ANTHELMINTHIC MACROLIDE SYNTHESIS

Abstract

Disclosed herein is a novel and inventive synthesis of amino-deoxyavermectins, and in particular, the economically significant, anthelminthic macrolide eprinomectin. The synthesis proceeds via reductive amination of an intermediate in which the allylic alcohol of the benzofuran ring is deprotected. Advantageously, the method of the present invention obviates the need for chromatographic purification.

Claims

1. A method of synthesising amino-deoxyavermectins of the general formula (I), or a stereoisomer thereof: ##STR00014## wherein p is 0 or 1; R.sup.1 is selected from the group consisting of C.sub.1-C.sub.10 alkyl, C.sub.2-C.sub.10 alkenyl, C.sub.2-C.sub.10 alkynyl, C.sub.3-C.sub.10 cycloalkyl, C.sub.3-C.sub.10 cycloalkenyl, C.sub.3-C.sub.10 cycloalkynyl, and combinations thereof; and R.sup.2 and R.sup.2′ are the same or different and are selected from the group consisting of H, C.sub.1-C.sub.10 acyl, C.sub.1-C.sub.10 alkyl, C.sub.2-C.sub.10 alkenyl, C.sub.2-C.sub.10 alkynyl, C.sub.3-C.sub.10 cycloalkyl, C.sub.3-C.sub.10 cycloalkenyl, C.sub.3-C.sub.10 cycloalkynyl, and combinations thereof; or R.sup.2 and R.sup.2′ and the nitrogen to which they are attached form a C.sub.3-C.sub.10 aliphatic heterocycle, the method comprising the step of: removing an ALLOC protecting group from a compound of the general formula (A), or a stereoisomer thereof to afford the corresponding deprotected compound of the general formula (B), or a stereoisomer thereof: ##STR00015## wherein, p is 0 or 1; R.sup.1 is selected from the group consisting of C.sub.1-C.sub.10 alkyl, C.sub.2-C.sub.10 alkenyl, C.sub.2-C.sub.10 alkynyl, C.sub.3-C.sub.10 cycloalkyl, C.sub.3-C.sub.10 cycloalkenyl, C.sub.3-C.sub.10 cycloalkynyl, and combinations thereof; and R.sup.3 is an oxo group.

2. The method of claim 1, further comprising the step of subjecting the oxo group of a compound of the formula (B), or a stereoisomer thereof to a reductive amination protocol to afford the corresponding amino compound (C), or a stereoisomer thereof, ##STR00016## wherein p is 0 or 1; R.sup.1 is selected from the group consisting of C.sub.1-C.sub.10 alkyl, C.sub.2-C.sub.10 alkenyl, C.sub.2-C.sub.10 alkynyl, C.sub.3-C.sub.10 cycloalkyl, C.sub.3-C.sub.10 cycloalkenyl, C.sub.3-C.sub.10 cycloalkynyl, and combinations thereof; R.sup.2 is selected from the group consisting of H, C.sub.1-C.sub.10 alkyl, C.sub.2-C.sub.10 alkenyl, C.sub.2-C.sub.10 alkynyl, C.sub.3-C.sub.10 cycloalkyl, C.sub.3-C.sub.10 cycloalkenyl, C.sub.3-C.sub.10 cycloalkynyl, and combinations thereof; and R.sup.3 is an oxo group.

3. The method of claim 1, wherein p is 1.

4. The method of claim 1, wherein R.sup.1 is C.sub.1-C.sub.10 alkyl.

5. The method of claim 1, wherein R.sup.2 is H.

6. The method claim 1, wherein when p is 1, R.sup.1 is selected from the group consisting of C.sub.1-C.sub.10 alkyl, C.sub.1-C.sub.10 alkenyl, C.sub.1-C.sub.10 alkynyl, C.sub.3-C.sub.10 cycloalkyl, C.sub.3-C.sub.10 cycloalkenyl, C.sub.3-C.sub.10 cycloalkynyl, and combinations thereof; and R.sup.2 is H, the method further comprises the step of acylating a compound of the general formula (C′), or a stereoisomer thereof to produce a compound of the general formula (E), or a stereoisomer thereof, ##STR00017##

7.-11. (canceled)

12. A pharmaceutical composition comprising a pharmaceutically effective amount of a compound obtained by the method of claim 1 combined with at least one pharmaceutically acceptable excipient.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0094] Additional features and advantages of the present invention are described in, and will be apparent from, the detailed description of the invention and from the drawings in which:

[0095] FIG. 1 illustrates a schematic of a synthesis of eprinomectin according to the method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0096] It should be readily apparent to one of ordinary skill in the art that the examples disclosed herein below represent generalised examples only, and that other arrangements and methods capable of reproducing the invention are possible and are embraced by the present invention.

Step 1. Protection with Allyl Chloroformate

5-O-(Allyloxycarbonyl)avermectin B1 (2)

[0097] Avermectin (1) (240 g, 0.275 mol) was dissolved in dry iPrOAc (900 mL) and cooled down to 0° C. N,N,N′,N′-Tetramethylethylenediamine (TMEDA) (41.0 mL, 1 eq) was added and a reaction mass was cooled down to −25° C. A solution of allyl chloroformate (36.1 ml, 1.25 eq) in iPrOAc (120 mL) was added drop wise maintaining internal temperature −25 to −20° C. The reaction was stirred for 1 hour and quenched with water (480 mL). The layers were separated and organic layer was washed with water (480 mL). The combined aqueous layers were washed with iPrOAc (2×120 mL) and all organic layers were pooled together, distilled off at <50° C. under vacuum to half of volume, and used directly in the next step. HPLC assay: 85.5%, yield assay: 90%.

Step 2. Oxidation

4″-Oxo-5-O-(allyloxycarbonyl)avermectin B1 (3)

[0098] To a solution of 2 (approx. 237 g, 0.247 mol) in iPrOAc (960 mL) were added triethylamine (240 mL, 6.95 eq) and DMSO (108 mL, 6.15 eq). The reaction mass was cooled down to −25° C. and a solution of phenyl dichlorophosphate (96.5 mL, 2.56 eq) in iPrOAc (180 mL) was added drop wise, while maintaining internal temperature between −25 and −20° C. The reaction mixture was stirred for an additional 1 hour and was quenched with water (480 mL). The layers were separated and the organic layer was washed with water (480 mL). The combined aqueous layers were washed with iPrOAc (2×120 mL) and all organic layers were pooled together, distilled off at <50° C. under vacuum to half of volume and used directly in the next step. HPLC assay: 87.24%, yield assay: 80%.

Step 3. Deprotection

4″-Oxo-avermectin B1 (4)

[0099] To a solution of 3 (approx. 189 g, 0.198 mol) in iPrOAc (960 mL) was added acetic acid (15.7 mL, 1.31 eq), sodium acetate (45 g, 2.75 eq) and tetrakis triphenylphosphine palladium (0) (7.2 g, 0.031 eq). The reaction mass was stirred at 20-25° C. for 4 hours or until completion of the reaction. 1% NaOH solution (960 mL) and activated charcoal (24 g) were added and the reaction mixture was stirred for 15 min at 20-25° C. Charcoal was filtered off, washed with iPrOAc (2×240 mL), and the layers were separated. The organic layer was washed with a mixture of water (240 mL) and brine (120 mL), and the combined aquatic layers were washed with iPrOAc (2×120 mL). Pooled together organic layers were distilled off to half of volume at <50° C. under vacuum and used directly in the next step. HPLC assay: 80.05%, yield assay: 90%.

Step 4. Reductive Amination

4″-epi-amino-4″-deoxoavermectin B1 (5)

[0100] To a solution of 4 (approx. 155 g, 0.178 mol) in iPrOAc (960 mL) were added HMDS (180 mL, 4.75 eq) and AcOH (18.5 mL, 1.8 eq) and all was stirred for 5 hours at 48-52° C. After cooling down to 5° C. ethanol (60 mL) was poured followed by drop wise addition of a precooled to 5° C. solution of sodium borohydride in ethanol (7.6 g, 1.1 eq in 190 mL of ethanol). The reaction mixture was next warmed up to 25° C., stirred for 3 hours and quenched at 5° C. with AcOH (72.0 mL). 5% NaOH solution (960 mL) and activated charcoal (24 g) were added and all was stirred at 20-25° C. for 15 min. Charcoal was filtered off, washed with iPrOAc (2×120 mL) and the layers were separated. The organic layer was washed with 2.5% NaOH (480 mL), and both aquatic layers were combined and washed with iPrOAc (2×120 mL). All organic layers were pooled together and treated with heptane to get a 1:1 iPrOAc:heptane mixture. Product was washed out with a solution of 1% HCl and EtOH (3:1, 2×600 mL) and stirred 1 hour at room temperature. Addition of 5M NaOH brought pH to 9 and product was extracted with iPrOAc (2×600 mL). The combined organic layers were distilled off to half of volume at <50° C. under vacuum and used directly in the next step. HPLC assay: 73.51%, yield assay: 85%.

Step 5. Acetylation

4″-epi-Acetylamino-4″-deoxoavermectin B1 (Eprinomectin)

[0101] To a solution of 5 (approx. 132 g, 0.151 mol) cooled down to 5° C. was added acetic anhydride (36 mL, 7.62 eq). The reaction mass was stirred for 1 hour at <5° C. and saturated NaHCO.sub.3 solution (600 mL) was added. The mixture was stirred for 15 min and the layers were separated. The organic layer was washed with brine (600 mL), treated with charcoal (24 g) and stirred for 30 min. Charcoal was filtered off, washed with iPrOAc (2×120 mL), and the combined organic phases were concentrated to half of volume at <50° C. under vacuum (HPLC assay: 74.98%, yield assay: 99%). iPrOAc was exchanged to acetonitrile by three times co-distillation with acetonitrile (3×720 mL) at <50° C. under vacuum to get a dense yellowish suspension. After addition of fresh acetonitrile (240 mL), stirred for 1 hour at room temperature, and for an additional 2 hours at 0° C., the suspension was filtered off, washed with cold acetonitrile (2×120 mL), and dried under vacuum at 40° C. to afford 110 g of crude Eprionomectin. HPLC assay: 97.14%, yield: 87%.

Crystallization of Eprinomectin

[0102] 110 g of crude Eprinomectin was dissolved in acetonitrile (1200 mL) under reflux, cooled down to room temperature, stirred for 1 hour and next for 2 hours at 0° C. A white suspension was filtered off, washed with cold acetonitrile (2×100 ml) and dried at 40° C. under vacuum to afford 95 g of Eprinomectin (82% yield). HPLC assay: 98.52%.

[0103] It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.