Title of Invention	A METHOD OF PROVIDING TETRAHYDRO-β-CARBOLINE RING SYSTEM
Abstract	A method of introducing a second stereogenic center into a tetrahydro-ß-carboline have two stereogenic centers using a modified Pictet-Spengler reaction is disclosed. The method provides a desired cis- or trans-isomer in high yield and purity, and in short processes times.

Full Text

MODIFIED PICTET-SPENGLER REACTION AND PRODUCTS PREPARED THEREFROM FIELD OF THE INVENTION The present inventicr. relates to a mod¬ified Pictet-Spengler reaction for introducing a second stereogenic center into a compound. More particularly, the present invention relates to a modified Pictet-Spengler reaction that provides a desired cis- or trans-diastereomer of a polycyclic compound having two stereogenic centers, in high yield and high purity. BACKGROUND OF THE INVENTION Compounds that exhibit biological activity typically contain at least one asymmetric carbon atom, i.e., at least one chiral center. A particu¬lar stereoisomer of such a compound usually exhibits excellent biological activity, whereas the other stereoisomers exhibit no or little biological activ¬ity. Accordingly, investigators strive to synthe¬size the biologically active stereoisomer, while minimizing or eliminating synthesis of the inactive or less active stereoisomer. Stereochemical .purity is important in the pharmaceutical field, where many of the most often prescribed drugs exhibit chirality. For example, the L-enantiomer of the p-adrenergic blocking agent, propranolol, is known to be 100 cimes more potent than its D-enantiomer. Additionally, optical purity is important in the pharmaceutical field because certain stereoisomers impart a deleterious effect, rather than an advantageous or inert effect. For example, it is believed that the D-enantiomer of thalidomide is a safe and effective sedative when i prescribed for the control of morning sickness dur¬ing pregnancy, whereas its corresponding L-enanti-omer is believed to be a potent teratogen. A stereoselective synthesis, therefore, permits the preparation of a more useful drug prod- ) act.. For example, the administered dose of a drug can be reduced because only the active stereoisomer is administered to an individual, as opposed to a mixture which contains a large amount of inactive " stereoisomer. This reduced dose of active stereo- ; isomer also reduces adverse side effects compared to a dose containing a mixture of stereoisomers. In addition, a stereoselective synthesis is more economical because a step of separating the desired stereoisomer from the undesired stereoisomer is I simplified or eliminated, and raw material wastes and costs are decreased because reactants are not consumed in the synthesis of undesired stereoiso¬mers. Many biologically active compounds contain j two asymmetric carbon atoms, i.e., two stereogenic centers, wherein each asymmetric carbon atom is a member of a ring system and each is bonded to a hydrogen atom and to a substituent different from a hydrogen atom. The nonhydrogen substituents of the ) asymmetric carbon atoms therefore can be in a cis or a trans configuration. A particularly difficult problem encountered in the synthesis of such bio¬logically active compounds is the high yield and high purity preparation of a particular stereoiso¬mer, i.e., the desired diasterecmer, wherein the ^ nonhydrogen substituents of the asymmetric carbon atoms are in the cis configuration, or the trans configuration, depending upon which diastereomer is the more biologically active. For such compounds, it is necessary to provide a synthetic pathway that provides each stereogenic center of correct stereochemistry, and thereby yield the desired diastereomer. The synthetic pathway also should provide a high yield of the desired diastereomer in as few steps as possible, with a minimum of diastereomer separation and purification. For example, U.S Patent No. 5,859,006, incorporated herein by reference, discloses the synthesis of (6R,12aR)-2,3,6,7,12,12a-hexahydro-2-methyl-6- (3, 4-methylenedioxyphenyl) -pyrazino-[2" , 1":6,l]pyrido[3,4-b]indole-1,4-dione having a structure (I): Compotind (I) has two asymmetric carbon atoms, each denoted by an asterisk, wherein the nonh drogen substituents of the asymmetric carbon atoms are in the cis configuration. Compound (I) can be prepared by the two synthetic path ways disclosed in U.S. Patent No. 5,859,005. Compound (I) is a potent and selective inhib¬itor of the phosphodiesterase enzyme PDE5, an has various therapeutic uses, for example, the treatment of male erectile dysfunction. The first synthetic pathway (A) , from D-tryptophan, has few steps, but the yield of the de¬sired diastereomer (i.e.. Compound II) is poor and requires a separation step from the trans-stereo-isomer (Compound Ila) . Pathway (A) also utilizes the highly corrosive trifluoroacetic acid (i.e., TFA or CF3CO2H) , The key step in pathway A is a classic Pictet-Spengler reaction using D-tryptophan methyl ester and piperonal to yield substituted tetrahydro-p-carboline Compounds (II) and (Ila). The second The overall yield of Compound (I) using syn¬thetic pathway (A) or (B) is about 25% to about 30%. Pathway (B) requires several synthetic steps, and, therefore, was considered inconvenient-A key step in the synthesis of Compound (I) is the preparation of Compound (II) in the shorter syn¬thetic pathway (A). The preparation of Compound (II) in pathway (A) utilizes a Pictet-Spengler cyclization between D-tryptophan methyl ester and piperonal in dichloromethane (CH2CI2) with two equivalents of trifluoroacetic acid at 4°C which xu would be an important advance in the art to provide a modified Pictet-Spengler cycliza-tion reaction that substantially improves the di-astereoselectivity of the reaction. In particular, it would be an advance in the art to improve pathway A, which utilizes the Pictet-Spengler reaction be¬tween commercially available D-tryptophan methyl ester and piperonal, or other aliphatic or aromatic aldehyde, in a straightforward method to prepare enantiomerically pure Compound (II), or similar tetrahydro-p-carboline, and that overcomes the iisadvantages of the classic Pictet-Spengler reac-:ion, such as use of TFA, long reaction times, and iifficult product separations. SUMMARY OF THE INVENTION The present invention is directed to a ethod of preparing a desired diastereomer, i.e., is or trans, of a polycyclic compound having two symmetric ring carbon atoms. More particularly, le present invention is directed to a method of reparing a desired diastereomer of a tetrahydro-(3-irboline compound having two asymmetric carbon ,oms utilizing a modified Pictet-Spengler reaction. Prior investigators attempted to prepare a sired diastereomer of a polycyclic ring system ntaining two asymmetric ring carbon atoms by per-rming a Pictet-Spengler cyclization reaction. 2se attempts generally were limited in success :ause the reaction was performed in a corrosive lium, led to mixtures of diastereomers that ad- several days to perform. The present method pro¬vides the desired diastereomer in good yield and short reaction times, and avcids the use of TFA. More particularly, -he present invention is directed to a method of preparing a desired di¬astereomer of a tetrahydro-^-carboline compound having two asyrru-etric carbons utilising a modified Pictet-Spengler cyclization reaction wherein the reaction is performed using a solvent in which only one of the diastereomers is soluble. In preferred embodiments, the desired diastereomer is insoluble in the solvent, and undesired diastereomer is sol¬uble. Another aspect of the present invention is to increase the yield of the desired diastereomer by allowing the undesired diastereomer to equilibrate Ln solution to provide additional desired diastereo-ler that precipitates from solution, and thereby in- xease the yield of the desired diastereomer at the xpense of the undesired diastereomer. These and other aspects and novel features f the present invention will become apparent from le following detailed description of the preferred nbodiments. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is directed to a thod of preparing a desired diastereomer of a lycyclic compound having two asymmetric carbon )ms as members of a ring system. The method utilises an improved Pictet-Spengler reaction that provides a desired tetrahydro-3-carboline dia; mer in high yield, high purity, and in a short cess time. The improved Picter-Spengler react also avoids the use of TFA in zhe reaction. Although the synthesis of Compo-unds and (ID are particularly discussed herein, th present method is not limited to these ccmpound The present method also can be used to synthes: the, desired diastereomer of other tetrahydro-β-carbolines by a judicious selection of starting tryptophan ester, e.g., the D- or L-form, the starting aldehyde, and the reaction solvents ut Lzed in the present modified Pictet-Spengler cy :ation reaction. Typically, the -Pictet-Spengler reactii proceeds through generation of an imine under nt tral conditions, then effecting cyclization usii trifluoroacetic acid (TFA) in dichloromethane (CH2CI2) at a low temperature (4°C) . In additioi starting with an imine, N-substitution of the tr tophan amino (-NH2) group often is used to previa cis-diastereomer. The Pictet-Spengler reaction closed in U.S. Patent No. 5,859,006 uses such co tions. As discussed above, the standard Pictet-Spengler reaction has the disadvantages of a lorn, cycle time, a low yield of the desired cis-diast( eomer, and use of the corrosive TFA. The present invention overcomes probler associated with the classic Pictet-Spengler reac¬tion, e.g., improves the yield and puritv of the synthetic route. In particular, the,present inv tion is directed to a simplified Pictet-Spengler reaction for generating a second ring stereogeni center, wherein the desired zis- or trans-diaste; eomer can be prepared in high yield and■purity b^ performing the reaction in a solvent in which th* desired disastereomer is inscluble and the undesi diastereomer is soluble. The modified Pictet- , Spengler reaction of the present invention also ■ , utilises an N-unsubstitutedscarting .itiaterial, e,. tryptophan, as the, hydrochloride salt/ andelim-,.inates, the use of TEA. The elimination of TFA fr the reaction has substantial ■advantages/ incl.uding improved isolation/identification of the tryptopha methyl hydrochloride and overcoming the corrosive properties of TFA. The selection, of,.a proper solvent for-us in the present modified Pictet-Spehglefregtctioh i well within the skill of persons in the"art. For example, in the prep.aratio,n;\of;".^Gpmpo\md","(,l:li).-"by"th^ Pictet-Spengler cyclization. reaction, isopropyl .alcohol was found to solubilize the undesired train -diastereomer, whereas the"""desired cis-diastereomer precipitated from the reaction mixture. In addi¬tion, the solubilized trans-diastereomer is in ■, dynamic equilibrium with the desired cis-diaster¬eomer. Accordingly,, as the- cis-diastereo"rrier" Corn-" pound (11) is formed in solution, and imrriediately precipitates, its concentration is lowered relative to the remaining trans-diasterebmer Compound (Ila), thereby providing a concentration differential that forces the equilibriiom to provide additional cis-diastereomer, This continuous driving of the reac¬tion increases both the yield and purity of the desired cis-diastereoraer. In particular, the present invention utilizes a modified Pictet-Spengler cyclization re¬action to form a tetrahydro-(2-carboline ring system having two stereogenic centers. The reaction is performed in a_solvent wherein the desired disaster-eomer is soluble at reflxxx temperature or below, and the undesired diastereomer is insoluble at reflux temperature or below. This solubility difference"■ illows a fast and easy separation of the desired iisastereomer from the undesired disastereomer. "urthermore, the dynamic cis~trans equilibrium in clution allows a more complete conversion of the tarting materials to the desired diastereomer, and more complete separation of the desired diastere-ner from the undesired diastereomer. Accordingly, mother advantage of the present invention is a jcrease in costs attributed to a more efficient use : reagents. As previously stated, the selection of a action solvent having the requisite solubility operties is within the ability of a person skilled the art. The selection merely requires deterini-::ion of the solubility of each diastereomer in a rticular solvent, and a solvent selection that its the above-described solubility/insolubility ameters for the two diastereomers. The following is a nor.limiting example of the present invention, illustrating the synthesis of Compound (II) by the modified Pictet-Spengler reac¬tion (Step 2), and the subsec-jent synthesis of Com¬pound (I) from Compound (II) ;Steps 3 and 4). In general, the synthesis of compound (I) using the method of the present invention involves a four-step synthetic pathway. The first step is an esterification in methanol (MeOH) using thionyl chloride (SOCI2) under reflux. The product is crys¬tallized and isolated by filtration. The second step involves the present novel and simplified variation of the Pictet-Spengler reaction, wherein D-tryptophan methyl ester hydrochloride is admixed v;ith piperonal in isopropyl alcohol (i-PrOH) and heated under reflux to form a mixture of diaster- ■^vjiiicixv- ^aaucts. Because the desired cis-diaster-eomer (Compound (II) ) is subs-ar.tially insoluble in isopropyl alcohol at reflux temperature and below, the cis-diastereomer crystallizes from solution leaving a dynamic els-trans equilibrium in solution. As the cis-diastereomer precipitates from the iso¬propyl alcohol, the equilibrium is driven towards the cis-diastereomer iintil the concentration of the cis-diastereomer is sufficiently low to remain in solution. The desired diastereomer is isolated in greater than 90% yield by crystallization and filtration. The third step involves an aqueous tetra-hydrofuran (THF) acylation of the amino (NH2) moiety of Compound (II), followed by crystallization and filtration. Ring closure with methylamine (MeNHa) completes the ring-forming sequence. After solvent e.xchange, the product is crystallized from aqueous isopropyl alcohol or other suitable solvent, and filtration provides Compound (I) in an overall yield of about 77%. In general, the present modified Pictet-Spengler reaction can be used to prepare the desired iiastereomer of tetrahydro-^-carboline-based com-jounds without limitation. For example, the present lodified Pictet-Spengler reaction can be used to lynthesize the desired diastereomer of classes of ompounds disclosed in U.S. Patent Nos. 5,859,006; ,981,527; 6,001,847. WO 02/28359, VJO 02/28865, 0 02/10166, WO 02/36593, WO 01/94345, WO 02/00658, D 02/00557, WO 02/38563, WO 01/94347, WO 02/94345, wo 02/00656, PCT/USOl/49393, PCT/US02/13719, PCT/US02/00017, PCT/US02/10367, PCT/USG2/13703, PCT/US02/11791, and PCT/US02/13397, each incorpo¬rated herein by reference, and ocher substituted tetrahydro-|3-carbolines. In addition to the preparation of tetra-hydro-p-carboline diketo-piperazines, like Compound (I) , the present method can be used to prepare tetrahydro-p-carboline hydantoins (III) of desired stereochemistry by reacting a compound such, as Compound (II) with an isocyana-e having a formula R^NCO, wherein R"* is aliphatic or aromatic. See U.S Patent No. 6,001,847, incorporated herein by refer¬ence. The following provides a detailed exem¬plary preparation of Compound (I) utilizing the method of the present invention. D-Tryptophan (50.0 kg, 245 mol) was sus¬pended in MeOH (270 L), then added to"a prepared solution of SOCI2 (67.0 kg, 563 mol) in MeOH (250 L) at ambient temperature under a nitrogen (N2) atmo¬sphere. The resulting solution was stirred at reflux for 1 to 2 hours, then MeOH was distilled" from the reaction mixture to about 50% of original volume. Methyl t-butyl ether (MTBE) (350 L) was added, and the solution was cooled to 0° to 5°C, with continued stirring for 1 hour. The product was filtered, washed with cold MTBE (150 L) , and dried in vacuum at 60°C to yield 57.6 kg (92.4%) of D-tryptophan methyl ester hydrochloride. "^H NMR (400 ^z DMSO) 5: 11.15 (IH, s) , 8.70 (2H, exch.), 7.50 (IH, d, J=8.2 Hz), 7.35 (IH, d, J=3.2 Hz), 7.24 (IH, ;), 7.08-7.05 (IH, m) , 7.00-6.97 (IH, m) , 4.18-4.16 IH, m), 3.61 (3H, s), 3.36-3.25 (2H, m) . HPLC Details: Column: SB-Phenyl 4.6 x 250 mm; Eluent: Isocratic 80% (K:0+0.1% T?A>720% ACN (acetonitrile) ; Temperature: 40°C; Flow Rate = 1 mL/min; UV Detec¬tion = 285 nm; Injection Vol"jme = 20 ]iL; Diluent = 1:1 ACN/H2O; and Retention Time = 10.0 min. D-Tryptophan methyl ester hydrochloride 0.0 kg, 196 mol) was suspended in isopropyl cohol (500 L) and treated with piperonal (32.4 kg, 5 mol) at ambient temperature under an N2 atmo-lere. The mixture was stirred betv/een 70°C and :lux (82°C) for 16 to 18 hours. At this time, the .ction mixture contained less than 3% Compound d. u"ne reaction mixture then was cooled to 0°C, filtered, and washed with cold isopropyl alcohol (150 L). The product was dried under vacuum at less than 60°C to yield 69.8 kg (92%) of cis-l-(l,3-benzodioxol-5-yl)-2,3,4,9-tetrahydro-lH-pyrido[3,4-b]indole-3-carboxylic acid mechyl ester (Compound ID). ^H NMR (400 MHz DMSO) 5: 10.81 (IH, s), 10.67 (IH, exch.), 10.21 (IH, exch.), 7.52 (IK, d, J=8.0 Hz), 7.27 (IH, d, J=8.0 Hz), 7.11 (IH, m), 7.05-6.95 (4H, mT, 6.08 (2H, sT,"^5.85 (IK, m) , 4.71 (IK, m) , 3.82 (3H, s), 3.39-3.23 (2K, m). HPLC Details: Column: SB-Phenyl 4.5 x 250 mm; ACN/(H2O+O . 1% TFA) gradient; Temperature: 40°C; Flow Rate = 1 mL/min; UV Det. = 285 nm; Injection Volume = 20 yL; Diluent =1:1 ACN/K2O; Sample concentration: about 0.1 mg/mL; and Retention time = £.0 min. In a preferred method of preparing Com¬pound (II) by the present method, a small seed amount of Compound (II), i.e., about 0.05% to about L%, and preferably about 0.05% to about 0.25%, based m the weight of D-tryptophan methyl ester hydro-:hloride, is added to the reaction mixture prior to .eating. This seed amoxmt induces crystallization f the cis-carboline Compound (II) in the reaction ixture. When isopropyl alcohol is used as the sol-gnt, it is preferred that the alcohol is anhydrous, .g., 0.1% water or less, by v;eight, because appre-".able amounts of water can adversely affect the ite of reaction. It is especially preferred that le isopropyl alcohol is essentially free of acectone. i.e., contains 0.3% acetone cries., by weight, to avoid formation of ar. undesired by-product. Use of a higher boili.-g solvent (eg n propanol. toluene, di.ethylfcr..a.-nide. acetonit.ile or acetic acid, leads to faster reaction ti^es with comparable product yield and purity. Other solvents useful in the preparation of co^ound ,11, using a Pictet-Spengler reaction (Step 2, Of the prasent invention include, but are not limited to, aromatic solvents (e.g., acetonitrile .enzena. or .xylene,, a nitrile (e.g., acetonitrile >r propionitrile,, an ester (e.g., ethyl acetate,, ,n alcohol (e.g., a propanol or butanol, . an ether e.g., THF, MTBE, or dioxane,, an aliphatic hydro-arbon (e.g., hexane, heptane,, an organic acid e.g.. acetic acid), ndxtures thereof, and agueous olutions thereof. -The substituted tetrahydro-(J-carboline . hydrochloride (II) (83.7 kg, 216 mol) was suspended in THF (335 L) and deionized water (84 L), and treated with triethylamine (EtaN) (57.0 kg, 560 mol at 0°C to 20°C under an N2 atmosphere. Chloroacety", chloride (C1CH2C(0)C1) (34.2 kg, 300 mol) in dry THI (0.6 volumes) then was added at a rate to maintain the temperature at 0°C to 10°C, followed by stirrin u-x^-x- vx, j-jDenzcdioxol-5-yl) -2 , 3 , 4, 9-tetrahydro-lK-pyrido[3,4-b]incole-3-carbox-/lic acid methyl ester. ^H NMR (400 MH2 DMSO) 5: 10.86 (IH, s) , 7.54 (IH, d, J=7.4), 7.27 (IH, d, J=8.0), 7.11-6.99 (2H, m) , j 6.81-6,75 (2H, m) , 6.63 (IH, s) , 6.45 (IH, d, J=8.2), 5.97 (2H, d, J=5.8), 5.19 (IH, d, J=6.6), 4.83 (IH, d, J=14), 4.43 (IK, d, J=14), 3.45 (IH, d, J=16), 3.10-3.03 (4H, m). Alternative solvents for Step 3 include ) lovf. molecular weight, alcohols, such as isopropyl alcohol or n-propyl alcohol; acetone; and methylene chloride. The chloroacetyl carboline (86.0 kg, 201 mol) was added to THF (430 L), and the resulting mixture was heated to 30°C to 55°C under an N2 atmosphere and stirred. The resulting solution then was filtered at a temperature of 45°C to 50°C to remove undissolved particles. Methylamine (78.2 kg, 1000 mol) then was added to the solution at a temperature of 5°C to 25°C- The resulting mixture was stirred at a temperature of 30°C to 55°C for about 1 hour, or until HPLC analysis indicated a complete reaction, i.e., less than 1% of the chloroacetyl carboline remained. The mixture was cooled to 0°C to 30°C, isopropyl alcohol (344 L) and water (175 L) then were added, followed by 12M hydrochloric acid (67 L) to neutralize the excess methylamine, i.e., to pH 2 to 9.4. Upon essentially complete removal of THF by distillation, the solu¬tion was treated with isopropyl alcohol (260 L) and v/ater (75 L) and cooled to -5°C to 30°C, followed by stirring for two hours to crystallize the product, "he product was filtered and v/ashed with cold (0°C to 5°C) 50% aqueous isopropyl alcohol. The wash solvent was filtered at -5°C to 30°C, and the prod¬uct was dried under vacuum at 80°C or less (e.g., 70°C to 80°C) to yield 75 kg (S4.6%) of Compound (I). For increased purity, Co-pound (I) optionally can be recrystallized from acetic acid. A reference standard was prepared in the same manner, with additional purification by double recrystallization from glacial acetic acid (HOAc). :ompoun.d -( I) was dissolved in-13 - volumes of HOAc at !0°C, and the solution was concentrated to one-third iriginal volume and then cooled to ambient tempera-ure. The product was filtered, washed with MTBE, nd dried in vacuum at 80°C. 2K NMR (400 MHz, DMSO) : 11.0 (IH, s), 7.52 (IH, d, J=7.3 Hz), 7.28 (IH, , 7.9 Hz), 7.28 (IH, d, J=7.9 Hz), 7.06-6.98 (2H, , 6.85 (IH, s), 6.76 (2H, s), 6.11 (IH, s), 5.91 IH, s), 4.40-4.35 (IH, dd, J=4.27, 11.6 Hz), 4.17 H, d, J=17-l Hz), 3.93 (IH, d; J=17.1), 3.54-3.47 H, dd, J=4.6, 11.3 Hz), 3.32 (IH, s), 3.00-2.91 H, m). HPLC Details: Column: Zorbax SB-Phenyl, 5 mm i.d. x 25 mm; 2.5 ^m particles; Mobile Phase: Btonitrile, 0.1% TFA in water; Flow rate =1.0 "min.; Detector wavelength =285 nm; Injection .ume = 20 βL; Column temperature = ambient; and ention time =9.0 min. Obviously, many modifications and varia-ns of the invention as set forth above can be 2 without departing from the spirit and scope reof, and, therefore, only such limitatione should be imposed as are indicated by the appended claims. WE CLAIM: A method of preparing a compound having a structural formula: comprising the steps of: (a) esterifying D-tryptophan in methanol and thionyl chloride to provide D-tryptophan methyl" ester hydrochloride; , (b) reacting the D-tryptophan methyl ester hydrochloride with piperonal in refluxing iso- propyl alcohol to provide cis-1-{1,3-benzodioxol-5- yl)-2, 3, 4,9-tetrahydro-lH-pyrido[3, 4-b]indole-3- carboxylic acid methyl ester; (c) reacting the product of step (b) with chloroacetyl chloride and triethylamine to provide cis-1-(1, 3-benzodioxo-5-yl)-2,3,4, 9-tetrahydro-lH- pyrido[3,4-b]indole-3-carboxylic acid methyl ester; and (d) reacting the product of step (c) with methylamine to provide the compound.

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