Use of t-butyldimethylsilyl cyanoacetate for preparation of α-cyano ketones

Use of t-Butyldimethylsilyl Cyanoacetate for Preparation of a-Cyano Ketones SOUMITRA S. GHOSH T h e Salk Institute Biotechnology/lndustrial Associates, Inc., P.O. Box 85200,San Diego, California 92138-9216 SYNOPSIS The synthesis of t-butyldimethylsilyl cyanoacetate and the reactions of its anion with acyl donors are described. The reagent was found to be the method of choice for the syntheses of a-cyano ketone substrate analogues for carboxypeptidase A. These compounds have been shown to be potent mechanism-based inactivators for the enzyme. INTRO D UCTlON A major objective of our research has been the design of mechanism-based inactivators for the enzyme, carboxypeptidase A (CPA). The key element of the strategy was the observation by Kaiser and his co-workers that CPA can catalyze the stereospecific abstraction of protons from activated methylene groups of ketonic substrate analogues.’-4 This enzyme-assisted enol formation was exploited to carry out a,P elimination reactions to produce Michael acceptors, which were chemically inert, however, in the active ~ i t e . More recently, we ~.~ have demonstrated that the a-cyano ketones, (R)2-benzyl-5-cyano-4-oxopentanoic (I),5and its acid regioisomer, (R)-3-benzyl-5-cyano-4-oxopentanoic acid (I1 are potent mechanism-based inactivators of CPA (Figure 1). ),‘I I I1 The mechanism of inactivation presumably occurs via an isomerization-acylation process (Scheme I). We propose that CPA-catalyzed enolization of the irihibi tors, followed by reprotonation at the nitrogen, toms the highly reactive a-ketoketeni- 1990 ,John Wiley & Sons, Inc. O(IOfi 3325 /90/OlOlO5-04 $04.00 Biopolyniers, Vol. 29, 105-108 (1990) ‘c CCC I I1 Figure 1 mine. This intermediate is then rapidly trapped by an active-site nucleophile to inactivate the enzyme. A crucial step in the synthesis of I1 required the regiospecific introduction of an acetonitrile anion equivalent to the precursor, 111, to form the acyano ketone derivative, IV. The alkali acetonitriles, such as the Li salt,7could not be considered because of their high reactivities and hence lack of discrimination. The condensation of t-butyl cyanoacetate anion7 with the acid chloride of 111 was successful in providing the desired addition product. However, attempts to remove the tbutoxycarbonyl group under a variety of acidic conditions, or by treatment with trimethylsilyl iodide,9 failed to give I V in any appreciable yield. In this report, we describe the use of t-butyldimethylsilyl (TBDMS) cyanoacetate anion as a superior acetonitrile anion equivalent and describe some of its applications. TBDMS cyanoacetate was prepared by reacting t-butyldimethylsilyl chloride (TBDMSC1) with cyanoacetic acid in the presence of triethylamine 105 106 GHOSH \I/ Zn++ \I/ zn++ Scheme I Scheme I1 (a) (COCI),, DMF (b) [NCCHCO,TBDMSI-Na' and (c) H'. NCCH,CO,H TBDlllS (21 NCCH,CO,TBDMS Figure 2 (Figure Z).'O The silyl ester was isolated in 90% yield by filtration and then concentration of the filtrate under vacuo. Reaction of the sodium salt of the reagent with freshly prepared acid chloride of 111, followed by acid quench, directly provides the a-cyano ketone derivative (Scheme 11; 48% overall yield from 111). Similarly, the a-cyan0 ketone derivative of the half ester of benzylmalonic acid was obtained in 38% overall yield. The reactivity of the TBDMS cyanoacetate anion toward anhydrides is modulated by the nature of its countercation. Thus, while its Na salt did not react a t all with (R)-benzylsuccinic anhydride, the Li derivative added facilely to give a 1:1 mixture of I and I1 in 87%isolated yield after silica gel flash chromatography (Figure 3). The two regioisomers are well resolved by C-8 semi-preparative reversephase high performance liquid chromatography (HPLC). The efficacy of the reagent was borne out in the subsequent syntheses of 14C-labeledI and 11. 14 C-labeled TBDMS cyanoacetate was prepared in two steps by reacting chloracetic acid with 14CNaCN," followed by esterification with TBDMSCI. Reaction of its Li salt with (R)-benzylsuccinic anhydride and then purification by reverse-phase chromatography afforded the desired radiolabeled compounds. This sequence of reactions is far more economical than the use of the very expensive 14 C-labeled acetonitrile. THDMS cyanoacetate was designed to allow a facile one-step introduction of a CH,CN group by taking advantage of the known acid lability of TBDMS esters. In its reactions with acid chlorides and anhydrides, the silyl esters of the initially formed products rapidly hydrolyze under the acidic work-up conditions and then decarboxylate to give rise to a-cyano ketones. The ease of its synthesis and application recommends its use as an attractive alternative to other synthetic methods for the preparation of cyanomethylated compounds. EXPERIMENTAL Infrared spectra were obtained on a Perkin Elmer 1420 spectrometer or a Mattson Cygnus 25 Fourier transform (FT) ir instrument. Optical rotations were taken on a Perkin Elmer 241 polarimeter. Nuclear magnetic resonance spectra were recorded on a Nicolet 360 MHz FT-nmr using tetramethylsilane as an internal standard. All mass spectra were USE OF TBDMS CYANOACETATE obtained a t the Rockefeller University Biotechnology Mass Spectrometric Research Resource. (TB DMS) Cyanoacetat e To a solution of cyanoacetic acid (1.7 g, 20 mmoles) and TBDMS chloride (3.14 g, 20 mmoles) in 22 mL of anhydrous ethyl acetate at 0°C and under N, was added 2.71 mL of triethylamine, resulting in the immediate precipitation of triethylamine hydrochloride. The reaction mixture was stirred a t 0°C for 30 min and then allowed to warm to an ambient temperature. The suspension was filtered, and the salt precipitate was washed with ethyl acetate (2 x 20 mL). The filtrates were combined and concentrated to afford 3.55 g (90%) of the TBDMS ester as a clear oil. 'H-nmr (CDC1,): 6 3.47 (s, 2H), 0.96 (s, 9H), 0.32 (s, 6H); ir (thin film) 3455, 2934, 2862, 2266, 1731, 1471 cm-'. (R)-2-Benzyl-3-Carbomethoxy-Propionyl Chloride To 0.33 g (1.5 mmoles) of (R)-2-benzyl-3-carbomethoxy-propionic acid in 19 mL of anhydrous benzene were added 0.019 mL of dimethylformamide (DMF) and 0.163 mL (1.88 mmoles) of oxalyl chloride, resulting in rapid gas evolution. After stirring for 30 min a t 23"C, the solution was concentrated, taken up in 20 mL tetrahydrofurane (THF), and evaporated once again to ensure removal of unreacted oxalyl chloride. Traces of solvent were removed under high vacuum, and the acid chloride was used immediately for the next step without further purification. (R)-Methyl 3-Benzyl-4-0~0-5-Cyano-Pentanoate (IV) To 0.072 g (3 mmoles) of sodium hydride in 30 mL of anhydrous THF under N, was added a solution of 0.591 g (3 m o l e s ) of TBDMS cyanoacetate in 5 mL THF over 5 min, and the reaction was allowed to proceed for 15 min. The reaction mixture was then cooled to - 78"C, and a solution of the (R)-2benzyl-3-carbomethoxy-propionyl chloride (max 1.5 mmoles) in 7 mL of THF was added dropwise over a period of 15 min. After stirring a t -78°C for 30 minutes, the mixture was warmed to 23°C over 30 minutes and then quenched with 27 ml of 0.06 N HCl. The solution was extracted with ethyl acetate (3 x 40 mL), and the organic phase was washed with brine, dried, and concentrated. Two drops of triethylamine were added to the residue, and the mixture was purified by flash chromatography. 107 Elution with 30% ethyl acetate in hexane containing 0.5% triethylamine separated a minor contaminant, which was followed by elution with ethyl acetate to afford 172 mg (48% from the acid) of the product as an oil. 'H-nmr (CDCI,): 6 7.37-7.14 (m, 5H), 3.64 (s, 3H), 3.54 (d, J = 19.7 Hz, lH), 3.31 (m, lH), 3.05 (d, J = 19.7 Hz, lH), 2.89 (m, lH), 2.71 (dd, J = 13.3, 7.1 Hz, lH), 2.52 (dd, J = 17.6, 3.5 Hz, 1H); ir (thin film) 3028, 2953, 2261, 1730, 1438 cm-'. HRMS (CI) calculated for C,,H160,N (M + 1) 246.1130, found 246.1126. [ a ] : + 75.2 (c 0.7, ethyl acetate). Syntheses of I and I1 To 0.414 mL of a 2.41M solution of butyl lithium (1 mmole) under argon were added 1 mL of anhydrous THF and then 0.14 mL (1 mmole) of diisopropylamine, and the mixture was stirred for 15 min a t 0°C. The solution was cooled to -78"C, and a solution of 0.203 g of TBDMS cyanoacetate in 2 mL of THF was added over 5 min. After stirring for 30 min, a solution of 0.094 g (0.5 mmoles) of (R)-benzylsuccinic anhydride in 3 mL of THF was added over a period of 5 min. The mixture was stirred a t -78°C for 1h and then a t 4°C for 3.5 h. The reaction was quenched by the addition of 0.5 mL of 10% HC1, taken up in 40 mL of ether, and washed with 15 mL of water. The aqueous layer was extracted with 2 x 15 mL of ether, and then the ethereal solutions were combined, washed with 20 mL of brine, dried over MgSO,, and concentrated under vacuo. The crude products were purified by flash chromatography using 50% ethyl acetate in hexane containing 0.5% acetic acid to afford 0.1 g of a 1:l mixture of I and I1 as an oil (87% yield). Spectral Data for I. 'H-nmr (CDCI,): S 7.34-7.16 (5H, m), 6 3.37 (2H, bs), 6 3.32-3.17 (2H, m), 6 2.83-2.76 (2H, dd, J = 9.3, 13.5 Hz), S 2.54--2.48 (lH, dd, J = 4.4, 17.4 Hz); ir (thin film) 3200 (br), 2916, 2264, 1731, 1713 cm-'. MS (CI) 232 (M + 1). Spectral Data for 11. 'H-nmr (CDCl,): 6 7.37-7.14 (m, SH), 3.41 (br.d, J = 18.6 Hz, lH), 3.28 (m, lH), 3.01 (d, J = 19.7 Hz, lH), 2.89 (m, 2H), 2.73 (dd, J = 13.4, 7.2 Hz, lH), 2.55 (dd, J = 17.9, 3.7 Hz, 1H); ir (thin film) 3500-3000 (br), 2263, 1730, 1393 cm-'. HRMS (CI) calculated for C,,H,,O,,N (M + 1) 232.0974, found 232.0970. [a]$ + 77.8 (c 0.7, ethyl acetate). 10s GHOSH This work was supported by funds from SIBIA. I thank Dr. Shahriar Mobashery for helpful discussions, and I owe a debt of gratitude to Dr. Tom Kaiser, without whom this work would not have been possible. REFERENCES 1. Sugimoto, T. & Kaiser, E. T. (1978) J . Am. Chem. SOC. 100,7750-7751. 2. Sugimoto, T. & Kaiser, E. T. (1979) J . Am. Chem. SOC.101, 3946-3951. 3. Nashed, N. T. & Kaiser, E. T. (1981) J . Am. Chem. SOC.103, 3611-3612. 4. Nashed, N. T. & Kaiser, E. T. (1986) J . Am. Chem. SOC.108, 2710-2715. 5. Mobashery, S., Ghosh, S. S., Tamura, S. Y. & Kaiser, E. T., Proc. Natl. Acad. Sci. U.S.A. (in press). 6. Ghosh, S. S., Spratt, T. E., Miller, W. T. & Kaiser, E. T., submitted. 7. Kaiser, E. M. & Hauser, C. R. (1968) J . Org. Chem. 33, 3402-3404. 8. Lawsson, S.-O., Larsen, E. H. & Jacobsen, H. J. (1965) Arkiu. Kemi. 23, 453-462. 9. Jung, M. E. & Lyster, M. A. (1977) J . Org. Chem. 99, 968-969. 10. Mobashery, S. &Johnston, M. (1985) J. Org. Chem. 50, 2200--2202. 11. Inglis, J. K. H. (1932) in Organic Syntheses collective Vol. I, Gilman, H. & Blatt, A. H., Eds., Wiley, New York, pp. 254-256 Received May 4,1989 Accepted June 9, 1989