WO1994027955A1

WO1994027955A1 - Intermediates and process of making dipeptide isosteres

Info

Publication number: WO1994027955A1
Application number: PCT/US1994/005862
Authority: WO
Inventors: David Malcolm Ryckman
Original assignee: Smithkline Beecham Corporation
Priority date: 1993-05-24
Filing date: 1994-05-24
Publication date: 1994-12-08

Abstract

Intermediates and a stereoselective process for preparing hydroxyethylene dipeptide isosteres and lactone compounds which process comprises, as an essential step, the stereoselective reduction of the keto group in a 5S-4-keto-5-dibenzylamino-6-phenylhexanal ethylene (or propylene) acetal. The intermediates and process of this invention are useful in the preparation of enzyme inhibitors, which are, in particular, active against the HIV-1 virus.

Description

INTERMEDIATES AND PROCESS OF MAKING DIPEPTIDE ISOSTERES.

This invention relates to new intermediates and a stereoselective process for preparing hydroxyethylene dipeptide isosteres. These isosteres are useful in the preparation of enzyme inhibitors which are, in particular, active against the HTV-1 virus. These isosteres are also useful as intermediates in the preparation of renin inhibitors.

Background of the Invention

There is interest in the therapeutic action of enzyme inhibitors. For example, aspartyl protease inhibitors are active against the HIV-1 virus, Dreyer et al., Proc. Natl. Ac d. Sci. TTSA 86:9752 (1989).

Attention has focused on preparation of enzyme inhibitors by replacing a hydrolyzable peptide bond with a non-hydrolyzable subunit such as a hydroxyethylene isostere that can mimic the transition state. Vara Prasad et al., Tetrahedron Letters 31 : 1803 (1990) and Fray et al., J. Org. Chem. 51:4828 (1986). One such structure, (4S, 5S)-4-hydroxy-5-amino-6-phenylhexanoic acid, which is an isosteric analog of the dipeptide Phe-Gly has the subunit of formula 1:

OH 1

Substitution of this subunit for the scissile Tyr-Pro dipeptide of oligopeptide enzyme substrate Ac-Ser-Ala-Ala-Tyr-Pro-Val-Val-OMe, as in the peptide of formula 2, yields a potent inhibitor of HTV-1 protease.

2

Structure activity relationships have shown that the 4S,5S stereochemistry in a hydroxyethylene isostere is essential for biological activity. Therefore, a stereoselective synthesis is required. Stereocontrolled methods of preparing hydroxyethylene isosteres have been reported, for example by Vara Prasad et al., and Fray et al., both cited hereabove, and De Camp et al., Tetrahedron Letters 32:1867 (1991) and Kano et al., Tetrahedron Letters 32:233 (1991). Also, Reetz et al., J. Chem. Soc. Chem. Commun. 1474 (1989) have reported stereospecific synthesis of statine and statine analogues by reduction of the N,N-dibenzyl-b-keto esters using NaBI followed by deprotection. The 3S,4S products were formed preferentially.

In European Patent Application 491,538 Al, published June 24, 1992, it is stated on page 7 that the reaction of N,N-dibenzyl α-amino aldehyde with a homoenolate anion vastly favored the formation of the 4R,5S diastereometric product over the 4S,5S product.

Description of the Invention

According to the process of this invention, shown in Scheme 1, the chirality of L-phenylalanine is relied upon to establish new chiral centers. Thus, N,N- dibenzyl L-phenylalanine is converted to the N-lower alkoxy-N-lower alkylamide, preferably the N-methoxy-N-methylamide. The amide is reacted with a Grignard reagent derived from 2-(2-haloethyl)-l,3-dioxolane (or 1,3-dioxane), preferably a 2- bromoethyl compound, to give 5S-4-keto-5-dibenzylamino-6-phenylhexanal ethylene (or propylene) acetal. The keto group is reduced using a reducing agent such as sodium borohydride in a solvent such as methanol at about 0°C to give the diastereometric alcohols 4S,5S-4-hydroxy-5-dibenzylamino-6-phenylhexanal ethylene (or propylene) acetal and the corresponding 4R,5S diastereomer. The process favors the preparation of the desired 4S,5S alcohol. The diastereomers are easily separated by flash chromatography or by crystallization. The undesired 4R,5S alcohol may be recycled to give more of the desired 4S,5S alcohol by an oxidation-reduction sequence, that is by oxidizing the 4R,5S alcohol to the 4-keto compound by Swern oxidation and then reducing the keto group as described hereabove. The 4S,5S-4-hydroxy-5-dibenzylamino-6-phenylhexanal ethylene

(propylene) acetal is converted to the 4S,5S-4-hydroxy-5-dibenzylamino-6- phenylhexanoic acid, gamma-lactol by treating with acid for example hydrochloric acid in tetrahydrofuran. Oxidation of the lactol gives the lactone, i.e. 4S.5S-4- hydroxy-5-dibenzylamino-6-phenylhexanoic acid gamma-lactone. A third chiral center is readily introduced into the lactone in a stereoselective manner. Treatment of the 4S,5S-4-hydroxy-5-dibenzylamino-6-phenylhexanoic acid gamma-lactone with lithium diisopropylamide (LDA) followed by the addition of an alkyl halide (R⁷-halide) gives the 2R,4S,5S-2-R⁷-4-hydroxy-5- dibenzylamino-6-phenylhexanoic acid gamma-lactone. Basic hydrolysis and neutralization of the lactones for example by reacting with aqueous sodium hydroxide and neutralizing with citric acid gives the 4S.5S-4- hydroxy-5-dibenzylamino-6-phenylhexanoic acid and the corresponding (2R)(4S)(5S)-2-R⁷ compounds.

The benzyl protecting groups on the amino may be removed by blocking the acid function, for example by esterifying the acid to a methyl or ethyl ester, and then subjecting the resulting compound to catalytic transfer hydrogenolysis and, optionally, the amino group may be protected with a group such as t-butoxycarbonyl (t-Boc). The ester group may be hydrolyzed to give the acid.

Alternatively, the protecting benzyl groups on the amino of the lactone may be removed and replaced with an amino protecting group, such as t-Boc; the alkyl group (R⁷) may optionally be introduced in the 2-position on the lactone either before or after replacing the dibenzyl groups with another protecting group such as t-Boc, and the 5-t-Boc amino lactone may be subjected to basic hydrolysis and neutralization to give the 4S,5S-4-hydroxy-5-t-Boc-amino-6-phenylhexanoic acid and the corresponding (2R) (4S)(5S)-2R⁷ compounds.

Thus, by the process of this invention hydroxyethylene isosteres of formula 3 are prepared:

in which Ph is phenyl; R! and R-** are both benzyl or hydrogen or one is an amino- protecting group such as t-butoxycarbonyl and the other is hydrogen; R*-^* is hydrogen, straight or branched lower alkyl or lower alkenyl, or benzyl; and R^ is hydrogen or lower alkyl.

The process of this invention described hereabove is shown in Scheme 1.

Grignard

6 8 (Major) 7 (Minor)

3

SCHEME 1 The novel intermediates of this invention are represented by structures 5, 6, 8 and 9 set forth in Scheme 1 wherein Ph, R¹, R², R- , R⁴ are defined above, Bn is benzyl and R*-^> and R^ are each lower alkyl. Preferably, R*-⁵ and R6 are methyl. The use of dibenzyl protecting groups on the amine in the process of this invention is preferable to other protecting groups such as t-butoxycarbonyl, as used in the process reported by Vara Prasad et al., cited hereabove, because the reduction of the keto compound (structure 6) to the hydroxy compound may be run advantageously at about 0°C instead of much lower temperature (-78°C) reported when t-Boc protected amino compounds are used. The dibenzylamino 4-keto compound (structure 6) is reduced preferentially to give the desired 4S,5S-4- hydroxy compound (structure 8) in a ratio of about 25 to 1 of the 4S,5S isomer to the 4R,5S isomer. Furthermore, the dibenzyl group plays a role in directing the introduction of the third chiral center (2R). Thus, the process of this invention is advantageous for preparing the 4S,5S and the 2R,4S,5S hydroxyethylene isosteres. The use of the 1,3-dioxane intermediates is advantageous because the intermediates of structures 6 and 8 in Scheme 1 when n is 2 are crystalline. This feature facilitates the separation of the desired 4S,5S compound (structure 8) from the undesired, minor 4R,5S isomer (structure7) by fractional crystallization.

A preferred process of this invention comprises the preparation of a 4S,5S or 2R,4S,5S lactone of the formula:

which comprises (1) reacting N,N-dibenzyl-L-phenyl-alanine with N-methyl-0- methylhydroxylamine to give the amide of the formula:

(2) reacting this amide with a Grignard reagent derived from 2-(2-bromoethyl)-l,3- dioxolane (or 1,3-dioxane) to give an acetal of the formula:

(3) reducing the keto group to give the 4S,5S alcohol of the formula:

OH

(4) treating the 4S,5S alcohol with acid to give the lactol of the formula:

(5) oxidizing the lactol to give the lactone of the formula:

and optionally alkylatmg the lactone using a halide alkalating agent such as a straight or branched lower alkyl halide, lower alkenyl halide or benzyl halide. As used herein, Ph is phenyl, Bn is benzyl, t-Boc is t-butoxycarbonyl, Ac is acetyl, Phe is phenylalanine, Gly is glycine, Ser is serine, Ala is alanine, Tyr is tyrosine, Pro is proline and Val is valine. Also, lower alkyl as used herein has 1-4 carbon atoms and lower alkenyl has preferably 3-4 carbon atoms. The following examples illustrate the process and intermediates of this invention. The examples are not intended to limit the scope of the invention as defined hereabove or as claimed herebelow.

Example 1

A. N'-methoxy-N'-methylamide of N.N-dihenzvl-L-phenvlalanine.

In a clean, dry 1L Erlenmeyer flask equipped with a magnetic stir bar, the following reagents were added in the order described: dimethylformamide (228 mL, 2.95 mol, 12.5 eq.), N,N-dibenzyl-L-phenylalanine (90.3 g, 0.24 mol, 1.0 eq.), 4-dimethylaminopyridine (31.8 g, 0.26 mol, 1.1 eq.), l-hydroxybenztriazole hydrate (35.2 g, 0.26 mol, 1.1 eq.), N,O-dimethylhydroxylamine hydrochloride (25.4 g, 0.26 mol, 1.1 eq.), diisopropylethylamine (67.2 g, 0.72 mol, 2.2 eq.), and l-(3- dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (68.0 g, 0.35 mol, 1.5 eq.). The clear, yellow solution was stirred 18 hours at room temperature. The reaction was slowly poured into vigorously stirring 5% aqueous sodium bicarbonate. The solution was stirred for 2.5 hours to allow the precipitation of the product. The white solid was filtered, rinsed with 1 L water, 500 mL 5% aqueous citric acid, 1 L water, 500 mL 5% aqueous sodium bicarbonate, and 1 L water. The product was dried (18 mmHg, 30°C) to give 84 g, 92% yield, of the title compound.

Product Characterization mp: 65-66°C

[α]o: +55.5, c^*=1.0 in CDCI3

B. 5S-4-keto-5-dibenzvlarnino-6-phenvlhexanal ethvlene acetal.

A clean dry 500 mL 3-neck flask equipped with an air stirrer, a thermometer, and an addition funnel was flushed with nitrogen and charged with 40 mL tetrahydrofuran (THF) and magnesium turnings (3.1 g, 129 mmol, 5.0 eq.). The addition funnel was charged with 2-(2-bromoethyl)-l,3-dioxolane (9.1 mL, 77.3 mmol, 3.0 eq.) and 70 mL THF. A 5 mL portion was added to the flask with stirring. After the reaction was initiated, the temperature was carefully maintained below 30°C. The remaining solution was added over 30 min. The reaction was stirred for an additional hour at 20°C then cooled to 0°C. The addition funnel was charged with a solution of the N'-methoxy-N'-methylamide of N,N-dibenzyl-L- phenylalanine (10 g, 25.8 mmol, 1.0 eq) in 70 mL THF, and this was added to the reaction over 15 min. The reaction was stirred at 0°C for 18 hours, then poured into a solution of 300 mL saturated NH4CI in 300 mL ice. The solution was concentrated under reduced pressure then extracted with CH2CI2 (2 x 250 mL). The organic layer was dried with MgSO filtered, and concentrated under reduced pressure to yield the title compound 5S-4-keto-5-dibenzylamino-6-phenylhexanal ethylene acetal (16 g) as a thick amber oil. The crude product was carried forward without further purification.

Product Characterization [<X]D: +108 c=1.0 in CHCl3

Anal. Calcd for C28H31NO3: C, 78.29; H, 7.27; N, 3.26. Found: C, 78.23; H, 7.19; N, 3.23.

C. 4S.5S rand 4R.5SV4-hvdroxv-5-dibenzvlamino-6-phenvlhexanal ethvlene

A clean, dry 500 mL flask equipped with a thermometer was charged with 250 mL anhydrous methanol. Sodium borohydride (NaBH 3.2 g, 84.6 mmol, 5.2 eq.) was added, and the solution was cooled to 0°C. A solution of 5S-4-keto-5- dibenzylamino-6-phenylhexanal ethylene acetal (7 g, 16.3 mmol, 1.0 eq.) in 25 mL methanol was added over 5 min while the temperature was maintained at 0°C. The reaction was stirred for 40 min, and then 3 g NH4CI was added to quench the excess NaBHφ The solution was concentrated under reduced pressure, and extracted with CH2CI2 (2 x 100 mL). The organic layer was extracted with water (2 x 100 mL), dried over Na2SO4, filtered, and concentrated to a clear oil (7.1 g crude) which is comprised of the diastereomeric alcohols in a ratio of 4S,5S to 4R,5S isomer of 30:1. Product Characterization [α]rj>: +18.8, c=1.25 in CHCI3

Anal. Calcd for C28H33NO3: C, 77.93; H, 7.71; N, 3.25. Found: C, 77.83; H, 7.71; N, 3.01.

D. 4S.5S-4-hvdroxv-5-dibenzvlamino-6-phenvlhexanoic acid pamma-lactol. A 500 mL flask equipped with a thermometer was charged with the above prepared hydroxy product (5.3 g, 12.3 mmol) in 130 mL tetrahydrofuran. Then 300 mL 3N hydrochloric acid was added. The reaction was stirred for 90 min, and then solid K2CO3 was added until the reaction was neutralized. The solution was concentrated under reduced pressure, extracted with diethyl ether, dried over Na2SO4, filtered, and concentrated to an oil of the title product which was 1 : 1 mixture α-β epimers and was used without further purification.

Product Characterization

IR (neat): 3700-3110, 3100-2740, 1950, 1880, 1810, 1600, 1490, 1450, 1360, 1125,

910, 745, 700 cm-¹ MS (DCI, CH4) for C26H29NO2: mlz 388 (100, M+H+), main fragments 371 (18.8), 370 (27.9, M+H+ -H₂O), 300 (31.05), 296 (38.72), 91 (18.99).

E. 4S.5S-4-hvdroxv-5-dibenzvlamino-6-phenvlhexanoic acid gamma-lactone. A I L flask was charged with above prepared lactol (4.5 g, 11.6 mmol) in

400 mL acetone. In a separate flask, 14 mL concentrated sulfuric acid was added to CTO3 (8 g, 80 mmol) in 40 mL water. A 50 mL portion of the chromic acid solution was slowly added to the IL flask containing the hemiacetal. The reaction was stirred at room temperature for one hour. Diethyl ether (100 mL) was added, and the reaction was neutralized by the slow addition of 10% NaOH. The thick, green solution was filtered through Celite. The organic layer was separated, extracted with brine (3 x 50 mL), dried over NaSO and concentrated under reduced pressure to a yellow semi-solid material. This material was crystallized from CH2θ2/hexanes to give 2.96 g (66% yield for two steps) of the title compound.

Product Characterization mp: 171.5-172.0 °C Exa le 2

4S.5S-4-hydroxy-5-dibenzylamino-6-phenylhexanoic acid.

The lactone from Example IE in tetrahydrofuran is reacted with aqueous sodium hydroxide suspended in tetrahydrofuran at room temperature overnight. The reaction mixture is neutralized with citric acid and the crystallized product is filtered off to give the title product, m.p. 137-138°C.

Example 3

4S.5S-4-hvdroxv-5-t-butoxvcarbonvlamino-6-phenvlhexanoic acid.

To prepare the 5-t-butyoxycarbonylamino compound, the above prepared dibenzylamino lactone from Example IE is subjected to catalytic transfer hydrogenolysis using Pd black/methanol/5% formic acid to give the formate salt of the deprotected amino compound which is then reacted with di-t-butylcarbonate and triethylamine in dimethylformamide to give 4S,5S-4-hydroxy-5-t- butoxycarbonylamino-6-phenylhexanoic acid gamma-lactone.

Basic hydrolysis and neutralization of this lactone by the procedure described in Example 2 gives 4S,5S-4-hydroxy-5-t-butoxycarbonylamino-6- phenylhexanoic acid.

Alternatively, the title compound is prepared from the corresponding 5- dibenzylamino hexanoic acid from Example 2 by blocking the acid group, e.g. by esterifying it with diazomethane in ethyl ether at 0°C, then removing the benzyl groups by catalytic transfer hydrogenolysis and reacting with di-t-butylcarbonate as described above.

Example 4

■Preparation of 2R compounds

A. 2R.4S.5S-2-benzvl-4-hvdroxv-5-dibenzvlamino-6-phenvlhexanoic acid gamma-lactpne,

A clean, dry 25 mL flask equipped with a magnetic stir bar, was charged with 6 mL tetrahydrofuran and diisopropylamine (0.364 mL, 2.6 mmol, 1.0 eq) under a nitrogen atmosphere. The solution was cooled to 0°C, and n-butyllithium (1.04 mL of 2.5 M, 2.6 mmol, 1.0 eq) was added over 5 min. The solution was stirred for 20 min at 0°C. In a separate flask, 4S,5S-4-hydroxy-5-dibenzylamino-6- phenylhexanoic acid gamma-lactone (1000 mg, 2.6 mmol, 1.0 eq) was dissolved in 14 mL tetrahydrofuran. This was added to the reaction and stirred at 0°C for 10 min. Benzyl bromide (0.309 mL, 2.6 mmol, 1.0 eq) was added as a solution in 0.6 mL tetrahydrofuran, and the reaction was stirred for 10 min. at 0°C. Saturated NH4CI was added to quench the reaction. The frozen mixture was warmed to room temperature, extracted into ethyl acetate, washed with water and brine, dried over Na2SO4, filtered, and concentrated under reduced pressure to give the crude product as a semi-solid material. The product was isolated via column chromatography (silica gel, 10% ethyl acetate in hexanes as eluent) to give 0.96 g, 78% yield, of title compound.

Product Characterization

[α]_D: - 18.2, c = 0.96 in CHCI3 Anal. Calcd for C33H33NO2: C, 83.33; H, 6.99; N, 2.94.

Found: C, 83.18; H, 6.99; N, 3.08

B. 2R.4S.5S-benzvl-4-hvdroxv-5-amino-6-phenvlhexanoic acid gamma-lactone formate.

A clean, dry 100 mL flask was flushed with nitrogen and charged with Pd black (65 mg). A solution of 4.4% formic acid in methanol was prepared, and 25 mL was added to the flask containing the palladium. With stirring, 2R,4S,5S-2- benzyl-4-hydroxy-5-dibenzylamino-6-phenylhexanoic acid gamma-lactone (225 mg, 0.47 mmol) was added as a solution in 7 mL methanol. After 25 min the solution was filtered through Celite, and concentrated under reduced pressure to give title compound.

C. 2R.4S.5S-2-benzvl-4-hvdroxv-5-t-butoxvcarbonvlamino-6-phenvlhexanoic acid gamma-lactone.

A clean, dry 25 mL flask, under a nitrogen atmosphere was charged with the above prepared de-protected lactone (0.47 mmol, 1.0 eq) in 1 mL dimethylformamide and di-t-butyl dicarbonate (103 mg, 0.473 mmol, 1.0 eq). Triethylamine (0.946 mmol, 1.0 eq) was added, and the reaction was stirred at room temperature for 18 hours. The product was extracted into ethyl acetate, washed with water (3 x 50 mL) and brine (1 x 50 mL), dried over Na2SO4, filtered, and concentrated under reduced pressure. The oil was purified via column chromatography (silica gel, 20% ethyl acetate in hexanes as eluent) to give title compound; mp. 89-91°C.

D. 2R.4S.5S-2-benzyl-4-hydroxy-5-t-butoxycarbonylamino-6-phenylhexanoic acid.

The lactone prepared in C is subjected to basic hydrolysis and neutralization by the procedure described above to give the title compound; mp. 123-125°C.

Example 5

A. 2R.4S.5S-2-methyl-4-hvdroxy-5-dibenzylamino-6-phenylhexanoic acid gamma-lactone.

By the above procedure of Example 4A, using methyl iodide in place of benzyl bromide, the title compound is prepared; mp. 127.2-127.6°C.

B. 2R.4S.5S-2-methvl-4-hvdroxv-5-t-butoxvcarbonvlamino-6-phenvlhexanoic asisL

The dibenzyl protecting groups are replaced by t-butoxycarbonyl and the resulting 2R,4S ,5S-2-methyl-4-hydroxy-5-t-butoxycarbonyl-amino-6- phenylhexanoic acid gamma-lactone (mp. 129-130°C) is subjected to basic hydrolysis and neutralization by the procedure of Example 3 to give 2R,4S,5S-2- methyl-4-hydroxy-5-t-butoxycarbonylamino-6-phenylhexanoic acid, m.p. 126-127°C.

Example 6

2R.4S ■5S-2-crotvl-4-hvdroxv-5-t-Boc-amino-6-phenvlhexanoic acid.

Similarly, using crotyl bromide in place of benzyl bromide, 2R,4S,5S-2- crotyl-4-hydroxy-5-dibenzylamino-6-phenylhexanoic acid gamma lactone is prepared. By the above procedure the dibenzyl protecting groups are replaced with t-Boc and the resulting lactone is converted to 2R,4S,5S-2-crotyl-4-hydroxy-5-t- Boc-amino-6-phenylhexanoic acid. Example 7

By the same procedure of Examples 1 and 2, using allyl bromide in place of benzyl bromide, the corresponding 2R,4S,5S-2-allyl lactone and hydroxy hexanoic acid compounds are prepared.

Example 8

A. 5S-4-keto-5-dibenzvlamino-6-phenvlhexanal propylene acetal.

A clean, dry 5 L 3-neck flask equipped with an air stirrer, a thermometer, and an addition funnel was flushed with nitrogen and charged with 1 L of tetrahydrofuran (THF) and magnesium turnings (48.6 g, 2.03 mol, 2.5 eq). The addition funnel was charged with 2-(2-bromoethyl)-l,3-dioxane (245 mL, 1.79mol) and 1.1 L THF. A 20 mL portion was added to the flask with stirring. After the reaction was initiated, the temperature was carefully maintained below 30°C. The remaining solution was added over 60 min. The reaction was stirred for an additional hour at 25°C, and then a titration was performed (1, 10-phenanthroline indicator, 1 N sec-butanol in xylenes) which showed a 71% conversion to Grignard. The addition funnel was charged with a solution of the N'-methoxy-N-methylamide of N,N-dibenzyl-L-phenylalanine (365 g, 0.94 mol) in 1.1 L of THF, and this was added to the reaction over 30 min. The reaction was stirred at 22°C for 2 hours. The reaction was poured into 3 L of saturated NH4CI. The layers were separated, and then the aqueous layer was re-extracted with hexanes (2 x 500 mL). The combined organic layers were washed with brine, dried with Na2SO4, filtered, and concentrated under reduced pressure to about 1.5 L volume. Hexanes (1 L) was added for recrystallization. The crystals were filtered and dried (18 mmHg, 30°C) to give 225 g, 66% yield of the title compound.

Product Characterization mp: 55-57°C [α]_D: -68.1, c=0.074 in CHCI3 B. 4S.5S-4-Hvo oxv-5-dibenzvl*mτino-6-phenvlhexanal propylene acetal.

A clean, dry 250 mL flask equipped with a thermometer was charged with 55 mL of anhydrous methanol. The solution was cooled to -5°C in an ice/acetone bath. Sodium borohydride (NaBH4, 2.2 g, 58.2 mmol, 5.1 eq) was added. The 4- keto compound prepared above (5.0 g, 11.3 mmol, 1.0 eq) was dissolved in a solution of 10 mL of methanol and 6 mL of THF, and this was added to the NaBH4 solution over 15 min while the temperature was maintained at -5°C. The reaction was stirred for 30 min, and then 50 mL of saturated NH4CI was slowly added to quench the excess NaBH4. The solution was concentrated under reduced pressure and extracted with CH2CI2 (2 x 100 mL). The organic layer was extracted with brine (1 x 100 mL), dried over Na2SO4, filtered, and concentrated to a clear oil (5.31 g crude) which is comprised of 4S,5S and 4R,5S-4-hydroxy-5-dibenzylamino- 6-phenylhexanal propylene acetal. The isomeric ratio was determined using reverse-phase HPLC area % analysis. A small sample of the isomeric mixture was separated by prep HPLC to give the major alcohol, the 4S,5S isomer, as a white crystalline solid and the minor alcohol, the 4R,5S isomer, as a thick clear oil. The isomers were shown to be separable by crystallization of the crude oil in hexanes/ethyl acetate to give 76% yield of the major 4S,5S alcohol with the enhanced ratio of 103:1.

Product Characterization m.p.: 78-79°C

[α]_D: +20.19, c=108 in CHCI3

C. 4S.5S.-4-hvdroxv-5-dibenzvlamino-6-phenvlhexanoic acid garnma-lactol. A 5 L flask equipped with a thermometer was charged with 190 g of 4S,5S- 4-hydroxy-5-dibenzylamino-6-phenylhexanal propylene acetal in 1.9 L of tetrahydrofuran. Then 1.5 L of 3N hydrochloric acid was added. The reaction was stirred for 60 min, and then a saturated solution of K2CO3 was added until the reaction was neutralized. The solution was concentrated under reduced pressure, then 1.0 L of CH2CI2 was added. The CH2CI2 layer was extracted with water (500 mL), dried over Na2SO4, filtered, and concentrated to give the title compound as an oil. Characterization of Product (mixture of α-β-isomers. approx. 1:1)

910, 745, 700 cm-¹ MS (DCI, CH4) for C26H29NO2: mlz 388(100, M+H+), main fragments 371 (18.8), 370 (27.9, M+H+ -H₂O), 300 (31.05), 296 (38.72), 91 (18.99).

Claims

What is claimed is:

1. A compound of the formula:

in which Ph is phenyl, Bn is benzyl and n is 1 or 2.

2. A compound of the formula:

OH

in which Ph is phenyl, Bn is benzyl and n is 1 or 2.

3. A compound of the formula:

in which Ph is phenyl and Bn is benzyl.

4. A compound of the formula:

in which Ph is phenyl, Bn is benzyl and R*⁵ and R^ are lower alkyl.

5. A compound of claim 4 in which R*-- and R^ are methyl.

6. A process for preparing a compound of the formula:

in which Ph is phenyl; R¹ and R-** are both benzyl or hydrogen or one is an amino protecting group such and the other is hydrogen; R3 is hydrogen, straight or branched lower alkyl or lower alkenyl, or benzyl; and R^ is hydrogen or lower alkyl, which comprises (1) reacting N,N-dibenzyl-L-phenylalanine having the following formula:

in which Ph is phenyl and Bn is benzyl, with a N-lower alkyl-O-lower alkylhydroxylamine to give the N-lower alkoxy-N-lower alkylamide of the formula:

in which Ph is phenyl, Bn is benzyl and R*-* and R^ are lower alkyl, (2) then reacting this amide with a Grignard reagent derived from a 2-(2-haloethyl)- 1,3-dioxolane (or 1,3-dioxane) to give a 5S-4-keto-5-dibenzylamino-6- phenylhexanal ethylene (or propylene) acetal of the formula:

in which Ph is phenyl, Bn is benzyl and n is 1 or 2,

(3) then reducing the keto group to give the 4S,5S alcohol of the formula:

OH

in which Ph is phenyl, Bn is benzyl and n is 1 or 2,

(4) treating the 4S,5S alcohol with acid to give a lactol of the formula:

in which Ph is phenyl and Bn is benzyl,

(5) then oxidizing the lactol to give a lactone of the formula:

in which Ph is phenyl and Bn is benzyl,

(6) optionally alkylating the lactone to give a 2R lactone of the formula:

Ph

in which Ph is phenyl, Bn is benzyl and R^ is straight on branched lower alkyl or lower alkenyl, or benzyl, (7) then hydrolyzing with base and neutralizing the lactones to give 4S,5S and 2R,4S,5S-2R^-4-hydroxy-5-N,N-dibenzylamino-6-phenylhexanoic acid; (8) optionally deprotecting the amino group by blocking the acid function and subjecting the resulting compound to catalytic transfer hydrogenolysis; and alternatively, removing the benzyl protecting groups from the amino group in the lactone and replacing with an amino protecting group such as t-Boc, optionally introducing R? in the 2-position on the lactone either before or after replacing the benzyl groups with another protecting group such as t-Boc, and then hydrolyzing with base and neutralizing the N-protected-amino-lactone to give the 4S,5S and 2R,4S,5S-2R-4-hydroxy-5-protected amino-6-phenyl hexanoic acid.

7. A process of claim 1 for preparing a compound in which R¹ is t-Boc and R*-* is hydrogen or R¹ and R--* are both benzyl, R3 is hydrogen or benzyl and R^ is hydrogen or methyl.

8. A process for preparing 4S,5S or 2R,4S,5S lactone of the formula:

in which Ph is phenyl, Bn is benzyl and R--^* is hydrogen, straight or branched lower alkyl or lower alkenyl, or benzyl, which comprises

(1) reacting N,N-dibenzyl-L-phenyl-alanine with N-methyl-O-methymydroxylamine to give the amide of the foπnula:

in which Ph is phenyl and Bn is benzyl,

(2) then reacting this amide with a Grignard reagent derived from 2-(2-bromoethyl)- 1,3-dioxolane (or 1,3-dioxane) to give an acetal of the foπnula:

in which Ph is phenyl, Bn is benzyl and n is 1 or 2,

(3) then reducing the keto group to give the 4S,5S alcohol of the formula:

OH in which Ph is phenyl, Bn is benzyl and n is 1 or 2,

(4) then treating the 4S,5S alcohol with acid to give the lactol of the formula:

in which Ph is phenyl and Bn is benzyl,

(5) then oxidizing the lactol to give the lactone of the formula:

in which Ph is phenyl and Bn is benzyl, and

(6) optionally alkylating the lactone using an alkyl halide of the formula R^-halide in which R? is straight or branched lower alkyl or lower alkenyl, or benzyl.