult to obtain,
R
ecently we had envisioned the synthesis of a number of aromatic antibiotics and antitumor agents which can be broadly classified as acetogenins. In this analysis the importance of 6-keto esters and their derivatives became increasingly obvious. Many reactions can be effected at the a carbon of a P-keto ester. These reactions proceed via the enol or enolate 1 and 0
0
O*
R
R
l , * = H or
-
follow from the well known ability of P-dicarbonyl systems to yield the enol or enolate, resulting from transfer of the hydrogen on the carbon flanked by the carbonyl groups and the reactivity of this system with a variety of electrophiles. The apparent greater versatility of P-keto esters compared to P-diketones encouraged us to further pursue the former type compounds in our synthesis of acetogenins. Whereas it is usually difficult to differentiate the two carbonyl carbons in a P-diketone, it is relatively easy to achieve reaction specifically at either one of the two carbonyl groups in a P-keto ester. Prior to our work, efforts to find a method to induce reactivity at the y carbon of a P-keto ester had produced very limited success. The dianion 2 of ethyl aceto0
II/CO,R
-
-
2, R = Et
3. R = Me
acetate has been generated by treating ethyl acetoacetate with potassium amide. However, alkylation of 2 proceeds in variable yield. For example, Wolfe, et al., found no alkylation of 3 with benzyl chloride or nbutyl bromide, and only fair yields (27-37 %) of alkylation with methyl iodide and ethyl bromide.2 Using a similar procedure Sugiyama, et al., were able to alkylate dianion 2 with benzyl bromide and n-propyl iodide in yields of 60-65 %. Unfortunately, these latter workers did not report the time for the alkylation reaction and ( 1 ) J. H. Richards and J. B. Hendrickson, “Biosynthesis of Steroids, Terpenes, and Acetogenins,” W. A. Benjamin, New York, N. Y., 1964. (2) J. F. Wolfe, T. M. Harris, and C. R. Hauser, J . Org. Chem., 29, 3249 (1964). (3) N. Sugiyama, M. Yamamoto, T. Takano, and C. Kashima, Bull. Cbem. SOC.Jap., 40,2909 (1967).
this appears to be a critical parameter (uide infra). Hagarty has reported that the dianion 2 can be alkylated with a variety of allylic chlorides in yields of 1366%.4 There are several possible reasons for the relatively low yields in the alkylation of dianion 3 compared to the high yield in the alkylation of P-diketo dianions and ,&keto aldehyde dianions.5 It is known that metal amides, the bases used in the generation of 2, can cause aminolysis of esters.6 Wolfe, et al., also suggested that formation of dianion 2 may not be complete under these conditions, namely potassium amide in liquid ammonia. A final complication may be the reaction temperature (-33’) and the reaction time (usually 1 hr). For these reasons we sought another base to generate the dianion 2. This base should be strong enough to lead t o complete formation of 2, it should be able to be used in solvents other than liquid ammonia, and finally it should be nonnucleophilic.
Results and Discussion Treatment of methyl acetoacetate with 2 equiv of n-butyllithium in hexane, benzene, or T H F at temperatures from -78 t o 25’ gave mainly carbonyl addition compounds and only traces of methyl acetoacetate were recovered on work-up. Brieger and Spencer have also noted that n-butyllithium adds to the carbonyl groups of P-keto esters.’ However, Mao, et al., overcame this difficulty in the case of phenylacetone by first generating the monoanion of phenylacetone with a metal amide which protected the carbonyl group from nucleophilic attack and then treatment of the monoanion with n-butyllithium gave the dianion in excellent yield.8 Our initial investigations of P-keto esters involved a similar approach. One equivalent of sodium hydride converted methyl acetoacetate into its monoanion ; treatment of this monoanion with n-butyllithium gave the dianion 3 (eq 1). As in the above example of phenylacetone, the carbonyl groups in the monoanion were protected from nucleophilic attack by the organometallic reagent. However, it is important to note that (4) J. D. Hagarty, U. S. Patent 3,565,928 (1971); Chem. Absrr., 74, 140999 (1971). ( 5 ) T.M. Harris and C. M. Harris, Org. React., 17,155 (1969).
( 6 ) (a) K.-W. Yang, J. G . Cannon, and J. G . Rose, Tetrahedron Lett., 1791 (1970); (b) B. Singh, TetrahedronLert., 321 (1971). (7) G . Brieger and D. G . Spencer, TerrahedronLett., 4585 (1971). (8) C. Mao, C. R. Hauser, and M. L. Miles, J. Amer. Chem. Soc., 89, 5303 (1967). (9) L. Weiler, J . Amer. Cbem. Soc., 92,6702 (1970).
Journal of the American Chemical Society / 96:4 / February 20, 1974
1083
n
Table I. Alkylation of Dianion 3 from Methyl Acetoacetate0
n
~~
3
excess organolithium reagent is not used.7~10111This same procedure has recently been applied to P-keto phosphonium salts12 and P-keto phosphonates. l 3 In the second step of the generation of 3, the n-butyllithium can be replaced by methyllithium. Using this procedure the dianion 3 could be generated in a variety of dipolar aprotic, ether, or hydrocarbon solvents, although solubility problems were occasionally encountered in the less polar solvents. Generally T H F proved very satisfactory and, if solubility problems arose, the addition of small amounts of HMP to the mixture usually alleviated these difficulties. When the dianion 3 was quenched with DzO and deuterated trifluoroacetic acid, it was found that the recovered methyl acetoacetate contained two to three deuterium atoms per molecule. The a-deuterium atoms were exchanged with aqueous sodium bicarbonate and, after these exchanges, the mass spectrum of the resulting methyl acetoacetate (4) showed that the sample of 4 contained 2 f 2 do, 96 f 2 % Q, and no detectable d2and d3 compound. The reaction sequence to give 4-dl and the mass spectral fragmentation pattern indicated that the deuterium atom was on the y carbon. This was shown beyond doubt by the nmr spectrum of the methyl acetoacetate-dl which has a three-proton singlet at 6 3.70 (OCH3), a two-proton singlet at 6 3.46 (CH2), and a 1:1:1 triplet and a very small singlet at 6 2.2 (DCH2C0 and CH3CO) which integrated t o two protons. The ratio of this triplet and singlet was determined in a variety of ways and found to be 49:l. J H D= 2.15 Hz and ~ C -H ~ ~C H ~=D -0.016 ppm, which is in good agreement with the values found for related systems. l 4 Following this evidence that we had generated the dianion 3 (eq l), we proceeded to investigate its chemistry. The first study involved the alkylation of the intermediate 3.9 When a solution of 3 in T H F was treated with a range of alkylating agents, a facile reaction often occurred and the monoalkylated products 5
z
0
0
3
R 5
(eq 6) were isolated in good yield (Table I). The alkylation does not proceed in all cases; for example, reaction of dianion 3 with cyclohexyl chloride and tert-butyl bromide, not unexpectedly, gave no alkylation products but only returned methyl acetoacetate after l hr at 25". Cyclohexyl bromide reacted (10) S. N. Huckin and L. Weiler, to be published. (11) (a) R. M. Coates and J. E. Shaw, J. Amer. Chem. Soc., 92, 5657 (1970); (b) E. Piers and R. D. Smillie, J. Org. Chem., 35,3997 (1970). (12) J. D. Taylor and J. F. Wolfe, J . Chem. SOC.,Chem. Commun., 876 (1972). (13) P. A. Grieco and C. S. Pogonowski, J. Amer. Chem. SOC.,95, 3071 (1973). (14) (a) G. V. D. Tiers, J. Chem. Phys., 29, 963 (1958); (b) H. J. Berstein and N. Sheppard, J. Chem. Phys., 37, 3012 (1962); (c) C. G. MacDonald, J. S . Shannon, and S. Sternhill, Ausr. J. Chem., 17, 38 (1964).
~~
Bp of5, "C (mm)
Me1 EtBr
81 (99) 84 (96)
i-PrI
73 (90) 72 (95) 0 0 (5P 83 (98) 81 (94)
39-40 (1 . 5 ) 77-79 (14) 52-54 (2) 66-67 (0.8)
z
RX 4
~
Yield of 5.b
n-BuBr r-BuBr C-c~Hncl c-C6Hl1Br CH2=CHCH2Br CsHjCH2Cl
99-10 (15) 102-103 (0.4)
See Experimental Section for details of conditions employed. to distilled products. The numbers in parentheses refer to vpc yields. c See text. a
* Yields refer
very slowly and a low yield of alkylation product could be obtained after 1 hr at 25". It should be noted that the yields of 5 reported in Table I are after 5-15 min reaction at 0-25". The reaction time could be shortened but for convenience we chose not to. When the dianion 3 was treated with allyl bromide at -23" for 1 hr, the monoalkylated product 5 (R = allyl) was isolated in 28-32x yield together with 60-65 recovered 4. The same reaction at -78" for 1 hr failed to yield any product and 9 2 z 4 was recovered. Hence the alkylation (eq 2 ) is quite temperature dependent, and this could be one source of the low yields reported in previous attempts to alkylate dianion 2.2-4 We also investigated the alkylation of 3 with allyl bromide at room temperature and found that after 15 min all the starting material had been consumed but a number of new products were appearing in the vpc of the crude reaction mixture. Hence it appears that maximum yields of alkylation can conveniently be obtained when the reaction is run in the range 0-25 O (see Experimental Section). The alkylation appears to proceed equally well in ether, tetrahydrofuran (THF), dimethoxyethane, and hexamethylphosphoric triamide (HMP), and we chose T H F for most of our subsequent studies. In some instances the dianions of complex 0-keto esters were not soluble in THF and it was found that addition of small amounts of HMP led to dissolution of these dianions. Alkylation of these dianions in the mixed solvent systems proceeded normally. A small range of bases was investigated to determine their efficiency in generating dianion 3 in THF. It was found that 3 could be formed directly from methyl acetoacetate using 2 equiv of lithium diisopropylamide, and alkylation with benzyl chloride gave 5 (R = benzyl) in 83 % yield (isolated). However, lithium bis(trimethylsily1)amide apparently is not a strong enough base to generate dianion 3 and, after the attempted alkylation with benzyl chloride, only starting j3-keto ester was recovered. This supports the earlier suggestion of Wolfe, et al., that perhaps potassium amide in ammonia is giving only partial formation of the dianion of ethyl acetoacetate. The recently discovered H+ arpoon bases15 also appear to be equally proficient in generating dianions of /3-dicarbonyl compounds. l6 It was important to show that the alkylation of 3 had in fact given 5 (eq 2). In all of these reactions only one al-
z
(15) R. A. Olofson and C. M. Dougherty, J . Amer. Chem. Soc., 95, 582 (1973). (16) J. F. Kingston andL. Weiler, unpublished results.
Huckin, Weiler 1 Dianions of @-KetoEsters
1084
kylation product could be isolated. The vpc of the crude reaction mixture occasionally showed the presence of other products; however, these were always present in less than 1 - 2 z of the major product. The spectral evidence clearly showed that the alkylation product was indeed 5. The ir spectra of these products had bands at -1740 and -1710 cm-' due to the 0-keto ester system, thus eliminating the possibility of the O-alkylation product. The nmr spectra of these products have a two-proton singlet at 6 3.4-3.5 (-C(=O)CH,C(=O)-) and the absence of a three-proton singlet at ca. 6 2.3 was definitive in showing that the alkylation had occurred at the y-carbon as shown in eq 2 . This was further collaborated by the mass spectra of 517 and by vpc differences between authentic a-alkylated methyl acetoacetate and the alkylation products 5. It was also found that the dialkylated products 6 could be obtained by alkylation of dianion 3 and generation of dianion 7 with an additional equivalent of nbutyllithium, followed by a second alkylation (eq 3). 0
0
0
R
3
7
1
1.
RX
2 H,O'
C0,Me
R
6
(3)
However, we generally obtained higher yields of dialkylated product 6 if we isolated and purified the monoalkylated product 5 before proceeding with the second alkylation. The results of several of these reactions are contained in Table 11. Tablea. Alkylation of Dianion from Substituted@Keto Esters RCHZCOCHR 'C02R ' ' R R ' R"
n-Bu H 12-BU H n-Bu H Me H CsHhCHz H H H H Me -(CH2)3a
Me Me Me Me Me Et Et Me
RX
Yield,a
z
BP, "C (mm)
81 77 63 76 86 77 78 67
54-56 (0.3) 82-84 (0.6) 122-1 23 (0.4) 104-105 (0.5) 99-100 (0.3) 106-108 (15) 102-105 (15) 110-112 (14)
Me1 CHFCHCHZBK CeHaCH2Cl Ce,HaCH*Cl Me1 CHFCHCH2Br EtBr CHFCHCH~BK
Yieldsrefer to distilled products.
As can be seen from these results, a variety of dianions from 0-keto esters undergo y alkylation and this reaction is not impaired by a-alkyl groups. As before (Table I), these reactions are very facile, being complete in less than 30 min at 0-25". This should be compared to the a alkylations of 0-keto esters which typically require several hours in refluxing solvents, such as ethanol. This would explain why we detect no a alkylation in reaction 2 or 3, even with an excess of alkylating agent. The alkylation of dianion 3 with dihaloalkanes was also investigated. In this reaction, there are two pos(17) L. Weiler, Can. J . Chem., 50,2707 (1972).
Journal of the American Chemical Society
96:4
sible products, 9 and 10, depending on the fate of the intermediate 8 (eq 4). This intermediate may undergo n
0
C0,Me 0 10
9
an intramolecular alkylation to give the cyclic keto ester 9, or it may react with a second molecule of dianion 3 to give 10. When 1 equiv of 1,3-dibromopropane was added to a solution of 3, two products were obtained in approximately equal amounts. These products were separated by chromatography and found to be methyl 2-oxocyclohexanecarboxylate1B (9, n = 3) and dimethyl 3,9-dioxoundecanedioate (10, n = 3). In view of the difference in reactivity of niono- and dianions of 0-keto esters (uide supra), formation of the bis adduct 10 should be favored by an excess of dianion 3. In fact, when only 0.5 equiv of 1,3-dibromopropane was added to a solution of 3, the only product isolated, in 77% yield, was the bis 0-keto ester 10 ( n = 3). Conversely, to favor the formation of the cyclic product 9, an excess of dianion should be avoided, and it was found that addition of a dilute solution of 3 to a large excess of 1,3-dibromopropane did give mostly the cyclic product 9. This procedure, however, produced low yields and would not be convenient if the dihaloalkane was difficult to obtain or costly. A double dilution experiment was performed and was found to give mainly the cyclic product 9 ( n = 3) in 68z yield. A small amount (1 1 %) of 10 ( n = 3) was also produced. Reaction of dianion 3 with 0.5 equiv of 1,lO-dibromodecane gave dimethyl 3,16-dioxoactadecanedioate (10, n = 10) in almost quantitative yield. The cyclization of this compound to a muscone intermediate is under investigation. When dibromomethane was employed in the alkylation (4), no evidence for the cyclic product 9 ( n = 1) or the bis @-keto ester 10 ( n = 1) was obtained. However, a new compound was produced in reasonable yield, and the spectral and analytical data indicated that the compound had structure 12, which arises from an 0
L C O , M e
CH,Br,
r o
1
0
internal aldol condensation of the expected intermediate 11. This allows an easy entry into the reduced (18) S. J. Rhoads, J. C . Gilbert, A. W. Desora, T. R. Garland, R . J. Spanger, and M. J. Urbrigkit, Tetrahedron, 19,1625 (1963).
1 February 20, 1974
1085
homophthalic acids of type 12 and should make these compounds more readily available. l9
Experimental Section All melting points, which were recorded on a Kofler micro hot stage, and boiling points are uncorrected and are reported in "C. The infrared spectra were recorded on a Perkin-Elmer Model 700 spectrometer and were calibrated with the 1601 cm-l band of polystyrene. The ultraviolet spectra were recorded on a Unicam Model SP 800 or a Cary Model 14 spectrophotometer. The proton nuclear magnetic resonance spectra were recorded on either a Varian T-60 or HA-100 spectrometer and the chemical shifts are reported in 6 units from internal tetramethylsilane. Low-resolution mass spectra were recorded on an AEI MS-9 or Atlas CH-4b mass spectrometer, and the high-resolution spectra were recorded on the MS-9 spectrometer. Both instruments were operated at an ionizing potential of 70 eV. Vapor phase chromatograms were obtained on a Varian Aerograph Model 90 P3 chromatograph using one of the following: column A, 5 ft X in. column of 3 % SE-30 on 60-80 mesh in. column of 20% DEGS Chromosorb W; column B, 5 ft X on 60-80 mesh Chromosorb W. The supports used for all micro-thin-layer and preparative thin-layer chromatography were silica gel PF254 (Merck). Merck silica
