976
J. Org. Chem. 1988,53,976-983
3-(3-Pyrrolyl)thiopyrrolidones as Precursors of Benzo[ 1,2-b:4,3-b')dipyrroles. Synthesis of Intermediates and Analogues Related to the Phosphodiesterase Inhibitors PDE I and PDE I1 and Antitumor Antibiotic CC-1065 Richard J. Sundberg,* Gregory S. Hamilton, and Joseph P. Laurino Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901
Received August 26, 1987
The 4-lithio derivative of ethyl 5-(3-methylisoxazol-5-yl)pyrrole-2-carboxylateadds to the 3-position of 1allylpyrrolidine-2,3-dione to give ethyl 4-(l-allyl-3-hydroxy-2-oxopyrrolidin-3-yl)-5-(3-methylisoxazol-5-yl)pyrrole-2-carboxylate in 53% yield. This adduct has been converted to ethyl 4 4 l-allyl-2-thioxopyrrolidin-3yl)-5-(1,3-dioxobutyl)pyrrole-2-carboxylate. This thiolactam is cyclized to ethyl 5-acetyl-6-allyl-4-hydroxy3,6,7&tetrahydrobenzo[1,2-b:4,3-b'jdipyrrole-2-carboxylate by reaction with methyl iodide at 80 "C. The cyclization product has been converted to methyl 5-acetyl-4-methoxy-3,6,7,8-tetrahydrobenzo[l,2-b:4,3-b'jdipyrrole-2carboxylate, an intermediate in a synthesis of PDE I1 methyl ester (Boger and Coleman, 1986). The thiolactam has also been converted to ethyl 7-allyl-4-oxo-3,4,5,6,7,8-hexahydrothiepino[2,3-b:5,4-b Idipyrrole-2-carboxylate. This thiepinone undergoes ring contraction to ethyl 6-allyl-4-hydroxy-3,6-dihydrobenzo[l,2b:4,3- b 'jdipyrrole2-carboxylate. This compound has been converted to ethyl 6-acetyl-4-methoxy-3,6,7,8-tetrahydrobenzo[ 1,2b:4,3-b 'ldipyrrole-2-carboxylate,the 5-deoxy analogue of PDE I1 ethyl ester. The benzo[ 1,2-b:4,3-b7dipyrrole ring occurs in the phosphodiesterase inhibitors PDE I and PDE I1 isolated by Umezawa and co-workers.' It is also found, with the same substitution pattern, in the potent antitumor antibiotic CC-1065.' There has been much activity directed toward the synthesis of this ring system r e ~ e n t l y . ~ In -~ earlier work it was shown that 3-(3-pyrrolyl)thiopyrrolidones could serve as intermediates in the synthesis of benzo[ 1,2-b:4,3-bldipyrrole~.~The most efficient reaction was the intramolecular condensation (path a), but two additional pathways to benzodipyrroles represented by b and c were also observed as summarized in Scheme I. In the initial study, the nitrogen substituent R was benzyl. We were unable to develop effective methods for debenzylation of that series of benzodipyrrole derivatives. (1) (a) Enomoto, Y.; Furutani, Y.; Naganawa, H.; Hamada, M.; Takeuchi, T.; Umezawa, H. Agric. BioE. Chem. 1978,42,1331. (b)Nakamura, H.; Ehomoto, Y.; Takeuchi, T.; Umezawa, H.; Iitaka, Y. Agn'c. Bid. Chem. 1978,42, 1337. (2) (a) Hanka, L. J.; Dietz, A.; Gerpheide, S. A.; Kuentzel, S. L.; Martin, D. G. J. Antibiot. 1978, 31, 1211. (b) Martin, D. G.; Chidester, C. G.; Duchamp, D. J. Mizsak, S. A. J . Antibiot. 1980,33,902. (c) Martin, D. G.; Biles, C.; Gerpheide, S. A.; Hanka, L. J.; Krueger, W. C.; McGovren, J. P.; Mizsak, S. A.; Neil, G. L.; Stewart, J. C.; Visser, J. J. Antibiot. 1981, 34,1119. (d) Chidester, C. G.; Krueger, W. C.; Mizsak, S. A.; Duchamp, D. J.; Martin, D. G. J . Am. Chem. SOC.1981, 103, 7629. (3) (a) Wierenga, W. J . Am. Chem. SOC.1981, 103, 5621. (b) Warpehoski, M. A. Tetrahedron Lett. 1986,27, 4103. (c) Warpehoski, M. A.; Bradford, V. S. Tetrahedron Lett. 1986, 27, 2735. (4) (a) Halazy, S.; Magnus, P. Tetrahedron Lett. 1984,25, 1421. (b) Magnus, P.; Gallagher, T. J. Chem. Soc., Chem. Commun. 1984,389. (c) Halazy, S.; Magnus, P. Tetrahedron Lett. 1985,26,2985. (d) Carter, P.; Fitzjohn, S.; Magnus, P. J . Chem. SOC.,Chem. Commun. 1986,1162. (e) Magnus, P.; Gallagher, T.; Schultz, J.; Or, Y.-S.; Ananthanarayan, T. P. J . Am. Chem. SOC. 1987,109,2706. (0 Magnus, P.; Carter, P.; Fitzjohn, S.; Halazy, S. J . Am. Chem. SOC.1987, 109, 2711. (5) Kraus, G. A.; Yue, S.; Sy, J. J . Org. Chem. 1985, 50, 283. (6) (a) Rawal, V. H.; Cava, M. P. J. Chem. SOC.,Chem. Commun. 1984, 1526. (b) Rawal, V. H.; Jones, R. J.; Cava, M. P. Tetrahedron Lett. 1986, 26,2423. (c) Rawal, V. H.; Cava, M. P. J . Am. Chem. SOC. 1986,108,2110. (d) Jones, R. J.; Cava, M. P. J . Chem. Soc., Chem. Commun. 1986,826. (e) Rawal, V. H.; Jones, R. J.; Cava, M. P. J. Org. Chem. 1987, 52, 19. (7) (a) Boger, D. L.; Coleman, R. S. J. Org. Chem. 1984,49, 2240. (b) Boger, D. L.; Coleman, R. S. J. Org. Chem. 1986,51, 3250. (c) Boger, D. L.; Coleman, R. S. J . Am. Chem. SOC.1987, 109,2717. (d) Boger, D. L.; Coleman, R. S.; Invergo, B. J. J. Org. Chem. 1987, 52, 1521. (8) (a) Bolton, R. E.; Moody, C. J.; Rees, C. W.; Tojo, G. J . Chem. SOC., Chem. Commun. 1985, 1775. (b) Moody, C. J.; Pass, M.; Rees, C. W.; Tojo, G. J . Chem. SOC.,Chem. Commun. 1986, 1062. (c) Bolton, R. E.; Moody, C. J.; Rees, C. W.; Tojo, G. J. Chem. Soc., Perkin Trans. 1 1987, 931. (d) Bolton, R. E.; Moody, C. J.;Rees, C. W.; Tojo, G. Tetrahedron Lett. 1987, 28, 3163. (9) Sundberg, R. J.; Pearce, B. C. J . Org. Chem. 1985, 50, 425.
Scheme I
..0
-
HO'
-
Acd
'SAC
A major problem was the facile dehydrogenation of the indoline ring by the usual debenzylation catalysts. In this paper we report the exploration of the analogous system in which the nitrogen substituent is allyl. Satisfactory methods for deallylation have been found and have led to the synthesis of methyl 5-acetyl-4-methoxy-3,6,7,8-tetrahydrobenzo[ 1,2-b:4,3-b 'ldipyrrole-2-carboxylate (13d), which is an intermediate in the synthesis of PDE I and PDE I1 methyl ester reported by Boger and Coleman,7b,c and ethyl 6-acetyl-4-methoxy-3,6,7,8-tetrahydrobenzo[ 1,2-b:4,3-b dipyrrole-2-carboxylate (24), the 5-deoxy analogue of PDE I1 ethyl ester. The early steps of the synthesis of the N-allyl thiolactam 4b parallel those used for the benzyl c o m p ~ u n d .The ~ addition of ethyl 4-lithio-l-sodio-5-(3-methylisoxazol-5yl)pyrrole-2-carboxylate(IC) to l-allylpyrrolidine-2,3-dione (2)'O proceeds in 50-55% yield. The remainder of the reactant is consumed by enolization, and the debrominated pyrrole la can be recovered and recycled. A modification of the method for reductive dehydroxylation of 3 was necessary because the hydrogenolysis used in the benzyl series is incompatible with the N-allyl substituent. It was found that the reduction could be effected with triethylsilane-trifluoroacetic acid.ll Dehydration giving 5a occurred in competition to the extent of up t o 20%. This deoxygenation sequence is an improvement over the hydrogenolysis used in the benzyl case since the isoxazole ring is unaffected. In the benzyl case this ring was opened by (10) Sundberg, R. J.; Pearce, B. C.; Laurino, J. P. J . Heterocycl. Chem. 1986, 23, 537. (11) Kursanov, D. N.; Parnes, Z. N.; Loim, N. M. Synthesis 1974,633.
0022-326318811953-0976$01.50/0 0 1988 American Chemical Society
J. Org. Chem., Vol. 53, No. 5, 1988 977
Precursors of Benzo[ 1,2-b:4,3-bqdipyrroles Scheme 11"
R
* CH2CH=CH2
CH 3
IC
X = L i (Na
salt)
6a Y = NH2 6b Y = OH
0
I
\
C!i
4a 4b
x - s
CH3
5b X = S
(a) NaH; (b) n-BuLi; (c) l-allylpyrrolidine-2,3-dione(2); (d) Et,SiH, TFA; ( e ) P2S,; (f) Mo(CO),; ( 9 ) HzO, H+.
hydrogenolysis, requiring an additional step to reform the isoxazole ring. The lactam 4a was not easily separated from the byproduct 5a. However, after conversion of the mixture to the corresponding thiolactams 4b and 5b,separation was readily achieved by chromatography. Molybdenum hexacarbonyl effected the reduction of the isoxazole ring of 4b,giving 6a. This was hydrolyzed to the key intermediate 6b. These transformations are summarized in Scheme 11. The overall yield of the cyclization substrate 6b from the adduct 3 is 50-60%. Cyclization of 6b to 7a occurred on heating to 80 O C in the presence of 2-3 equiv of methyl iodide. The yield was >go%. It was found that 7a was thermally unstable toward a Claisen rearrangement to give 8a. The identifi-
R"6 8a
R3 ,R6 = H
8b 8c
~3 =
9b
~3
9C
C H ~ , R6 = H = H, R6 = CH 3
9a
CH,; ;
R3,R4 = H R 3 = H , R" = CH 3 R3 = H , R" = COCH3
tion using methyl sulfate or methyl iodide with the potassium salt of 7a or under phase-transfer conditions was also tried. Various mixtures were obtained, which included the enone 8a,its N(3)-methyl (8b)and N(G)-methyl (8c) derivatives, the aromatized indolic phenol 9a,its 0-methyl derivative 9b,recovered starting material, and its N(3)methyl derivative 7b. None of these conditions were preparatively useful, and they served only to confirm the previously noted resistance of the o-hydroxyphenyl ketone moiety to m e t h y l a t i ~ n . ~ J In ~ contrast, 7a was easily acetylated to 7c (Scheme 111). Two general approaches for removal of the N-allyl group were examined. These were acylative dealkylati~n'~ and metal-catalyzed isomerization to a hydrolytically unstable N-propenyl isomer.15 The 0-acetyl derivative 7c was allowed to react with cyanogen bromide, vinyl chloroformate,14aand 2,2,2-trichloroethyl c h l ~ r o f o r m a t e ' in ~~ attempts to effect deallylation. The major product in each case was the result of ring opening, loa-c, rather than deallylation. The cleanest reaction was with trichloroethyl chloroformate, which gave 1Oc in 85% yield. The preference for reaction at the ring carbon rather than the allyl group must reflect steric encumbrance of the allyl group since normally the allylic bond would be preferentially ~1eaved.l~~
cation of 8a was based on spectral data. The allyl methylene group appears as an AB quartet, J = 14 at 2.31 and 2.39. Each doublet is further coupled to the vinyl proton AcO' CCH3 a t 5.59. The indoline protons which appear as a pair of triplets in 7a become a series of complex multiplets in 8a 'i due to the nonequivalence induced by the chiral center at loa X = CII, Y = Br C-8a. The four protons appear a t 3.92 (a), 3.85 (b), 2.55 lob X = C02CH=CH2, Y C1 (c), and 2.20 (d). The coupling is as follows: Jab= 9, Jac 1Oc X = C02CH2CC13, Y = C1 = 6.5, Jad= 9, Jbc = 0, JM= 9, and Jcd = 12.5 Hz. The UV maximum of 7s a t 308 nm is replaced by maxima at 284 and 336 nm. It is interesting that the nonbenzenoid (12) Voncken, W. G.;Buck, H. M. Red. Trau. Chim. Pays-Bas 1974, structure is thermodynamically stable. It is stabilized by 93, 210. both the 2-acylpyrrole conjugation and the vinylogous (13) Schonberg, A.; Mustafa, A. J. Chem. SOC.1946, 746. amide structure (enaminone), which is presumably also (14) (a) Olofson, R. A.; Schnur, R. C.; Bunes, L.;Pepe, J. P. Tetrahedron Lett. 1977, 1567. (b) Montzka, T. A.; Matiskella, J. D.; Partyka, stabilized by an intramolecular hydrogen bond. R. A. Tetrahedron Lett. 1974, 1325. ( c ) Kapnang, H.; Charles, G.TetNumerous attempts to methylate 7a a t the 4-hydroxy rahedron Lett. 1983, 24, 3233. group were unsuccessful. These included diazomethane (15) (a) Corey, E. J.; Suggs, J. W. J. Org. Chem. 1973, 38, 3224. (b) Hubert, A. J.; Georis, A.; Warin, R.; Teyssie, P. J. Chem. SOC.,Perkin in methanol, in methylene chloride, or in ether in the Trans. 2 1972, 366. (c) Moreau, B.; Lavielle, S.;Marquet, A. Tetrahedron presence of boron trifluoride etherate. Methylation was Lett. 1977, 2591. (d) Stille, J. K.; Becker, Y. J. Org. Chem. 1980,45, 2139. also attempted with 2,2,2,3-tetrahydro-2,2,2-trimethoxy- (e) Guibe, F.; Saint M'Leux, Y. Tetrahedron Lett. 1981, 22, 3591. (D 5-methyl-l,2-dioxaphosphole.12 Base-catalyzed methylaGuibe, F.; Dangles, 0.;Balavoine, G. Tetrahedron Lett. 1986,27, 2365.
Sundberg et al.
978 J. Org. Chem., Vol. 53, No. 5, 1988 Scheme I11 Et02C CH2CH=CH2 R"0
jCH3
0'
7a
Ila R4 = H l l b R4 = COCH3
R3,R4 = H
7b R 3 = CH3,
R4
= H
llc
R 4 = COCH3
Eto2cw R202cv 7c
R 3 = H, R 4 = COCH3
R6
-
H
Me0
13a R 2 = C2H5, R * = C2H5,
(7,8-dehydro)
R6
\ I
CCH3
d
R6 =
d COCH3
R 6 = C02CI12cc13
13c R2 = C2H5, R 6 = H H 13d R2 = CH3, R6
Metal-catalyzed deconjugation-hydrolysis was then examined. Preliminary experiments in which 7c was heated with tetrakis(tripheny1phosphine)rhodium hydride16at 100 "C in ethanol gave mixtures of the desired deallylation product llb with the thermal rearrangement product 8a. The latter was presumably formed from 7a after solvolysis of the 0-acetyl group. Better results were achieved by use of 1.0 equiv of trifluoroacetic acid and 0.25 equiv of the rhodium complex in refluxing ethanol. The desired product llb was formed in 70% yield under these conditions. Acetylation gave the more stable compound 12a. The diacetyl derivative 12a was readily deacetylated to 12b,which reacted with diazomethane to give 13a. The efficient 0-methylation of 12b is in striking contrast to the N-allyl compound 7a, which was unreactive to diazomethane. We attribute the change in reactivity to steric disruption of the hydrogen bond of 12b. In contrast to 7a, where rotation of the allyl group is available to minimize steric interaction with the 5-acetyl substituent, the amide group in 12b presumably prefers one of the two possible planar amide rotamers. (The NMR spectrum shows a single unbroadened methyl group indicating a preference for one rotamer.) This would force the 5-acetyl group to adopt a conformation perpendicular to the benzene ring and thus weaken the hydrogen bond. Reaction of 1 lb with 2,2,2-trichloroethyl chloroformate gave the expected carbamate 12c. It too was easily 0deacetylated and methylated to give 13b. The structure of 13b was proven after conversion to 13c by removal of the (trich1oroethoxy)carbonyl group with zinc. Ester interchange then gave the methyl ester 13d, which was identical by NMR, IR, MS, and TLC with the material prepared by Boger and C ~ l e m a n . ~ The ~ , ~ preparation ,~~ of 13d constitutes a formal total synthesis of PDE I and PDE I1 ethyl esters. These transformations are summarized in Scheme 111. Although it did not prove to be preparatively reliable, an interesting result was achieved when a bis(tripheny1phosphine)palladium(II) chloride-tin hydride systemlkSf was used as a deallylation catalyst with 7a. The yields were (16) Ahmad, N.; Robinson, S. D.; Uttley, M. F. J. Chem. Soc., Dalton Trans. 1972, 843. (17)We thank Prof. Boger and Dr. Coleman for providing a sample of their material.
12a
R4, R6 = COCH3
12b R4 = H, R6 = COCH 3 12c R4 COCH3, R6 = C 0 2 C H 2 C C 1 3 12d R4 = H, R6 = C02CH2CC13
variable, but the major identifiable products were 8a and 14,a nonbenzenoid tautomer of the expected product lla.
H 14
This compound was identified by spectroscopic data. In particular, the NMR spectrum reveals a series of five upfield multiplets whose mutual coupling is consistent with that expected for the nonaromatic protons in structure 14. The coupling pattern is analogous to that for 8a, with additional coupling of 7 and 14 Hz between the proton at position 8a and the C-8 methylene group. The isolation of 14 is consistent with the apparent thermodynamic preference for the nonbenzenoid system revealed by the Claisen rearrangement of 7a. When 14 was acetylated, it was converted to a mixture containing the aromatized indole llc and the indolines 12a and 12b. Boger and Coleman have investigated the use of the Baeyer-Villiger oxidation of compounds such as 13d to establish the naturally occurring oxygen substitution pattern. Such methods were not very satisfactory, and the oxidation procedure that was successfully employed to complete their synthesis involves formation and rearrangement of a hydroperoxide intermediate.7b*cJ6We were interested in examining the conditions described by Nishinaga in which arylhydrazones of o-hydroxyacetophenones are catalytically oxidized to catechol derivat i v e ~ . ' ~Reaction of the p-bromophenylhydrazone of 12b with O2in the presence of the Co(SALPR12catalystm gave a complex mixture. The Co catalyst and oxygen converted 12b itself to a mixture of 15a and 15b. Treatment of the mixture with trimethyl phosphite converted it all to 15b. These compounds were identified on the basis of the coupling pattern of the four indoline ring protons in 15b, which is very similar to the pattern of 8a. The four in(18)Boger, D. L.; Coleman, R. S. J. Org. Chem. 1986,51, 5436. Boger, D. L.; Coleman, R. S. Tetrahedron Lett. 1987,28, 1027. (19) Nishinaga, A,; Yamazaki, S.; Matsuura, T. Tetrahedron Lett. 1984, 25, 5805. (20) Nishinaga, A.; Tomita, H.; Nishizawa, K.; Matsuura, T.;Ooi, S.; Hirotsu, K. J. Chem. Soc., Dalton Trans. 1981, 1504.
Precursors of Benzo[ 1,2-b:4,3-b Idipyrroles
J . Org. Chem., Vol. 53, No. 5, 1988 979
bromopyrrole lb (3.31 g, 11.1mmol) in 150 mL of dry THF. The mixture was stirred for 10 min a t room temperature and then cooled to -98 "C (liquid nitrogen-methanol bath). A solution of tert-butyllithium (1.7 M in hexane; 14 mL; 23.0 mmol) was added, giving rise to a deep red solution. There was quickly added a (2.01 g, 14.4 mmol) in 10 solution of l-allylpyrrolidine-2,3-dione mL of T H F precooled to -78 "C. The mixture was stirred for 1h a t -98 "C, allowed to come to room temperature, and stirred 15a X = DOH for an additional hour. Several mL of acetic acid was added, and 15b X = OH the mixture was poured into 200 mL of water. Dilute HC1 was doline protons occur at 4.19 (a), 3.98 (b), 2.66 (c), and 2.16 added until the color lightened to a pale brownish orange (pH -5). The product was extracted into ethyl acetate. Chroma(d) with coupling constants similar t o those in Sa: J a b = tography (1:l hexanes-ethyl acetate) gave 3 (2.10 g, 53%) as a 9, J,, = 6, Jad = 9, Jh = 0, JM = 9, and Jbc = 1 4 Hz. T h e white crystalline compound, mp 174 "C. There was also recovered C-3 proton and t h e ester peak are unchanged in intensity from the column 1.03 g of debrominated pyrrole la. 3: 'H NMR or multiplicity from 12b. T h e N- and C-acetyl peaks a r e (360 MHz) l.40 (t,J = 7, 3 H), 2.25 (s, 3 H), 2.35-2.50 (m, 2 H), present at 2.23 and 2.63 ppm. Compound 12b was i n e r t 3.40 (m, 1 H), 3.53 (m, 1 H), 4.00-4.20 (AB m, J A B = 14, 2 H), t o oxygen u n d e r similar conditions i n t h e absence of Co4.35 (9, J = 7, 2 H), 5.36 (m, 2 H), 5.90 (m, 1H), 6.55 (s, 1 H), (SALPR)t. 6.74 (d, J = 2, 1 H); E1 MS, m / z 359, 341, 263, 247, 201. The N-allyl series of compounds was also examined with Anal. Calcd for ClJ-IzlN305: C, 60.16; H, 5.89; N, 11.69. Found: respect to path b i n Scheme I. The thiolactam 6b was C, 60.33; H, 5.93; N, 11.60. converted to diazoacetyl thiolactam 16a b y t h e m e t h o d Ethyl 4-(l-Allyl-2-oxopyrrolidin-3-yl)-5-(3-methylisox~ azol-5-yl)pyrrole-2-carboxylate (4a). Adduct 3 (129 mg, 0.36 previously developed in the benzyl series involving diazo "01) was dissolved in 5 mL of trifluoroacetic acid. Triethylsilane transfer and d e a ~ y l a t i o n .T~r e a t m e n t of 16a with boron (1.0 mL) was immediately added and the mixture was stirred trifluoride followed b y (dimethy1amino)pyridine gave t h e vigorously at room temperature for 2 h. Several additional 0.5-mL thiepinone 17a in quantitative yield (Scheme IV). Dealiquots of triethylsilane were added at 30-min intervals until the sulfurization and ring contraction to 18a occurred on reactant was consumed. Evaporation left a tan gummy substance, heating in ethanol with Raney nickel. This transformation which was purified by flash chromatography (1:l hexanes-ethyl parallels that developed earlier for the benzyl compound acetate). There was obtained 108 mg (0.31 mmol, 86%) of 4a as 16b.9 Small amounts of spiro thiolactams 20a and 20b a snowy white crystalline material, mp 122-124 "C. 4a: 'H NMR were also isolated from these reactions. Both the N-benzyl (360 MHz) 1.37 (t,J = 7, 3 H), 2.15 (m, 1 H), 2.32 (s, 3 H), 2.55 (m, 1H), 3.50 (m, 2 H), 3.90-4.10 (m, 3 H), 4.35 (q, J = 7 , 2 H), and N-allyl phenolic compounds could be methylated with 5.25 (m, 2 H), 5.80 (m, 1 H), 6.45 (s, 1 H), 6.80 (d, J = 2, 1 H), diazomethane, giving 18c and 19c, respectively. 9.65 (br, 1 H); E1 MS, m/z 343,228, 145, 123. Variable amounts B o t h the N-allyl and N-benzyl ethers 18c and 19c were (
