Nov. 5, 1952
5365
THENITROGEN-CONTAINING RINGOF CARPAINE
[CONTRIBUTION FROM THE DEPARTMENT O F CHEMISTRY, UNIVERSITY O F CALIFORNIA, AND THE XAVAL
MEDICAL RESEARCH
INSTITUTE]
The Nitrogen-Containing Ring of Carpaine BY HENRYR A P O P O R T
AND
HENRYD. BALDRIDGE, JR.’
RECEIVED MAY9. 1952 Examination of carpanlic acid, the hydrolysis product from carpaine, by ninhydrin reaction and potentiometric titration indicates the carboxyl group and nitrogen-containing ring are at opposite ends of the molecule. Liquid phase catalytic acid ( X ) . Thus dehydrogenation of carpaine gives desoxycarpyrinic acid, identified as 8-[2‘-(6’-methylpyridyl)]-octanoic carpaine most reasonably contains a piperidine rather than a pyrrolidine ring and structure XI is proposed for the alkaloid.
In a preceding publication,* the structure (I) proposed3 for carpaine, the alkaloid from papaya leaves, was shown to be untenable, since from a two-stage Hofmann degradation the straight chain, fourteen-carbon acid, myristic acid, was obtained. In this report evidence is presented on a second structural feature, the position of the nitrogen atom.
H
L--l
I
I
That the nitrogen of carpaine is present as a secondary amine and in a ring has been established by the formation of typical secondary amine derivat i v e ~and ~ ~the , ~ necessity of a two-stage Hofmann degradation for its elimination. Two points, however, on which further data would be highly desirable are the position of the nitrogen ring relative to the carboxyl group and the size of the nitrogen ring. For example, in regard to the relative position of carboxyl group and nitrogen-containing ring, a structure such as I1 would be completely compatible with the previously established structural H?C-CHz CHa-CH--(
I
I
CHZ),-HC,~, CH-C=O H
I
PH . Fig. 1.-Potentiometric
titration of carpamic acid
from which the $ K f of the carboxyl group was found to be 4.6.j Since i t has been shown that, in amino acids, the @K‘of the carboxyl group does not reach values of about 4.5 until the amino group is beyond the y-position,6 structures similar to I1 may be eliminated and only those formulations in which the carboxyl group and nitrogen ring are a t opposite ends of the molecule need be considered. If two other structural elements are incorporated, viz., the presence of one C-CHa group3b and the oxidation of carpaine to azelaic acidIaaa generalized formula for carpamic acid, C~H27N03,may be written as in 111.
I1
features. This possibility may be easily investigated, since the amino acid resulting from lactone ring opening, carpamic acid, would be distinguishable as an a-amino acid. Carpamic acid was therefore subjected to the ninhydrin reaction and found to evolve no carbon dioxide, whereas proline readily evolved one mole. Further information has been obtained by potentiometric titration of carpamic acid (Fig. l ) (1) Naval Medical Research Iustitute, National Naval Medical Center, Bethesda, Md. The opinions contained herein are the writers’ and are not necessarily those of the Navy Department. (2) H. Rapoport and H. D. Baldridge, TEIS JOURNAL, ‘73, 343
(1951). (3) (a) G . Barger, A. Girardet and R. Robinson, Hclv. Chim. Acto, 16, 90 (1933); (b) G. Barger, R. Robinson and T . S. Work, J . Chcm. Soc., 711 (1937).
_. H
I11
Previous conclusions as to the size of the nitrogen ring were based on dehydrogenations effected with selenium a t 280” or above. The products were oils which gave positive Ehrlich tests and appeared to be substituted pyrroles. Hence a pyrrolidine nucleus was presumed to be present in the parent In order to have more confidence that the dehydrogenation products reflect the structure present originally, we have sought to find milder dehydrogenation conditions, and a t the same time to establish the structure of the products. Since a t this stage, IV was considered to be a reasonable alternative to I as the structure of carpaine, the
(5) For leading references and a discussion of the method used for (4) Although an N-H band was not detectable in the infrared absorption spectrum of carpaine in solution, a weak band was present this calculation see C. L. A. Schmidt, Editor, “Chemistry of the at 3.0 p when observed in an oil mull. Similar observations have been Amino Acids and Proteins,” 2nd Ed., Charles C. Thomas, Springfield, reported by L. Marion, D. A. Ramsay and R. N. Jones, THISJOURNAL, Ill., 1949, Chapter XI (by D. I. Hitchcock), especially pp. 604-609. 18, 305 (1951). (6) Reference 5, p. 614.
HENRYK ~ r o ~ .om o ~ HENRY r L). BALDRIDGE, J R.
5360
pyrrolidine V was synthesized as a model cornpound for studying dehydrogenation conditioiis.
c’ol. i-1
of a hydrochloride, indicated desoxycarpyrinic acid was not a substituted pyrrole. X potentiometric titration of desoxycarpyrinic acid hydrochloride gave the curve shown in Fig. 2 from which the presence of two groups of PK’ 1.6and 7.0 can be ~ a l c u l a t e d . ~The value of 4.6 is that expected for the carboxyl group; however, Iv a value of 7.0 for the amine hydrochloride indicates 1 the presence of a nitrogen with apparent basic I 1 dissociation constant, Kf3, equal to lo-’. Thus the nitrogen cannot be present as a pyrrole. -1 very reasonable alternative is an CY, a‘-dialkylpyri1dine since 2,6-lutidine has been reported to have a The synthesis of V proceeded from ?-inethyl- Kf3 of 1 0 - 7 , 7 a 7.9 x 10-8,’b and 4.2 X 10-8.7c However, if desoxycarpyrinic acid is a substipyrrole (VI) as outlined in the sequence below. The pyrryl Grignard reagent was added to S-cyano- tuted pyridine, an explanation for the origin of the third double bond must be found, since only two 1, EtMgBr irioles of hydrogen were evolved on dehydrogenai, 1, tion. The loss of water, as indicated by the enipirical formula, was a t first thought to account for this, but exainination of the dehydrogenation reaction mixture revealed the product was present as a carboxylic acid and the saponification step, applied on the assumption that the product was a lactone, was unnecessary. Therefore, the lactone I /I originally present in carpaine had cracked to give a ,T, I~--(CH,)XOOH f- /j,Sj--c--(cH,j7cooI-r carboxylic acid during the dehydrogenation, thus CH,,’ C I 4 11 0 accounting for the third unsaturation. It is interIX VI11 esting in this regard to compare the facile dehydrooctanoyl chloride and 2- (8’-cyanooctanoyl)-5-meth- genation of carpaine with that of methyl hexaThe latter compound, ylpyrrole (VII) was obtained in 38% yield. hydrodesoxycarpyrinate. a piperidine in which the cracking possibility does This could be hydrolyzed to the keto acid (VIII) not exist, dehydrogenated very slowly, and 12 which was then reduced, or VI1 could be directly hours was needed for the evolution of 2.9 moles of converted to IX in 89% yield by modified Wolff- hydrogen. Kishner reduction. Hydrogenation of 9- [2’-(5’These observations lead to the proposal of X as methylpyrryl) 1-nonanoic acid (1x1 in glacial acetic the most reasonable structure for desoxycarpyrinic acid with a platinum oxide catalyst ceased after acid.8 Support for this structure is found in the absorption of two moles, and the pyrrolidine acid (V) after conversion to methyl ester and without attempting to separate isomers, was sub jected to dehydrogenation experiments. The mildest dehydrogenation conditions were .found to be liquid phase dehydrogenation in boiling A p-cymene with a 5% palladium-on-carbon catalyst. Under these conditions, the methyl ester of V close similarity of the ultraviolet absorption specevolved two moles of hydrogen in three hours after trum of desoxycarpyrinic acid hydrochloride, with which evolution ceased completely and the pyrrole that of 2,6-lutidine hydrochloride, and the marked acid, IX, was isolated after saponification in an difference from that of 9- [2’-(5’-methylpyrryl) 1nonanoic acid (IX) (Fig. 3). Further evidence 527, yield. Carpaine was then subjected to these dehydro- has been obtained by oxidation of desoxycarpyrinic acid. genation conditions and found to smoothly evolve acid to pyridine-2,6-dicarboxylic Since desoxycarpyrinic acid has been shown to two moles of hydrogen in two hours, whereupon hydrogen evolution practically ceased. Exaniina- contain a pyridine ring, the inference is very strong tion of the reaction mixture by the usual procedure that carpaine is a substituted piperidine rather than gave as the only product a water soluble hydro- a pyrrolidine. I n order to maintain a pyrrolidine chloride, C14HZ1NO2HCl, to which the name de- ring for carpaine, a rearrangement during the soxycarpyrinic acid hydrochloride has been assigned. dehydrogenation reaction must be postulated. Two features of desoxycarpyrinic acid were (7) (a) H. C. Brown and R . B. Johannesen, THISJOURNAL, 73,2934 quite unexpected. Since carpamic acid, the sub- (1950), footnote 1 ; (b) C . Golumbic and M. Orchin, ibid., 73, 4145 ( c ) A. Gero and J. J. Markham, J . Org. Chern., 16, 1835 stance that results from opening the lactone ring of (1950); (1951). carpaine, is C14H27N03, the saponified dehydrogena(8) Previous investigators (ref. 3a) have derived support for the tion product of carpaine, assuming a pyrrolidine presence of a pyrrole nucleus in the dehydrogenation products from nucleus, was expected to be C14H23N03. Appar- the Ehrlich test ( H . Fischer and H. Orth, “Die Chemie des Pyrrols,” Val. I, Akad. Verlag., Leipzig, 1934, p. 66). However, we have ently, desoxycarpyrinic acid, C14H21N02,had lost found that 2,6-lutidine and desoxycarpyrinic acid give positive tests the clenients of water as well. Secondly, the at room temperature. T h u s the test may be misleading in the presprescncc of a basic nitrogen, as shown by formation cncc ol a-alkylpyridincs.
2
,
THENITROGEN-CONTAINING RINGOF CARPAINE
Nov. 5, 1952
- -[ 0
2
- 1 5'-
AI
E THI LP I R R I L
I]
N Q N A H O I C A C I D IN M E T H A N O L
DESOXICAIPIRINIC
4ClQ
H Y O R O L U L O R I Q E lii W A T E R
2000
2200
2400
5367
.\,
2600
Wave length in ingstronis. Fig. 3.-ultraviolet absorption spectra: 0
2
4
PH Fig. 2.--Potentiometric
6
8
IO
0
titration of desoxycarpyrinic acid hydrochloride.
Conceivably, dehydrogenation may proceed to a pyrrole which then rearranges to a pyridine. However, this seems unlikely and would result in a 2,5-dialkylpyridine whereas desoxycarpyinic acid is a 2,6-substituted pyridine, as established by oxidation (see above). Another rearrangement to consider is that which might possibly occur if carpaine had the structure IV. Cracking might then give an intermediate which could rearrange to a six-membered ringlo and dehydrogenate to a 2,6disubstituted pyridine. However, a structure such as I V for carpaine, which would make carpamic acid an a,p-amino alcohol, seems unlikely on the basis of preliminary periodate oxidation experiments. l 1 Considering all the above evidence, the structure for carpaine which best accounts for its chemistry is XI, with the point of attachment of the lactone ring a t some as yet undetermined position in the piperidine nucleus.
XI
Experimental12 Carpamic Acid.-After heating under reflux for two hours, a solution of 2.07 g. of carpaine in 50 ml. of 12 N hydrochloric acid was concentrated t o dryness in z'acuo. The concentration was repeated with two 20-ml. portions of water and the residue, after drying in a desiccator, was crystallized from a n absolute ethanol-acetone mixture t o give (9) Purified by the method of W. Koenigs and G . Happe, Bcr.. 36, 2904 (1903). . . (10) Similar rearrangements have been observed with 1.2-aminohalides, e x . . R. C. Fuson and C. L. Zirkle, THISJOURNAL, 70, 2760 (1948). (11) H. D . Baldridge, unpublished results: to be discussed in a subsequent publication. (12) -411 melting points are corrected and those above ZOOo were taken in evacuated capillaries; microanalyses were performed by the Microchemical Laboratory, University of California.
2800
3000
.if sohtions.
2.18 g., 86%, of carpamic acid hydrochloride, 1n.p. 160.6161.4' (reportedl3m.p. 161'). To obtain the free amino acid, the crude hydrochloride from 2 g. of carpaine was dissolved in 30 ml. of water, shaken with silver oxide (freshly prepared from 5 g. of silver nitrate) and the mixture filtered. Evaporation of the filtrate after treatment with hy h o g e n sulfide left a residue which was dried (vacuum desiccator) and crystallized several times from 95% ethanol, yielding 0.8 g. of carpamic acid, 1n.p. 225-226' (dec.) (reported13 m.p. 224'). Ninhydrin Reaction with Carpamic Acid.-Using the procedure of Van Slyke, et a1.,14carpaniic acid was treated with ninhydrin at 100" in an aqueous solution buffered at pH 4.7. A very light yellow color appeared and only 2% of the theoretical quantity of carbon dioxide was evolved. Under identical conditions, a solution of proline rapidly developed a deep red color and evolved 99.4% of the calculated amount of carbon dioxide. 2-Methylpyrrole (VI).-2-Methylpyrrole was prepared from pyrrole and ethyl diazoacetate through ethyl 2-pyrrylacetate according to the method of Senitzescu and Solomonicals; b.p. 147-149", T Z ~ ~I .5021 D (reported15 b.p. 149'). 2-(8'-Cyano6ctanoyl)-5-methylpyrrole (VII).-A solution of 2-methylpyrrylmagnesium bromide (0.116 mole, freshly prepared by addition of 9.4 g. of 2-methylpyrrole in 413 tnl. of anhydrous ether t o 65.5 ml. of 1.8 A I ethereal ethylmagiiesium bromide) was added over a three-hour period to a stirred solution of 21.6 g. (0.115 mole) of 8-cyanooctanoyl chloride2 in 25 ml. of anhydrous ether cooled in an ice-bath. Care had to be taken t o prevent stoppage of the stirrer by the lumps produced during the addition period. \Vith continuous stirring, the mixture was allowed to rise slowly to room temperature as the ice-bath melted (cu. four hours), after which stirring was continued for 15 hours. The reaction mixture was then heated under reflux for one hour, and, after cooling, was decomposed with 100 g. of ice and 150 ml. of saturated ammonium chloride solution. The layers were separated, the aqueous phase was washed with 100 ml. of ether and the combined ether extracts were washed with three 50-ml. portions of 3 'V potassium carbonate solution and water. Evaporation of the ethereal solution after drying over magnesium sulfate left an oil (23.5 g.) which was distilled at reduced prcssure. The fraction of b.p. 120-130' (0.1 mm.) solidified in the receiver aiitl was crystallized from 95% ethanol t o give 10.1 g. (38%) of material, m.p. 63-64". Anal. Calcd. for ClrHnoOSz: C, 72.4; H , 8.7; N. 12.1. Found: C, 72.2; H, 8.6; N, 12.1. 2-(8'-Carboxyoctanoyl)-5-methylpyrrole (VIII).-A solution of 221 mg. (0.95 millimole) of 2-(8'-cyanooctanoyl)-5methylpyrrole in 2 ml. of 2 N potassium hydroxide in diethylene glycol was heated at 170-180" (bath temperature) for '
(13) G. Barger, J. Chcm. Soc., 67, 466 (1910). (14) D. D. Van Slyke, D. A. MacFayden and P. IIamilton, J. Biul. Ciiem., 141, 671 (1941). (15) C. D. Nenitzescu and li. Sululuonica, Bey., 64, 1024 (1931).
5368
HENRY KAI'OPORT AND HENRY
D. BALDRIDGE, JR.
VOl. 74
four hours a t which time hydrolysis was 95% cornpletctl as prcciablc rcsidues on cvaporatioii to tlryucss. 'L'hc combined bicarbonate extracts were acidified to congo n i t h hydroindicated by titrationI6 of the evolved ammonia. rlfter adding 50 nil. of water to the reaction inixturc, it \vas chloric acid and concentrated to dryncss in z'acuo at 50". ivashed thoroughly with ether, acidified to congo re11 xvith IZepeated digestion of the dry residue with chloroform and hydrochloric acid and extracted with cthcr. The cxtracts cvaporation of the combined chloroform extract lcft a rcsiwere washed free from mineral acid with water, dried ovcr due, in 85% yield, which crystallized 011 standing in a vacuum desiccator. Recrystallizatioii from anhydrous rriagncsiuni sulfatc and concentrated. Crude keto acid ~rcctoiieand drying a t 80" in 'I'UCUO overnight gave desoxy(0.21 g., 86%) resulted which after crystallization from fiO% carpyrinic acid hydrochloride, 1n.p. 96-97'. aqucous ethanol and sublimation a t 115-150" (0.1 I I I I I I . ), nieltcd at 143-144". .4?zal. Calctl. for C14H2~OJC1: C, 61.9; I I , S.2; cl, 13.1; cquiv. w t . , 1.36. I.'ountl: C, 61.8; I%,8.2; C1, 13.1; . 1 d . Calcd. for C1&108S: C, 66.9; 11, S . I ; S , 3.6. cquiv. ivt., 1 3 j . l'oulltl: C, 67.1; H, 8.3; s,5.6. C. Methyl Hexahydrodesoxycarpyrinate.-A sltniplc uf 9- [ 2'-( 5'-Methylpyrryl)] -nonanoic Acid (IX).-Iii sccordtlcsoxycarpyrinic acid hydrochloride in glacial acctic acid ;iiicc with tlic general procedure of H u a n g - M i n l o i ~a, ~solu~ tion of 2.0 g. (8.6 millinioles) of ~ - ( 8 ' - c y ~ l l O ~ C ~ a l l O ~ ~\\as ) - ~ hydrogenated with 10% of platinum oxide as catalyst ;it room teinpcrature and atmospheric pressure. Hydrogen tnethylpyrrolc, 2.5 g. of potassium hydroxide and 2 nil. of absorption ccased after the uptake of three moles in two 85% hydrazinc hydrate in 12 ml. of diethylctic glycol was hours. After removal of the catalyst and distillation of the hcatetl under reflux iii a nitrogen atmosphere at 120-130" (internal temperature) for 1.5 hours and then at 19&20n0 acetic acid, the rcsidue was esterified with diazomethane and the oily methyl hexahydrodesoxycarpyrinate was dehydrofor four hours. The cooled solution was added to 50 nil. genated as above. Hydrogen evolution proceeded slowly uf watcr, washeti with two $0-id. portions of ether, acidified untl after 12 hours 2.9 inolcs had been evolved. The reacto congo red with hydrochloric acid and extracted with ether. Ev:il)oration of the ether extracts after washing free of miti- tion mixture was refined as in the case of the pyrrolidyl ester ithove, and a 51 % yield of recrystallized desoxycarpyrinic cral :wid with nxtcr and drying ovcr magnesiuni sulfatc lcft 1.9 g . (93%) of a crystalline solid. After recrystallizing acid hydrochloride, m.p. 95-96', was obtained. Potentiometric Titrations.-A Beckman Model C pH froni GOYo aqueous ethanol. a samplc was distilled onto a iiictcr equipped with glass and calomel electrodes was used. cold linger a t 130-14Oo (0.1 i n i n . ) to give a \vhitc solitl, Thc carpainic acid solution contained 0.1 millimole of cart11.1). 118.5-119°. :Inal. Calcd. for ClnII&S: C, 70.9; 11, 9.S; S ,5.9; paunic acid antl slightly more than 0.1 millimole of hydroequiv. wt., 237. Found: C, 71.0; H, 9.7; S , \5.6; cquiv. chloric acid in 3 ml. of solution. T o this was added small aliquots of 1 17' sodium hydroxide from a microburet and the \ v t . , 237. pIi of the resulting solution determined. From the data, Dehydrogenation Experiments.-Catalytic liquid phase the following values were calculated? pK'( COOH), 4.6; tichydrogenation, as described by Fieser,I8 was employed PI', 7.5; pK'( -SHz+), 10.4. i l l all the cases given bclow. A solution of one millimole of I compound in 20 ml. of p-cymene (purified by distillation Desoxycarpyrinic acid hydrochloride, 0.05 millimole in 7 . 5 from phosphorus pentoxide) was heated under reflux with nil. of water, was titrated as above and gave the following 70 mg. of 5 % palladium-on-carbon and the evolved gas was values: pK'(COOH), 4 . 6 ; P I ' , 5.8; pK'( + S H + ) , 7.0. collcctcd iii an eudionietcr over 50% aqueous potassium Oxidation of Desoxycarpyrinic Acid19 (X).-To a stirred hvdrosidc, using carbon dioxide as carrier. The collecterl solution of 272 mg. (1 millimole) of desoxycarpyrinic acid gas was shown to be entirely hydrogen hy using it to hydro- hydrochloride in 20 nil. of water, maintained at 70°, was gcnate cinnamic acid. added 1.58 g. (10 milliinoles) of potassium permanganate in A. Methyl 9- [2 '-(5'-Methylpyrrolidyl)] -nonanoate.-Hyten equal portions over a period of 15 hours, each successive drogenation of 9-[2'-(5'-mcthylpyrryl)]-nonanoicacid ( I S ) portion being added only after the previous one had been in glacial acetic acid a t room temperature and three atmos- consumed. The hot solution was filtered, the precipitate pheres pressure using 10% by weight of platinum oxide ivas digested with two 10-ml. portions of water and the comas catalyst proceeded rapidly with absorptioti of two nioles of bined filtrate and digests concentrated t o 2 ml. After carehydrogen, then ceased. Filtration, evaporation of the sol- ful acidification with 12 N hydrochloric acid to pH 1.5, vent and esterification i n methanol with diazomethanc rc- the solution was cooled and filtered. The dried, precipisulteti in crudelnethyl9-[2'-( 5'-methylpyrrolidyl)] -norianoate tated crystalline material (containing potassium chloride) ('is ;in oilj which was dehydrogenated according to the was heated under reflux for one hour with 5 ml. of purified gciicral procedure above. Hydrogeii evolution reached 2 thionyl chloride, the excess thionyl chloride was renioved moles in three hours and then ceased. After removing thc under reduced pressure and the residue was treated with 10 catalyst by filtration, 1 IVcthanolic potzssiuin hydrosidc was mi. of cold, concentrated aqueous ammonia. Crystalliza;ddcti, the solution was heated under reflux for riiiie hours, tion or thc resulting amide from nitrobenzene gave material aiid the alcohol was evaporated on the stcam-bath. \\.ater nicltirig a t 30307" (dcc.). \vas added, thc mixture was washed with ether anti thc '411 authentic sample of 2,6-pyridinedicarboxamidc (di:iqueous phase was acidified and extracted thoroughly w-ith yicoliuainidc), prepai ed by the above procedure from dirther. Evaporation of the ether solution after washing aiid picolinic acid,20nicltetl a t 304-306' (dec.) (reported2' m.p. drying gave a n 82% yicld of crude and a 60% yield of pure 302") anti on mixing did not depress the melting point of the 9-12'-(5-methylpyrryl)]-nonanoicacid, 1n.p. 118-1 19'. obtained from desoxycarpyrinic acid above. HowR. Carpaine.--When subjected to the above dehydru- inaterial ?\-et-, 3,.i-pyridincdicarboxamide(isochinchomeronamide), gciiatioii conditions, carpaine (in three different rxperi- prcparctl as ahove froin isochinchomeronic acid,M melted inents) cvolverl from 2 to 2.04 inolcs of hydrogen in two 314-316" (dcc.) (reported22 melting point 310-313') and hours, after which gas cvolution practically ceased. Ben- aitt rnixturc with the amide of the desoxycarpyrinic acid oxizene, 50 ml., mas added t o the cooled mixture which was dation product melted a t 275281'. A mixture of authentic then filtered free of catalyst and washed successively with thrce 5 0 - i d . portions of 5% sodium bicarbonate solution, samples of the 2,B- and 2,j-diarnides melted a t 277-280'. BERKELEY, CALIFORXIA thrcr 50-ml. portions of I N acetic acid and 5U mi. of mater. Scithcr the heiizcne or acetic acid solutiolis gave ally ap(iti) T.S. Ala a n d G. ZuaJaga. , l n d Chert!,, 14,280 (1942). i l 7 ) €Juang-X,Linlun, THISJ O U R N A L , 68, 2487 (1946). ( 1 % [.. F. Fieser, " E x p e r i m e n t s in Organic Chemislry." Second Edil i o u , D. C . H e a t h and Cumpany, New York, N. Y.,1941, p. 461.
(1'J) T h e s e experiments were performed by Ann Hollins. (20) G . Black, E. D e p p a n d B. B. Corson, J. 0 i . g . C h e m . , 14, 11 (1949). (21) 15. M e y e r , Afunalsii., 24, 207 (1903). ( 2 2 ) T. 0 . Soine, J . A m . Phavm. Assoc., 33, 223 (1944).
