Diastereomeric Purity Research Articles

New methods for determining the enantiomeric purity of erythro-sphingosine

The enantiomeric purity of erythro-sphingosine samples can be determined simply, reliably, and accurately from 1H or 19F nuclear magnetic resonance spectra of the α-methoxy-α-(trifluoromethyl)phenylacetate (MTPA) derivative. As little as 0.1% of the minor enantiomer could be observed in a 1-mg sample, and detection limits of 1% and 5% were estimated for samples of 100 μg and 10 μg. The two threo-sphingosine enantiomers and four dihydrosphingosine stereoisomers were also differentiated by this technique, which served as an effective method for assessing the purity of sphingosine and dihydrosphingosine samples. Enantiomeric and diastereomeric purities could also be determined by normal-phase high performance liquid chromatographic analysis of the MTPA derivatives.—Li, S., W. K. Wilson, and G. J. Schroepfer, Jr. New methods for determining the enantiomeric purity of erythro-sphingosine. J. Lipid Res. 1999. 40: 764–772.

On the synthesis, isolation, and radiochemical studies for the preparation of in-house kits for 99mTc- meso- and d,l-HMPAO: a few additional observations

The simple procedure documented for the synthesis of d,l-hexamethyl propylene amineoxime ( d,l-HMPAO), the widely used radiopharmaceutical precursor for cerebral perfusion studies, has provided no clue to the inherent difficulties associated with the isolation of this desired isomer in moderate yields from the diastereomeric mixture. In addition, determination of the diastereomeric purity of the isolated fractions with the help of suitable analytical techniques has posed a major challenge. Herein, we report the protocol standardized for the purification and isolation of d,l-HMPAO in significantly improved yields from the crude diastereomeric mixture by fractional crystallization using a dual solvent system. In addition to the use of high resolution 1H-NMR, 13C-NMR as a suitable analytical tool for conclusive determination of the diastereomeric composition of HMPAO is being reported for the first time. Intermediate stages of crystallisation in the process of purification yielded a third product resulting in reduction of the yield of the desired product. Attempts made to characterize this product are discussed. Labelling of d,l-HMPAO with 99mTc was standardized in our laboratory using stannous tartrate as the reducing agent. The radiochemical studies of the meso isomer, which is also isolated in the process of crystallization in adequate amounts, have been standardized with a view to investigating its potential use for leukocyte labelling studies. The biodistribution studies of the 99mTc complexes prepared from the pure meso and d,l isomer in Swiss mice are reported. Diastereomeric forms of HMPAO were isolated in increased yields using an improved protocol by fractional crystallization involving a dual solvent system. High resolution 13C-NMR spectroscopy has been utilized to provide a conclusive analytical tool for efficient and simple screening of the diastereomeric composition. An additional compound exhibiting a different crystal habit and spectroscopic features was isolated in the process of crystallization. 99mTc complexes of both the diastereomers were standardized and used in animal biodistribution studies.

Class I fructose-1,6-bisphosphate aldolases as catalysts for asymmetric aldol reactions

The activity of two commercially available bacterial class I fructose-1,6-bisphosphate aldolases (FruA) towards a number of aldehydes has been compared with rabbit muscle aldolase (RAMA), which is the most widely used enzyme for aldol reactions with dihydroxyacetone phosphate. The kinetic properties of the three aldolases are very similar, but the bacterial aldolases were much more stable than RAMA. Reaction of butanal and dihydroxyacetone phosphate catalyzed by FruA from Staphylococcus carnosus was performed on a 5 mmol scale in 53% isolated yield. Enantiomeric and diastereomeric purity of the major reaction product [90% (3 S,4 R)] was determined by chiral gas chromatography.

Atorvastatin, an HMG-COA reductase inhibitor and effective lipid-regulating agent. Part II. Synthesis of side-chain-labeled [14C]atorvastatin

[14C]Atorvastatin was synthesized in a ten-step sequence with an overall yield of 5.7%. The label was introduced as sodium [1-14C]acetate, which was converted via the acid chloride to the (S)-2-hydroxy-1,2,2-triphenylethyl ester 4. Chiral condensation of 4 with aldehyde 5 gave the chiral ester intermediate 6 with a yield of about 70%. Following transesterification of 6 to a methyl ester and condensation with tert-butyl lithioacetate to give (R)-β-ketoester 8, a second chiral center was generated by reduction of the hydroxyketone 3, giving the (R,R)-dihydroxy ester 9, which was then converted via the acid to the lactone 11. The desired pure diastereomer 11, obtained from the mother liquor during crystallization, was then converted to the corresponding calcium salt (2:1) 13 (atorvastatin). Copyright © 1999 John Wiley & Sons, Ltd.

Formation of Stable Spiro[4.4] Ortho Ester Aminals during the Synthesis of the C26−C32 Oxazole Fragment of Calyculin C

Ortho ester aminals (compounds with two nitrogen atoms and an oxygen atom bonded to a sp3 carbon) are occasionally encountered as side products or intermediates in heterocycle syntheses,1 in nucleotide chemistry,2 and in flavonoid chemistry.3 Open-chain ortho ester aminals are prone to disproportionation and further hydrolysis by water or alcohols, but the corresponding polycyclic ortho ester aminals are often stable enough to be isolated. In contrast to the ortho esters, which are rapidly gaining importance in organic synthesis,4 the corresponding aminals have only scarcely been used, possibly due to their instability. The reasons for the hydrolytic stability of the polycyclic ortho ester aminals relative to their open-chain counterparts have not been discussed in detail in the literature, although stereoelectronic effects are likely to play a central role.5 The exo-anomeric effect has been invoked as an explanation for the stability of three simpler derivatives, a monocyclic ortho ester aminal and two spirocyclic analogues.6 Herein, we report the facile formation of stable spirocyclic [4.4] ortho ester aminals and characterization of one isomer (4b) with X-ray crystallography. These compounds cannot benefit from anomeric stabilization (vide infra), and an alternative explanation for their stability as well as the lability of the open-chain derivatives is therefore required and presented. The final step of the synthesis of the C26-C32 oxazole fragment (2a) of calyculin C (1)7 (Scheme 1) requires the oxidation of the oxazolines 3a/3b to the corresponding oxazoles 2a/2b (Scheme 2).8 Along with the desired oxazoles, another set of diastereomeric compounds were formed in up to 52% yield (Table 1). To our surprise, these compounds proved to be the isomeric spirocyclic ortho ester aminals (diastereomeric due to the presence of an additional chiral spiro atom) (4a, pure diastereomer; 4b: 4b′, 2:1 ratio).9 Recrystallization afforded the pure diastereomer 4b from the mixture 4b/4b′. Both oxidation methods employed10 depend on the generation of the ester enolate and subsequent oxidative removal of the oxazoline ring hydrogen.11 The formation of ortho ester aminals 4a/4b in both cases indicates that * To whom correspondence should be addressed. Phone: +358 8 553 1630. Fax: +358 8 553 1629. E-mail: Ari.Koskinen@oulu.fi. (1) For examples, see: (a) Feinauer, R. Angew. Chem. 1966, 78, 938. (b) Doleschall, G.; Lempert, K. Tetrahedron 1968, 24, 5529-5545. (c) Fryer, R. I.; Earley, J. V.; Blount, J. F. J. Org. Chem. 1977, 42, 22122219. (d) Gotthardt, H.; Schenk, K.-H. Angew. Chem. 1985, 97, 604606. (e) Knolker, H.-J.; Boese, R.; Doring, D.; El-Ahl, A.-A.; Hitzemann, R.; Jones, P. G. Chem. Ber. 1992, 125, 1939-1951. (e) Kappe, C. O.; Peters, K.; Peters, E.-M. J. Org. Chem. 1997, 62, 3109-3118. (f) Couture, P.; Warkentin, J. Can. J. Chem. 1997, 75, 1264-1280, 12811294. (2) For an example, see: Anzai, K.; Uzawa, J. J. Org. Chem. 1984, 49, 5076-5080. (3) Lee, Y. T.; Fisher, J. F. J. Org. Chem. 1993, 58, 3712-3721. (4) For a review of ortho esters, see: DeWolfe, R. H. Synthesis 1974, 153-172. For recent use of ortho esters in synthesis, see: Charette, A. B.; Chua, P. Tetrahedron Lett. 1997, 38, 8499-8502 and references therein. (5) Poje, N.; Palkovic, A.; Poje, M. J. Heterocycl. Chem. 1997, 34, 477-483. (6) (a) Bertolasi, V.; Ferretti, V.; Gilli, G.; Marchetti, P.; D’Angeli, F. J. Chem. Soc., Perkin Trans. 2 1990, 2135-2140. (7) Isolation and characterization: Kato, Y.; Fusetani, N.; Matsunaga, S.; Hashimoto, K.; Koseki, K. J. Org. Chem. 1988, 53, 39303932. For references to recent total or partial syntheses of calyculins, see ref 8. (8) Pihko, P. M.; Koskinen, A. M. P. J. Org. Chem. 1998, 63, 9298. (9) The isomer ratios were determined by 1H NMR. At 365 K, 4a gave only a single set of signals in 1H and 13C NMR. We are grateful to an anynomous reviewer for drawing our attention to this point. (10) (a) Method A: Barrish, J. C.; Singh, J.; Spergel, S. H.; Han, W.; Kissick, T. P.; Kronenthal, D. R.; Mueller, R. H. J. Org. Chem. 1993, 58, 4494-4496. (b) Method B: see ref 8. Scheme 1

Diastereoselective synthesis, binding affinity for vitamin D receptor, and chiral stationary phase chromatography of hydroxy analogs of 1,25-dihydroxycholecalciferol and 25-hydroxycholecalciferol.

A series of analogs of 1,25-dihydroxycholecalciferol and 25-hydroxycholecalciferol were obtained with an additional hydroxyl in the aliphatic side chain at carbon atom C-24. These analogs were synthesized by direct and diastereo-selective alpha-hydroxylation of enolates derived from respective vitamin D esters using Davies chiral oxaziridines. The use of (+)-(2R,8aS)-(8, 8-dichlorocamphoryl)sulfonyl oxaziridine resulted in (R) stereochemistry of the new asymmetric center for both series of analogs. Similarly, (-)-(2S,8aR) oxaziridine gave (S) analogs. The diastereomeric purity of hydroxy analogs was determined by high-performance liquid chromatography on a chiral stationary phase. High diastereopurity of hydroxylation of vitamin D esters was obtained without the use of any chiral auxiliary. The binding affinity of (24R)-1,24,25-trihydroxycholecalciferol for the calf thymus intracellular vitamin D receptor was one order of magnitude higher than that of the respective (24S)-diastereomer.

Diastereoselective synthesis, binding affinity for vitamin D receptor, and chiral stationary phase chromatography of hydroxy analogs of 1,25‐dihydroxycholecalciferol and 25‐hydroxycholecalciferol

A series of analogs of 1,25-dihydroxycholecalciferol and 25-hydroxycholecalciferol were obtained with an additional hydroxyl in the aliphatic side chain at carbon atom C-24. These analogs were synthesized by direct and diastereo-selective α-hydroxylation of enolates derived from respective vitamin D esters using Davies chiral oxaziridines. The use of (+)-(2R,8aS)-(8,8-dichlorocamphoryl)sulfonyl oxaziridine resulted in (R) stereochemistry of the new asymmetric center for both series of analogs. Similarly, (−)-(2S,8aR) oxaziridine gave (S) analogs. The diastereomeric purity of hydroxy analogs was determined by high-performance liquid chromatography on a chiral stationary phase. High diastereopurity of hydroxylation of vitamin D esters was obtained without the use of any chiral auxiliary. The binding affinity of (24R)-1,24,25-trihydroxycholecalciferol for the calf thymus intracellular vitamin D receptor was one order of magnitude higher than that of the respective (24S)-diastereomer. Chirality 11:701–706, 1999. © 1999 Wiley-Liss, Inc.

Convergent synthesis, chiral HPLC, and vitamin D receptor affinity of analogs of 1,25-dihydroxycholecalciferol.

A series of analogs of 1,25-dihydroxycholecalciferol was obtained with an additional chiral center at the terminus of the aliphatic side chain (C-25). The analogs were obtained from (+)-(R)- and (-)-(S)-2-methylglycidols, by opening of the oxirane ring with the carbanions derived from vitamin D C23a,24- or C22-sulfones. The diastereomeric purity of the analogs was determined by high-performance liquid chromatography on a chiral stationary phase. The binding affinity of analogs for the calf thymus intracellular vitamin D receptor (VDR) was two orders of magnitude lower than that of the lead compound of this group, 24a,24b-dihomo-1,25-dihydroxycholecalciferol, and it was comparable to the affinity of analogs of 24-nor-1,25-dihydroxycholecalciferol. However, a twofold difference was observed for analogs diastereomeric at C-25 in their affinity for VDR. The diastereodifferentiation of the binding affinity was found to be specific for vitamin D vicinal 25,26-diols as it disappears for analogs where 26-hydroxyl, neighboring the C-25 chiral center, is replaced with methyl.

Stereoselective synthesis of (−)-(1 R,2 S)-2-aminocyclobutane-1-carboxylic acid, a conformationally constrained β-amino acid

The title compound as well as some derivatives have been synthesized for the first time in optically active form by means of a chemoenzymatic transformation used to induce asymmetry in achiral precursors. The enantio- and diastereomeric purity has been determined by HPLC and NMR techniques.

Zinc-mediated asymmetric epoxidation of nitro alkenes with oxygen

The asymmetric epoxidation of nitro alkenes using oxygen in the presence of diethylzinc and N-methyl pseudo-ephedrine as a chiral additive is reported. This method provides an access to 3-substituted trans-2-nitro oxiranes of excellent diastereomeric purity (de≥98%) and with medium to good enantiomeric excesses (ee=36–82%).

High yield preparation of a novel tetrakis[ruthenium tris(bipyridine)]calix[6]arene derivative with good diastereomeric purity

In order to develop methodology for the selective preparation of complex molecular structures with potential photochemical or electron transfer functions, the diastereoselective synthesis of multiple ruthenium tris(bipyridine) complexes tethered to a central calix[6]arene core was investigated. Applying recently developed methodology, the resolved precursor cis-Δ-[Ru(bpy) 2(DMSO)Cl]PF 6 (98.6% ee) was efficiently reacted with a novel calix[6]arene derivative, to give the tetrakis[Ru(bpy) 2(bpy′)]calix[6]arene derivative ( 9) with almost complete retention of absolute stereochemistry at each of the four metal centres, as seen by the unusually strong molar circular dichroism (CD) spectrum. The synthesis of racemic 9 was also carried out, and demonstrated to have an inactive CD spectrum.

The enantiospecific synthesis of novel lysine analogues incorporating a pyrrolidine containing side chain

Two novel analogues of lysine have been prepared in high enantiomeric and diastereomeric purity. These unnatural α-amino acids possess modified aminoalkyl side chains incorporating a pyrrolidine nucleus as a cyclic constraint.

Zirconocene Mediated Diastereo- and Enantioselective Synthesis of 4,5,5-Trisubstituted 2-Alkoxytetrahydrofurans

1-Aza-2-zirconacyclopent-4-enes 3, prepared from α,β-unsaturated imines 2 were used as homoenolate-equivalents to generate 3-aza-1-oxa-2-zirconacyclohept-4-enes 4 via the diastereoselective insertion of prochiral, sterically demanding ketones. Hydrolysis of the zirconacycles using our pyridinium-N-oxide method led to the formation of 4,4,5-substituted 4-phenyltetrahydro-2-furanyl-Narylamines 5 in high diastereomeric purity. A simple method for their conversion into α-methoxytetrahydrofurans 6a-g by treatment with methanol/THF and catalytic amounts of TsOH was developed. The α-methoxytetrahydrofurans 6a-g were easily transformed into 2,2,3-substituted 2,3-dihydrofurans 7 in good overall yields using an acidic elimination step. Benzyl protected γ-butyrolactols 6h-j could be generated in a one-pot procedure. Additionally, first efforts to perform this synthesis enantioselectively are described.

A concise synthesis of aza-dipeptide isosteres

Aza-dipeptide isosteres as potent HIV-protease inhibitors containing a (hydroxyethyl)-hydrazine moiety are synthesised in >98% diastereomeric and enantiomeric purity starting from ( L)-phenylalanine aldehyde.

A Flexible Stereocontrolled Synthesis of β-Hydroxy-α-methyl Esters: Application to the Synthesis of Stegobiol and Serricorole

β-Hydroxy-α-methyl esters have been obtained in a stereocontrolled manner and high enantiomeric and diastereomeric purity from commercially available methyl 3-oxopentanoate and methyl 3-oxobutanoate. The key step is the catalytic hydrogenation of the carbonyl group using (R)- or (S)-BINAP-Ru as chiral catalyst followed by asymmetric alkylation. Stegobiol and serricorole, components of the sex pheromone of the drugstore beetle, Stegobium paniceum (L.) and cigarette beetle, Lasioderma serricorne (F.), have been prepared from these chiral building blocks without the need for stoichiometric amounts of chiral auxiliaries.

New amino acids derived from L-pyroglutamic acid: Synthesis of Trans-4-benzyl- cis-5-phenyl- L-proline, L- α-(2-benzyl-3-phenylpropyl)-glycine and L- α-(3-phenylpropyl)-glycine

A convenient synthesis of three new amino acids, L- α-(3-phenylpropyl)-glycine, L- α-(2-benzyl-3-phenylpropyl)-glycine and its conformationally constrained analog, trans-4-benzyl- cis-5-phenyl- L-proline is reported. All compounds were prepared in good diastereomeric or enantiomeric purity from L-pyroglutamic acid. Trans-4-benzyl- cis-5-phenyl- L-proline was prepared by benzylation of Boc- L-Pyr-OBn, ring opening with phenyllithium and subsequent cyclisation. Hydrogenolysis under mild conditions then furnished L- α-(2-benzyl-3-phenylpropyl)-glycine. In a similar way, L- α-(3-phenylpropyl)-glycine was prepared from cis-5-phenyl- L-proline by catalytic hydrogenolysis.

Biocatalytic and chemical preparation of all four diastereomers of methionine sulfoxide

Biocatalytic or chemical oxidations can be used in a complementary manner for the preparation of all four diastereomers of methionine sulfoxide with high diastereomeric purity in overall isolated yields of 20–55% from methionine. The N-phthaloyl derivatives of L- and D-methionine were selectively oxidised to the (SSSC) and (SSRC) sulfoxides respectively by biotransformation using the fungus Beauveria bassiana ATCC 7159. Hydrogen peroxide oxidation of the same materials gave mixtures from which the (SSSC) and (RSRC) isomers can be readily isolated by crystallisation. Chromatography of the residual material then afforded the (RSSC) and (SSRC) isomers.

Large-Scale Synthesis of Enantio- and Diastereomerically Pure (R,R)-Formoterol

(R,R)-Formoterol (1) is a long-acting, very potent β2-agonist, which is used as a bronchodilator in the therapy of asthma and chronic bronchitis. Highly convergent synthesis of enantio- and diastereomerically pure (R,R)-formoterol fumarate is achieved by a chromatography-free process with an overall yield of 44%. Asymmetric catalytic reduction of bromoketone 4 using as catalyst oxazaborolidine derived from (1R, 2S)-1-amino-2-indanol and resolution of chiral amine 3 are the origins of chirality in this process. Further enrichment of enantio- and diastereomeric purity is accomplished by crystallizations of the isolated intermediates throughout the process to give (R,R)-formoterol (1) as the pure stereoisomer (ee, de >99.5%).

Syntheses of optically active α-amino nitriles by asymmetric transformation of the second kind using a principle of O. Dimroth

A mixture of solids As and Bs in equilibrium with the dissolved compounds A1 and B1 is transformed completely into one pure solid, say Bs, if the dissolved compounds A1⇌B1 are equilibrating in solution. This is applied to transform 1∶1 mixtures of solid diastereomeric amygdalates (2-hydroxy-2-phenylacetates; mandelates) (R,R)-3 + (S,R)-3 prepared from racemic α-amino nitriles (R,S)-1 with (R)-mandelic acid 2 into stereochemically pure single diastereomers (R,R)-3, or (S,R)-3 (de > 97%) (‘asymmetric transformation of the second kind by application of Dimroth’s principle’). Decomposition of the amygdalates (R,R)-3, or (S,R)-3, with aqueous base affords the enantiomerically pure α-amino nitriles (R)-1, or (S)-1 (ten examples). The chiral auxiliary (R)-mandelic acid is recovered almost quantitatively. The optically active α-amino nitriles are hydrolyzed to amides 6, and further to α-N-alkylamino acids 7. N-Benzylamino acids 7 are hydrogenated to α-amino acids 8. Some of the optically active α-amino nitriles 1 are reduced to optically active 1,2-diamines 9. In most cases, absolute configurations could be assigned by comparison of the specific rotations observed with those of authentic compounds.

Enantiospecific synthesis of 4-(4′-methoxyphenyl)-hexan-3-one as precursor for optically active (p S) or (p R) isomer of ( Z) or ( E)-3-(2′-(( N, N-dimethylamino)methylferrocenyl)-4-(4″-methoxyphenyl)-hex-3-ene

We describe herein an original method for the preparation of enantiomerically pure ( Z)- or ( E)-3-(2′-(( N, N-dimethylamino)methylferrocenyl)-4-(4″-methoxyphenyl)-hex-3-ene possessing a p( S) or p( R) plane of chirality. The key step of the synthesis lies in obtaining enantiomerically pure ( R) or ( S) 4-(4′-methoxyphenyl)-hexan-3-one whose reaction with the lithiated N, N-dimethylaminomethylferrocene leads to two enantiomerically pure amino-alcohol diastereomers (p S,3 S,4 R) and (p R,3 S,4 R), or (p S,3 R,4 S) and (p R,3 R,4 S) respectively. Subsequent dehydration yields a mixture of three olefins, namely, two trisubstituted olefins and either the ( Z)- or ( E)-tetrasubstituted olefin with respect to the starting amino-alcohol diastereomer. Additionally we obtained the enantiomerically pure ( R)- and ( S)-4-phenyl-hexan-3-one and the corresponding diastereomeric amino-alcohols.