J . Am, Chem. SOC.1984, 106, 6049-6060
product is not thiocyanate but either the product of its further reaction with BrCN according to eq 1 or a further decomposition product of S(CN)2. BrCN
+SCN
-
Br-
+ S(CN),
(1)
The reactions of ADPaS and ATPPS are not useful for synthesizing adenosine 5'- ['80]dipho~phate~ or -triphosphates with chiral Pa or Po because of the oxygen rearrangements in the polyphosphate systems described herein. These can be prevented by protecting the terminal phosphoryl groups with removable alkyl substituents, as demonstrated by the reactions of the R p and Sp epimers of 6-(cyanoethy1)-ADPaS with BrCN in HZ1'0to produce high yields of the Sp and RP epimers of P-(cyanoethyl)-[aL80]ADP.z2The displacement of thiocyanate by HZl80proceeded with inversion of configuration at P,, and the P-cyanoethyl groups were easily removed by treatment with base to produce the Sp and RP epimers of [ C U - ~ ~ O ] A D P . ~ ~ Eckstein and Lowe and their collaborators have been able to desulfurize nucleoside phosphorothioates with electrophilic brominating agents in acidic solutions with inversion of configuration and without rearrangements in p o l y p h o ~ p h a t e s . ~At ~ ~neutral ,~~ pHs these reactions also involved rearrangements, suggesting that our observations do not represent an isolated phenomenon observable only in the special case of reactions of BrCN with ADPaS or ATPPS. This paper and our earlier communication4 provide the first evidence for involvement of cyclo-diphosphates in chemical reactions. Dimeric phenylphosphonic anhydride is the only fourmembered ring organophosphorus compound reported in the literature that has two P-0-P bond^.^^,^^ Recently a cyclic phosphoric acid anhydride was postulated as a possible inter(22) Sammons, R. D.; Frey, P. A. J . Biol. Chem. 1982,257, 1138-1141. (23) Connolly, B. A.; Eckstein, F.; Fiilder, H. H. J . Biol. Chem. 1982,257, 3382-3384. (24) Michaelis, A.; Rotte, F. Ber. Dtsch. Chem. Ges. 1892, 25, 1747-1752. (25) Fluck, V. E.; Binder, H. Z . Anorg. Alleg. Chem. 1967,354, 139-148.
6049
mediate in the equilibration of the phosphorus in H332P04with that of POC13.26 cyclo-Dephosphates have never been proposed as intermediates in enzymatic reactions. In view of their involvement in nonenzymatic reactions proceeding to completion within a few minutes in aqueous solutions at physiological pHs and moderate temperatures, they should be considered as possible intermediates in enzymatic reactions. However, the substrates for most nucleoside di- and triphosphate-dependent enzymes are the metal complexes of the nucleotides rather than the nucleotides themselves, and it is not known what effect metal complexation would have on the rearrangements described here. It is possible that these rearrangements would be inhibited or prevented by metal chelation. Moreover, there is no reason to consider invoking cyclo-diphosphates as intermediates in ordinary phosphotransferase or nucleotidyl transferase reactions. Nevertheless, the ease and speed with which cyclo-diphosphates appear and react under mild conditions demonstrate their potential as transient intermediates that might be involved in more complex biological reactions.
Acknowledgment. This research was supported by Grant GM30480 from the National Institute of General Medical Sciences. Purchase and operation of the Bruker WP-200 N M R spectrometer was supported by Grant GM2743 1. Spectra were also obtained at The Ohio State University Chemical Instrument Ceiiter using a Bruker WM-300 spectrometer funded in part by NSF Grant C H E 7910019. We are grateful to Me1 Micke for his assistance in obtaining mass spectral data. Registry No. 4,91711-81-8;5,91741-70-7;10, 91741-71-8; cyanogen bromide, 506-68-3; (Sp)-[P-'*O]ATPBS,87226-46-8;(Sp)-[P,y-bridging-'80]ATPBS, 87883-26-9;(Rp)-[P-180]ADPBS,69182-10-1; (Sp)[P-180]ADPBS,68973-41-1; (Sp)-ATPBS, 59261-36-8;(Sp)-ADPaS, 59286-20-3; (Sp)-[a-1802]ADP~S, 91711-80-7; [a-180,@-180]ADP, 9171 1-82-9; [fi-180,?-180]ATP, 91711-83-0. (26) Keenan, R. W.; Martinez, R. A,; Williams, R. F. J . Biol. Chem. 1982, 257,
14817-14820.
Solvent -Induced Fragmentation of Prostaglandin Endoperoxides. New Aldehyde Products from PGH2 and a Novel Intramolecular 1,2-Hydride Shift during Endoperoxide Fragmentation in Aqueous Solution' Robert G . Salomon,* Donald B. Miller, Michael G. Zagorski, and Daniel J. Coughlin Contribution from the Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106. Received December 5, 1983
Abstract: The prostaglandin (PG) endoperoxide nucleus, 2,3-dioxabicyclo[2.2.11heptane, is three orders of magnitude less stable in aqueous vs. cyclohexane solution. Water-induced fragmentation of 2,3-dioxabicyclo[2.2.llheptane-Z-dl exhibits a deuterium kinetic isotope effect k H / k D= 1.5. Although rate-determining cleavage of a C-D bond occurs, deuterium is not lost to the protic solvent. Rather, the bridgehead deuterium migrates intramolecularly to the incipient methyl group of the levulinaldehyde product. Dimethyl sulfoxide (MezSO) also induces rapid decomposition of the PG endoperoxide nucleus at 37 O C . However, a different mechanism is suggested by kH/kD= 3.3 for the fragmentation reaction induced by Me2S0. The PG endoperoxide PGH2 [9a,l1a-epidioxy-15(S)-hydroxy-5(Z),13(E)-prostadienoicacid] rearranges spontaneously at 37 " C in Me2S0 or aqueous solution to form levulinaldehyde derivatives, levuglandin (LG) E2 [8(R)-acetyl-9(R)-formyl-l2(S)hydroxy-5(2), 1O(E)-heptadecadienoic acid] and LGDz [9(R)-acetyL8(R)-forrnyl-12(S)-hydroxy-S(Z),1O(E)-heptadecadienoic acid]. The Me2SO-induced fragmentation affords levuglandins in 70 to 80% yield according to 'H NMR analysis of the decomposition product mixture while the yield of levuglandins in aqueous solution is about 22% at pH 7.9.
Prostaglandin (PG) endoperoxides, e.g., PGHz (la), are pivotal intermediates in the biosynthesis of a vast array of natural products
(Scheme I).2 Most of the primary derivatives, 2a-loa, exhibit potent physiological a~tivities.~With the exception of 6a,they
0002-7863/84/ 1506-6049%01.50/0 0 1984 American Chemical Society
6050 J . Am. Chem. SOC.,Vol. 106, No. 20, 1984
Salomon et al.
Scheme I
kI
OHC
2
'8"' 3
too. Q' Ho
9
4
are generated by rearrangements of the unusually reactive bicyclic peroxide nucleus l b of la. Detailed study of these rearrangements is complicated, inter alia, by the availability of only minute quantities of la, by the possibility of competing solvent-induced and enzymic processes, as well as by facile reactions of the primary derivatives. Thus, prostacyclin (9a) and thromboxane A, (loa) are rapidly hydrated (tIl2. = 2 min and 30 s, respectively) under the conditions of their biosynthesis. Prostaglandin E, (sa), a &hydroxy ketone, and the vinylogous 8-hydroxy carbonyl compounds 2a-4a readily undergo dehydration. Additional complication is caused by the possible existence of 15-hydroperoxy analogues of 2a-1Oa which are produced directly from PGG2 (IC), the 15-hydroperoxy precursor of PGH2 (la).4 One of the remarkable properties of PGH, is its extraordinary instability in the aqueous environment of its biosynthesis ( t l l z = 5 min at 37 0C).5 Furthermore, solvent-induced rearrangement of PGH, in aqueous solution at p H 8 is generally considered to afford 66% PGE2 (sa) and 28% PGD2 (4a), while 2% PGFh (6a) is produced by reductive cleavage of the peroxide bond.5 However, our studies of a readily available6 model, the PG endoperoxide
(1) Prostaglandin Endoperoxides. 14. For previous publications in this (a) Zagorski, M. G.; Salomon, R. G. J. Am. Chem. Soc. 1983,105, 1750. (b) 1982, 104, 3498 and references cited therein. (2) For recent work see: van Dorp, D.A. In 'Chemistry, Biochemistry and Pharmacological Activity of Prostanoids"; Roberts, S. M., Scheinmann, F., Eds.; Pergamon: New York, 1979; pp 233-242 and references cited therein. (3) For reviews see: (a) Nicolaou, K. C.; Gasic, G. P.; Barnette, W. E. Angew. Chem., Int. Ed. Engl. 1978, 17, 293-378. (b) Samuelsson, B.; Goldyne, M.; Granstrom, E.; Hamberg, M.; Hammarstrom, S.; Malmsten, C. Annu. Reo. Biochem. 1978, 47, 997-1029. (4) Raz, A,; Schwartzman, M.; Kenig-Wakshal, R. Eur. J. Biochem. 1976, 70, 89. (5) (a) Nugteren, D. H.; Hazelhof, E. Biochim. Biophys. Acta 1973, 326, 448-461. (b) Nugteren, D. H.; Christ-Hazelhof, E. Ado. Prostaglandin Thromboxane Res. 1980, 6 , 129-13. (c) Raz, A,; Kenig-Wakshal, R.; Schwartzman, M. Biochim. Biophys. Acta 1977,488, 322-9. series see:
Table I. Solvent Effects for Decomposition of Endoperoxides l b and 11 relative rates dielectric reaction solvent constant" l b a t 73 'C 11 at 130 'C cyclohexane-dlz 1.94 1.Ob 1.OC benzene-d6 2.18 1.4 0.8 chlorobenzene 4.85 2.4 1.1 CD 3 C00D 6.63 26.0 2.7 CICDZCDZCI 7.94 2.7 1.5 2-butanone 14.35 2.8 1.3 4.4 1.8 CD3CN 28 63 1.1 x 103 d 6.2 D20 'Estimated for 73 'C. b4.4 X s-I. c4.8 X lo-' s-'. dExtrapolated from rates measured between 21 and 46 'C.
nucleus lb, revealed an alternative rearrangement p a t h ~ a y . ~ Thus, l b gives 28% 3-hydroxycyclopentanone (5b) and 72% lev5b disulinaldehyde (2b) in D 2 0 at 40 O C . ' l a While the l b proportionation parallels the generation of PGD, (4a) and PGE2 (5a) from PGH2 ( l a ) , the fragmentation of l b to give levulinaldehyde (2b) suggested a previously undetected rearrangement of l a to give levulinaldehyde derivatives 2a and 3a. We now report a quantitative evaluation of a remarkable solvent effect on the stability of the PG endoperoxide nucleus l b . A thorough understanding of the mechanisms of solvent-induced rearrangement of l b is gleaned from kinetic and product studies with the monodeuterated peroxide Id. These model studies with
-
(6) (a) Coughlin, D. J.; Salomon, R. G. J . Am. Chem. SOC.1977, 99, 655-7. (b) Coughlin, D. J.; Brown, R. S.; Salomon, R. G. Ibid. 1979, 101, 1533-9. (c) Salomon, R. G.; Salomon, M. F. Ibid. 1977, 99, 3501-3. (d) Eggelte, Porter, N. A.; Gilmore, D.W. Ibid. 1977, 99, 3503-5. ( e ) Adam, W.; H. J. J . Org. Chem. 1977, 42, 3987-8. (f) Wilson, R. M.; Geiser, F. J . Am. Chem. SOC.1978, 100, 2225-6. (7) (a) Salomon, R. G.;Salomon, M. F.;Coughlin, D. J. J . Am. Chem. SOC.1978, 100, 66C-2. (b) Salomon, R. G.; Coughlin, D.J. Ibid. 1979, 101, 2761-3.
J . Am. Chem. SOC.,Vol. 106, No. 20, 1984 6051
Prostaglandin Endoperoxides Table 11. Temperature Dependence for Rate of Decomposition of l b in D 2 0 T , oc 1 0 3 ( 1 / n , K-' In k 104k, s-I 476" 73.0 2.89 -3.05" -5.76" 31.5" 48.4 3.11 -6.07 23.2 45.8 3.14 -7.19 7.56 37.0 3.22 -8.13 2.95 30.1 3.30 -9.30 0.91 21.0 3.40 "Values extrapolated from the relationship in k = -12338/T 32.61, see Figure 1.
+
Table 111. Solvent-Induced Rearrangement of Id" product yields,b % product ratios solvent H20
2d
2h
5d
5h
2d/2h
5d/5h
41 43
26 26
26 24
7.3 7.6
3.5 3.2
60
19
17
4
1.5 1.6 1.9 1.8 3.3
CH,COOHC Me2S0
" T = 37 OC, [Id]
4.6
0.4 M. bYields reported for 5d and 5h are the sums of the final yields of these products and their corresponding dehydration products (see ref 1) determined from ratios since overall yields are quantitative. CSincesolvent peak overlaps with determination of 5d and 5h, only relative yields of 2d and 2h were determined in this solvent. N
greater than expected for solvents of similar dielectric constant. To provide a mechanistic basis for understanding this unique behavior of the prostaglandin endoperoxide nucleus lb, kinetic and product studies were performed with 2,3-dioxabicyclo[2.2.l]heptane-1-d, (Id) and 2,3-dioxabicyclo[2.2.l]heptane1,4,5,6,7,7-d6 (Id6). Kinetic Isotope Effects in Solvent-InducedRearrangements of 2,3-Dioxabicyclo[2.2.1]heptane-d6(Id6). Data for determination of deuterium kinetic isotope effects was obtained by measuring the rates of decomposition of the perprotio endoperoxide l b and the hexadeuterio endoperoxide Id6 in Me2S0 and D,O solutions.
2.9
3.1 VT
3.3 IOK
3.5
x1031
Figure 1. Temperature dependence for rate of decomposition of l b in D2O.
simple bicyclic peroxides inspired and guided parallel studies on PGH,. Thus, presumptive evidence is presented now for the formation of levulinaldehyde derivatives 2a and 3a from the prostaglandin endoperoxide PGH, (la) by solvent-induced fragmentation. These new aldehyde products from FGH2were named' levuglandin (LG) E, (2s) and LGD, (3a) since these derivatives of levulinaldehyde are related to PGEz (5a) and PGD, (4a) respectively by aldol condensation.
Results and Discussion Solvent Effects on Endoperoxide Stability. Thermal decomposition of the prostaglandin endoperoxide nucleus lb and the less strained homologue, 2,3-dioxabicyclo[2.2.2]octane (ll),were
11
conveniently monitored by 'H N M R in a variety of solvents at 73 and 130 O C , respectively (Table I).7b However, in aqueous solution the decomposition of l b is too rapid to measure by this method. Therefore the rate of spontaneous decomposition of l b in aqueous solution was determined at various temperatures over the range of 21 to 46 O C (Table 11). These data show an excellent linear correlation (r = 0.999) expressed by the relationship In k = -12338/T 32.61, as shown in Figure 1. Extrapolation to 73 OC gives k = 4.76 X lo-, s-]. The data presented in Table I reveal two important facts. Protic solvents exert a profound destabilizing influence on the prostaglandin endoperoxide nucleus l b compared with the aprotic solvents of similar dielectric constant, and a similar effect is not observed for the closely homologous peroxide 11. Thus, the rate of decomposition of l b increases slightly with solvent polarity as measured by dielectric constant. However, the rate of decomposition in acetic acid is an order of magnitude higher, and the rate of decomposition in aqueous solution is at least two orders of magnitude greater than expected for solvents of similar dielectric constant. This contrasts with the decomposition of 11 for which the rate in acetic acid or aqueous solution is not substantially
+
2b,X:H 2dg,X:D
lb,X=H ldg,X:D
5b,X:H 5d6,X:D
For decompositions in MezSO the rate constants for solvent-ind u d disappearance of 1 were measured at 37.0 OC by monitoring the appearance of 2.8 In MezSO the rate constant for disappearance of l b is k-] = 4.55 X lo4 s-l and the product yields are 2b (21%) and 5b (79%) while the rate constant for disappearance sC1 and the product yields are 2 d 6 (20%) of Id6 is k-] = 6.78 X and 5d6 (80%). The rate constants for solvent-induced decomposition of 1 in D,O solution were determined at 48.4 OC by monitoring the disappearance of 1. In DzO, the rate constant for disappearance of lb is kWl= 3.15 X s-l while the rate constant for disappearance of Id6 is k-l = 7.83 X s-l. Clearly the solvent-induced decomposition of l b is much faster than decomposition of Id6 in both Me2S0 and D 2 0 solutions. The data may be expressed as deuterium kinetic isotope effects ( k H / k D ) -=] 6.7 and 4.0 in MezSO and DzO, respectively, for disappearance of l b VS. Ids. Isotope Effects on Product Ratios from the Solvent-Induced Rearrangementsof 2,3-Dioxabicyclo[2.2.1]heptane-dl(Id). The monodeuterated peroxide Id was dissolved in water, acetic acid, or dimethyl sulfoxide (Me2SO) and kept at 37 "C. Although decomposition of undeuterated peroxide l b in water or M e 2 S 0 is complete within an hour at 37 OC, the solutions of monodeuterated peroxide Id were heated at 37 "C for 24 h to ensure complete decomposition. For the rearrangement in acetic acid, 'H N M R analysis of the solution after 2 days at 37 OC revealed incomplete decomposition. After 5 days at 37 OC no trace of Id could be detected. Yields of the four primary products 2d, 2b, 5d, and 5h presented in Table I11 were determined as described previously.Ia
Id
2h
2d
5d
5h
(8) Since 1 decomposes by concurrent first-order pathways to 2 and 5, the rate constant obtained by monitoring appearance of 2 or 5 is equal to the rate constant k-, for disappearance of 1 and k-, = k2 + k5 (see: Jencks, W.P. "Catalysis in Chemistry and Enzymology"; McGraw-Hill: New York, 1959; p 571).
6052 J . Am. Chem. SOC.,Vol. 106, No. 20, 1984
Salomon et al.
Table IV. Distribution of Deuterium in Levulinaldehvde Products"
solvent H20 CH3COOH
CDH2 CDH 36
