Uec. 5, 1962
COMMUNICATIONS TO
THE HYDROLYSIS OF METHYL O-FORMYLBENZOATE. PARTICIPATION OF THE NEIGHBORING ALDEHYDE GROUP I N THE -HYDROXIDE ION AND MORPHOLINECATALYZED REACTIONS Sir :
The carboxylate ion, imidazole, carboxylic acid, carboxamide, and aromatic and aliphatic hydroxyl groups can participate in intramolecular catalyses which may serve as models for the intracomplex reactions in enzymic catalysis.2 Recently Newman and Hishida3 explained the exceptional hydrolytic reactivity of certain methyl substitutedo-benzoylbenzoates in terms of the initial attack of hydroxide ion on the keto group of the substrate. We have investigated the hydrolytic reactions of the similar compound, methyl oformylbenzoate, using hydroxide ion and morpholine as catalysts. Both reactions are exceedingly facile; kZoH- = 2000 M-lsec.-' (hi2 a t pH 8.5 = 115 seconds)4 ; the morpholine catalysis is kinetically a complex reaction involving an intermediate formed in appreciable concentration (Fig. 1). These are the fastest nonenzymatic hydrolyses of methyl esters known in aqueous solution a t 25', and presumably involve the participation of the o-formyl group in the hydrolytic process. The rate constant for the alkaline hydrolysis of methyl o-formylbenzoate may be calculated to be 1.3 X l o p 2 mole-lsec.-l, using the ratio of rate constants of the o-nitro- and p-nitrobenzoate esters, and the rate constant of the p-formylbenzoate ester (corrected to aqueous solution). Since the experimental rate constant is over lo5 faster than that calculated for the compound solely on the basis of its substituent (electronic and steric) effect, the direct involvement of the formyl group as suggested by Newman and Hishida3 must be operative. The formation and subsequent intramolecular reaction of the intermediate I, containing an alkoxide ion in a position directly adjacent to an ester grouping, presumably will account for the enhanced reactivity found here. The reaction H
.OH
458'3
THE EDITOR
0.17
2 0.16 Q,
a
c)
e,
B
2 0.15 2
0.14
0 1 2 3 4 1 0 50 100 Time, seconds. Fig. 1.-The morpholine-catalyzed hydrolysis of methyl o-formylbenzoate; [morpholine] = 0.242 M ; p = 1.0; T 25.0'; pH 8.74; measurements 0 taken on a spectrophotometer equipped with a stopped-flow mixing device (cf. F. J. Kezdy and M. L. Bender, Biochemistry, in press (1962)); solid line, calculated curve (see text).
hydroxylamine, bisulfite ion, and Tris gave complex side reactions; while morpholine was a strong catalyst for hydrolysis. The morpholine catalysis was shown spectrophotometrically to produce o-formylbenzoate. I t was possible to observe (at the isosbestic point of the reactant and product) the formation and decomposition of an unstable intermediate (Fig. l ) . Two simple possibilities may explain the behavior in Fig. 1; Equation 2 which postulates the reversible formation of the intermediate B, or a similar system in which both reactions are irreversible. k,
kiW1
A
a
~
B
+
C
(2)
kt
Equation 2 can explain the dependence of the decay of the intermediate on morpholine concentration of I is suggestive of the intra-complex reaction of approaching a saturation value while two irreversseroxide ion (the anion of the serine hydroxyl group) ible steps cannot. The reaction is not general in the catalytic steps of the enzyme chymotrypsin.6 base-catalyzed; therefore morpholine acts as a Since nucleophiles other than hydroxide ion can nucleophile in step k l , a conclusion supported by add to carbonyl groups, such reagents were tested the observation of an intermediate of lower extincas catalysts for this reaction. Of the bases and/or tion coefficient (presumably an adduct of morphonucleophiles tested, imidazole, diethanolamine, line and the aldehyde group). Equation 2 has carbonate, bicarbonate, monohydrogen phosphate, been tested by catalyses using four morpholine dihydrogen phosphate, and azide ions were with- concentrations from 0.25 to 0.025 144. Using the out significant effect; N-methylhydroxylamine, general kinetic solution for Eq. 2,6 a consistent I
(1) This research was supported by a grant from the National Science Foundation. (2) M. L. Bender, Chem. Revs.,60, 53 (1960). (3) M. S. Sewman and S. Hishida, J. Am. Chem. SOC.,84, 3582 (1962). The authors are grateful for the opportunity of seeing this manuscript before publication. (4) Aqueous solution a t 2 5 ' . Excellent pseudo first-order plots were obtained in 11 run4 from p H 5 3 t o 9 3. ( 5 ) M. L. Bender, J .4m. C h c m . Soc., 84, 2582 ( 1 S G O ) .
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(6) The general solution of Eq.2 is of the form B = Q ( e - a l e-@) where i3 and a,the apparent rate constant of the decay of the intermediate, are functions of k i , M, ka and kr. See s. W. Benson, "The Foundations of Chemical Kinetics," McGraw-Hill Book Co., New York, 1960, p. 41. Kinetically Eq. 2 cannot be distinguished from ka[M] kl['] f- A R, but mechanistically this scheme does nut kz seem probable.
c
I -
4590
CORlhlUNlC.%TIONST O
THE
EDITOR
Vol. 84
set of data is found, yielding kl = 21.5 M-l, indicated the multicomponent nature of the alkakz = 0.24 and k3 = 0.033 set.-' a t p 1.0 and pH loid extract.1,3--6 Evidence is presented herewith 8.7. These data yield the calculated solid line for assignment of structure I to an alkaloid isolated in Fig. 1, in good agreement with experiment.’ from the acetone-insoluble portion of the strong The calculated and experimental curves for the base fraction, for which we propose the name other data are in reasonable agreement. cyclobuxi?ae.’ Cyclobuxine represents a novel type On the basis of the above evidence mechanism of steroidal alkaloid; i t appears to be the first ( 3 ) for the rnorpholine catalysis can be p o s t ~ l a t e d . ~recognized ~~ to contain a cyclopropane ring and thc first with a substitution pattern a t C-4 and C-14 which is intermediate in the biogenetic scheme, between lanosterol- and cholesterol-type steroids. Cyclobuxine (I), C25H420NZ, m.p. 245-247’ dec., 9S0,’shows Amax 6.00, 11.20 p* (terminal methylene) and n.m.r. peaksY 5.20 and 5.43 (2H, doublets, J < 1 c./s.; terminal inethylene), 5.92 (lH, octuplet, J’s 3, 7 , 9.5 c./s.; CHICHOH-CH), 7.53 and 7.57 (6H, two XCHa), 8.87 and 9.03 (6H, two tertiary CH3);8.92 (3H, doublet, J 6 c./s.; one sec. CH,,) and 9.12 arLd9.95 11 Ii 7 (2H, doublets, J 4 c./s. ; cyclopropyl methylene). Cyclobuxine was converted to several crystalline derivatives: e.g., the diliydrobromide, CZHW ON2Br2, m.p. 288-292’ dec. ; the N.N‘-diniethyl derivative,, CZiH460N2, 1n.p.20-1-205O dec., [cy]% f 99O, n.m.r. peaks a t 5.07, 5,3S, 5.98, 8.87, 9.03, The morpholine catalysis (like the hydroxide ion 9.72, 9.97 (as for cyclobuxine), 5.60 (lH,OH), catalysis) is, on this basis, an example of nucleo- 7.68 and 7.77 ( U H , two x(CH3)Z) and 9.13 r philic catalysis of ester hydrolysis in which the (3H, doublet, J 6.5 c./s.; sec. CH, near N(CH3)d; nucleophile does not react directly with the ester the N,N’-dimethyl-O-a~etate,~C ~ ~ H ~ ~ O Z mN . pZ , linkage to form an unstable intermediate (the usual 173-175’, [ c y ] ? B 69’, A,, 5.81 p , n.m.r. peaks type of nucleophilic catalysis) but rather reacts a t 4.95 (lH, octuplet; CH2-CHOXc--CII), 8.03 with a neighboring group to form an unstable (3H, CH3COO-) ; the triacetate, C ~ I H ~ B O ~ N ~ , intermediate which leads eventually to the hydro- m.p. 256-258’ dec., [ a I z 4 ~- 12’, Amax 5.7% lytic products and the regeneration of the catalyst. 6.14, 11.08,~;the dihydro derivative, C ~ ~ H M O X ~ , -4 similar example in the hydrolysis of a keto-sub- m.p. 208-209’, [ ~ t ] * ~ ~4Bo, , infrared showed 110 stituted phosphoric acid ester is consistent with bands a t 6.08 or 11.20 p , r1.m.r. peaks a t 9.22 1311, this interpretation.10 doublet, J 7 c./s.; new sec. CH,$),9.43 anti 0.73 r (2H, cyclopropyl methylene). (7) T h e theoretical curve was calculated using an Applied Dynamics AD-232PB analog computer by Dr. Kenneth A. Connors of the School The gross skeletal structure was indicated hy the of Pharmacy, University of Wisconsin. I t is assumed in this comstructures suggested for the products of selenium putation t h a t the intermediate has an extinction coe5cient a t 290 mp dehydrogenation : an S-methyl-1,kyclopentenoof approximately two-thirds t h a t of the reactant. phenanthrene (11),C21H2~, n1.p. 145-147’, trinitro(8) T h e constant kl is increased and k: is decreased by increasing p , as predicted b y eq. 3. benzene complex, C27H250,&3, m.p. 165-168’; the (9) Compound I 1 1 is analogous to an intermediate in Schiff base corresponding 5-methyl- 1,2-cyclopentenoant11raformation: E. H. Cordes and W. P. Jencks, J. Am. Chcm. Sac., 84, 832 cene (111), C21HZ2, m.p. 110-112’, trinitrobenzene (1962). complex, C2iH2506N3, m.p. 16s-169’; and two (10) F. Ramirea, B. Hansen and N. B. Dcsai, ibid,. 84, 4588 (1962). (11) Alfred P. Sloan Foundation Research Fellow. naphthalenes, IV, C21H28, m.p. 134-135’, trinitro(12) National Science Foundation Postdoctoral Fellow on leave from benzene complex, C2?H31O6N3, m.p. 143-143° ; and Amherst College. V, C22H28, m.p. 111-117’, trinit,robenzene complex, (13) The authors acknowledge the assistance of Drs. F. 1. Kezdy C28H310&r3, m.p. 139-141 ’. The hydrocarbons and G. E. Clement d t h the morpholine kinetics. were characterized by analysis, infrared, ultraviolet, DEPARTMENT OF CHEMISTRY
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SORTHWESTERN UNIVERSITY MYRONL. BENDER^^ EVANSTON, ILLINOIS MARCS. SILVER^**^^ RECEIVED OCTOBER 6, 1962
THE STRUCTURE OF CYCLOBUXINE
Sir : The medicinal properties of Buxus sempervirens L. have been known since ancient times; extracts of the plant have been used in the treatment of a wide variety of diseases, including malaria and venereal disease. * More recently, an alkaloidal extract of the plant has been reported to possess antitubercular properties. Earlier studies have (1) E. Schlittler, E. Heusler and W. Friedrich, Hclw. Chim. Adas 31, 2209 (1949).
(2) L. E. Weller, C. T.Redemann, R. Y.Gottsball, J. hl. Roberts, E. H. Lucas, and H. M. Sell, Anlibiolics and Chemolheiapy. 3, 603 (1953); Merck 8r Co., Inc., British Patent 782,469 (1957). (3) K. Heusler and E. Schlittler, Helu. Cliim. Acla, 32, 2226 (1949). (4) W. Friedrich and E. Schlittler, i h i d . , 33, 873 (1950). (5) E. Schlittler and W. Friedrich, ibid., 3 3 , 878 (1950). (6) K. S. Brown, Jr., and S. hi. Kupchan, J. Chromulogtuphy, 9, 71 (1962). (7) Cyclobuxine is “111,” t h e alkaloid of Rf0.76 in Fig. 2 uf reference 6. It is most probably the same as “Alkaloid A” of reference 3; although no comparison sample of “A” is available, the phy5ical CUDstants of cyclobuxine and its derivatives correspond clusely tu those of “A” and the respective derivatives. (8) All rotations and infrared spectra are in chloroform. (9) All n.m.r. spectra were determined on a Varian Associates recording spectrometer (A-60) a t 60 hlc. in deuterated chloroform or carbon tetrachloride. Chemical shifts are reported in 7 values (p.p.m.) [G. V. D. Tiers, J. Phys. C h e m . , 62, 1151 (1958)l. We thank Mr. Roy Matsuo and Mr. Arnold Kruhsack for these determinations.
