BioconJugate Chem. 1002, 3, 167-175
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Vinylstannylated Alkylating Agents as Prosthetic Groups for Radioiodination of Small Molecules: Design, Synthesis, and Application to Iodoallyl Analogues of Spiperone and Diprenorphine John L. Musachio and John R. Lever' Division of Radiation Health Sciences, Department of Environmental Health Sciences, The Johns Hopkins University School of Hygiene and Public Health, Baltimore, Maryland 21205-2179.Received December 2, 1991 The preparation and synthetic utility of p-toluenesulfonate esters of (E)and (2)-3-(tri-n-butylstannyl)prop-Zen-1-01 as bifunctional reagents for radioiodination are described. These vinylstannylated alkylating agents are prepared in two steps from propargyl alcohol, and readily couple with nucleophilic functionality (amide nitrogen, secondary amine, tertiary alcohol) in good yields (48-955%) to provide derivatives of the neuroreceptor ligands spiperone and diprenorphine. Regio- and stereospecific radioiododestannylation with retention of configuration occurs under mild, no-carrier-added conditions to give the corresponding radiolabeled N- or O-iodoallyl analogues in good radiochemical yields (55955% ) with high specific radioactivities. The methodology is versatile and well-suited to selective labeling of small molecules with radioisotopes of iodine such as lZ5Ior 1231.
INTRODUCTION Radioiodinated tracers have diverse applications in biomedical research, and there are a number of methods for incorporation of radioiodine into organic substrates (Seevers & Counsell, 1982;Coenen et al., 1983;Baldwin, 1986). Direct electrophilic radioiodination is most commonly employed, but many compounds are not suited to this procedure because they lack activating functionality, are not stable upon iodination, or lose biological activity when iodinated at the chemically favored site. In such circumstances, the use of a prosthetic group is advisable in order to enhance reactivity, promote stability, control regiospecificity, and avoid harsh reaction conditions. A variety of radioiodinated aromatics, exemplified by the Bolton-Hunter reagent (Chart I, A), have been tailored for conjugation to proteins, antibodies, and glycosides (Bolton & Hunter, 1973;Lowndes et al., 1988;Wilbur et al., 1989;Lin et al., 1989;Vaidyanathan & Zalutsky, 1990). However, these techniques are of limited applicability to small molecules due to the relatively large size of the reagents. Notwithstanding, the Bolton-Hunter method is quite useful in some instances, including the preparation of certain neuroreceptor ligands (Korner et al., 1986;Ponchant et al., 1988). p-Iodobenzyl bromide (Chart I, B), one of the few prosthetic groups chiefly intended for ra(Wilson diolabeling small molecules with either lZ5Ior 1231 et al., 1986),has been applied to radioiodinated glucose analogues (Saji et al., 1987). This reagent is particularly appropriate when the benzyl moiety is an integral feature of the target, as in the case of the muscarinic cholinergic receptor antagonist 4-iododexetimide (Wilsonet al., 1989). Prosthetic groups which impart less steric bulk and lipophilicity are especially desirable for radioiodination of small molecules, since minimal perturbation of structural and physicochemical parameters increases the likelihood of preserving favorable properties of the parent compound. The smallest prosthetic group for radioiodination is a lithiated vinylstannane (Chart I, c),which, after nucleophilic
* Address correspondence and offprint requests to John R. Lever, Ph.D., Department of Environmental Health Sciences, The Johns Hopkins University School of Hygiene and Public Health, 2001 Hume, 615 N. Wolfe St.,Baltimore,Maryland 212052179.
Chart I.8 Prosthetic Groups for Radioiodination 0
0
n-Bu3Sna Li C
(I*I = radioisotopes of iodine.
addition to carbonyl groups and halodemetalation, gives iodovinyl derivatives (Hanson & Seitz, 1982;Hanson et al., 1982). Since radioiododestannylations are fast and reliable (Kabalka & Varma, 1989),complementary electrophilic reagents for the introduction of vinylstannylated prosthetic groups would be of considerable synthetic value. In a recent communication, we described bifunctional alkylating agents 1E and 12,which render vinylstannanes for radioiodination as depicted in Scheme I (Musachio & Lever, 1989). Several factors provided the rationale for selection of 1E and 12 as prosthetic groups. The allylic framework, in conjunction with the leaving-group ability of tosylate, allows ready coupling to nucleophilic functionality. The intermediacy of vinylstannanes permits radioiodination in high yield under mild, no-carrier-added conditions. Radiolabeling at the end of the synthetic procedure mitigates exposure to and handling of radioactive materials. Finally, the sequence gives a vinyl carbon-iodine bond which is stronger than the often used aryl carbon-iodine bond (Coenen et al., 1983). Of note, the allylic linkage is the smallest unit which accommodates sites which facilitate conjugation as well as stable incorporation of radioiodine. In our initial studies, 1E and 12 were employed for syntheses of N-iodoallyl analogues of spiperone radiolabeled with lZ5I(tip = 60 d) or the diagnostic imaging radioisotope l23I (tl/2 = 13.3 h). These ligands display high affinities for serotonin 5-HTz and dopamine Dz receptors in vitro, and are suited for in vivo studies of D2 0 1992 American Chemical Society
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Bloconjugate Chem., Vol. 3, No. 2, 1992
Scheme I. Radioiodination Alkylating Agents
via
Vinylstannylated
) Nuc:(
*I&"
)
p N u c
*I
*I =
127, 1251
or 1231
sites including single photon emission computed tomography (Lever et al., 1989; Lever et al., 1990a). Here we provide full details for the preparation of 1E and 12,as and (2)-3-N-(iodoallyl)spiperone.Further, well as for (E)we illustrate the general utility of the approach through syntheses of N - and O-iodoallyl analogues of diprenorphine, a potent opioid receptor ligand. EXPERIMENTAL PROCEDURES
Chemicals and solvents were reagent grade and used as received from commercial sources unless otherwise stated. Dry DMF was prepared by sequential distillations under reduced pressure from CaH2 and then BaO. No-carrieradded [12511NaIwas obtained from Amersham Corp. (1 mCi/lO pL of dilute NaOH, pH 7-11). [12311NaI,produced by the (p, 5n) reaction and dispensed in 0.1 N NaOH (50200 pL), was purchased from Nordion Intl. Uncorrected melting points were determined with a Thomas-Hoover capillary apparatus. lH NMR spectra were obtained with a Bruker WM-300 (300.13 MHz) unless otherwise stated. Chemical shifts are reported in ppm (6) relative to internal Me& in CDCl3. Proton assignments for spiperone derivatives (4-7)were made by analogy to previous reports (Burns et al., 1984; Chi et al., 1987) while assignments for morphinans (8-16)were based upon a 2D NMR study of diprenorphine (Mazza et al., 1990). HRMS via EI, CI, or FAB was performed at the University of Minnesota Mass Spectrometry Facility. Elemental analyses were determined by Atlantic Microlab, Inc. Analytical TLC was conducted on Macherey Nagel silica gel 60 F-254 plates (250 pm). Short-path column chromatography was performed using E. Merck 7729 (92 % while glacial HOAc (100 pL; 95:5 v/v) and aqueous chloraminethe specific radioactivity was >5050 mCi/pmol based upon T trihydrate (25 pL, 3.5 mM). After 1.5 min at ambient the UV absorbance detection limit. temperature, the reaction was quenched with NazSzO5 (100 (E)17-[3-(Tri-n-butylstannyl)prop-2-enyl]-4,5a- pL, 6.3 mM). Semipreparative reverse-phase HPLC (70: 30 MeOH/0.05 N NH4HCOz containing 1%v/v glacial epoxy-18,19-dihydro-3,6-dimethoxy-7a-( 1-hydroxy-1HOAc; 6 mL/min) gave [12511-8( t R = 24.2 min, k' = 16.9) methylethyl)-6,14-endo-ethenomorphinan(11). A mixin a 20-mL volume. Dilution with water (20mL) and solidture of 10 (301 mg, 0.78 mmol), prepared as previously phase extraction provided an ethanolic solution of [125I]described (Bentley & Hardy, 1967a,b;Bentley et al., 1967), 8 (0.71 mCi) in 72 % radiochemical yield. An aliquot (130 and 1E(600 mg, 1.20 mmol) was treated with Na2C03 (176 pCi) was reconstituted in HPLC mobile phase (75:25 mg, 1.66 mmol) in absolute ethanol (14 mL) at reflux for MeOH/0.05 N NH4HCOz) for examination by analytical 13h, cooled to room temperature, and filtered. The filtrate reverse-phase HPLC (2 mL/min). The radioproduct ( t R was concentrated under reduced pressure, and the residue = 6.9 min, 12' = 5.0) was of >99% radiochemical purity purified by short-path chromatography (hexane/EtOAc/ with a specific radioactivity of 1647 mCi/pmol. EtsN, 9:l:O.Ol) to give 11(528mg; 0.74 mmol) as a colorless oil in 95% yield. 'H NMR: (CDCl3, 6) 0.75-1.10 (m, 18 (E)17-(Cyclopropylmethyl)-4,5a-epoxy- 18,19-dihyH, n-Bu 15 H, H-18 2 H, H-8a 1 H), 1.17 (8, 3 H, CH3), dro-3-methoxy-6-[ [3-(tri-n-butylstannyl)prop%-enyl]1.22-1.32 (m, 6 H, n-Bu), 1.35 (s,3 H, CH3), 1.43-1.56 (m, oxyl-7a-( 1-hydroxy-1-methylet hyl)-6,14-endo-et he6 H, n-Bu), 1.63-1.70 (m, 1H, H-15eq),1.73-1.79 (m, 2 H, nomorphinan (14). To a 5-mL conical vial containing 30 H-19), 1.90 (t, 1H, J = 10.0 Hz, H-78), 2.01 (m, 1 H, Hmg (0.070 mmol) of 13 (Lever et al., 1987) in DMF (2 mL) 15,),2.19(dd,lH,J=l8.4,6.4Hz,H-lOa),2.25-2.35(m, was added NaH (17 mg, 0.71 mmol). The mixture was 1H, H-16,), 2.49 (dd, 1H, J = 11.6,4.9 Hz, H-16eq),2.78 stirred for 1min before addition of 1E (177 mg, 0.35 mmol) (d, 1 H, J = 6.4 Hz, H-ga), 2.87 (m, 1 H, H-801, 3.03 (d, in DMF (1mL). After stirring for 90 min at ambient tem1H, J =18.4 Hz, H-lop), 3.10 (d, 2 H, J =5.2 Hz, NCHzperature, saturated NHdCl(O.8 mL) was added. Extractive CHCHSn), 3.52 (s, 3 H, 6-0CH3), 3.86 (s, 3 H, 3-Oc&), workup followed by semipreparative reverse-phase HPLC 4.39 (8, 1 H, H-5/3), 5.06 ( 8 , 1 H, Om, 5.89 (dt, 1 H, J = (Alltech Econosil (2-18, 10 mm X 25 cm, 10 pm) using 19.0 Hz, 5.2 Hz, NCHZCHCHSn), 6.08 (d, 1 H, J = 19.0 MeOH/0.05 N NH4HCOz (95:5 v/v) at 8 mL/min provided Hz, NCHzCHCHSn), 6.54 (d, 1H, J = 8.1 Hz, H-l),6.69 27 mg (0.036 mmol) of 14 ( t R = 51.6 min, k' = 41.0) in 51 % (d, 1H, J = 8.1 Hz, H-2). HRMS-CI: m / z calcd, 715.3622; yield. lH N M R (CDCl3,6)0.10 (m, 2 H, c - C ~ H ~0.50 ) , (m, found, 716.3664 (M+ + 1, 15). Anal. Calcd for C38H612 H, c - C ~ H ~0.78-0.81 ), (m, 2 H, H-18, c-C&), 0.84-0.90 N04Sn: C, 63.87; H, 8.60; N, 1.96. Found: C, 63.78; H, (m, 15 H, n-Bu), 1.04-1.12 (m, 2 H, H-8a, H-18), 1.21 (s, 8.64; N, 1.93. 3 H, CH3), 1.23-1.35 (m, 6 H, n-Bu), 1.41 (8, 3 H, CH3), (E)-17-[3-(Tri-n-butylstannyl)prop-2-enyl]-4,5a- 1.42-1.53 (m, 6 H, n-Bu), 1.64-1.70 (m, 1H, H-15,), 1.80 epoxy-18,lI)-dihydr0-3-hydroxy-6-methoxy-7a-( l-hy(m, 2 H, H-19),1.98 (m, 1H, H-7@),2.04 (m, 1H, H-15,), droxy-1-met hylethyl)-6,14-endo-ethenomorphinan 2.18-2.41 (m, 4 H, CHZ-C-C~H~; 2 H, H-loa, H-16,), 2.63 (12). To a solution of 11 (65 mg, 0.091 mmol) in DMF (5 (dd, 1H, J = 11.8Hz, 4.9 Hz, H-16,), 2.87 (m, 1H, H-881,
+
Vinylstannylated Prosthetlcs for Radlolodlnation
2.97-3.02 (m, 2 H, H-108, H-Sa), 3.89 (s,3 H, C-3 Oc&), l H-5@), 4.23 (dd, 1 H, J = 12.9,4.8 Hz, CHzO), 4.43 ( ~ ,H, 4.48 (dd, 1H, J = 12.9, 4.8 Hz, CHzO), 5.2 (s, 1H, OH), 6.03 (dt, 1H, J = 19.1,4.8 Hz, SnCHCH), 6.20 (d, 1H, J = 19.1 Hz, SnCHCH), 6.52 (d, 1H, J = 8.0 Hz, H-l), 6.70 (d, 1H, J = 8.0 Hz, H-2). HRMS-CI (C41H65N04Sn): m/z calcd, 755.3935; found, 756.4025 (M+ + 1, lo), 698.3208 (M+ - WBU,100). (E)17-(Cyclopropylmethyl)-4,5a-epoxy18,lg-dihy[3-(tri-n-butylstannyl)prop-2-enyl]dro-3-hydroxy-6-[ oxy]-7a-(l-hydroxy-l-methylethyl)-6,14-endo-ethenomorphinan (15). To a solution of 14 (52 mg, 0.069 mmol) in DMF (5 mL) under argon was added sodium propanethiolate (339 mg, 3.45 mmol). The mixture was heated at reflux for 8 min, cooled to ambient temperature, and quenched by addition of saturated NH4C1 (2 mL). Extractive workup followed by semipreparative reversephase HPLC (Alltech Econosil C-18,lO mm X 25 cm, 10 pm) using 95:5 MeOH/0.05 N NH4HC02 at 8 mL/min provided 20.0 mg (0.027 mmol, 39%) of 15 ( t =~ 13.16 min, k' = 9.36) and 13.5 mg (0.030 mmol, 43%) of 16 ( t ~ = 2.70 min, k' = 1.12). Characteristic 'H NMR (CDC13, 6) for 15: 4.19 (ddd, 1 H, J = 12.1, 4.9, 1.2 Hz, CHzO), 4.41-4.46 (overlapping s and ddd, 2 H, H-50, CHzO), 4.80 (brs, 1H, 3-0H), 6.04 (dt, 1H, J = 19.1,4.9 Hz, SnCHCH), 6.21 (d, 1 H, J = 19.1 Hz, SnCHCH). HRMS-CI (15, C40H63N04Sn): m/z calcd, 741.3779; found 742.3818 (M+ + 1). Characteristic 'H NMR (CDC13,6)for 16: 5.93 (m, 1 H, HzCCHCHzO), 5.28 (dd, 1 H, J = 17.2, 1.5 Hz, H2CCHCHzO),5.15 (dd, 1H, J = 10.5,1.5H~,H2CCHCH20), 4.43-4.46 (overlapping s and dd, 2 H, H-50, H2CCHCH20), 4.19 (dd, 1H, J = 12.2,5.4 Hz, HzCCHCH20). HRMS-CI (16,Cz&I37N04): m/z calcd, 451.2722;found, 452.2802 (M+ + 1). (E)-17-(Cyclopropylmethyl)-4,5a-epoxy-18,19-dihydro-3-hydroxy-6-[ (3-iodoprop-2-enyl)oxy]-7a-( l-hydroxy-l-methylethyl)-6,14-endo-ethenomorphinan (9). To 15 (16 mg, 0.022 mmol) in CHzClz (1.0 mL) was added 12 (0.024 mmol) in CHzClz (0.55 mL, 0.043 M). The mixture was stirred at room temperature for 10 min and then applied to a preparative TLC silica plate (Analtech, 20 cm X 20 cm, 1000 pm) which was developed in hexane/ EtOAc/EtsN (2:l:O.Ol). The major component (Rf= 0.54) was desorbed using EtOAc containing 1% Et3N. Filtration followed by concentration gave 7.0 mg (0.012 mmol, 55 5%) of 9 as an oil. Characteristic 'H NMR (CDCl3, 6) for 9: 4.14 (ddd, 1 H, J = 12.7, 5.8, 1.4 Hz, CHzO), 4.37-4.44 (overlapping s and ddd, 2 H, H-58, CHZO),6.41 (dt, 1H, J = 14.6,1.4 Hz, ICHCH), 6.65 (dt, 1H, J = 14.6, 5.8 Hz, 577.1688; ICHCH). HRMS-CI ( C Z ~ H ~ ~ I Nm/z O ~calcd, ): found, 578.1767 (M+ + 1). [l251]-9. To a solution of 15 (0.30 mg, 0.40 pmol) in MeOH (50 pL) in a glass vial sealed with a Teflon-faced septum was added [lz5I1NaI (5 pL, ca. 0.25 nmol; 0.54 mCi) followed by MeOH containing glacial HOAc (50 pL; 955 v/v) and aqueous chloramine-T trihydrate (25 pL, 4.3 mM). After 1min at ambient temperature, the reaction was quenched withNazS2O5(100pL, 50 mM). Semipreparative reverse-phase HPLC using MeOH/0.05 N NHdHCOz (6040 v/v) at 6 mL/min gave [I25I]-9 ( t =~30.7 min, k' = 21.0) in a 20-mL volume. Dilution with water (35 mL) and solid-phase extraction provided an ethanolic solution of [12511-9(0.38 mCi) in 71% radiochemical yield. An aliquot was reconstituted in mobile phase (MeOH/O.O5 N NHdHC02, 7525 v/v) for examination by analytical reverse-phase HPLC at 3 mL/min. The radioproduct ( t ~ = 6.1 min, k' = 4.4) was of >99% radiochemical purity with specific radioactivity of 1405 mCi/pmol.
Bloconlugate Chem., Vol. 3, No. 2, 1992
171
Scheme 11. Synthesis of Bifunctional Reagents 1E and 12 n - B u 3 S n ~ O H 2E TsCl I KOSiMe3
Chart 11. Alkyl Derivatives of Spiperone at the N-3 Position
R -
4E 42 5E 52 6E 6Z 7
H n-Bu3Sn H
R' -
n-Bu3Sn H 'I
*I
H
H CI
CI
H
H
H
* I = 127I, 125I or lZ31 RESULTS AND DISCUSSION
Allylic alcohols 2 E and 22,the precursors to 1E and 12, were prepared by treatment of propargyl alcohol with trin-butyltin hydride and AIBN as described by Jung and Light (1982). This route allows convenient access in one step to both 2 E and 2 2 along with structural isomer 3 (Scheme 11). Chromatographic isolation afforded pure 2E,while 22and 3 were obtained as an inseparable mixture. Alternatively, 2 E and 2 2can be individually obtained from propargyl alcohol in stereospecific fashion by palladiumcatalyzed syn hydrostannylation (2E,Miyake & Yamamura, 1989) or by hydroalumination followed by stannylation (22; Corey & Eckrich, 1984). Standard conditions for esterification with TsCl in pyridine (Tipson, 1944)proved unsuccessfulfor 2 E even when using aqueous copper sulfate for removal of pyridine. A procedure for tosylation of allylic alcohols (Johnson & Dutra, 1973) was then modified by substitution of the organic-soluble potassium trimethylsilanolate (Laganis & Chenard, 1984) for freshly ground sodium hydroxide. Treatment of 2 E with the silanolate and TsCl in diethyl ether at -25 "C gave 1E in 65% yield (Scheme 11). In similar fashion, the mixture of 2 2 and 3 gave 12 (42%) without concomitant esterification of 3. Once purified, 1E and 12suffer negligible decomposition upon storage for at least 6 months at -20 "C. To ascertain the utility of vinylstannylated alkylating agents 1E and 12 for selective radioiodination of drugs having multiple functionality, we investigated the preparation of (E)- and (Z)-3-N-(iodoallyl)spiperone(Chart 11). These targets were selected because spiperone derivatives with bulky substituents at the amide nitrogen maintain high affinity for dopamine DZreceptors (Agui et al., 1988;Welch et al., 1988). Accordingly,vinylstannanes 4E and 4 2 were prepared as precursors for iodination in approximately 50 % yield by NaH-promoted alkylation with either 1E or 12in DMF. By contrast to fluoroalkylations of spiperone (Chi et al., 19871,iminoester formation
Musachio and Lever
172 Bloconlugate Chem., Vol. 3, No. 2, 1992
by 0-alkylation was not detected in the reaction mixture by 1H NMR. In fact, no evidence was observed for competitive pathways, such as C-alkylation adjacent to the carbonyl group (Burns et al., 1984) or 3-N-alkylation ' in S N ~fashion. Routes to allylic vinylstannanes frequently involve reaction of organotin hydrides with alkynes. For instance, 4E has been prepared by hydrostannylation of 3-N-propargylspiperone (Hanson & Ranade, 1989; Lisic et al., 1989). Hydrostannylations are not always well-defined stereochemically, and a benefit of the present method is controlled availability of both E- and 2-isomers. In addition, some multifunctional substrates which are not compatible with hydrostannylation reaction conditions would be suited to alkylation with 1E or 12. A structural congener of lE, (E)-l-chloro-3-(tri-n-butylstannyl)prop2-ene,has also been reported by Hanson (1989). The better leaving-group ability of tosylate with respect to chloride should make 1E or 12 preferred for alkylation of weak nucleophiles. Vinylstannanes 4E and 4 2 were converted to the corresponding vinyl iodides 5E and 5 2 (Chart 11)with iodine in CH2C12 in 40% and 84% yields, respectively. Even though the anilino rings are activated toward electrophiles, only ipso-substitution of the vinylstannane was observed. Retention of configuration was confirmed by appropriate 'H NMR vicinal couplings for the ABX2 spin systems of 14.6 Hz) and 52 ( 3 J =~7.3 Hz). On the basis 5E ( 3 J= ~ of normal-phase HPLC, which allows detection of 97 5%. Specific radioactivities ranged from 1329to 2000 mCi/pmol, and were comparable to that of the batches of [12511NaIemployed. During reverse-phase HPLC isolation of the [125I]labeled products, N-allyl derivative 7 was observed as the
Chart 111. Structures of N-and 0-Iodoallyl Analogues of Diprenorphine (DPN)
CH3 DPN
major nonradioactive product, while chloroallyl analogues (6E, 62) were not detected. Side reactions can complicate radiotracer purification, and it is noteworthy that chlorination is averted with these vinylstannanes. Extensive protodestannylation does not adversely effect radioiododestannylation, and 7 is readily separated from [12511-5E or [lz5II-52. Nevertheless, conditions were sought which would minimize protodemetalation and thereby expedite radiotracer purification in general. With a less acidic solvent system, methanol containing 1% glacial acetic acid, 4E suffers no protodemetalation (0.2% HPLC detection limit) while incorporation of radioiodide is >95 5%. The procedures were easily adapted to radiolabeling with lZ3I by neutralizing the variable amounts of base contained in dispensing solutions of [lz3I1NaIwith aqueous acid (1 equiv). For a typical preparation on a 20 mCi scale, [12311-5Ewas synthesized, purified, and formulated within 2 h in 55% yield (not corrected for decay). The radiochemical purity of [l23I1-5Ewas >92% while the specificradioactivity was >5050 mCi/pmol. Normal-phase HPLC confirmed retention of configuration. To further explore the scope and limitations of radioiodination via vinylstannylated alkylating agents, we selected N- and 0-iodoallyl analogues 8 and 9 of diprenorphine as targets (Chart 111). Coupling to secondary amine and tertiary alcohol residues would serve as a measure of the alkylating ability of 1E and 12. Further, the highly activated aromatic ring provides a stringent test of the ability of the stannyl group to direct iodination. In addition to testing synthetic constraints, the preparation of 8 and 9 might lead to radioligands for opioid receptor studies. A prosthetic group is necessary since direct substitution of the aromatic ring with heavy halogens leads to a substantial reduction in potency of 4,5-epoxymorphinans such as diprenorphine (Casy & Parfitt, 1986).The N-[l8Flfluoroalkyl analogues of diprenorphine similar to 8 retain affinity for opioid receptors, and show promise for positron emission tomography (Bai et al., 1990; Chesis et al., 1990). Moreover, 9 was of interest because structurally related oxymorphones with bulky substituents at C-6 display good biological activity at opioid receptors (Hazum et al., 1982).
Bloconjugate Chem., Voi. 3, No. 2, 1992 173
Vlnylstannylated Prosthetics for Radioiodination
Scheme 111. Synthetic Route to 8, the N-Iodoallyl Analogue of Diprenorphine CH30
0. @NH
E
CH3O , HOtCH,
c12j+N
Jsn(*B~~rsNa-Hl&N
CH3O , HOYCH, CH3 11
CH3 10
Jsn(*Bu)3
CH30 , HO+CH~ CH3
7
12 T-en:a! : :< tZ51]-8
8
Scheme IV. Synthetic Route to 9, the 0-Iodoallyl Analogue of Diprenorphine
J
+ r CH3 16
n-Bu3Sn
oH O t Cy H 3 CH3 15
99 % radiochemicalpurity. With the fortuitous availability of 16 as a reference standard, protodestannylation of 15 to give 16 was not observed (ca. 1% HPLC detection limit) during purification of the reaction mixture. Using the same reaction conditions except for the omission of radioiodide, no indication of side-product formation (e.g., chlorination) was found by HPLC. In summary, 1E and 12are versatile bifunctional alkylating agents which couple readily with nucleophilic residues, including amide nitrogen, secondary amine, and tertiary alcohol,to provide vinylstannylated intermediates
174
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i n good yields (48-95 ?% ). Subsequent regio- and stereospecific radioiododestannylation with retention of configuration occurs under mild, no-carrier-added conditions to give products with high specific radioactivities i n good isolated radiochemical yields (55-95 9% 1. The radioiodination is rapid, efficient, and dependable, and is suited to preparation of [lZSI] - or [lz3I] -labeled tracers without requiring extensive optimization of reaction conditions. Oxidant-promoted chlorination is not a significant side reaction, and conditions have been established which minimize protodemetalation. Thus, 1E and 1.2 allow access to iodoallyl analogues from highly functionalized structural classes. The stability of such adducts has yet to be determined, and is likely to depend upon overall structure. For example, iodovinyl estradiols (Hanson et al., 1982) and iodovinyl fatty acids (Knapp et al., 1984) are quite stable in vivo, while certain iodovinyl antibody conjugates suffer metabolic degradation (Hadley & Wilbur, 1990). The novel reagents 1E and 12 complement the arsenal of existing prosthetic groups and should prove useful for radioiodination of a variety of biologically active
small molecules. ACKNOWLEDGMENT
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