Methods for Biosynthesis & Asymmetric Synthesis of Chiral Acetylenic Acid Derivative, Study notes of Stereochemistry

Download Methods for Biosynthesis & Asymmetric Synthesis of Chiral Acetylenic Acid Derivative and more Study notes Stereochemistry in PDF only on Docsity! ASYMMETRIC SYNTHESIS OF PROSTAGLANDINS by Rachael White A Thesis Submitted to the Faculty of the WORCESTER POLYTECHNIC INSTITUTE in partial fulfillment of the requirements for the Degree of Master of Science in Chemistry May 2005 APPROVED: ___________________________________________ Dr. James P. Dittami Advisor & Chemistry/Biochemistry Department Head Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. ii Abstract Prostaglandins (PGs) are medicinally interesting because of the wide variety of roles they play in the body. PGs are ubiquitous and can be found in the reproductive system, the nervous system, the cardiovascular system, and the immune system. Accordingly, PGs are an important therapeutic target for pharmaceutical companies, and an efficient synthesis is highly desirable. Past research indicates that an approach to prostaglandins via a chiral acetylenic ester or amide provides a promising method for control of C-15 geometry. This project seeks to validate a key stereospecific reduction of an enantiomerically pure cyclopentenone intermediate. This is in turn available from a chiral acetylenic ester or amide via a formal [3+2] cycloaddition step. Several methods have been investigated for asymmetric synthesis of the requisite chiral acetylenic acid derivative including asymmetric conjugate addition, CBS-oxazaborolidine reduction of a ketone, and the separation of diastereomers of a chiral amide. With the optically pure cyclopentenone in hand, we will investigate hydroxyl directed conjugate reduction of the cyclopentenone double bond as shown in the figure. O C5H11 O SiMe3 Cu H H CO2Et Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. v Table of Schemes Scheme 1. Retrosynthesis 7 Scheme 2. Racemic Alcohol 8 Scheme 3. [3+2] Cycloaddition (Adapted from Ref. 9) 9 Scheme 4. Stereospecific Reaction Conditions 10 Scheme 5. [3,3] Sigmatropic Shift 11 Scheme 6. Separation of Stereoisomers 12 Scheme 7. Similar Substrate in Novozym 435 Hydrolysis 13 Scheme 8. Acylation 14 Scheme 9. Novozym 435 Reaction Conditions 14 Scheme 10. Cycloaddition with Amides 14 Scheme 11. Formation of Chiral Acetylene 15 Scheme 12. Addition (Adapted from Ref. 17) 15 Scheme 13. Proposed Synthesis of 16a 15 Scheme 14. Direct Asymmetric Synthesis 16 Scheme 15. Direct Synthesis of 6 16 Scheme 16. Route to Unsubstituted Propargyl Alcohols (From Ref. 20) 16 Scheme 17. Addition of Acetylene 20 to Benzaldehyde 17 Scheme 18. Asymmetric Reduction of 24 18 Scheme 19. Formation of MTPA Esters 18 Scheme 20. Cyclization to Provide 5 19 Scheme 21. Cyclization 19 Scheme 22. Cycloaddition using Methyl Octynoate (27) 20 Scheme 23. Formation of Zinc Homoenolate 21 Scheme 24. Preparation of Optically Pure 6 21 Scheme 25. Evaluation of Conjugate Reduction 21 Scheme 26. Creation of Chiral Amides 22 Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 1 Introduction Prostaglandins (PGs) are medicinally interesting due to their broad spectrum of biological activity. PGs act in many parts of the body, including (but not limited to): the reproductive system (in both males and females), the nervous system, the cardiovascular system, the immune system and gastrointestinal system.1 Due to their diverse biological activity, there is potential for prostaglandin analogs (prostanoids) to function as effective therapeutic agents. Indeed, there are already PG analogs used as drugs for the treatment of ulcers, hypertension and other conditions. In the 1960’s and 1970’s, soon after the core structure of the PGs was determined, many different routes for prostaglandin synthesis were explored.2,3 However, achieving an asymmetric synthesis of the PGs still presents unique challenges. A wide variety of strategies have been employed, however further investigation is still warranted. Development of a robust, asymmetric route to prostaglandins would greatly benefit development of prostanoids by pharmaceutical companies. Structure In the most basic sense, PGs are twenty-carbon molecules made up of a five- membered (cyclopentane) ring with two aliphatic sidechains. The numbering of these molecules is straightforward, and is shown in representative example 1 (Figure 1). Two other structural features are found in all prostaglandins: C-15 is a stereo center with an attached hydroxyl group, and C-9 always has an attached oxygen function (present as a hydroxyl group, ketone, or part of a bridged peroxide moiety). There are a few other distinguishing characteristics of PGs that are not ubiquitous: In most cases, the carbons to which the side chains are attached are stereo centers, and some PGs have a second oxygen function attached to either the C-10 or C-11 positions. Also, PGs can have up to three double bonds present in the sidechains. These structural details separate the known prostaglandins into nine families (or “classes”), while the absence or presence of double bonds in the sidechains separates the PGs into three “series”. Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 2 O CO2H C5H11 HO 1 8 12 15 1 Figure 1. Prostaglandin A2 As mentioned, prostaglandins are categorized into “series” and “classes”. The “series” classification divides PGs up by general structural elements conserved across all classes of prostaglandin as well as indicating from which fatty acid the PG is derived. The “classes” categorize different types PGs by their specific structural elements. There are three series of prostaglandins, labeled as: PGX1, PGX2 and PGX3, where “X” is a letter indicating the class of a given prostaglandin. The subscripts indicate the number of double bonds in the side chains coming off of C-8 and C-12, which indirectly indicates from what precursor a PG has been generated. Prostanoids are derived from certain essential fatty acids, which are known as the eicosanoic acids. Each series of prostaglandin is derived from a different eicosanoic acid: the 1-, 2-, and 3- series are derived from dihomo-γ-linolenic acid, arachidonic acid (2), and 5, 8, 11, 14, 17-eicosapentaenoic acid respectively (Figure 2). The 1-, 2-, and 3- series, “class A” prostaglandins (also known as “PGAs”) are shown below in Figure 3 were derived from the corresponding fatty acid in Figure 2. COOH dihomo-γ-linolenic acid COOH arachidonic acid (2) COOH 5,8,11,14,17-eicosapentaenoic acid Figure 2. C-20 Fatty Acid PG Precursors Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 5 Stimulus Activation of phospholipases phospholipid COOH arachidonic acid 2 O2 cyclooxygenase COOH HOO 2 e- hydroperoxidase PGG2 O O COOH HO O O PGH2 PGD synthase PGE synthase PGF2 synthase COOH HO COOH HO COOH HO HO O O HO HO HO PGD2 PGE2 PGF2 Figure 5. Biosynthesis (Modified from Ref. 7) Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 6 Once released, arachidonic acid (2) is acted upon sequentially by two enzymes: a cyclooxygenase and a peroxidase (collectively known as “prostaglandin synthase”). The cyclooxygenase enzyme begins to develop the structure of the future prostaglandin; the familiar five-membered ring with trans-stereochemistry is formed, and two molecules of oxygen are added to the molecule to form the intermediate PGG2. The first molecule of oxygen exists in the form of an endoperoxide, while the second molecule of oxygen is added (stereospecifically) as a peroxide group in the C-15 position. The second enzyme, a peroxidase, converts the C-15 peroxide group in to a C-15 hydroxyl group leading to PGH2 which is both a naturally occurring prostaglandin with its own biological effects and a precursor for further biological derivation. Once the action of the prostaglandin synthase machinery is complete, the newly formed PGH2 may be acted on by other enzymes, such as the PGD2 or PGE2 synthases to create the other classes of prostaglandin. Previous Work Previous research8 in the laboratory of Professor Dittami has afforded a fairly efficient route to a prostaglandin core (3) framework (Figure 6). Key points to note in the structure are the stereo center at C-15 positiona, the trans-stereochemistry across the bond between C-8 and C-12 positions, and the ester function at position 8 which serves as a precursor to the C1-C7 sidechain. The stereo centers are common to many PGs and are important to their biological activity, while the ester on C-8 provides a versatile handle for later conversion to analogs. O C5H11 HO O OEt 8 12 15 3 Figure 6. Desired Prostaglandin Core The retrosynthetic approach is shown below in Scheme 1. In Step 4, a stereospecific [3,3] sigmatropic shift moves the hydroxyl group from the C-13 position to the C-15 position with retention of configuration, providing 3, the desired core structure. a For simplicity, the same numbering is used for both PGs and for this desired PG core. Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 7 Step 3 employs stereospecific reduction of the double bond in the ring, directed by the hydroxyl group on the C-15 stereocenter, yielding 4. A formal [3+2] cycloaddition of a chiral acetylenic ester creates the substituted cyclopentenone ring in Step 2 (5). 9 Step 1 involves formation of an enantiomerically pure chiral acetylenic ester or amide (6). Racemic 6 is easily obtained from the commercially available starting materials ethyl propiolate (7) and trans-2-octenal (8). Steps 4 and 2 have been previously carried out and optimized using racemic mixtures or model compounds with good yields. Previous attempts to asymmetrically synthesize the acetylenic ester intermediate 6 in Step 1 have resulted in inadequate enantiomeric excess (ee). Because of this, the stereospecificity in Step 3 remains to be proven, and is therefore the focus of this and future research. O C5H11 HO O OEt O C5H11 O OEt HO O C5H11 O OEt HO O O OH C5H11 Step 4 Step 3 Step 2 Step 1 O O H H O C5H11 + 3 4 5 6 7 8 Scheme 1. Retrosynthesis Establishing the Latent C-15 Hydroxyl Stereochemistry A key element of our synthesis is to establish the stereochemistry of the hydroxyl center in acetylenic ester 6. This functional group is expected to serve two roles. (1) It can participate in directing the reduction of the double bond in 5, giving rise to two new stereo centers at C-8 and C-12. (2) It serves as latent functionality for the C-15 stereocenter via [3,3] sigmatropic shift. The stereochemical integrity of the C-15 center is vital for the biological activity of the PG’s. Previous strategies for obtaining this intermediate included derivatization of the racemic intermediate by attaching a chiral group to the hydroxyl moiety (followed by separation of diastereomers), reduction of the corresponding ketone with Alpine borane, and taddol-assisted asymmetric addition of ethyl propiolate (7) to trans-2-octenal (8). None of these methods, however, provided the intermediate with both adequate yield and ee. Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 10 O O O TMSO C5H11 LiAlH4 / 2 CuI THF or Ether -78 oC O O O TMSO C5H11 O O O TMSO C5H11 O O O TMSO C5H11 O O O TMSO C5H11 4 4b 4d4c 5 HH H H H H H H Scheme 4. Stereospecific Reaction Conditions The proposed intermediate (9) for the reaction is shown in Figure 8. The lithium aluminum hydride and copper bromide react to form a copper hydride species which complexes the TMS-ether oxygen in the C-13 position. The complex positions a hydride group in an ideal position for the reduction. O C5H11 O O O SiMe3Cu H H 9 Figure 8. Copper Hydride Complex Directing Reduction (Adapted from Ref. 8) However, if the oxygen bearing sidechain is not rotationally constrained, the reduction can occur from both faces of the cyclopentenone ring. Given enantiomerically pure starting material, this would lead to two diastereomeric products (Figure 9). The experimental evidence reported above suggests that a conformation leading mainly to only one of the diastereomers is preferred. The two possible diastereomeric products 4 and 4c are shown in Figure 9. As noted above, the reduction of racemic 5 provided mainly product 4. By extension a reaction on only one enantiomer should have the same distribution of products as in the reaction with a racemic mixture product: 99.5% desired product 4 and 0.5% undesired diastereomer 4c. Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 11 O CO2Et O HR sidechain rotation O CO2Et O H RCuH H SiMe3 SiMe3 Cu H H 4c O CO2Et O H R SiMe3 H H sidechain rotation O CO2Et H H O SiMe3 R H 4 O CO2Et H H O SiMe3 R H via attack at ß face via attack at a face Figure 9. Diastereomeric Products Formed by the Reduction Stereoselective [3,3] Sigmatropic Shift The last step of the synthesis is a stereospecific [3,3] sigmatropic rearrangement of 4 to 3 to establish the C-15 hydroxyl with correct stereochemistry. This reaction is performed using the acylated alcohol. This rearrangement is known to provide product with retention of configuration (Scheme 5).11 It proceeds via a concerted mechanism involving a six-electron rearrangement. Previous results have shown that the yields are relatively good (70%), and that remaining starting material can be recovered for further use. O O C5H11 OEt O O [Pd(MeCN)2]Cl2 THF O O C5H11 OEt O O 1513 4 3 Scheme 5. [3,3] Sigmatropic Shift Current Work The key point of optimization for our PG synthesis is to obtain the intermediate 6 in high yield and good stereochemical purity so that it can be cyclized and used to test the stereospecificity of the reduction of 5 to 4. Current work in our laboratory involves investigation of several methods for obtaining the intermediate 6 or a similar acetylenic Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 12 acid derivative 10 (Figure 10). The methods used are broken down into two categories: direct asymmetric synthesis and separation of diastereoisomers. X O OH C5H11 10 R X = -NH-, -O- R =Alkyl Figure 10. Acetylenic Acid Derivative Advantages of the separation of stereoisomers include ease of separation (eg. conventional chromatography) and reactions that are relatively insensitive to water and air. The obvious disadvantage: the maximum yield of the desired enantiomer is only 50% (Scheme 6). In this case, however, it is worth investigation: even a small amount of the intermediate can be used to show stereospecificity in the conjugate reduction of 5 (Scheme 4). Two methods for separation of stereoisomers are being investigated: selective deacylation of the intermediate 6 by the commercially available enzyme Novozym 435 and separation of the diastereomers of a chiral acetylenic amide intermediate. R O OH C5H11 resolution R O OH C5H11 R O OH C5H11 + 50% 50% 6 Scheme 6. Separation of Stereoisomers Novozym 435 consists of a lipase isolated from Candida antarctica bound to a solid phase. Past research on the lipase in Novozym 435 has shown that the similar substrate 11 undergoes stereospecific deacylation of one enantiomer with good yield and ee (>95%) as shown in Scheme 7.12 Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 15 acetylene 16 to 8 may provide an easily separable set of diastereomers that can be resolved and taken on to the cyclization and reduction steps (Scheme 13). HOOC H N-hydroxysuccinimide DCC, dimethoxyethane N O O H O O Intermediate a N O O H O O Intermediate a NH2 N H O H 14 15 16 Scheme 11. Formation of Chiral Acetylene N H O H LDA R1(CO)R2 N H O OH R2 R117 18 Scheme 12. Addition (Adapted from Ref. 17) N H O H LDA N H O OH 16 16aH O C5H11 8 C5H11 Scheme 13. Proposed Synthesis of 16a Direct asymmetric synthesis of 6 has the distinct advantage of 100% maximum yield. Two methods were investigated: asymmetric addition of an alkyne to an aldehyde and asymmetric reduction of the ketone of 19 (Scheme 14). Literature searches on asymmetric addition lead to work done in the laboratory of E.M. Carreira.18 Using N- methyl ephedrine as a chiral auxiliary, Carreira has shown that a number of both substituted and unsubstituted acetylenes undergo asymmetric addition to aldehydes.19,20 In a well-known reaction pioneered by E.J. Corey, ketones such as 19 undergo asymmetric reduction directed by chiral oxazaborolidines in the presence of a reducing agent such as borane-dimethylsulfide.21 This reaction has since been used in stereospecific syntheses of natural products such as discodermolide and petrofuran.22,23 Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 16 EtO2C H + H O C5H11 O O O C5H11 7 8 19 O O OH C5H11 reduction addition 6 Scheme 14. Direct Asymmetric Synthesis Using the work of Carreira as a guide, we attempted the addition of ethyl propiolate 7 to trans-2-octenal 8 to synthesize intermediate 6 directly (Scheme 15). Formation of product was not observed. Carreira noted improved performance with the acetylene 2-methylbut-3-yn-2-ol (20) in this addition. Furthermore, he devised fragmentation conditions for removal of the acetylenic substituent, providing unsubstituted propargylic alcohols (Scheme 16).20 Attempts to add the Zn-alkynilide of 20 to aldehyde 8 however, did not result in formation of the desired product. Though reactions using unsubstituted aliphatic aldehydes have been reported, the yields are lower than reactions using shorter substituted aliphatic aldehydes or cyclic aldehydes. (-)-N-methylephedrine Zn(OTf)2, toluene, triethylamine 23 oC OH EtO2CHEtO2C C5H11 H O C5H11 8 6 7 Scheme 15. Direct Synthesis of 6 R O H HHO R OH OH R OH H 20 cat. 18-C-6 K2CO3 toluene reflux (-)-N-methylephedrine Zn(OTf)2, toluene, triethylamine, 23 oC Scheme 16. Route to Unsubstituted Propargyl Alcohols (From Ref. 20) As a check on our technique, we attempted to repeat one of the reactions reported in the Carreira paper. The addition of acetylene 20 to benzaldehyde (21) had been reported to provide propargyl alcohol 22 in quantitative yield. This reaction was repeated successfully in our laboratory, indicating that the aldehyde 8 was the source of the trouble in our reactions. We concluded that the aldehyde 8 does not easily submit to this type of addition and other methods were explored. Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 17 (-)-N-methylephedrine Zn(OTf)2, toluene, triethylamine 23 oC OH HHO HO O H 20 21 22 Scheme 17. Addition of Acetylene 20 to Benzaldehyde Asymmetric reduction of acetylenic enones such as 23 by oxazaborolidines has been reported previously, and there is ample precedent for the successful reduction of compounds similar to our compound 19 (Figure 12).22,23 However, commercially available oxazaborolidine catalyst is relatively expensive, so a model compound was employed to optimize the reaction conditions. The reduction of similar enone 23 has been reported in quantitative yield and 95% ee.24 It should be noted that the trimethylsilyl (TMS) group can be easily removed and replaced with the desired ester function. As a model compound, we chose a hybrid between the desired compound 19 and the known compound 23. In the model compound 24, the length of the aliphatic chain on the aldehyde is preserved from 19 and the TMS group from 23 is retained (Figure 12). O C5H11 TMS 24 O C3H7 TMS 23 O C5H11 EtO2C 19 Figure 12. Similar Enone, Model Compound and Desired Compound Compound 24 was obtained via oxidation of racemic alcohol 26 either by manganese oxide or Jones’ reagent.25,26 Compound 26 was prepared in turn from n-butyl lithium promoted addition of trimethylsilylacetylene to trans-2-octenal (8).24 Product 24 was subjected to reduction by (S)-2-methyl-CBS-oxazaborolidine 25 and borane- dimethylsulfide quantitatively yielding the enantioenriched alcohol 26. The absolute configuration and ee of this compound were determined using the modified Mosher method as described below.27 Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 20 MeO2C OEt OTMS ZnCl2, ether, CuBr-Me2S THF, HMPA O CO2Me 27 28 Scheme 22. Cycloaddition using Methyl Octynoate (27) Several factors could be responsible for the low yield. Accordingly, we set out to systematically test each variable. Suspecting the copper bromide complex may be bad, we attempted to synthesize and purify fresh material via the published procedure. 30,31 Eventually we bought new commercially available copper bromide-dimethylsulfide complex which was colorless and appeared to be of good purity. Unfortunately, yields for the reaction remained low. With good copper bromide in hand and confident of the purity of our reaction solvents, we attempted to evaluate the first step – formation of zinc homoenolate 29 via sonication of (1-ethoxycyclopropyloxy)trimethylsilane (Scheme 23). It has been reported that 29 is stable in deuterochloroform solution and can be analyzed by NMR.32,33 Thus, (1-ethoxycyclopropyloxy)trimethylsilane was sonicated in dry ether in the presence of zinc chloride. Analysis by NMR, however, did not show formation of the expected homoenolate 29.32,33 A conversation with Dr. Crimmins confirmed our suspicion that perhaps the zinc chloride reagent was bad.34 Dr. Crimmins mentioned that in early iterations of the reaction, commercial zinc chloride solutions provided adequate formation of the homoenolate (29). He went on to say that later reactions using commercial zinc chloride were unsuccessful, and laboratory prepared zinc chloride was used instead, with success. The zinc chloride employed in our laboratory was obtained commercially as a 1 M solution in diethylether and had been recently purchased. An equivalent solution can be made in the laboratory by fusing solid zinc chloride under vacuum and subsequently sonicating the solid with an appropriate amount of dry diethylether to facilitate dissolution. Alternatively, new zinc chloride solution in ether can be purchased from a different vendor for use in the reaction. Both methods are currently being tested in the laboratory. Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 21 OEt OTMS ZnCl2 Et2O ultrasound EtO2C Zn 2 TMSCl+ 29 Scheme 23. Formation of Zinc Homoenolate Future Work Future work will involve: (1) Preparation of optically pure 6 via either direct reduction of intermediate 19 or conversion of 26 (Scheme 24).35 OH C5H11 TMS 26 O C5H11 EtO2C 19 O O OH C5H11 reduction 6 MeLi Methyl chloroformate Scheme 24. Preparation of Optically Pure 6 (2) Optimization of the Crimmins conditions for preparation of 5 with racemic 6. (3) Synthesis of optically pure 5 from optically pure 6 (Scheme 21). OEt OTMS ZnCl2, ether, CuBr-Me2S THF, HMPAOH EtO2C C5H11 6 O C5H11HO OEt O 5 Scheme 20. Cyclization (4) Evaluation of the asymmetric conjugate reduction of 5 to provide the cyclopentenone skeleton 4 (Scheme 25). O O O TMSO C5H11 LiAlH4 / 2 CuI THF or Ether -78 oC O O O TMSO C5H11 O O O TMSO C5H11 4 4c5 H H H H + Scheme 25. Evaluation of Conjugate Reduction (5) Evaluation of the corresponding amide analogs of 6 in the overall synthesis plan. These are reported to provide better yields in the Crimmins procedure. There are also a Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 22 number of readily available chiral amines which could be used for preparation of chiral amide intermediates 16a (Scheme 26). HOOC H NH2 N H O H 14 15 16 2 LDA H O C5H11 8 N H O 16a OH C5H11 Scheme 26. Creation of Chiral Amides Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 25 Synthesis of ethyl 4-hydroxyundec-5-en-2-ynoate (racemic 6) O O H H O C5H11 O O OH C5H11 1)LDA/THF 7 6 82) Method B To a stirred solution of LDA (1.6 mL, 2.8 mmol) in dry THF (1 mL) at –78 °C under nitrogen was added dropwise ethyl propiolate (7) (0.11 mL, 0.11 g, 1.1 mmol). The brown solution was stirred for 30 minutes at –78 °C and trans-2-octenal (8) (0.15 mL, 0.13 g, 1.1 mmol) was added dropwise. The reaction mixture was stirred at –78 °C, and monitored by TLC (9:1 hexane: ethyl acetate) for disappearance of the aldehyde starting material. After 40 minutes a saturated solution of ammonium chloride (0.5 mL) was added and the mixture was allowed to warm to room temperature. The organic phase was extracted with ethyl ether (20 mL). The combined organic phases were washed with water and brine and dried over MgSO4. The crude product was purified by flash chromatography (9:1 hexane: ethyl acetate) to yield 6 as a light yellow oil (0.21 g, 94%). Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 26 Synthesis of ethyl 4-hydroxyundec-5-en-2-ynoate (racemic 6) O O H H O C5H11 O O OH C5H11 1)n-Bu Li/THF 7 6 82) Method C To a flame dried round bottom flask was added dry tetrahydrofuran (10 mL) and ethyl propiolate (7) (1.0 mL, 10 mmol). The clear solution was cooled to -78 °C and a 2.0 M of n-butyl lithium in cyclohexane (5.6 mL, 11 mmol) was added dropwise. As the base was added, the solution turned from clear and colorless to dark brown. The solution was allowed to stir at -78 °C for 10 minutes and trans-2-octenal (8) (1.2 mL, 8.2 mmol) was added dropwise. The reaction mixture was stirred for 45 minutes and monitored by TLC (1:9 ethyl acetate: hexanes, p-anisaldehyde stain, UV light). After 45 minutes, some of the aldehyde starting material remained. The reaction mixture was warmed to room temperature after which it was poured over pH 7.0 phosphate buffer. The organic layer was extracted with ether (2 x 50 mL) and the combined organic layers were washed with water and brine, dried over MgSO4. Solvent was removed under reduced pressure to give 6 as a brown oil. 1H NMR of the crude oil showed that it contained desired product and aldehyde starting material. The crude product was used directly in the next step without further purification. Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 27 Synthesis of ethyl 4-acetoxyundec-5-en-2-ynoate (13) O O O OO Pyridine O O O C5H11 O C5H11 OH6 13 To a stirred solution of pyridine (0.75 mL) and acetic anhydride (0.25 mL) was added ethyl 4-acetoxyundec-5-en-2-ynoate (6) (0.12 g, 0.50 mmol). The reaction mixture was allowed to stir at room temperature for one hour and was monitored by TLC (1:9 ethyl acetate: hexanes, uv light) for disappearance of starting material after which the reaction was cooled to 0 °C and saturated aqueous ammonium chloride solution was added. The organic layer was diluted with ether, washed with ammonium chloride solution and brine, dried over magnesium sulfate. Solvent was removed under reduced pressure and excess pyridine was removed by azeotropic distillation with toluene providing 13 as a brown oil (82 mg, 60%). 1H NMR (CDCl3, 400 MHz): δ 0.82 (t, 3H, J = 7.1 Hz); 1.19-1.35 (m, 11 H); 2.00 (s, 3 H); 4.18 (q, 2 H); 5.42-5.48 (m, 1 H); 5.84 (d, 1 H, J = 6.8 Hz); 5.90-5.97 (m, 1 H). The 1H NMR of this product is found in the Appendix as D. Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 30 Synthesis of 1-(trimethylsilyl)dec-4-en-1-yn-3-one (24) MnO2 hexane TMS O C5H11 TMS OH C5H11 26 24 To a crude mixture of trans-2-octenal and 26 (1:5 mixture, 0.51 g total, 2.3 mmol) in hexanes (5.0 mL) was added MnO2 (2.7 g, 31 mmol). The heterogeneous mixture was stirred overnight at room temperature. TLC analysis (1:9 ethyl acetate: hexanes, p- anisaldehyde stain, UV light) showed disappearance of the starting material. The mixture was filtered through celite and the solvent removed to give the crude product as a yellow oil, 1H NMR of the crude mixture showed desired product, and trans-2-octenal left over from the previous step. The product was purified by flash chromatography (1:9 ethyl acetate: hexanes) to yield 24 as a light yellow oil (0.12 g, 29% yield based on conversion of the starting material 26). 1H NMR (CDCl3 400 MHz): δ 0.28 (s, 9 H); 1.28-1.61 (m, 11 H); 6.17 (d, 1 H, J = 15.8 Hz); 7.18-7.28 (m, 1 H). The 1H NMR of the product 24 can be found in the appendix as K. Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 31 Synthesis of (S)-1-(trimethylsilyl)dec-4-en-1-yn-3-ol (26) TMS O C5H11 TMS OH C5H11 N B O H Ph Ph BH3-Me2S, THF, Ar, 0 oC 24 25 26 To an oven dried round bottom flask under argon at 0 °C was added tetrahydrofuran (2 mL), a 1 M solution of (S)-2-methyl-CBS-oxazaborolidine in toluene (0.50 mL, 0.50 mmol) and 2 M solution of borane-dimethyl sulfide in tetrahydrofuran (0.29 mL, 0.57 mmol). The mixture was stirred for 5 minutes at 0 °C and a solution of 1- (trimethylsilyl)dec-4-en-1-yn-3-one (24) (0.12 g, 0.50 mmol) in tetrahydrofuran (1 mL) was added dropwise over 10 minutes. Upon completion of addition, TLC analysis (1:9 ethyl acetate: hexanes, p-anisaldehyde stain, UV light) revealed complete disappearance of the starting material. The reaction was cautiously quenched with methanol (1 mL) at 0 °C and then stirred for 15 minutes at room temperature. Solvent was removed under reduced pressure to provide a viscous yellow oil. 1H NMR of the crude mixture showed formation of desired product. The crude oil was purified by flash chromatography (1:9 ethyl acetate: hexanes) to yield 26 quantitatively and with 80% ee. 1H NMR (CDCl3 400 MHz): δ 0.0 (s, 9 H); 1.1-1.4 (m, 11 H); 4.62-4.65 (m, 1 H); 5.37-5.43 (m, 1 H); 5.68- 5.72 (m, 1 H). The ee of the product was analyzed using the modified Mosher method. The 1H NMR data for the product, R-MTPA ester and S-MTPA ester are found in the appendix as L, M, and N respectively. Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 32 Synthesis of methyl 5-oxo-2-pentylcyclopent-1-enecarboxylate (28) MeO2C OEt OTMS ZnCl2, ether, CuBr-Me2S THF, HMPA O CO2Me 27 28 To a flame dried round bottom flask under dry nitrogen was added dry ether (9 mL), (1- ethoxycyclopropoxy)trimethylsilane (2.4 mL, 12 mmol), and a 1 M solution of ZnCl2 in ether (9.0 mL, 9.0 mmol). The solution was sonicated for 40 minutes and stirred for an additional 10 minutes at room temperature. To the heterogeneous mixture was added CuBr-Me2S (0.15 g, 0.75 mmol), a solution of methyl oct-2-ynoate (27) (0.77 g, 5 mmol) in tetrahydrofuran (18 mL) and hexamethylphosphoramide (2.1 mL, 12 mmol). The reaction mixture was stirred at room temperature and monitored by TLC (1:9 ethyl acetate: hexanes, p-anisaldehyde stain, UV light). The reaction was quenched by addition of saturated aqueous ammonium chloride solution. The organic layer was washed with half saturated ammonium hydroxide solution until no blue color appeared in the wash. The organic layer was then washed with water and brine, dried over MgSO4. Solvent was removed under reduced pressure to leave a yellow oil. 1H NMR of the crude mixture showed the alkyne starting material present as well as many other peaks. The product was purified by flash chromatography (1:3 ethyl acetate: hexanes) to give 28 as a light yellow oil (0.17 g, 16%). 1H NMR (CDCl3 400 MHz): δ 0.78-0.81 (m, 3 H); 1.22- 1.25 (m, 4 H); 1.38-1.50 (m, 2 H); 2.36-2.39 (m, 2 H); 2.56-2.57 (m, 2 H); 2.63-2.65 (m, 2H); 3.73 (s, 3 H). 13C NMR (CDCl3 100 MHz): δ 14.2, 22.7, 27.8, 30.8, 32.2, 33.0, 35.3, 52.3, 133, 164, 190, 204. The 1H NMR of the (1-ethyoxycyclopropoxy)-trimethylsilane and methyl octynoate starting materials are found in the appendix as O and P respectively. The 1H and 13C NMR for the product 28 are found in the appendix as Q and R respectively. Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 35 Appendix A O O H Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 36 B H O C 5H 11 Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 37 C E tO 2C O H C 5H 11 Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 40 F H H O Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 41 G O H N Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 42 H H O O H Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 45 K T M S O C 5H 11 Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 46 L T M S O H C 5H 11 Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 47 M T M S O R M T P A C 5H 11 Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 50 P O O Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 51 Q O C O 2M e Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 52 R O C O 2M e Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 55 V N O O O H Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 56 U N H 2 Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now. 57 W H O O H Evaluation notes were added to the output document. To get rid of these notes, please order your copy of ePrint IV now.