An Efficient Synthesis of Hydroxyethylene Dipeptide Isosteres: The Core Unit of Potent HIV-1 Protease Inhibitors

Summary:

An efficient and stereocontrolled synthesis of hydroxyethylene dipeptide isosteres 1 from commercially available, optically pure D-mannose is described. This synthesis represents a practical and enantioselective entry to a range of other dipeptide isosteres, which are not limited to amino acid derived substituents.

Since the advent of acquired immunodeficiency syndrome (AIDS) and the discovery of its causative agent, human immunodeficiency virus (HIV-l),¹ the design and synthesis of mechanism-based HIV-1 protease inhibitors has intensified tremendously. The HIV-1 protease, an aspartic protease, is a proteolytic enzyme that cleaves the gag and gag-pol precursor polyproteins into the functional proteins of infectious virus particles² In vitro disruption of this protease-dependent processing step results in the production of protease-defective virions which are immature and noninfectious.³ Therefore, therapeutic intervention of this enzyme represents a powerful strategy for the treatment of AIDS and related ailments.

Recently, we⁴ and others⁵ have reported a series of potent and selective HIV-1 protease inhibitors which are designed based on the transition-state mimetic concept.⁶ This approach incorporates the natural amino acid statine⁷ and hydroxyethylene dipeptide isostere^8,9 at the scissile site, an approach that has been successfully utilized in the design and synthesis of potent inhibitors of renin¹⁰ and other aspartic proteases.¹¹ Since the first synthesis of hydroxyethylene dipeptide mimics, by Szelke⁸ and Rich⁹ in 1983, several other syntheses of dipeptide isosteres have appeared in the literature.¹² The majority of previous syntheses, however, have limitations with regard to stereochemical controls and variations of the substituents at C-2 and C-5 positions. In connection with the synthesis of potent and selective inhibitors of HIV-1 protease, we required an efficient, flexible and enantioselective synthesis of hydroxyethylene dipeptide mimics which were not limited to amino acid derived substituents. We describe here a new synthesis of Phe-Phe isostere 1 through the inter-mediacy of lactone 2, with absolute stereocontrol at the C-4 and C-5 positions, utilizing commercially available and optically pure D-mannOSe as starting material. This synthesis is potentially versatile and represents a practical and enantioselective entry to other dipeptide isosteres with a wide variety of substituents at C-2 and C-5 positions.

As shown in Scheme I, optically active D-mannose presents a unique opportunity for the synthesis of enantiomerically pure Phe-Phe dipeptide isostere 1 and related analogues. D-Mannose 3 already possesses the C-4 hydroxy group of hydroxyethylene isostere 1 with appropriate absolute configuration. Synthesis of isosteric lactone 2 requires deoxygenation of the C-2 and C-3 hydroxy groups, incorporation of phenyl and benzyl groups at C-6 and C-2, and introduction of nitrogen with inversion of configuration at C-5. Thus, D-mannOSe 3 was converted to 2,3:5,6-di-O-isopropylidene-α-D-mannofuranose 4 according to the reported procedure of Freudenberg.¹³ Required deoxygenation of 4 at the C-2 position was readily achieved utilizing Ireland’s procedure.¹⁴ Ãccordingly, reaction of mannofuranose 4 with carbon tetrachloride (1.2 equiv) and tris(dimethy1amino)phosphine (1.2 equiv) in tetrahydrofuran (−78 °C for 30 min and then 23 °C for 30 min) and subsequent reduction of the resulting furanosyl chloride with lithium in liquid ammonia at −78 to 0 °C for 5 h afforded the glycal 5 (72% yield) along with a small amount of dehalogenation byproduct 6 (3% yield) after flash chromatography over silica gel. Although the reduction of furanosyl chloride with di-tert-butylbiphenyl radical anion resulted in 5 in excellent yield (85%), use of lithium in liquid ammonia was found to be operationally more convenient for large-scale preparation.

Scheme Ia.

^aReagents: (a) Me₂C0, 3%H₂SO₄, 23 °C, 12 h; (b) CC1₄, (Me₂N)₃,P, THF, −78 to 23 °C, 1 h; then Li, NH₃, −78 to 0 °C, 5 h; (c) MeOH, PPTS, CH₂C1₂, 0–23 °C, 12 h; (d) 10% Pd-C, H₂, Et-OAC-MeOH (4:1), 5 h; (e) 40% aqueous AcOH, 90 °C, 3 h; (f) p-TsCl, pyridine, 0–23 °C, 12 h; (g) NaOMe, CHCl₃, 0 °C for 10 min; then 23 °C, 4 h; (h) PhMgBr, CUI, THF, −40 to 0 °C, 3 h; (i) Ph₃P, EtO₂CN=NCO₂Et, Ph₂P(O)N₃, PhMe, −10 to 23 °C, 12 h.

Further dehydroxylation of glycal 5 at the C-3 position was efficiently carried out employing Ferrier-type rear-rangement¹⁵ with methanol in the presence of pyridinium p-toluenesulfonate in methylene chloride (0–23 °C for 12 h) to provide a mixture (52:48 by ¹H NMR) of methyl glycoside 7 in 97% isolated yield. Reaction of 5 with 2-propanol in methylene chloride in the presence of p-toluenesulfonic acid at 0 °C or stannic chloride promoted reaction at −78 °C offered no enhancement of the anomeric effect;^l6 the ratio of anomers remained the same (1:l) after workup. Since the methyl glycoside 7 would eventually be converted to the corresponding γ-lactone, all subsequent reactions were carried out with the mixture of anomers. Catalytic hydrogenation of 7 with 10% palladium on charcoal under atmospheric pressure in 4:l ethyl acetate-methanol afforded the corresponding saturated glycoside in quantitative yield. Removal of the isopropylidene group was then effected by heating the resulting glycoside with 40% aqueous acetic acid at 90 °C for 3 h, followed by evaporation of solvents under reduced pressure and silica gel chromatography provided diol 8 (84% yield). Glycosidic diol 8 was then converted to the desired epoxide 9 in the following two-step sequence: (1) selective O-tosylation of the primary alcohol with p-toluenesulfonyl chloride (1.1 equiv) in pyridine at 0 °C for 12 h and (2) treatment of the resulting crude tosylate with sodium methoxide (5 equiv) in chloroform at 23 °C for 4 h to provide the epoxide 9 in 72% yield after silica gel chromatography. Regiospecific epoxide ring opening of 9 with phenylmagnesium bromide (2.2 equiv) in the presence of cuprous iodide (1.1 equiv) at −40 to 0 °C for 3 h afforded glycosidic alcohol 10 in 91 % yield after flash chromatography over silica gel. Thus, nucleophilic opening of epoxide 9 allows the incorporation of a variety of substituents at the C-5 position, an option not afforded by previous syntheses of hydroxyethylene isosteres.

Conversion of glycosidic alcohol 10 to the corresponding azide 11 was readily accomplished by a Mitsunobu reaction.¹⁷ Thus, reaction of 10 with triphenylphosphine (1.2 equiv), diethyl azodicarboxylate (1.2 equiv), and diphenylphosphoryl azide (1.2 equiv) in toluene at −10 to 23 °C for 12 h furnished the azide 11 (92% yield) along with a small amount (<3%) of elimination product. Interestingly, formation of the mesylate of 10 with masy1 chloride in pyridine and subsequent displacement of the mesylate with sodium azide in DMF at 90 °C or tetramethylguanidinium azide in DMF at 80 °C resulted in 11 (65–70% yield) and 15–20% elimination product resulting from E₂ reaction. Grieco oxidation¹⁸ of methyl furanoside 11 with m-chloroperbenzoic acid (1.2 equiv) in dry methylene chloride at 0 °C for 3 h in the presence of boron trifluoride etherate (0.25 equiv) afforded the corresponding azido γ-lactone in 72% yield after silica gel chromatography. Catalytic hydrogenation¹⁹ of the resulting azido lactone with 10% palladium on charcoal in the presence of di-tert-butyl dicarbonate for 6 h furnished the tert-butyloxycarbonyl-protected amino lactone 12 (white solid, mp 95 °C) exclusively in 91 % yield. The γ-lactone 12 is a versatile intermediate for the synthesis of suitably substituted hydroxyethylene dipeptide isosteres.

Introduction of a benzyl group at C-2 was accomplished by stereoselective alkylation^12b of 12 with benzyl halide. Thus, generation of the dianion of lactone 12 with lithium hexamethyldisilazide (2.2 equiv) in tetrahydrofuran at −78 °C (30 min) and alkylation with benzyl iodide (1.1 equiv) for 30 min at −78 °C, followed by quenching with propionic acid (5 equiv), provided the desired alkylated lactone 2 (84% yield, mp 76–78 °C) along with a small amount (<4% by HPLC) of undesired cis isomer 13 (mp 112–115 °C). The minor cis lactone was conveniently removed by column chromatography over silica gel using a mixture (1:l) of ethyl acetate-hexanes as the solvent system. The stereochemical assignment of alkylated lactones 2 and 13 were made based on extensive ¹H NMR NOE experiments as well as by comparison of ¹H NMR spectra with that reported previously.¹² Weinreb amidation²⁰ of lactone 2 with benzylamine (2 equiv) and trimethylaluminum (2 equiv) in methylene chloride (23 °C for 10 min, then 40 °C for 3 h) afforded hydroxyamide 14 (74% yield, mp 179–182 °C) after silica gel chromatography.²¹ Lactone 2 was converted to potent and selective inhibitors of HIV-1 protease according to the previous literature procedure.⁴

In conclusion, an efficient, stereocontrolled, and economical synthetic route to dipeptide isostere 1 has been developed. Since the starting material of this synthesis is D-mannose rather than an amino acid, the present methodology should provide convenient access to other dipeptide isosteres with a great deal of structural diversity at C-2 and C-5 positions. Synthesis of a number of HIV-1 protease inhibitors containing hydroxyethylene isosteres and their biological evaluation is currently under investigation.

Scheme IIa.

^aReagents: (j) MCPBA, BF₃.OEt₂, CH₂Cl₂, 0 °C, 3 h; (k) 10% Pd-C, H₂, EtOAc, BOC₂O, 6 h; (1) (TMS)₂NLi, THF, −78 °C, 30 min; PhCH₂I, −78 °C, 30 min; then MeCH₂CO₂H, −78 to 23 °C, 15 min; (m) Me₃A1, PhCH₂NH₂, CH₂C1₂, 40 °C, 3 h.