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Principles and Applications of Asymmetric Synthesis

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1.4 DETERMINING ABSOLUTE CONFIGURATION

39

1.4.5Horeau's Method

Another method for determining the absolute con®gurations of secondary alcohols is Horeau's method, which is based on kinetic resolution. As shown in Scheme 1±14, an optically active alcohol reacts with racemic 2-phenylbutanoic anhydride (54), and an optically active 2-phenylbutanoic acid (52) is obtained after hydrolysis of the half-reacted anhydride.

It has been found experimentally that, as with Prelog's rule, there is a relationship between the sign of optical rotation of the isolated 2-phenylbutyric acid and the absolute con®guration of the alcohol involved. If the 2-phenylbutanoic acid isolated is levorotatory, the secondary alcohol 53 will be con®gured such that, in a Fischer projection, the hydroxy group is down, the hydrogen atom is up, and the larger group of the two remaining substituents is on the right. If the isolated acid is dextrorotatory, the secondary alcohol will arrange with the larger substituent on the left. Accordingly, the absolute con®guration of the secondary alcohol being studied can be de®ned as (R) or (S) according to the CIP rule (Scheme 1±14).6 Interested readers may consult the review written by Horeau.65

Scheme 1±14. Horeau's method.

2-Phenylbutanoic anhydride can be easily prepared as shown in Scheme 1±1566:

40 INTRODUCTION

Scheme 1±15. Preparation of 2-phenylbutanoic anhydride.

2-Phenylbutanoic anhydride can be used for assigning the absolute con®guration of most chiral secondary alcohols. Unfortunately this reagent fails to give a satisfactory result when the alcohol contains very bulky groups.67 A modi®cation of Horeau's method, using PhCH(C2H5)COCl as the resolving reagent, has been shown to be e¨ective for alcohols with bulky groups. In fact, with this modi®cation following a similar esteri®cation procedure, even very hindered neopentyl alcohols react with the acid chloride. (Acid anhydride did not react in this case.) The result is consistent with that achieved through the use of phenylbutanoic acid anhydride.68

1.4.6 Nuclear Magnetic Resonance Method for Relative Configuration Determination

1.4.6.1 Nuclear Overhauser Effect for Con®guration Determination.

When one resonance in an NMR spectrum is perturbed by saturation or inversion, the net intensities of other resonances in the spectrum may change. This phenomenon is called the nuclear Overhauser e¨ect (NOE). The change in resonance intensities is caused by spins close in space to those directly a¨ected by the perturbation. In an ideal NOE experiment, the target resonance is completely saturated by selected irradiation, while all other signals are completely una¨ected. An NOE study of a rigid molecule or molecular residue often gives both structural and conformational information, whereas for highly ¯exible molecules or residues NOE studies are less useful.

The NOE is a product of double magnetic resonance. The intensity of an NMR signal depends on the rate of spin relaxation (i.e., the rate at which the nuclei return from a high magnetic energy level to a low one). The process of relaxation of one nucleus can be signi®cantly in¯uenced by the magnetic events occurring in the other nuclei in the vicinity.

Because the NOE falls o¨ with the inverse sixth power of distance, an observed NOE between two protons implies that the two protons are reasonably close to each other. Thus, it is possible to determine their relative position using the NOE. The following examples illustrate the use of NOE for stereochemical assignments.

In Figure 1±20, the protected amino acids 56a and 56b are synthetic precursors of the two diastereomers of aminostatine,69 and they can be distinguished by the NOE spectra. The spatial interaction between H-2 and H-3 causes the di¨erence in the NOE in the cyclized derivatives 56aM and 56bM. For compound 56aM, no NOE is observed between these two vicinal protons, while for 56bM a signi®cant NOE between H-3 and H-2 can be observed (Fig. 1±20).

 

1.4 DETERMINING ABSOLUTE CONFIGURATION

41

 

 

 

 

 

 

 

 

 

 

Figure 1±20. Compounds with di¨erent con®gurations may have di¨erent nuclear Overhauser e¨ects.

Another example is in the determination of the absolute con®guration of the antibiotic analogue GV 129606X (57).70 From the NOE, the relative positions of H-4, H-5, H-9, H-10, and H-11 can easily be established, and the con®g- urations of the corresponding chiral carbon centers to which the protons are attached can also be deduced unambiguously by relating them to the known absolute con®guration.

1.4.6.2 Modi®ed Mosher's Method for Determining the Absolute Con- ®guration. Among a number of methods used to determine the absolute con®guration of organic compounds, Mosher's method71 using 2-methoxy-2-

42 INTRODUCTION

tri¯uoromethyl-2-phenylacetic acid (MTPA) has been the most frequently used. This method involves converting chiral alcohols (or amines) to their corresponding MTPA esters (or amides), followed by NMR analysis of the resulting derivatives. With a high-®eld FT-NMR technique, the absolute con®guration of these chiral alcohols (or amines) can be deduced from the corresponding chemical shift di¨erence, and this method is now known as Mosher's method.

Mosher proposed that, in solution, the carbinyl proton and ester carbonyl, as well as the tri¯uoromethyl group of an MTPA moiety, lie in the same plane (Fig. 1±21). Calculations on this MTPA ester demonstrate that the proposed conformation is just one of two stable conformations.72 X-ray studies on the (R)-MTPA ester of 4-trans-t-butylcyclohexanol73 and (1R)-hydroxy-(2R)- bromo-1,2,3,4-tetrahydronaphthalene74 reveal that the position of the MTPA moiety is almost identical with that proposed by Mosher, as shown in Figure 1± 21A. As a result of the diamagnetic e¨ect of the benzene ring, the NMR signals of HA, HB, HC of the (R)-MTPA ester should appear up®eld relative to those of the (S)-MTPA ester. The reverse should be true for the NMR signals of HX, HY, HZ. Therefore, for Dd ˆ dS ÿ dR, protons on the right side of the MTPA plane A must have positive values …Dd > 0†, and protons on the left side of the plane must have negative values …Dd < 0†, as illustrated in Figure 1±21.

Procedures for using this method to determine the absolute con®guration of secondary alcohols can be outlined as follows (Fig. 1±21B):

1.Assign as many proton signals as possible with respect to each of the (R)- and (S)-MTPA esters.

2.Calculate the Dd …dS ÿ dR† values for these signals of (R)- and (S)-MTPA esters.

Figure 1±21. Model to determine the absolute con®gurations of secondary alcohols. Reprinted with permission by Am. Chem. Soc., Ref. 75.

1.4 DETERMINING ABSOLUTE CONFIGURATION

43

3.Put the protons with positive Dd on the right side and those with negative Dd on the left side of the model.

4.Construct a molecular model of the compound in question and con®rm that all the assigned protons with positive and negative Dd values are actually found on the right and left sides of the MTPA plane, respectively. The absolute values of Dd must be proportional to the distance from the MTPA moiety. When these conditions have all been met, the model will indicate the correct absolute con®guration of the compound.75

The following are some examples of the application of Mosher's method. Combretastatins D-1 (58) and D-2 (59) are two 15-membered macrocyclic

lactone compounds isolated from the South African tree Cambretum ca¨rum and have been shown to inhibit PS cell line growth with ED50 values of 3.3 and 5.2 mg/ml, respectively.76 Compound …‡†-58 can be prepared via asymmetric epoxidation of the acetate of compound 59 catalyzed by the (S,S)-Mn-salen complex (Mn-salen catalyzed epoxidation is discussed in Chapter 4). The absolute con®guration of this compound was determined according to Mosher's method. At ®rst, the epoxide was hydrogenated to give 3-ol 60 as the major product. It was then esteri®ed to give the di-MTPA ester 61 and was subjected to NMR analysis. The DdH indicates that C-3 of the major product has an (S)- con®guration; thus the synthetic combretastatin …‡†-58 is concluded to have a (3R,4S)-con®guration, and this is consistent with the expected result of epoxidation catalyzed by the (S,S)-Mn-salen complex (Scheme 1±16, note the change of priority for the two carbon atoms attached to C-3 in 58 and 60.).

Scheme 1±16. Deduction of the absolute con®guration of 58.

44 INTRODUCTION

Figure 1±22. Determining the absolute con®guration of chiral centers in compounds 62 and 63 by measuring Dd values.

Because the natural 58 exhibited a negative value of optical rotation, the stereochemistry of natural …ÿ†-combrestatin can be assigned as (3S,4R).77

Penaresidins A 62 and B 63 are sphingosine-related compounds isolated from the Okinawa marine sponge Penares sp. that have shown potent actomyosin ATPase-activating activity.78 Through detailed analysis of the 11H COSY and HOHAHA spectra, the 1H NMR spectrum for the MTPA ester (acetamide) can be assigned.78 As shown in Figure 1±22, the chemical shift di¨erences Dd ˆ dS ÿ dR for the (S)- and (R)-MTPA esters of N-acetyl penaresidins A and B reveal that the absolute con®gurations of these compounds are as follows79: C-15: S, C-2: S, C-3: R, C-4: S, C-16: S. [C-16-(S ) is only for penaresidin A.]

Similarly, 2-anthrylmethoxyacetic acid (2ATMA 64) can also be used as a chiral anisotropic reagent (Fig. 1±23).80 The advantage of using this compound is that the absolute con®guration assignment can be accomplished from only one isomer of the ester without calculating the Dd value …dS ÿ dR†:

Figure 1±23. 2-Anthrylmethoxyacetic acid in absolute con®guration deduction.

1.4 DETERMINING ABSOLUTE CONFIGURATION

45

This is illustrated by the determination of the absolute con®gurations of C-7 and C-13 in taurolipid B (65). The absolute con®guration can be deduced by analyzing the 1H NMR spectrum of either the (R)- or the (S)-2ATMA ester without calculating the corresponding Dd values. Taurolipids A, B, and C were isolated from the freshwater protozoan Tetradymena thermophila. Taurolipid B (65) had shown growth-inhibitory activity against HL-60.81 Mild hydrolysis of 65 gave a tetrahydroxy compound that possesses four chiral centers. Subsequent derivatization gave an acetonide with two free hydroxyl groups at C-7 and C-13 for the formation of 2ATMA ester. Their 2ATMA esters exist in a conformation similar to that of its MTPA esters, in which the carbinyl proton, carbonyl oxygen, and methoxy groups are oriented on the same plane. Up®eld shifts are observed for protons shielded by the anthryl group, and the shifted signals are wide ranging. Thus, the C-7 can be assigned an (R)-con®guration and C-13 an (S)-con®guration (Fig. 1±24):

Figure 1±24. Deduction of absolute con®guration of 65.

The result of this method is consistent with that obtained by using the corresponding Mosher's ester method …DdH ˆ d…R†-ATMA ÿ d…S†-ATMA† (Fig. 1±25). Note that in the method of Mosher's ester, …dS ÿ dR† was applied to calculate DdH, the di¨erence in the 2ATMA method is only due to the con®guration nomenclature di¨erence caused by the CIP rule.

The absolute con®guration of a secondary alcohol can also be determined through the NMR spectra of a single methoxyphenylacetic ester derivative

46 INTRODUCTION

Figure 1±25. Mosher's method for determining the absolute con®guration of compound

65.

(MPA, either the [R]- or the [S ]-form) recorded at two di¨erent temperatures. This approach, requiring just one derivatizing reaction, also simpli®es the NMR-based methodologies for absolute con®guration determination.82

For a given chiral secondary alcohol L1L2CHOH …L1 0 L2†, its (R)- or (S)- methoxyphenylacetic acid ester (or MPA ester) may exist in two conformations, sp and ap, in equilibrium around the CR±CO bond, with the CO-O-C10 HL1L2 fragment virtually rigid (Fig. 1±26).83 Experimental and theoretical data83 indicate that the sp conformation is the more stable one in both the (R)- and the (S)-MPA esters. In addition, the relative population of the sp conformer is practically una¨ected by the nature of the alcohol.

Figure 1±27 depicts the application of this method. According to Boltzman's law, a selective increase of the sp conformation population over the ap is expected at lower temperatures. Thus, an up®eld shift should be observed for the L1 group of an (R)-MPA ester (positive DdT1T2 ) with decreasing temperature, because the fraction of molecules having L1 under the shielding cone of the phenyl ring is being increased. Analogously, a down®eld shift for L2 should be expected (negative DdT1T2 ). Similar analysis of the (S)-MPA ester leads to op-

Figure 1±26. Two conformations of secondary alcohol MPA ester. Reprinted with permission by Am. Chem. Soc., Ref. 82.

1.5 GENERAL STRATEGIES FOR ASYMMETRIC SYNTHESIS

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Figure 1±27. Absolute con®guration deduction using MPA. Reprinted with permission by Am. Chem. Soc., Ref. 82.

posite shifts, which are equally useful. L1 should be shifted down®eld (negative DdT1T2 ) at decreased temperatures, while L2 should be moved up®eld, and thus the corresponding DdT1T2 values must become positive.

As a result, the relative position of L1 or L2 in relation to the aromatic ring in the sp conformation can be established from the sign of the variations in the chemical shifts of substituents L1 and L2 with temperature (positive or negative DdT1T2 ). Assigning the con®guration of the chiral center is then straightforward.

1.5GENERAL STRATEGIES FOR ASYMMETRIC SYNTHESIS

Asymmetric organic reactions have proved to be very valuable in the study of reaction mechanisms, in the determination of relative and absolute con®gurations, and in the practical synthesis of optically active compounds. The pharmaceutical industry, in particular, has shown markedly increased interest in asymmetric organic reactions. Currently, an expanding number of drugs, food additives, and ¯avoring agents are being prepared by synthetic methods. Most often, the desired compound is obtained through resolution of the corresponding racemic species performed at the end of the synthetic sequence. Because only one optical antipode is useful, half of the synthetic product is often discarded. Obviously, this is a wasteful procedure from the preparative point of view. Even if the wrong isomer can be converted to the active form via race-

48 INTRODUCTION

mization and resolution, extensive work is required. Also, resolution is usually a tedious, repetitious, and laborious process. It is economically appealing to exclude the unwanted optical isomers at the earliest possible stage through the asymmetric creation of chiral centers. In the interest of e¨ective use of raw material, it is wise to choose an early step in the synthetic sequence for the asymmetric operation and to consider carefully the principles of convergent synthesis.

Asymmetric synthesis refers to the conversion of an achiral starting material to a chiral product in a chiral environment. It is presently the most powerful and commonly used method for chiral molecule preparation. Thus far, most of the best asymmetric syntheses are catalyzed by enzymes, and the challenge before us today is to develop chemical systems as e½cient as the enzymatic ones.

The resolution of racemates has been an important technique for obtaining enantiomerically pure compounds. Other methods involve the conversion or derivatization of readily available natural chiral compounds (chiral pools) such as amino acids, tartaric and lactic acids, terpenes, carbohydrates, and alkaloids. Biological transformations using enzymes, cell cultures, or whole microorganisms are also practical and powerful means of access to enantiomerically pure compounds from prochiral precursors, even though the scope of such reactions is limited due to the highly speci®c action of enzymes. Organic synthesis is characterized by generality and ¯exibility. During the last three decades, chemists have made tremendous progress in discovering a variety of versatile stereoselective reactions that complement biological processes.

In an asymmetric reaction, substrate and reagent combine to form diastereomeric transition states. One of the two reactants must have a chiral element to induce asymmetry at the reaction site. Most often, asymmetry is created upon conversion of trigonal carbons to tetrahedral ones at the site of the functionality. Such asymmetry at carbon is currently a major area of interest for the synthetic organic chemists.

1.5.1``Chiron'' Approaches

Naturally occurring chiral compounds provide an enormous range and diversity of possible starting materials. To be useful in asymmetric synthesis, they should be readily available in high enantiomeric purity. For many applications, the availability of both enantiomers is desirable. Many chiral molecules can be synthesized from natural carbohydrates or amino acids. The syntheses of …‡†- exo-brevicomin (66) and negamycin (67) illustrate the application of such naturally occurring materials.

6,8-Dioxalicyclo[3.2.1]octane, or …‡†-exo-brevicomin (66), is the aggregating pheromone of the western pine beetle. It has been prepared from glucose using a procedure based on the retro synthesis design shown in Figure 1±2884:

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