# Patent

## Title
Synthesis of derivatives of ginkgolide C

## Abstract

The subject invention provides ginkgolide C derivatives compounds having the structure:




    wherein R is H or -A-Ar,
    
        where A is an alkyl group; and
        Ar is an aryl group, which may contain heteroatoms and may be unsubstituted or substituted by one to five substituents each selected from the group consisting of hydrogen, alkoxy, —CH2CO2R4, and —CH2CONR5R6;
        
            where R4 is an alkyl group; and
            R5 and R6 are each, independently, hydrogen or a branched or unbranched alkyl group;
        
        
    
    
    wherein R1 is H or —COR7,
    
        where R7 is alkyl, aryl or amino;
    
    
    wherein R2 is present or absent, and when present is H, —COR8 or —CO—Z—R8;
    
        where R8 is alkyl, aryl or amino; and
        Z is oxygen;
    
    
    wherein R3 is present or absent, and when present is —COR9;
    
        where R9 is alkyl or aryl;
    
    
    wherein only one of R2 or R3 is present in the compound;
    wherein only two of R, R1, R2 and R3 are H; and
    wherein each of a and b designates a single covalent bond which is present or absent,
    
        where bond a is present when R3 is absent and bond b is present when R2 is absent;

or an optically pure enantiomer of the compound. Additionally, the subject invention provides methods of inhibiting the activity of a glycine receptor using these compounds.

## Background
This application claims the benefit of U.S. Provisional Application No. 60/498,014, filed Aug 27, 2003, the contents of which are hereby incorporated by reference.


This invention has been made with government support under National Institutes of Health grant MH068817. Accordingly, the U.S. Government has certain rights in the invention.


Throughout this application various publications are referenced in parenthesis. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this invention pertains.
BACKGROUND OF THE INVENTION
The Ginkgo biloba tree is among the oldest living plants and is referred to as a “living fossil”. It was introduced into the Western world in the 18th century and admired for its unique beauty, especially the leaves, as exemplified by a 1815 poem by Wilhelm von Goethe referring to its beauty (Goethe, 1819). The Ginkgo tree has a long history of use in traditional Chinese medicine, but it was not until the 1960's that a standardized extract of G. biloba leaves, EGb 761, was introduced into the European markets (Defeudis, 1998). Today G. biloba extract is one of the most popular botanical medicines worldwide.
Numerous beneficial effects of EGb 761 has been postulated including improving neuroprotection in Alzheimer's disease. Alzheimer's disease is a progressive, inexorable loss of cognitive function associated with an excessive number of senile plaques in the cerebral cortex and subcortical gray matter, which also contains β-amyloid and neurofibrillary tangles consisting of tau protein. The cause of Alzheimer's disease is not known (Merck Manual, 1999).
EGb 761 has also been postulated in improving peripheral vascular function, inhibition of thrombosis and embolism cognitive disorders, anti-inflammatory and antiproliferative activities, as well as antioxidant activities (Defeudis, 2000). EGb 761 is a complex mixture of compounds, the main ingredients being flavonoids and terpene trilactones (ginkgolides and bilobalide) that comprise 24% and 6%, respectively, of the total extract (Ahlemeyer, 1998; Drieu, 1986). It is considered that the flavonoids act as antioxidants, while the terpene trilactones are involved in anti-inflammation and prevention of blood clotting associated with the antagonistic activity at the platelet-activating factor (PAF) receptor (McKenna, 2001). However, the neuroprotective effects of EGb 761 have so far not been associated with specific components of the extract (Peskind, 1998; Simonson, 1998).
The terpene trilactones, the ginkgolides (FIG. 1) and bilobalide, are unique components of EGb 761, the structures of which were elucidated in 1967 (Maruyama, 1967; Maruyama, 1967; Maruyama, 1967; Maruyama, 1967; Woods, 1967; Nakanishi, 1967; Okabe, 1967). The ginkgolides are diterpene trilactones with a cage-like skeleton consisting of six 5-membered rings, i.e., a spiro[4.4]nonane carbocyclic ring, three lactones, and a tetrahydrofuran moiety. Terpene trilactones from G. biloba are also among the very few natural products containing a tert-butyl group.
In contrast to many studies on the neuroprotective effects of EGb 761, the ginkgolides have not been extensively studied partly due to limited availability of pure ginkgolides. The finding in 1985 that ginkgolide B (GB, 2) was a potent antagonist of the PAF receptor (PAFR) (Braquet, 1986; Braquet, 1985) led to extensive structure-activity relationship (SAR) studies on this receptor (Rapin, 1998; Braquet, 1987; Braquet, 1991; Hu, 2001; Hu, 2000; Hu, 2000; Corey, 1991; Park, 1993; Park, 1995; Stromgaard, 2002). However, the significance of these effects in relation to the neuroprotective effects of EGb 761 is not clear (Stromgaard, 2003).
The first indication of a direct interaction of ginkgolides with important targets in the brain was discovered when ginkgolides were shown to be potent and highly selective antagonists of the inhibitory glycine receptor (GlyR) (Stromgaard, 2003; Kondratskaya, 2002). The GlyR is a ligand-gated ion channel found primarily in spinal cord and brain stem, but also in higher brain regions such as hippocampus and developing cortex that consists of α1-α4 and β subunits (Betz, 2001). Only a few ligands for GlyRs have been available, the classical example being the convulsant strychnine, a competitive antagonist. However, since the neuropharmacology and functional importance of GlyRs in higher brain regions is not well characterized (Chattipakorn, 2002) new potent and selective ligands are needed for further investigations of the GlyR.
The introduction of combinatorial chemistry and application of related techniques such as parallel or split-and-pool synthesis, solid and solution-phase strategies have opened avenues to libraries of various compounds (Marcaurelle, 2002; Arya, 2001; Ganesan, 2002; Abel, 2002; Krchnak, 2002; Schreiber, 2000). These compounds could be entirely artificial or derivatives of natural products with possible enhanced bioactivities. Recently diversity-oriented semi-synthetic methods have been used for the preparation of library of natural product derivatives for biological activity studies (Abel, 2002). In the following we describe a parallel solution-phase synthesis of a ginkgolide combinatorial library. Diversity is achieved by modification of traditional synthetic methods to prepare novel ginkgolide derivatives in a selective fashion. The ginkgolide derivatives have been evaluated as antagonists of the GlyR using a fluorescence-based high-throughput screening assay.
SUMMARY OF THE INVENTION
The subject invention provides compounds having the structure:




    wherein R is H or -A-Ar,
    
        where A is an alkyl group; and
        Ar is an aryl group, which may contain heteroatoms and may be unsubstituted or substituted by one to five substituents each selected from the group consisting of hydrogen, alkoxy, —CH2CO2R4, and —CH2CONR5R6;
        
            where R4 is an alkyl group; and
            R5 and R6 are each, independently, hydrogen or a branched or unbranched alkyl group;
        
        
    
    
    wherein R1 is H or —COR7,
    
        where R7 is alkyl, aryl or amino;
    
    
    wherein R2 is present or absent, and when present is H, —COR8 or —CO-Z-R8;
    
        where R8 is alkyl, aryl or amino; and
        Z is oxygen;
    
    
    wherein R3 is present or absent, and when present is —COR9;
    
        where R9 is alkyl or aryl;
    
    
    wherein only one of R2 or R3 is present in the compound;
    wherein only two of R, R1, R2 and R3 are H; and
    wherein each of a and b designates a single covalent bond which is present or absent,
    
        where bond a is present when R3 is absent and bond b is present when R2 is absent;
    
    
    or an optically pure enantiomer of the compound.


The invention also provides a method of inhibiting the activity of a glycine receptor comprising contacting the glycine receptor with the compound having the structure




The invention further provides a method of treating Alzheimer's disease in a subject in need of such treatment, comprising administering to the subject a compound having the structure




The subject invention also provides a pharmaceutical composition comprising a compound having the structure




The subject invention also provides a process of preparing the compound having the structure




comprising the step of reacting a compound having the structure





with an electrophile to form the above-indicated compound.

The subject invention still further provides a compound having the structure




    wherein Y is oxygen or —NH—.





DESCRIPTION OF THE FIGURES
FIG. 1. Ginkgolides A, B, C, J, and M.
FIG. 2. 10-Alkyl-iso-GC-1,6-bisacetates.
FIG. 3. Benzoates of Ginkgolide C.



DETAILED DESCRIPTION OF THE INVENTION
The subject invention provides compounds having the structure:




    wherein R is H or -A-Ar,
    
        where A is an alkyl group; and
        Ar is an aryl group, which may contain heteroatoms and may be unsubstituted or substituted by one to five substituents each selected from the group consisting of hydrogen, alkoxy, —CH2CO2R4, and —CH2CONR5R6;
        
            where R4 is an alkyl group; and
            R5 and R6 are each, independently, hydrogen or a branched or unbranched alkyl group;
        
        
    
    
    wherein R1 is H or —COR7,
    
        where R7 is alkyl, aryl or amino;
    
    
    wherein R2 is present or absent, and when present is H, —COR8 or —CO-Z-R8;
    
        where R8 is alkyl, aryl or amino; and
        Z is oxygen;
    
    
    wherein R3 is present or absent, and when present is —COR9;
    
        where R9 is alkyl or aryl;
    
    
    wherein only one of R2 or R3 is present in the compound;
    wherein only two of R, R1, R2 and R3 are H; and
    wherein each of a and b designates a single covalent bond which is present or absent,
    
        where bond a is present when R3 is absent and bond b is present when R2 is absent;
    
    
    or an optically pure enantiomer of the compound.


In one embodiment, the compound has the structure




In another embodiment, the compound has the structure




In a further embodiment, the compound has the structure




    wherein R is H or -A-Ar-;
    
        where A is —CH2—,
        Ar is a phenyl group or a picolyl group, either of which may be substituted or unsubstituted by a substituent selected from the group consisting of hydrogen, methoxy, —CH2CO2Me, and —CH2CONH2;
    
    


In another embodiment, R is H, benzyl, para-methoxybenzyl or picolyl.
In one embodiment, the compound has the structure:




    wherein R1 is H or —COR7;
    
        where R7 is methyl, phenyl or —NR5R6;
        where R5 and R6 are each, independently, hydrogen, methyl, or t-butyl, or wherein R5 and R6 together with the nitrogen to which they are attached form a morpholine or piperidine.
    
    


In a further embodiment, R1 is H, benzoyl, —COMe, —CONHMe, —CONHtBu, piperidyl, or morpholinyl.
In yet another embodiment, the compound has the structure




    wherein R2 is H, —COR8 or CO-Z-R8;
    
        where R8 is —NR5R6 or phenyl, which may be unsubstituted or substituted by one to five substituents which are each, independently, halogen or —NO2.
        
            where R5 and R6 are each, independently, hydrogen, methyl, or t-butyl, or wherein R5 and R6 together with the nitrogen to which they are attached together form a morpholine or piperidine.
        
        
    
    


In yet a further embodiment, R2 is H, benzoyl, —COMe, —CO2C6H4NO2, —CONHMe, —CONHtBu, piperidyl or morpholinyl.
In still another embodiment, the compound has the structure




    wherein R is H or -A-Ar—;
    
        where A is —CH2—,
        Ar is a phenyl group or a picolyl group, either of which may be substituted or unsubstituted by a substituent selected from the group consisting of hydrogen, methoxy, —CH2CO2Me, and —CH2CONH2.
    
    


In one embodiment, R is H, benzyl, p-methoxybenzyl or picolyl.
In yet another embodiment, the compound has the structure




    wherein R1 is H or —COR7,
    
        where R7 is methyl, phenyl or —NR5R6;
        
            where R5 and R6 are each, independently, hydrogen, methyl, or t-butyl, or wherein R5 and R6 together with the nitrogen to which they are attached together form a morpholine or piperidine.
        
        
    
    


In a further embodiment, R1 is H, phenyl, —COMe, —CONHMe, —CONHtBu, —CONC5H10 or CONC4H8O.
In a yet further embodiment the compound has the structure




    wherein R3 is benzoyl, —COMe or —CO2C6H4NO2.


In another embodiment, the compound has the structure




In an even yet further embodiment, the compound has the structure




In still yet another embodiment, the compound has the structure




In yet another embodiment, the compound has the structure




In still another embodiment, the compound has the structure




In yet another embodiment, the compound has the structure




In still another embodiment, the compound has the structure




The invention also provides a method of inhibiting the activity of a glycine receptor comprising contacting the glycine receptor with the compound having the structure




In another embodiment of the method, the compound has the structure




The invention further provides a method of treating Alzheimer's disease in a subject in need of such treatment, comprising administering to the subject a compound having the structure




The subject invention also provides a pharmaceutical composition comprising a compound having the structure




and a pharmaceutical carrier.

The subject invention also provides a process of preparing the compound having the structure




comprising the step of reacting a compound having the structure





with an electrophile to form the above-indicated compound.

In another embodiment, the process provides the compound having the structure




comprising the steps of:

    
    
        i) reacting a compound having the structure
    
    






with either 4-methoxybenzyl chloride or benzyl chloride, and a suitable base to form a compound having the structure





    wherein R is benzyl or p-methoxybenzyl; and
    
        ii) reacting the product of step i) with an electrophilic reagents to form the above-indicated compound.
    
    


The subject invention even further provides a process of preparing the compound having the structure




comprising the steps of:

    
    
        i) reacting a compound having the structure
    
    






with 4-methoxybenzyl chloride or benzyl chloride, and a suitable base to form a compound having the structure





    wherein R is benzyl or p-methoxybenzyl; and
    
        ii) reacting the product of step i with the appropriate electrophilic reagents to form the above-indicated compound.
    
    


The subject invention still further provides a process of preparing the compound having the structure




comprising the steps of:

    
    
        i) reacting a compound having the structure
    
    






in dimethylformamide with a resin and an appropriate base to form a compound having the structure





    
    
        ii) cleaving the product of step i) from the resin by mixing the product of step i) with trifluoracetic acid in a polar solvent to form a compound having the structure
    
    






    wherein R10 is —H, —CH2CO2H or CH2CONH2; and
    
        iii) dissolving the product of step ii) and a suitable amine base in an appropriate solvent and reacting with benzoic anhydride, thereby forming the above-indicated compound.
    
    


The subject invention yet even further provides a process of preparing the compound having the structure




comprising the step of reacting a compound having the structure





with 2-picolyl chloride in a polar solvent with a suitable base, thereby forming the above-indicated compound.

The subject invention provides a process of preparing the compound having the structure




comprising the steps of:

    
    
        i) reacting a compound having the structure
    
    






with 4-methoxybenzyl chloride in a polar solvent with a suitable base, providing a compound having the structure





    
    
        ii) dissolving the product of step i) and a suitable amine base in a polar solvent and reacting with benzoic anhydride, thereby forming the above-indicated compound.
    
    


The subject invention still further provides a process of preparing the compound having the structure




comprising the steps of:

    
    
        i) reacting a compound having the structure
    
    






with 4-methoxybenzyl chloride in an appropriate polar solvent with a suitable base, providing a compound having the structure





and

    
    
        ii) dissolving the product of step i) and iPr2EtN base in a suitable solvent and reacting with p-nitrophenyl chloroformate and then with MeNH2, forming a compound having the structure
    
    






    
    
        iii) dissolving the product of step ii) in a suitable solvent and reacting with (NH4)2Ce(NO3)6, thereby forming the above-indicated compound.
    
    


The subject invention yet further provides a process of preparing the compound having the structure




comprising the steps of:

    
    
        i) reacting a compound having the structure
    
    






with 4-methoxybenzyl chloride in an a polar solvent with a suitable base, providing a compound having the structure





    
    
        ii) dissolving the product of step i) and iPr2EtN base in a suitable solvent and reacting with p-nitrophenyl chloroformate and then with tBuNH2, thereby forming the product
    
    






    
    
        iii) dissolving the product of step ii) in a suitable solvent and reacting with (NH4)2Ce(NO3)6, thereby forming the above-indicated compound.
    
    


The subject invention still further provides a compound having the structure




    wherein Y is oxygen or —NH—.


Experimental Details
Materials and Methods. All reactions were performed under argon at ambient temperature in dry solvents and yields refer to isolated products, unless otherwise stated. GC (3) originated from early structural studies (Maruyama, 1967; Maruyama, 1967; Maruyama, 1967; Maruyama, 1967; Woods, 1967), and was recrystalized from ethanol/water and dried in dessicator. All other reagents were purchased and used as received. Polystyrene resin, Wang resin and bromopolystyrene resin were obtained from Advanced ChemTech, DES resin and sulfonyl chloride resin were obtained from Aldrich, Tentagel® resin was purchased from Rapp-Polymere Gmbh. Reactions were monitored by analytical TLC with silica gel 60 F254 and spots were visualized by heating and UV light (254 nm). Flash chromatography was performed using silica gel (230-400 mesh).
1H-NMR and 13C-NMR spectra were recorded with Bruker (300, 400 or 500 MHz) spectrometers. The chemical shifts are expressed in ppm (δ)downfield from tetramethylsilane (in CDCl3) or calibrated according to residual solvent peak as an internal standard (DMSO-d6, δ 2.50, MeOD, δ 3.30). Assignment of peaks was achieved using 2D methods (COSY), by comparison with published data where available and comparison with 1H-NMR spectra of parent ginkgolides. In certain cases, HSQC was used instead of 13C-NMR. High-resolution mass spectra (HRMS) were measured on JEOL JMS-HX110/100A HF mass spectrometer under FAB conditions with NBA as the matrix. In general, all ginkgolide derivatives decomposed above 250° C.










Abbreviations


















Ac =
CH3CO



Bn =
—CH2C6H5



Bz =
—COC6H5










Synthesis
Representative procedure for attachment of GC (3) to polystyrene resin with silyl linker. To a chlorodiethylsilylpolystyrene resin (100 mg, 0.052 mmol) in glass vial with PTFE septa in the stopper was added a solution of GC (3, 45.8 mg, 0.104 mmol, 2 equiv), iPr2EtN (18.2 mL, 0.104 mmol, 2 equiv) and dimethylaminopyridine (DMAP, 1.3 mg, 0.01 mmol, 0.2 equiv) in THF/CH2Cl2 (1:1, 0.52 mL). The reaction mixture was agitated gently on a rotary shaker at ambient temperature for 24 h and then filtered and washed with CH2Cl2 (3×2 mL) and THF (3×2 mL) alternately, then with Et2O (2×2 mL) and dried under high vacuum.
7-pipsyl-GC (6). To a suspension of GC (3, 99.8 mg, 0.227 mmol) and 4-iodobenzenesulfonyl chloride (205.7 mg, 0.680 mmol, 3 equiv) in CH2Cl2 (2.25 mL) was added pyridine (73 mL, 0.906 mmol, 4 equiv) and the reaction mixture was stirred for 26 h. The solvent was removed under reduced pressure, the residue dissolved in EtOAc and washed with 1M aq. HCl, brine and dried with MgSO4. The crude product was purified by flash column chromatography (gradient 40-99% EtOAc/hexanes, 1% AcOH) to obtain 113.8 mg of pure 6 (73% yield) and 24.3 mg of recovered GC (25%). 1H-NMR (400 MHz, DMSO-d6) δ 8.18-8.10 (m, 2HAR), 7.78-7.70 (m, 2HAR), 7.57 (d, J=5.5 Hz, 1H, 10-H), 6.52 (s, 1H, 3-OH), 6.13 (s, 1H, 12-H), 5.30 (d, J=4.2 Hz, 1H, 1-OH), 5.16 (d, J=4.1 Hz, 1H, 6-H), 5.01 (d, J=5.5 Hz, 1H, 10-H), 4.85 (dd, J=12.4, 4.1 Hz, 1H, 7-H), 4.60 (d, J=6.5 Hz, 1H, 2-H), 4.09 (dd, J=6.5, 4.2 Hz, 1H, 1-H), 2.80 (q, J=7.1 Hz, 1H, 14-H), 1.95 (d, J=12.4 Hz, 1H, 8-H), 1.10 (d, J=7.1 Hz, 3H, 16-CH3), 0.98 (s, 9H, tBu); 13C-NMR (75 MHz, MeOD) δ 8.16, 29.24, 33.20, 43.17, 50.03, 64.78, 68.71, 70.24, 75.03, 76.67, 81.10, 84.50, 93.47, 100.06, 103.62, 111.14, 130.49, 137.04, 140.39, 171.66, 174.40, 178.05. HRMS (FAB) calcd for C26H28O13SI 707.0295, found 707.0265.
Attachment of 6 to a resin (7). To a sample tube was added TentaGel® HL-NH2 (93 mg, 42.6 mmol), 6 (30.1 mg, 42.6 mmol, 1 equiv), Et3N (17 mL, 0.122 mmol, 2.9 equiv), Pd(PPh3)4 (1 mg, 0.9 mmol, 0.02 equiv) and DMF (0.626 mL). The tube was placed into autoclave, which was purged with carbon monoxide three times before applying a pressure of 15 atm. After being left at 80-90° C. for 14 h, the resin was transferred into a plastic syringe with porous PE plug and consecutively washed with DMF (3×3 mL), CH2Cl2(2×3 mL), DMF (2×3 mL), acetonitrile (2×3 mL), acetonitrile/H2O (1:1, 4×3 mL), MeOH (3×3 mL), CH2Cl2 (3×3 mL), and Et2O (3×3 mL) and dried under vacuum to obtain 7.
Attachment of 4-(bromomethyl)phenylacetic acid to a Wang resin (8). To a solution of 4-(bromomethyl)phenylacetic acid (185.7 mg, 0.811 mmol, 8 equiv) in THF (1.6 mL) was added DIC (127 mL, 0.811 mmol, 8 equiv) and mixture was agitated for 5 min until white crystals developed. More THF (1.6 mL) was added to redissolve the precipitate and the solution was mixed with Wang resin (169 mg, 0.101 mmol, 1 equiv). The reaction mixture was gently agitated on a rotary shaker for 9 h and then filtered, washed with THF (4×5 mL) and CH2Cl2 (4×5 mL) alternately, then with CH2Cl2 (2×5 mL), MeOH (2×5 mL) and dried under vacuum to obtain 8.
Attachment of 4-(bromomethyl)phenylacetic acid to a Rink-amide resin (9). To a solution of 4-(bromomethyl)phenylacetic acid (47.4 mg, 0.207 mmol, 8 equiv) in THF (0.4 mL) was added 2-dimethylaminoisopropyl chloride hydrochloride (DIC, 32.4 mL, 0.207 mmol, 8 equiv) and mixture was agitated for 5 min until white crystals developed. More THF (0.6 mL) was added to re-dissolve the precipitate and the solution was mixed with Rink-amide TentaGel® resin (108 mg, 0.026 mmol, 1 equiv). The reaction mixture was gently agitated on a rotary shaker for 2.5 h and then filtered, washed with THF (4×3 mL) and CH2Cl2 (4×3 mL) alternately, then with CH2Cl2 (2×3 mL), Et2O (2×3 mL) and dried under vacuum to obtain 9.
Synthesis of silicon-linked solid-phase anchored benzyl bromide (14). To a solution of 4-(allyldimethylsilyl)benzyl alcohol (475 mg, 2.30 mmol) in THF (4.6 mL) at 0° C. was slowly added 9-borabicyclo[3.3.1]nonane (9-BBN, 9.2 mL, 0.5M in THF, 4.6 mmol, 2 equiv). The reaction mixture was warmed up slowly to room temperature and then stirred for 10 h. To the mixture was then added bromopolystyrene (1.075 g, 1.5 mmol/g, 1.61 mmol, 0.7 equiv), K2CO3 (2.3 mL, 3M in H2O, 6.9 mmol, 3 equiv), DMF (6.3 mL) and finally Pd(PPh3)4 (53.3 mg, 0.046 mmol, 0.02 equiv). The resulting suspension was slowly stirred in closed vial at 70-75° C. for 24 h. Pd(PPh3)4 (53.3 mg, 0.046 mmol, 0.02 equiv) was added to the reaction mixture, which was again reacted at 70-75° C. for additional 24 h. The resin was filtered and washed with DMF (3×), H2O (3×), DMF (3×), then with CH2Cl2 (3×) and MeOH (3×) alternately and with MeOH (1×) and dried under high vacuum to obtain 1.265 g of dry resin. The resin (≦1.61 mmol) was placed into solid-phase reactor and PPh3 (846 mg, 3.22 mmol, 2 equiv), CBr4 (1.070 g, 3.22 mmol, 2 equiv) and CH2Cl2 (16 mL) were added at 0° C. The mixture was agitated on rotary shaker at room temperature for 6 h and then filtered and washed with CH2Cl2 (3×), then MeOH (2×) and CH2Cl2 (2×) alternately and then with MeOH (2×) and dried under vacuum to obtain 1.371 g of resin 14. Loading of the resin was determined by cleavage with Br2 to obtain 4-bromobenzyl bromide (1.17 mmol/g, 85% yield from bromopolystyrene).
General procedure for attachment of GC (3) to solid-phase anchored benzyl bromide (10, 11, 15). To a mixture of a resin (8, 9, or 14) (0.1 mmol), GC (3) (79.2 mg, 0.18 mmol, 1.8 equiv) and finely grounded K2CO3 (41.5 mg, 0.3 mmol, 3 equiv) in a plastic syringe with porous PE plug was added DMF and the suspension was gently agitated on rotary shaker for 30 h. The resin was filtered and washed with DMF (3×) for GC (3) recovery and the resin was further washed with H2O (3×), DMF (3×), then CH2Cl2 (3×) and MeOH (3×) alternately and dried under high vacuum. A test for the presence of alkyl halides (using 4-nitrobezylpyridine) was negative (Kuisle, 1999).
Cleavage from Wang resin (12). To a sample of resin 10 (12 mg) in a vial was added TFA/CH2Cl2 (1:1, 0.2 mL). After standing for 30 min, resin was filtered and washed with TFA/CH2Cl2 (2:3, 0.17 mL), TFA/CH2Cl2 (1:3, 0.22 mL), and TFA/CH2Cl2 (1:4, 0.2 mL). Combined washing solutions were mixed with filtrate and volatiles were removed under reduced pressure. Residue was dissolved in MeOH (0.1 mL) and treated with CH2N2 (1 mL,  ̃0.2M in Et2O) for 10 min. Volatiles were removed under reduced pressure to provide 12 ( ̃90% purity based on 1H NMR). 1H-NMR (400 MHz, CDCl3) δ 7.40-7.30 (m, 4HAR), 5.99 (s, 1H, 12-H), 5.48 (d, J=9.3 Hz, 1H, benzyl), 5.13 (d, J=4.4 Hz, 1H, 6-H), 4.93 (s, 1H, 10-H), 4.61 (d, J=9.3 Hz, 1H, benzyl), 4.53 (d, J=7.8 Hz, 1H, 2-H), 4.22 (dd, J=3.3, 7.8 Hz, 1H, 1-H), 4.18-4.08 (m, 1H, 7-H), 3.71 (s, 3H, —OCH3), 3.66 (s, 2H, CH2), 3.05 (q, J=7.0 Hz, 1H, 14-H), 2.82 (d, J=3.4 Hz, 1H, 1-OH), 2.73 (bs, 1H, 3-OH), 2.11 (d, J=11.5 Hz, 1H, 7-OH), 1.69 (d, J=12.4 Hz, 1H, 8-H), 1.30 (d, J=7.0 Hz, 3H, 16-CH3), 1.23 (s, 9H, tBu); HRMS (FAB) calcd for C30H35O13 603.2078, found 603.2081.
Cleavage from Rink-amide resin (13). To a sample of resin 11 (27.6 mg) in a vial was added TFA/H2O (9:1, 0.4 mL). After standing for 50 min, the resin was filtered and washed with TFA/H2O (9:1, 2×0.28 mL). The combined washing solutions were mixed with filtrate and volatiles were removed under reduced pressure to provide 13 (95% purity based on 1H NMR). 1H-NMR (400 MHz, DMSO-d6) δ 7.48 (s, 1H, NHAHB), 7.35-7.28 (m, 2HAR), 7.28-7.21 (m, 2HAR), 6.89 (s, 1H, NHAHB), 6.48 (s, 1H, 3-OH), 6.19 (s, 1H, 12-H), 5.65 (bs, 1H, 7-OH), 5.34 (d, J=10.9 Hz, 1H, benzyl), 5.22 (s, 1H, 10-H), 4.94 (d, J=4.1 Hz, 1H, 6-H), 4.80 (d, J=4.0 Hz, 1H, 1-OH), 4.63 (d, J=10.9 Hz, 1H, benzyl), 4.59 (d, J=6.5 Hz, 1H, 2-H), 4.13 (dd, J=4.0, 6.4 Hz, 1H, 1-H), 3.97 (dd, J=4.1, 12.5 Hz, 1H, 7-H), 3.37 (s, 2H, CH2), 2.84 (q, J=7.1 Hz, 1H, 14-H), 1.55 (d, J=12.5 Hz, 1H, 8-H), 1.14 (d, J=7.1 Hz, 3H, 16-CH3), 1.09 (s, 9H, tBu); HRMS (FAB) calcd for C29H34O12N 588.2081, found 588.2083.
Cleavage from silicone-linker resin (16). To a sample of resin 15 (12.6 mg) in a vial was added TFA/CH2Cl2 (1:1, 0.5 mL).
After standing for 24 h, resin was filtered and washed with TFA/CH2Cl2 (1:1, 2×0.3 mL). Combined washing solutions were mixed with filtrate and volatiles were removed under reduced pressure to provide 16 (85% purity based on 1H NMR). Analytical data as previously described (Vogensen, 2003).
10-Benzyloxy-GC (16). Synthesis and analytical data as previously described (Vogensen, 2003).
10-Benzyloxy-isoGC-6-benzoate (17). To a solution of 16 (18.0 mg, 0.034 mmol) in CH2Cl2 (0.87 mL) and iPr2EtN (0.145 mL, 0.829 mmol, 24 equiv) was added benzoic anhydride (23 mg, 0.102 mmol, 3 equiv). The mixture was stirred for 11 h and then quenched with phosphate buffer (pH  ̃2-3, 1 mL) washed with brine and dried with MgSO4. The crude product was purified by flash chromatography (30-50% EtOAc/hexanes) to obtain 17 as a white powder (16.6 mg, 77%). 1H-NMR (400 MHz, CDCl3) δ 8.15-8.09 (m, 2HAR), 7.66-7.56 (m, 1HAR), 7.52-7.38 (m, 7HAR), 5.75 (s, 1H, 12-H), 5.48 (d, J=9.5 Hz, 1H, benzyl), 5.44 (d, J=4.0 Hz, 1H, 6-H), 5.17 (s, 1H, 10-H), 5.06 (d, J=3.9 Hz, 1H, 7-H), 4.91 (d, J=7.7 Hz, 1H, 2-H), 4.83 (d, J=9.5 Hz, 1H, benzyl), 4.28 (dd, J=7.6, 2.3 Hz, 1H, 1-H), 3.83 (s, 1H, 3-OH), 3.22 (q, J=6.9 Hz, 1H, 14-H), 3.22 (d, J=2.7 Hz, 1H, 1-OH), 2.30 (s, 1H, 8-H), 1.35 (d, J=7.07 Hz, 3H, 16-CH3), 1.25 (s, 9H, tBu); 13C-NMR (100 MHz, CDCl3) δ 6.99, 29.71, 33.74, 41.47, 61.78, 62.86, 67.73, 69.90, 73.11, 74.29, 75.21, 78.81, 84.07, 91.96, 95.53, 110.02, 127.82, 128.80, 129.23, 129.46, 129.90, 130.19, 133.65, 134.18, 165.09, 166.56, 170.70, 176.38; HRMS (FAB) calcd for C34H35O12 635.2129, found 635.2097.
10-Benzyloxy-isoGC-1,6-bisacetate (19). To a solution of 16 (8.5 mg, 0.016 mmol) in CH2Cl2 (0.15 mL) and iPr2EtN (28 mL, 0.16 mmol, 10 equiv) was added acetic anhydride (15 mL, 0.16 mmol, 10 equiv). The mixture was stirred for 12 h and then quenched with aq. HCl (1 mL, 1M), extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (40-50% EtOAc/hexanes) to obtain 19 as a white powder (8.0 mg, 81%). 1H-NMR (300 MHz, CDCl3) δ 7.43-7.38 (m, 5HAR), 5.66 (d, J=3.7 Hz, 1H, 1-H), 5.63 (S, 1H, 12-H), 5.51 (d, J=4.1 Hz, 1H, 6-H), 5.30 (d, J=10.6 Hz, 1H, benzyl), 5.01 (s, 1H, 10-H), 4.83 (d, J=4.1 Hz, 1H, 7-H), 4.72 (d, J=10.6 Hz, 1H, benzyl), 4.48 (d, J=3.7 Hz, 1H, 2-H), 3.31 (q, J=7.5 Hz, 1H, 14-H), 3.30 (S, 1H, 3-OH), 2.20 (S, 1H, 8-H), 2.12 (S, 3H, CH3-acetyl), 1.31 (d, J=7.5 Hz, 3H, 16-CH3), 1.30 (S, 3H, CH3-acetyl), 1.19 (s, 9H, tBu); 13C-NMR (75 MHz, CDCl3) δ 9.56, 19.52, 20.59, 29.26, 33.63, 41.21, 61.55, 63.67, 67.20, 70.13, 73.29, 74.25, 74.98, 78.99, 84.47, 92.04, 97.31, 108.60, 128.76 (2 C), 128.93, 135.22, 166.51, 167.87, 168.74, 170.07, 175.87; HRMS (FAB) calcd for C31H35O13 615.2078, found 615.2083.
10-(4-Methoxy-benzyloxy)-isoGC-1,6-bisacetate (20). To a solution of 24 (13.4 mg, 0.024 mmol) in CH2Cl2 (0.24 mL) and iPr2EtN (42 mL, 0.24 mmol, 10 equiv) was added acetic anhydride (23 mL, 0.24 mmol, 10 equiv). The mixture was stirred for 12 h and then quenched with aq. HCl (1 mL, 1M), extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (40-70% EtOAc/hexanes) to obtain 20 as a white powder (12.1 mg, 79%). 1H-NMR (400 MHz, CDCl3) δ 7.31-7.24 (m, 2HAR), 6.93-6.86 (m, 2HAR), 5.64 (d, J=3.8 Hz, 1H, 1-H), 5.63 (S, 1H, 12-H), 5.45 (d, J=4.1 Hz, 1H, 6-H), 5.18 (d, J=10.4 Hz, 1H, benzyl), 5.00 (s, 1H, 10-H), 4.82 (d, J=4.1 Hz, 1H, 7-H), 4.69 (d, J=10.4 Hz, 1H, benzyl), 4.50 (d, J=3.8 Hz, 1H, 2-H), 3.81 (s, 3H, —OMe), 3.34 (s, 1H, 3-OH), 3.30 (q, J=7.5 Hz, 1H, 14-H), 2.20 (s, 1H, 8-H), 2.12 (s, 3H, CH3-acetyl), 1.39 (s, 3H, CH3-acetyl), 1.30 (d, J=7.5 Hz, 3H, 16-CH3), 1.17 (s, 9H, tBu); 13C-NMR (100 MHz, CDCl3) δ 9.40, 19.62, 20.53, 29.27, 33.52, 41.08, 55.17, 61.28, 63.41, 66.87, 69.90, 72.62, 73.84, 74.18, 78.70, 84.15, 91.75, 96.83, 108.29, 113.65, 126.79, 130.36, 159.46, 165.97, 167.33, 168.21, 169.65, 175.34; HSQC correlation data in the Supporting Information; HRMS (FAB) calcd for C32H36O14 644.2105, found 644.2082.
10-Benzyloxy-GC-7-benzoate (21). To a solution of 16 (22.9 mg, 0.043 mmol) in CH2Cl2 (0.9 mL) and pyridine (0.15 mL, 1.85 mmol, 43 equiv) was added benzoyl chloride (25 mL, 0.216 mmol, 5 equiv). The mixture was stirred for 5 h and then quenched with aq. HCl (1 mL, 1M), washed with brine and dried with MgSO4. The crude product was purified by flash chromatography (30-40% EtOAc/hexanes) to obtain 21 as a white powder (21.6 mg, 79%). 1H-NMR (400 MHz, CDCl3) δ 8.12-8.05 (m, 2HAR), 7.68-7.58 (m, 1HAR), 7.53-7.40 (m, 7HAR), 6.08 (s, 1H, 12-H), 5.50 (dd, J=12.9, 4.3 Hz, 1H, 7-H), 5.48 (d, J=9.4 Hz, 1H, benzyl), 5.41 (d, J=4.3 Hz, 1H, 6-H), 5.02 (s, 1H, 10-H), 4.76 (d, J=9.4 Hz, 1H, benzyl), 4.51 (d, J=7.9 Hz, 1H, 2-H), 4.26 (dd, J=7.9, 3.2 Hz, 1H, 1-H), 3.07 (q, J=7.0 Hz, 1H, 14-H), 2.86 (d, J=3.2 Hz, 1H, 1-OH), 2.9-2.7 (bs, 1H, 3-OH), 2.25 (d, J=12.9 Hz, 1H, 8-H), 1.31 (d, J=7.0 Hz, 3H, 16-CH3), 1.20 (s, 9H, tBu); 13C-NMR (100 MHz, CDCl3) δ 7.26, 29.36, 32.13, 41.57, 48.60, 64.03, 67.83, 73.93, 74.01, 74.15, 75.39, 76.88, 83.41, 90.53, 98.34, 109.86, 128.60, 128.74, 128.80, 129.57, 129.85, 130.06, 133.95, 134.12, 164.84, 170.50, 170.97, 175.13; HRMS (FAB) calcd for C34H35O12 635.2129, found 635.2156.
10-Benzyloxy-isoGC-1,6-bisbenzoate (22). To a solution of 16 (9.3 mg, 0.015 mmol) in CH2Cl2 (0.38 mL) and iPr2EtN (63 mL, 0.36 mmol, 24 equiv) was added benzoic anhydride (19.9 mg, 0.090 mmol, 6 equiv). The mixture was stirred for 20 h and then quenched with aq. HCl (1 mL, 1M), washed with brine and dried with MgSO4. The crude product was purified by flash chromatography (30-50% EtOAc/1% AcOH/hexanes) to obtain 22 as a white powder (7.1 mg, 64%). 1H-NMR (400 MHz, CDCl3) δ 8.25-8.18 (m, 2HAR), 7.66-7.58 (m, 3HAR), 7.52-7.37 (m, 3HAR), 7.21-7.13 (m, 2HAR), 7.11-7.03 (m, 2HAR), 7.02-6.95 (m, 3HAR), 6.02 (d, J=5.1 Hz, 1H, 1-H), 5.79 (d, J=4.0 Hz, 1H, 6-H), 5.69 (s, 1H, 12-H), 5.03 (d, J=4.0 Hz, 1H, 7-H), 5.01 (s, 1H, 10-H), 4.88 (d, J=5.1 Hz, 1H, 2-H), 4.86 (s, 2H, benzyl), 3.66 (s, 1H, 3-OH), 3.41 (q, J=7.3 Hz, 1H, 14-H), 2.25 (s, 1H, 8-H), 1.39 (d, J=7.3 Hz, 3H, 16-CH3), 1.08 (s, 9H, tBu); 13C-NMR (75 MHz, CDCl3) δ 8.60, 29.21, 33.73, 41.42, 61.61, 64.01, 67.11, 71.02, 72.94, 73.78, 74.05, 79.05, 84.72, 92.51, 96.69, 109.00, 127.96, 128.25, 128.29, 128.62, 128.91, 128.98, 129.22, 129.58, 130.21, 133.08, 133.92, 134.37, 163.78, 165.61, 166.74, 170.05, 175.91; HRMS (FAB) calcd for C41H39O13 739.2391, found 739.2381.
10-Benzyloxy-GC-1-benzoate (23). To a solution of 17 (15.6 mg, 0.025 mmol) in DMF (0.40 mL) was added iPr2EtN (50 mL, 0.29 mmol, 12 equiv) and mixture was stirred for 1.5 h at 100° C. Solvent was removed under reduced pressure, a residue was treated with phosphate buffer (pH 2-3, 1 mL), then extracted with EtOAc (3×) and combined organic layers were dried with MgSO4. The crude product was purified by flash chromatography (30-50% EtOAc/hexanes) to obtain 23 as a white solid (12.2 mg, 78%). 1H-NMR (400 MHz, CDCl3) δ 7.64-7.59 (m, 2HAR), 7.51-7.45 (m, 1HAR), 7.31-7.25 (m, 2HAR), 6.98-6.91 (m, 2HAR), 6.88-6.83 (m, 3HAR), 6.03 (s, 1H, 12-H), 5.89 (d, J=6.1 Hz, 1H, 1-H), 5.46 (d, J=4.4 Hz, 1H, 6-H), 5.17 (d, J=11.0 Hz, 1H, benzyl), 4.85 (s, 1H, 10-H), 4.70 (d, J=6.1 Hz, 1H, 2-H), 4.56 (d, J=10.9 Hz, 1H, benzyl), 4.42-4.33 (m, 1H, 7-H), 3.28 (q, J=7.2 Hz, 1H, 14-H), 3.4-2.9 (bs, 1H, 3-OH), 2.65-2.3 (bs, 1H, 7-OH), 1.76 (d, J=12.3 Hz, 1H, 8-H), 1.33 (d, J=7.2 Hz, 3H, 16-CH3), 1.15 (s, 9H, tBu); 13C-NMR (100 MHz, CDCl3) δ 7.94, 29.13, 32.34, 41.20, 50.57, 65.09, 66.47, 73.55, 74.30, 75.25, 75.64, 80.32, 84.27, 92.14, 99.62, 109.34, 128.02, 128.11, 128.24, 128.34, 128.70, 129.56, 133.23, 134.66, 163.47, 170.25, 170.70, 174.91; HRMS (FAB) calcd for C34H35O12 635.2129, found 635.2148.
10-(4-Methoxy-benzyloxy)-GC (24). To a solution of GC (3) (201 mg, 0.456 mmol) in DMF (2.28 mL) was added powdered K2CO3 (315 mg, 2.28 mmol, 5 equiv) and 4-methoxybenzyl chloride (620 mL, 4.56 mmol, 10 equiv). The mixture was stirred at 60° C. for 3 h and then stirred at room temperature for 5 h. The solution was concentrated under reduced pressure, an aq. phosphate buffer (pH 2-3, 10 mL) was added and the resulting solution was extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (40-100% EtOAc/hexanes) to obtain 24 as a white powder (240 mg, 94%). 1H-NMR (300 MHz, CDCl3) δ 7.35-7.27 (m, 2HAR), 6.99-6.89 (m, 2HAR), 5.98 (s, 1H, 12-H), 5.41 (d, J=9.2 Hz, 1H, benzyl), 5.12 (d, J=4.4 Hz, 1H, 6-H), 4.92 (s, 1H, 10-H), 4.57 (d, J=9.2 Hz, 1H, benzyl), 4.52 (d, J=7.9 Hz, 1H, 2-H), 4.21 (dd, J=3.4, 7.9 Hz, 1H, 1-H), 4.13 (m, 1H, 7-H), 3.83 (s, 3H, —OCH3), 3.05 (q, J=7.1 Hz, 1H, 14-H), 2.88 (d, J=3.4 Hz, 1H, 1-OH), 2.75 (s, 1H, 3-OH), 2.12 (d, J=11.5 Hz, 1H, 7-OH), 1.69 (d, J=12.4 Hz, 1H, 8-H), 1.30 (d, J=7.0 Hz, 3H, 16-CH3), 1.23 (s, 9H, tBu); 13C-NMR (100 MHz, CDCl3) δ 7.23, 29.08, 32.19, 41.65, 50.38, 55.31, 64.03, 67.08, 73.79, 75.31, 75.59, 79.33, 83.47, 90.70, 98.49, 110.13, 114.73, 126.36, 130.65, 160.56, 170.90, 171.03, 175.83; HRMS (FAB) calcd for C28H32O12 560.1894, found 560.1866.
10-(2-Pyridinyl-methoxy)-GC (25). A mixture of 2-picolylchloride hydrochloride (401.4 mg, 2.45 mmol, 6 equiv) and EtOAc was washed with sat. aq. NaHCO3 (2×), brine and dried with MgSO4. After filtration and removal of EtOAc, free 2-picolyl chloride was added to a solution of GC (3, 179.6 mg, 0.408 mmol, 1 equiv) in DMF (2.04 mL) and powdered K2CO3 (338 mg, 2.45 mmol, 6 equiv) was added. The mixture was briefly heated to about 60° C. and then stirred at room temperature for 15 h. The solution was concentrated under reduced pressure, an aq. phosphate buffer (pH 2-3, 10 mL) was added and resulting solution was extracted with EtOAc (3×) and dried with MgSO4.
The crude product was purified by flash chromatography (50-100% EtOAc/hexanes) to obtain 25 as a white powder (211.2 mg, 97%). 1H-NMR (400 MHz, CDCl3) δ 8.54 (d, J=4.9 Hz, 1HAR), 8.20 (d, J=4.1 Hz, 1H, 1-OH), 7.79-7.71 (m, 1HAR), 7.35-7.28 (m, 1HAR), 7.19-7.11 (m, 1HAR), 6.00 (s, 1H, 12-H), 5.67 (d, J=13.0 Hz, 1H, benzyl), 5.43 (d, J=4.3 Hz, 1H, 6-H), 4.96 (s, 1H, 10-H), 4.82 (d, J=13.0 Hz, 1H, benzyl), 4.66 (d, J=7.6 Hz, 1H, 2-H), 4.47-4.38 (m, 1H, 1-H), 4.27-4.17 (m, 1H, 7-H), 3.10 (s, 1H, 3-OH), 3.08 (q, J=7.0 Hz, 1H, 14-H), 2.32 (d, J=11.3 Hz, 1H, 7-OH), 1.73 (d, J=12.4 Hz, 1H, 8-H), 1.30 (d, J=7.0 Hz, 3H, 16-CH3), 1.18 (s, 9H, tBu); 13C-NMR (100 MHz, CDCl3) δ 7.33, 29.12, 32.27, 41.74, 50.55, 64.70, 67.62, 70.67, 73.81, 74.89, 75.36, 80.14, 83.06, 92.61, 98.74, 109.99, 120.37, 123.29, 137.41, 148.74, 154.52, 170.96, 171.45, 175.79; HRMS (FAB) calcd for C26H30O11N 532.1819, found 532.1844.
10-(4-Methoxy-benzyloxy)-GC-7-benzoate (26). To a solution of 24 (11.3 mg, 0.0202 mmol) in CH2Cl2 (0.34 mL) and pyridine (56 mL, 0.69 mmol, 34 equiv) was added benzoyl chloride (11.7 mL, 0.101 mmol, 5 equiv). Mixture was stirred for 7 h and then quenched with aq. HCl (1 mL, 1M), extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (30-50% EtOAc/hexanes) to obtain 26 as a white powder (12.3 mg, 92%). 1H-NMR (300 MHz, CDCl3) δ 8.13-8.03 (m, 2HAR), 7.69-7.58 (m, 1HAR), 7.54-7.44 (m, 2HAR), 7.44-7.34 (m, 2HAR) 7.03-6.94 (m, 2HAR), 6.07 (s, 1H, 12-H), 5.48 (dd, J=12.8, 4.3 Hz, 1H, 7-H), 5.40 (d, J=9.3 Hz, 1H, benzyl), 5.40 (d, J=4.3 Hz, 1H, 6-H), 4.99 (s, 1H, 10-H), 4.70 (d, J=9.3 Hz, 1H, benzyl), 4.50 (d, J=7.9 Hz, 1H, 2-H), 4.25 (dd, J=7.9, 3.4 Hz, 1H, 1-H), 3.84 (s, 3H, —OMe), 3.06 (q, J=7.0 Hz, 1H, 14-H), 2.92 (d, J=3.4 Hz, 1H, 1-OH), 2.90 (s, 1H, 3-OH), 2.24 (d, J=12.8 Hz, 1H, 8-H), 1.31 (d., J=7.0 Hz, 3H, 16-CH3), 1.20 (s, 9H, tBu); 13C-NMR (75 MHz, CDCl3) δ 7.28, 29.34, 32.10, 41.61, 48.57, 55.33, 64.01, 67.81, 73.70, 73.94, 73.97, 75.05, 76.84, 83.41, 90.54, 98.31, 109.87, 114.82, 126.18, 128.59, 128.73, 130.05, 130.57, 133.94, 160.61, 164.84, 170.65, 171.00, 175.28; HRMS (FAB) calcd for C35H36O13 664.2156, found 664.2129.
10-(4-Methoxy-benzyloxy)-isoGC-6-benzoate (27). To a solution of 24 (24.6 mg, 0.0438 mmol) in CH2Cl2 (0.83 mL) and iPr2EtN (46 mL, 0.263 mmol, 6 equiv) was added benzoic anhydride (29.7 mg, 0.131 mmol, 3 equiv). Mixture was stirred for 11 h and then quenched with phosphate buffer (pH  ̃2-3, 1 mL), extracted with EtOAc and dried with MgSO4. The crude product was purified by flash chromatography (30-50% EtOAc/1% AcOH/hexanes) to obtain 27 as a white powder (16.5 mg, 57%). 1H-NMR (400 MHz, CDCl3) δ 8.15-8.08 (m, 2HAR), 7.62-7.55 (m, 1HAR), 7.48-7.39 (m, 2HAR)., 7.38-7.30 (m, 2HAR), 6.96-6.88 (m, 2HAR), 5.74 (s, 1H, 12-H), 5.42 (d, J=4.0 Hz, 1H, 6-H), 5.40 (d, J=9.4 Hz, 1H, benzyl), 5.16 (s, 1H, 10-H), 5.06 (d, J=4.0 Hz, 1H, 7-H), 4.91 (d, J=7.7 Hz, 1H, 2-H), 4.76 (d, J=9.4 Hz, 1H, benzyl), 4.27 (dd, J=7.7, 2.6 Hz, 1H, 1-H), 3.84 (s, 1H, 3-OH), 3.79 (s, 3H, —OMe), 3.30 (d, J=2.7 Hz, 1H, 1-OH), 3.22 (q, J=7.1 Hz, 1H, 14-H), 2.30 (s, 1H, 8-H), 1.35 (d, J=7.1 Hz, 3H, 16-CH3), 1.25 (s, 9H, tBu); 13C-NMR (75 MHz, CDCl3) δ 6.99, 29.39, 33.76, 41.49, 55.31, 61.79, 62.91, 67.76, 69.96, 73.12, 73.86, 74.94, 78.86, 84.11, 91.98, 95.51, 110.06, 114.76, 125.78, 127.88, 128.80, 130.23, 130.98, 134.17, 160.71, 165.11, 166.61, 170.84, 176.40; HRMS (FAB) calcd for C35H36O13 664.2156, found 664.2165.
10-(4-Methoxy-benzyloxy)-GC-1-benzoate (28). To a solution of 27 (9.9 mg, 0.015 mmol) in DMF (0.25 mL) was added iPr2EtN (30 mL, 0.17 mmol, 12 equiv) and the mixture was stirred for 3 h at 100° C. Solvent was removed under reduced pressure, a residue was treated with phosphate buffer (pH  ̃2-3, 1 mL), then extracted with EtOAc (3×) and combined organic layers were dried with MgSO4. The crude product was purified by flash chromatography (30-50% EtOAc/hexanes) to obtain 28 as a white solid (7.9 mg, 80%). 1H-NMR (400 MHz, CDCl3) δ 7.64-7.58 (m, 2HAR), 7.52-7.45 (m, 1HAR), 7.32-7.25 (m, 2HAR), 6.91-6.85 (m, 2HAR), 6.39-6.33 (m, 2HAR), 6.02 (s, 1H, 12-H), 5.84 (d, J=6.2 Hz, 1H, 1-H), 5.44 (d, J=4.4 Hz, 1H, 6-H), 5.10 (d, J=10.5 Hz, 1H, benzyl), 4.83 (s, 1H, 10-H), 4.68 (d, J=6.2 Hz, 1H, 2-H), 4.47 (d, J=10.5 Hz, 1H, benzyl), 4.42-4.33 (m, 1H, 7-H), 3.60 (s, 3H, —OMe), 3.28 (q, J=7.1 Hz, 1H, 14-H), 3.09 (s, 1H, 3-OH), 2.37 (d, J=12.0 Hz, 1H, 7-OH), 1.74 (d, J=12.3 Hz, 1H, 8-H), 1.33 (d, J=7.1 Hz, 3H, 16-CH3), 1.18 (s, 9H, tBu); 13C-NMR (125 MHz, CDCl3) δ 7.85, 29.17, 32.34, 41.17, 50.65, 54.81, 65.09, 66.39, 73.31, 74.24, 74.99, 75.62, 80.32, 84.25, 92.11, 99.43, 109.33, 113.44, 126.84, 128.18, 128.71, 129.57, 129.88, 133.01, 159.24, 163.43, 170.31, 170.67, 174.72; HRMS (FAB) calcd for C35H36O13 664.2156, found 664.2170.
10-(2-Pyridinyl-methoxy)-GC-7-benzoate (29). To a solution of 25 (13.2 mg, 0.0248 mmol) in CH2Cl2 (0.52 mL) and pyridine (87 mL, 1.08 mmol, 43 equiv) was added benzoyl chloride (14.4 mL, 0.124 mmol, 5 equiv). Mixture was stirred for 7 h and then quenched with phosphate buffer (pH  ̃2-3, 1 mL), extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (40-50% EtOAc/hexanes) to obtain 29 as a white powder (13.4 mg, 85%). 1H-NMR (400 MHz, CDCl3) δ 8.58 (d, J=5.0 Hz, 1H, 6-py), 8.42 (bs, 1H, 1-OH), 8.08-8.00 (m, 2HAR), 7.79-7.71 (m, 1H, 4-py), 7.65-7.56 (m, 1HAR), 7.51-7.42 (m, 2HAR), 7.35-7.28 (m, 1H, 5-py), 7.15 (d, J=7.9 Hz, 1H, 3-py), 6.08 (s, 1H, 12-H), 5.75 (d, J=4.4 Hz, 1H, 6-H), 5.68 (d, J=13.1 Hz, 1H, benzyl), 5.48 (dd, J=12.8, 4.4 Hz, 1H, 7-H), 5.03 (s, 1H, 10-H), 4.89 (d, J=13.1 Hz, 1H, benzyl), 4.64 (d, J=7.5 Hz, 1H, 2-H), 4.48 (bd, J=7.5 Hz, 1H, 1-H), 3.09 (q, J=7.0 Hz, 1H, 14-H), 2.88 (bs, 1H, 3-OH), 2.28 (d, J=12.8 Hz, 1H, 8-H), 1.31 (d, J=7.0 Hz, 3H, 16-CH3), 1.18 (s, 9H, tBu); 13C-NMR (100 MHz, CDCl3) δ 7.34, 29.39, 32.24, 41.76, 48.89, 64.61, 68.43, 70.65, 73.84, 74.35, 74.93, 77.53, 83.04, 92.62, 98.68, 109.94, 120.22, 123.28, 128.67, 128.84, 130.00, 133.78, 137.37, 148.85, 154.47, 164.87, 170.71, 171.53, 175.63; HRMS (FAB) calcd for C33H34O12N 636.2081, found 636.2058.
GC-7-benzoate (30). To a solution of 26 (8.0 mg, 12.0 mmol) in acetonitrile (150 mL) and CHCl3 (49 mL) was added aq. solution of (NH4)2Ce(NO3)6 (14.7 mL, 24 mmol, 2 equiv, 1.63M). The mixture was stirred for 14 h, volatiles were removed under reduced pressure and residue was purified by flash chromatography (30-100% EtOAc/1% AcOH/hexanes) to obtain 30 as a white powder (4.4 mg, 68%). 1H-NMR (400 MHz, CD3OD) δ 8.11-8.05 (m, 2HAR), 7.70-7.62 (m, 1HAR), 7.57-7.49 (m, 2HAR), 6.20 (s, 1H, 12-H), 5.54-5.46 (m, 2H, 6-H & 7-H), 5.19 (s, 1H, 10-H), 4.57 (d, J=7.6 Hz, 1H, 2-H), 4.22 (d, J=7.6 Hz, 1H, 1-H), 3.02 (q, J=7.1 Hz, 1H, 14-H), 2.29-2.22 (m, 1H, 8-H), 1.23 (d, J=7.1 Hz, 3H, 16-CH3), 1.18 (s, 9H, tBu); HSQC correlation data in the Supporting Information; HRMS (FAB) calcd for C27H28O12Na 567.1478, found 567.1472.
IsoGC-6-benzoate (31). To a solution of 27 (3.3 mg, 5.0 mmol) in acetonitrile (63 mL) and CHCl3 (20 mL) was added aq. solution of (NH4)2Ce(NO3)6 (6.2 mL, 10 mmol, 2 equiv, 1.63M). The mixture was stirred for 3 h, volatiles were removed under reduced pressure and residue was purified by flash chromatography (30-70% EtOAc/1% AcOH/hexanes) to obtain 31 as a white powder (2.2 mg, 81%). 1H-NMR (400 MHz, CDCl3) δ 8.18-8.11 (m, 2HAR), 7.63-7.55 (m, 1HAR), 7.48-7.40 (m, 2HAR), 5.75 (s, 1H, 12-H), 5.64 (d, J=3.9 Hz, 1H, 6-H), 5.30 (s, 1H, 10-H), 5.06 (d, J=3.9 Hz, 1H, 7-H), 5.00 (d, J=7.7 Hz, 1H, 2-H), 4.26 (d, J=7.7 Hz, 1H, 1-H), 3.78 (s, 1H, 3-OH), 3.40 (q, J=7.1 Hz, 1H, 14-H), 2.29 (s, 1H, 8-H), 1.37 (d, J=7.1 Hz, 3H, 16-CH3), 1.26 (s, 9H, tBu); HSQC correlation data in the Supporting Information; HRMS (FAB) calcd for C27H29O12 545.1659, found 545.1666.
GC-1-benzoate (32). To a solution of 28 (5.3 mg, 8.0 mmol) in acetonitrile (100 mL) and CHCl3 (32 mL) was added aq. solution of (NH4)2Ce(NO3)6 (9.8 mL, 16 mmol, 2 equiv, 1.63M). The mixture was stirred for 1.5 h, aq. solution of (NH4)2Ce(NO3)6 (4.9 mL, 8 mmol, 1 equiv, 1.63M) was added and mixture was further stirred for 12 h. Volatiles were removed under reduced pressure and residue was purified by flash chromatography (40-70% EtOAc/hexanes) to obtain 32 as a white powder (2.6 mg, 60%). 1H-NMR (300 MHz, CDCl3) δ 7.98-7.90 (m, 2HAR), 7.67-7.58 (m, 1HAR), 7.51-7.41 (m, 2HAR), 6.05 (s, 1H, 12-H), 5.77 (d, J=6.8 Hz, 1H, 1-H), 5.47 (d, J=4.3 Hz, 1H, 6-H), 5.09 (d, J=1.9 Hz, 1H, 10-H), 4.77 (d, J=6.8 Hz, 1H, 2-H), 4.40-4.28 (m, 1H, 7-H), 3.56 (d, J=1.9 Hz, 1H, 10-OH), 3.22 (q, J=7.1 Hz, 1H, 14-H), 2.86 (s, 1H, 3-OH), 2.25 (d, J=11.7 Hz, 1H, 7-OH), 1.75 (d, J=12.3 Hz, 1H, 8-H), 1.33 (d, J=7.1 Hz, 3H, 16-CH3), 1.19 (s, 9H, tBu); HSQC correlation data in the Supporting Information; HRMS (FAB) calcd for C27H29O12 545.1659, found 545.1641.
7-(4-Nitro-phenoxycarbonyloxy)-10-Benzyloxy-GC (33). To a solution of 16 (34.7 mg, 0.065 mmol) in CH2Cl2 (0.82 mL) and pyridine (187 mL, 2.31 mmol, 35 equiv) was added solution of p-nitrophenyl chloroformate (39.6 mg, 0.195 mmol, 3 equiv) in CH2Cl2 (0.3 mL). The mixture was stirred for 50 min and then quenched with aq. HCl (2 mL, 1M), extracted with EtOAc (3×) and dried with MgSO4. The crude product was recrystalized from CHCl3 to obtain 33 as a white crystals (27.8 mg, 61%). 1H-NMR (300 MHz, CDCl3) δ 8.36-8.27 (m, 2HAR), 7.50-7.32 (m, 7HAR), 6.07 (s, 1H, 12-H), 5.47 (d, J=9.4 Hz, 1H, benzyl), 5.47 (d, J=4.3 Hz, 1H, 6-H), 5.08 (dd, J=12.8, 4.3 Hz, 1H, 7-H), 4.99 (s, 1H, 10-H), 4.72. (d, J=9.4 Hz, 1H, benzyl), 4.51 (d, J=7.8 Hz, 1H, 2-H), 4.25 (dd, J=7.8, 3.5 Hz, 1H, 1-H), 3.05 (q, J=7.0 Hz, 1H, 14-H), 2.88 (bs, 1H, 3-OH), 2.79 (d, J=3.5 Hz, 1H, 1-OH), 2.18 (d, J=12.8 Hz, 1H, 8-H), 1.32 (d, J=7.0 Hz, 3H, 16-CH3), 1.23 (s, 9H, tBu).
7-Methylcarbamoyloxy-10-benzyloxy-GC (34). To a solution of 33 (6.4 mg, 9.3 mmol) in THF (0.17 mL) was added MeNH2 in THF (14 mL, 28 mmol, 3 equiv, 2M). The mixture was stirred for 25 min and then quenched with sat. aq. NH4Cl, extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (40-50% EtOAc/hexanes) to obtain 34 as a white solid (4.8 mg, 88%). 1H-NMR (400 MHz, CDCl3) δ 7.50-7.37 (m, 5HAR), 6.02 (s, 1H, 12-H), 5.44 (d; J=9.4 Hz, 1H, benzyl), 5.28 (d, J=4.4 Hz, 1H, 6-H), 5.21 (dd, J=12.8, 4.4 Hz, 1H, 7-H), 4.96 (s, 1H, 10-H), 4.84 (bq, J ̃4.8 Hz, 1H, —NH—), 4.73 (d, J=9.4 Hz, 1H, benzyl), 4.49 (d, J=7.8 Hz, 1H, 2-H), 4.21 (dd, J=7.8, 3.4 Hz, 1H, 1-H), 3.03 (q, J=7.0 Hz, 1H, 14-H), 2.92 (s, 1H, 3-OH), 2.86 (d, J=4.9 Hz, 3H, CH3), 2.80 (d, J=3.4 Hz, 1H, 1-OH), 2.00 (d, J=12.8 Hz, 1H, 8-H), 1.30 (d, J=7.0 Hz, 3H, 16-CH3), 1.16 (s, 9H, tBu); 13C-NMR (75 MHz, CDCl3) δ 7.25, 27.72, 29.31, 32.12, 41.58, 48.50, 63.90, 67.65, 74.02, 74.10, 74.20, 75.36, 77.30, 83.41, 90.61, 98.37, 109.92, 128.82, 129.53, 129.78, 134.15, 154.68, 170.59, 171.04, 175.17; HRMS (FAB) calcd for C29H34O12N 588.2081, found 588.2069.
7-tert-Butylcarbamoyloxy-10-benzyloxy-GC (35). To a solution of 33 (5.5 mg, 7.9 mmol) in THF (0.16 mL) was added t-BuNH2 (3.4 mL, 32 mmol, 4 equiv). The mixture was stirred for 40 min. and then quenched with aq. HCl (1 mL, 1M), extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (30-50% EtOAc/hexanes) to obtain 35 as a white solid (4.4 mg, 88%). The product was further purified by reverse phase HPLC. 1H-NMR (500 MHz, CDCl3) δ 7.48-7.38 (m, 5HAR), 6.02 (s, 1H, 12-H), 5.43 (d, J=9.5 Hz, 1H, benzyl), 5.28 (d, J=4.2 Hz, 1H, 6-H), 5.18 (dd, J=12.5, 4.2 Hz, 1H, 7-H), 4.96 (s, 1H, 10-H), 4.79 (s, 1H, —NH—), 4.75 (d, J=9.5 Hz, 1H, benzyl), 4.50 (d, J=7.8 Hz, 1H, 2-H), 4.21 (dd, J=7.9, 3.4 Hz, 1H, 1-H), 3.04 (q, J=7.0 Hz, 1H, 14-H), 2.81 (d, J=3.4 Hz, 1H, 1-OH), 2.69 (s, 1H, 3-OH), 1.98 (d, J=12.8 Hz, 1H, 8-H), 1.35 (s, 9H, tBu), 1.30 (d, J=7.0 Hz, 3H, 16-CH3), 1.15 (s, 9H, tBu); 13C-NMR (125 MHz, CDCl3) δ 7.21, 28.82, 29.33, 32.10, 41.53, 48.65, 50.93, 63.95, 67.65, 73.38, 74.07, 74.10, 75.39, 77.56, 83.43, 90.56, 98.34, 109.97, 128.85, 129.50, 129.73, 134.16, 152.11, 170.62, 171.09, 175.08; HSQC correlation data in the Supporting Information; HRMS (FAB) calcd for C32H39O12NNa 652.2370, found 652.2368.
7-(Morpholine-4-carbonyloxy)-10-benzyloxy-GC (36). To a solution of 33 (5.2 mg, 7.5 mmol) in THF (0.15 mL) was added morpholine (2 mL, 23 mmol, 3 equiv). The mixture was stirred for 25 min and then quenched with aq. HCl (1 mL, 1M), extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (0.40-50% EtOAc/hexanes) to obtain 36 as a white solid (6.0 mg, 92%). 1H-NMR (400 MHz, CDCl3) δ 7.49-7.38 (m, 5HAR), 6.03 (s, 1H, 12-H), 5.44 (d, J=9.4 Hz, 1H, benzyl), 5.32-5.23 (m, 2H, 6-H & 7-H), 4.97 (s, 1H, 10-H), 4.74 (d, J=9.4 Hz, 1H, benzyl), 4.48 (d, J=7.8 Hz, 1H, 2-H), 4.22 (dd, J=7.8, 3.5 Hz, 1H, 1-H), 3.77-3.33 (m, 8H, morph.), 3.04 (q, J=7.0 Hz, 1H, 14-H), 2.93 (s, 1H, 3-OH), 2.80 (d, J=3.5 Hz, 1H, 1-OH), 2.09-2.00 (m, 1H, 8-H), 1.30 (d, J=7.0 Hz, 3H, 16-CH3), 1.17 (s, 9H, tBu); 13C-NMR (100 MHz, CDCl3) δ 7.27, 29.31, 32.13, 41.57, 44.21 (morph.), 44.48 (morph.), 48.62, 63.94, 66.48 (morph.), 66.59 (morph.), 67.56, 73.97, 74.06, 74.68, 75.30, 77.14, 83.40, 90.58, 98.39, 109.88, 128.80, 129.51, 129.78, 134.10, 152.99, 170.53, 171.02, 175.16; HRMS (FAB) calcd for C32H38O13N 644.2343, found 644.2333.
7-(Piperidine-carbonyloxy)-10-benzyloxy-GC (37). To a solution of 33 (5.0 mg, 7.2 mmol) in THF (0.15 mL) was added piperidine (2.1 mL, 22 mmol, 3 equiv). The mixture was stirred for 30 min and then quenched with aq. HCl (1 mL, 1M), extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (30-40% EtOAc/hexanes) to obtain 37 as a white solid (3.8 mg, 81%). 1H-NMR (300 MHz, CDCl3) δ 7.50-7.37 (m, 5HAR), 6.03 (s, 1H, 12-H), 5.43 (d, J=9.4 Hz, 1H, benzyl), 5.32-5.23 (m, 2H, 6-H & 7-H), 4.97 (s, 1H, 10-H), 4.74 (d, J=9.4 Hz, 1H, benzyl), 4.48 (d, J=7.8 Hz, 1H, 2-H), 4.21 (dd, J=7.8, 3.4 Hz, 1H, 1-H), 3.57-3.33 (m, 4H, piper.), 3.04 (q, J=7.0 Hz, 1H, 14-H), 2.79 (d, J=3.4 Hz, 1H, 1-OH), 2.78 (s, 1H, 3-OH), 2.10-1.99 (m, 1H, 8-H), 1.71-1.40 (m, 6H, piper.), 1.30 (d, J=7.0 Hz, 3H, 16-CH3), 1.17 (s, 9H, tBu); 13C-NMR (75 MHz, CDCl3) δ 7.25, 24.22 (piper.), 25.55 (piper.), 25.89 (piper.), 29.31, 32.12, 41.56, 45.20 (piper.), 48.66, 63.97, 67.57, 74.01, 74.04, 74.37, 75.33, 77.23, 83.39, 90.58, 98.37, 109.93, 128.80, 129.50, 129.73, 134.13, 152.94, 170.63, 171.12, 175.14; HRMS (FAB) calcd for C33H40O12N 642.2551, found 642.2579.
1-Methylcarbamoyloxy-10-benzyloxy-GC (39). To a solution of 16 (9.3 mg, 17.6 mmol) in CH2Cl2 (110 mL) and iPr2EtN (18.4 mL, 0.105 mmol, 6 equiv), was added a solution of p-nitrophenyl chloroformate (10.6 mg, 52.6 mmol, 3 equiv) in CH2Cl2 (50 mL). The mixture was stirred for 16 min and MeNH2 (2M in THF, 26 mL, 52.6 mmol, 3 equiv) and THF (80 mL) were added. The mixture was then stirred for additional 25 min and then quenched with aq. HCl (1 mL, 1M), extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (50-100% EtOAc/hexanes) to obtain 39 as a white solid (6.6 mg, 64%). 1H-NMR (300 MHz, 320K, CDCl3) δ 7.42-7.2 (m, 5HAR), 5.96 (s, 1H, 12-H), 5.55 (d, J=6.0 Hz, 1H, 1-H), 5.39 (d, J=11.5 Hz, 1H, benzyl), 5.19 (d, J=4.3 Hz, 1H, 6-H), 4.82 (s, 1H, 10-H), 4.64 (bd, J=11.5 Hz, 1H, benzyl), 4.62 (d, J=6.0 Hz, 1H, 2-H), 4.38-4.25 (m, 1H, 7-H), 4.26 (m, 1H, —NH—), 3.20 (q, J=7.2 Hz, 1H, 14-H), 2.92 (s, 1H, 3-OH), 2.29 (bd, J=4.1 Hz, 3H, CH3), 2.16 (d, J=11.5 Hz, 1H, 7-OH), 1.69 (d, J=12.3 Hz, 1H, 8-H), 1.31 (d, J=7.2 Hz, 3H, 16-CH3), 1.15 (s, 9H, tBu); HSQC correlation data in the Supporting Information; HRMS (FAB) calcd for C29H34O12N 588.2081, found 588.2103.
1-tert-Butylcarbamoyloxy-10-benzyloxy-GC (40). To a solution of 16 (3.7 mg, 7.0 mmol) in CH2Cl2 (50 mL) and iPr2EtN (7.3 mL, 42 mmol, 6 equiv), was added solution of p-nitrophenyl chloroformate (4.2 mg, 21 mmol, 3 equiv) in CH2Cl2 (20 mL). The mixture was stirred for 16 min and tBuNH2 (7.4 mL, 70 mmol, 10 equiv) and THF (40 mL) were added. The mixture was then stirred for additional 12 h and then quenched with aq. HCl (1 mL, 1M), extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (40-50% EtOAc/hexanes) to obtain 40 as a white solid (2.6 mg, 59%). 1H-NMR (300 MHz, CDCl3) δ 7.40-7.22 (m, 5HAR), 5.93 (s, 1H, 12-H), 5.55 (d, J=5.1 Hz, 1H, 1-H), 5.32 (d, J=12.3 Hz, 1H, benzyl), 5.22 (d, J=4.3 Hz, 1H, 6-H), 4.81 (bd, J=11.5 Hz, 1H, benzyl), 4.79 (s, 1H, 10-H), 4.67 (d, J=5.1 Hz, 1H, 2-H), 4.53 (bs, 1H, —NH—), 4.36-4.24 (m, 1H, 7-H), 3.21 (q, J=7.3 Hz, 1H, 14-H), 3.17 (bs, 1H, 3-OH), 2.13 (bd, J=12.0 Hz, 1H, 7-OH), 1.65 (d, J=12.3 Hz, 1H, 8-H), 1.32 (d, J=7.3 Hz, 3H, 16-CH3), 1.12 (s, 9H, tBu), 1.03 (s, 9H, tBu); 13C-NMR (100 MHz, CDCl3) δ 8.57, 28.49, 28.97, 32.16, 40.94, 50.43, 50.75, 65.05, 66.53, 74.61, 74.94, 75.19, 80.10, 84.08, 91.90, 99.96, 108.52, 127.85, 128.12 (2), 135.55, 151.09, 169.47, 170.34, 174.20; HRMS (FAB) calcd for C32H40O12N 630.2551, found 630.2554.
1-(Morpholine-4-carbonyloxy)-10-benzyloxy-GC (41). To a solution of 16 (6.3 mg, 12.0 mmol) in CH2Cl2 (350 mL) and iPr2EtN (12.5 mL, 72 mmol, 6 equiv), was added solution of p-nitrophenyl chloroformate (7.2 mg, 36 mmol, 3 equiv) in CH2Cl2 (50 mL). The mixture was stirred for 30 min and morpholine (6.3 mL, 72 mmol, 6 equiv) was added. The mixture was then stirred for additional 2 h and then quenched with aq. HCl (1 mL, 1M), extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (50-100% EtOAc/hexanes) to obtain 41 as a white solid (6.0 mg, 78%). 1H-NMR (400 MHz, CDCl3) δ 7.41-7.31 (m, 3HAR), 7.29-7.21 (m, 2HAR), 5.99 (s, 1H, 12-H), 5.60 (d, J=5.6 Hz, 1H, 1-H), 5.40 (d, J=11.5 Hz, 1H, benzyl), 5.15 (d, J=4.4 Hz, 1H, 6-H), 4.85 (s, 1H, 10-H), 4.71 (d, J=5.6 Hz, 1H, 2-H), 4.62 (d, J=11.5 Hz, 1H, benzyl), 4.38-4.28 (m, 1H, 7-H), 3.60 (s, 1H, 3-OH), 3.5-3.2 (m, 6H, morph.), 3.21 (q, J=7.2 Hz, 1H, 14-H), 2.78-2.67 (m, 1H, morph.), 2.50 (d, J=11.7 Hz, 1H, 7-OH), 2.49-2.38 (m, 1H, morph.), 1.72 (d, J=12.3 Hz, 1H, 8-H), 1.32 (d, J=7.2 Hz, 3H, 16-CH3), 1.18 (S, 9H, tBu); 13C-NMR (75 MHz, CDCl3) δ 8.30, 29.16, 32.40, 41.19, 43.48, 43.73, 50.40, 65.07, 65.87, 66.15, 66.51, 73.24, 75.65, 75.85, 80.21, 84.12, 92.16, 99.90, 109.11, 127.44, 128.42, 128.65, 136.20, 152.15, 169.99, 170.60, 175.04; HRMS (FAB) calcd for C32H38O13N 644.2343, found 644.2355.
1-(Piperidine-carbonyloxy)-10-benzyloxy-GC (42). To a solution of 16 (11.3 mg, 21.3 mmol) in CH2Cl2 (710 mL) and iPr2EtN (22.4 mL, 128 mmol, 6 equiv), was added solution of p-nitrophenyl chloroformate (13 mg, 64 mmol, 3 equiv) in CH2Cl2 (80 mL). The mixture was stirred for 30 min and piperidine (12.6 mL, 128 mmol, 6 equiv) was added. The mixture was then stirred for additional 2 h and then quenched with aq. HCl (1 mL, 1M), extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (35-70% EtOAc/hexanes) to obtain 42 as a white solid (6.8 mg, 50%). 1H-NMR (400 MHz, CDCl3) δ 7.39-7.28 (m, 3HAR), 7.28-7.21 (m, 2HAR), 5.97 (S, 1H, 12-H), 5.57 (d, J=5.3 Hz, 1H, 1-H), 5.38 (d, J=11.5 Hz, 1H, benzyl), 5.14 (d, J=4.4 Hz, 1H, 6-H), 4.84 (s, 1H, 10-H), 4.72 (d, J=5.3 Hz, 1H, 2-H), 4.65 (d, J=11.5 Hz, 1H, benzyl), 4.37-4.28 (m, 1H, 7-H), 3.57 (s, 1H, 3-OH), 3.50-3.38 (m, 1H, piper.), 3.35-3.23 (m, 1H, piper.), 3.22 (q, J=7.3 Hz, 1H, 14-H), 2.60-2.49 (m, 1H, piper.), 2.47-2.37 (m, 1H, piper.), 2.36 (d, J=12.0 Hz, 1H, 7-OH), 1.70 (d, J=12.3 Hz, 1H, 8-H), 1.61-1.50 (m, 1H, piper.), 1.45-1.25 (m, 5H, piper.), 1.32 (d, J=7.3 Hz, 3H, 16-CH3), 1.16 (S, 9H, tBu); 13C_NMR (100 MHz, CDCl3) δ 8.53, 23.89, 24.96, 25.52, 29.17, 32.38, 41.15, 44.49, 44.64, 50.47, 65.17, 66.80, 73.12, 75.44, 75.64, 76.07, 80.39, 84.26, 92.23, 100.23, 108.98, 127.75, 128.17, 128.54, 136.16, 152.22, 170.02, 170.80, 175.00; HRMS (FAB) calcd for C33H40O12N 642.2551, found 642.2521.
7-(4-Nitro-phenoxycarbonyloxy)-10-(4-methoxy-benzyloxy)-GC (43). To a solution of 24 (47.6 mg, 0.085 mmol) in CH2Cl2 (0.53 mL) and pyridine (121 mL, 1.50 mmol, 18 equiv) at 0° C. was added solution of p-nitrophenyl chloroformate (34.2 mg, 0.170 mmol, 2 equiv) in CH2Cl2 (0.2 mL). The mixture was allowed to warm up slowly to room temperature while stirring. After 2.5 h the reaction was quenched with aq. HCl (2 mL, 1M), extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (30-50% EtOAc/hexanes) to obtain 43 as a white powder (42.6 mg, 69%) and unreacted starting material 24 (9.5 mg, 20%). 1H-NMR (400 MHz, CDCl3) δ 8.35-8.27 (m, 2HAR), 7.46-7.38 (m, 2HAR), 7.35-7.28 (m, 2HAR), 6.95-6.88 (m, 2HAR), 6.06 (s, 1H, 12-H), 5.45 (d, J=4.3 Hz, 1H, 6-H), 5.38 (d, J=9.3 Hz, 1H, benzyl), 5.05 (dd, J=12.8, 4.3 Hz, 1H, 7-H), 4.98 (s, 1H, 10-H), 4.66 (d, J=9.3 Hz, 1H, benzyl), 4.51 (d, J=7.8 Hz, 1H, 2-H)., 4.25 (dd, J=7.8, 3.5 Hz, 1H, 1-H), 3.80 (s, 3H, —OMe), 3.06 (q, J=7.0 Hz, 1H, 14-H), 3.04 (s, 1H, 3-OH), 2.87 (d, J=3.5 Hz, 1H, 1-OH), 2.17 (d, J=12.8 Hz, 1H, 8-H), 1.30 (d, J=7.0 Hz, 3H, 16-CH3), 1.22 (s, 9H, tBu); 13C-NMR (100 MHz, CDCl3) δ 7.46, 29.42, 32.24, 41.70, 48.48, 55.38, 63.85, 67.77, 73.72, 73.90, 74.93, 75.69, 78.17, 83.43, 90.48, 98.23, 109.65, 114.71, 121.48, 125.28, 125.94, 130.36, 145.52, 150.82, 154.72, 160.41, 170.07, 170.11, 174.90.
7-Methylcarbamoyloxy-10-(4-methoxy-benzyloxy)-GC (44). To a solution of 43 (14.2 mg, 19.5 mmol) in THF (0.20 mL) was added MeNH2 in THF (29 mL, 59 mmol, 3 equiv, 2M). The mixture was stirred for 20 min and then quenched with aq. HCl (1 mL, 1M), extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (40-70% EtOAc/hexanes) to obtain 44 as a white solid (8.9 mg, 74%). 1H-NMR (500 MHz, CDCl3) δ 7.36 (d, J=8.5 Hz, 2HAR), 6.95 (d, J=8.5 Hz, 2HAR), 6.01 (s, 1H, 12-H), 5.35 (d, J=9.3 Hz, 1H, benzyl), 5.26 (d, J=4.3 Hz, 1H, 6-H), 5.19 (dd, J=12.8, 4.4 Hz, 1H, 7-H), 4.94 (s, 1H, 10-H), 4.83 (bq, J ̃4.8 Hz, 1H, —NH—), 4.67 (d, J=9.3 Hz, 1H, benzyl), 4.49 (d, J=7.8 Hz, 1H, 2-H), 4.20 (dd, J=7.8, 3.3 Hz, 1H, 1-H), 3.82 (s, 3H, —OMe), 3.03 (q, J=7.0 Hz, 1H, 14-H), 2.90 (s, 1H, 3-OH), 2.89-2.80 (m, 4H, CH3 & 1-OH), 1.99 (d, J=12.8 Hz, 1H, 8-H), 1.29 (d, J=7.0 Hz, 3H, 16-CH3), 1.15 (s, 9H, tBu); 13C-NMR (100 MHz, CDCl3) δ 7.24, 27.61, 29.21, 32.01, 41.47, 48.35, 55.14, 63.70, 67.43, 73.40, 73.77, 73.96, 74.78, 83.15, 90.31, 98.01, 109.55, 114.45, 125.81, 130.13, 154.19, 160.10, 170.15, 170.52, 174.69; HRMS (FAB) calcd for C30H34O13N 616.2030, found 616.2056.
7-tert-Butylcarbamoyloxy-10-(4-methoxy-benzyloxy)-GC (45). To a solution of 43 (14.2 mg, 19.5 mmol) in THF (0.20 mL) was added t-BuNH2 (6.2 mL, 59 mmol, 3 equiv). The mixture was stirred for 40 min and then quenched with aq. HCl (1 mL, 1M), extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (30-70% EtOAc/hexanes) to obtain 45 as a white solid (10.0 mg, 77%). 1H-NMR (400 MHz, CDCl3) δ 7.41-7.35 (m, 2HAR), 6.99-6.91 (m, 2HAR), 6.01 (s, 1H, 12-H), 5.34 (d, J=9.3 Hz, 1H, benzyl), 5.26 (d, J=4.3 Hz, 1H, 6-H), 5.15 (dd, J=12.8, 4.2 Hz, 1H, 7-H), 4.94 (s, 1H, 10-H), 4.81 (s, 1H, —NH—), 4.69 (d, J=9.3 Hz, 1H, benzyl), 4.50 (d, J=7.9 Hz, 1H, 2-H), 4.20 (dd, J=7.9, 3.4 Hz, 1H, 1-H), 3.82 (s, 3H, —OMe), 3.03 (q, J=7.0 Hz, 1H, 14-H), 2.94 (s, 1H, 3-OH), 2.88 (d, J=3.4 Hz, 1H, 1-OH), 1.97 (d, J=12.8 Hz, 1H, 8-H), 1.35 (s, 9H, tBu), 1.29 (d, J=7.0 Hz, 3H, 16-CH3), 1.15 (s, 9H, tBu); 13C-NMR (75 MHz, CDCl3) δ 7.27, 28.79, 29.31, 32.08, 41.60, 48.60, 50.89, 55.30, 63.90, 67.61, 73.35, 73.62, 74.01, 75.02, 77.55, 83.42, 90.58, 98.34, 109.96, 114.76, 126.24, 130.63, 152.11, 160.56, 170.78, 171.18, 175.29; HRMS (FAB) calcd for C33H41O13N 659.2578, found 659.2582.
7-(Morpholine-4-carbonyloxy)-10-(4-methoxy-benzyloxy)-GC (46). To a solution of 43 (11.4 mg, 15.7 mmol) in THF (0.20 mL) was added morpholine (6.8 mL, 78 mmol, 5 equiv). The mixture was stirred for 25 min and then quenched with aq. HCl (1 mL, 1M), extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (30-70% EtOAc/hexanes) to obtain 46 as a white solid (9.5 mg, 90%). 1H-NMR (300 MHz, CDCl3) δ 7.36 (d, J=8.6 Hz, 2HAR), 6.95 (d, J=8.6 Hz, 2HAR), 6.02 (s, 1H, 12-H), 5.36 (d, J=9.3 Hz, 1H, benzyl), 5.30-5.19 (m, 2H, 6-H & 7-H), 4.95 (s, 1H, 10-H), 4.68 (d, J=9.3 Hz, 1H, benzyl), 4.48 (d, J=7.8 Hz, 1H, 2-H), 4.21 (d, J=7.8 Hz, 1H, 1-H), 3.82 (s, 3H, —OMe), 3.78-3.33 (m, 8H, morph.), 3.04 (q, J=7.0 Hz, 1H, 14-H), 2.85 (bs, 2H, 3-OH & 1-OH), 2.09-1.99 (m, 1H, 8-H), 1.30 (d, J=7.0 Hz, 3H, 16-CH3), 1.16 (s, 9H, tBu); 13C-NMR (100 MHz, CDCl3) δ 7.24, 29.23, 32.04, 41.45, 44.08 (morph.), 44.35 (morph.), 48.54, 55.14, 63.78, 66.29 (morph.), 66.36 (morph.), 67.3.8, 73.40, 73.76, 74.47, 74.76, 76.92, 83.15, 90.29, 98.04, 109.53, 114.45, 125.79, 130.13, 152.52, 160.11, 170.08, 170.48, 174.59; HRMS (FAB) calcd for C33H39O14N 673.2371, found 673.2402.
7-(Piperidine-carbonyloxy)-10-(4-methoxy-benzyloxy)-GC (47). To a solution of 43 (14.2 mg, 19.5 mmol) in THF (0.20 mL) was added piperidine (5.8 mL, 59 mmol, 3 equiv). The mixture was stirred for 30 min and then quenched with aq. HCl (1 mL, 1M), extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (30-50% EtOAc/hexanes) to obtain 47 as a white solid (11.3 mg, 86%). 1H-NMR (400 MHz, CDCl3) δ 7.36 (d, J=8.7 Hz, 2HAR), 6.94 (d, J=8.7 Hz, 2HAR), 6.01 (s, 1H, 12-H), 5.35 (d, J=9.3 Hz, 1H, benzyl), 5.29-5.20 (m, 2H, 6-H & 7-H), 4.95 (s, 1H, 10-H), 4.68 (d, J=9.3 Hz, 1H, benzyl), 4.48 (d, J=7.9 Hz, 1H, 2-H), 4.20 (dd, J=7.9, 3.4 Hz, 1H, 1-H), 3.81 (s, 3H, —OMe), 3.53-3.37 (m, 4H, piper.), 3.04 (q, J=7.0 Hz, 1H, 14-H), 2.95 (s, 1H, 3-OH), 2.86 (d, J=3.4 Hz, 1H, 1-OH), 2.03 (d, J=12.2 Hz, 1H, 8-H), 1.68-1.42 (m, 6H, piper.), 1.30 (d, J=7.0 Hz, 3H, 16-CH3), 1.16 (s, 9H, tBu); 13C-NMR (100 MHz, CDCl3) δ 7.51, 24.39 (piper.), 25.71 (piper.), 26.07 (piper.), 29.47, 32.27, 41.74, 45.25 (piper.), 48.80, 55.38, 64.03, 67.62, 73.60, 74.04, 74.44, 75.01, 77.41, 83.39, 90.59, 98.32, 109.85, 114.68, 126.09, 130.39, 152.74, 160.31, 170.47, 170.85, 174.96; HRMS (FAB) calcd for C34H40O13N 670.2500, found 670.2526.
1-Methylcarbamoyloxy-10-(4-methoxy-benzyloxy)-GC (49). To a solution of 24 (11.1 mg, 19 mmol) in CH2Cl2 (120 mL) and iPr2EtN (20.8 mL, 0.114 mmol, 6 equiv), was added solution of p-nitrophenyl chloroformate (12.0 mg, 57 mmol, 3 equiv) in CH2Cl2 (56 mL). The mixture was stirred for 15 min and MeNH2 (2M in THF, 30 mL, 57 mmol, 3 equiv) and THF (50 mL) were added. The mixture was then stirred for additional 25 min and then quenched with aq. HCl (1 mL, 1M), extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (50-100% EtOAc/hexanes) to obtain 49 as a white solid (8.2 mg, 67%). 1H-NMR (400 MHz, CDCl3, T=330K) δ 7.25-7.15 (m, 2HAR), 6.90-6.80 (m, 2HAR), 5.94 (s, 1H, 12-H), 5.53 (d, J=5.7 Hz, 1H, 1-H), 5.24 (d, J=11.1 Hz, 1H, benzyl), 5.18 (d, J=4.3 Hz, 1H, 6-H), 4.79 (s, 1H, 10-H), 4.69-4.54 (m, 2H, 2-H & benzyl), 4.39 (bs, 1H, —NH—), 4.32-4.21 (m, 1H, 7-H), 3.80 (s, 3H, —OMe), 3.19 (q, J=7.2 Hz, 1H, 14-H), 3.01 (s, 1H, 3-OH), 2.42 (bs, 3H, —CH3), 2.19 (d, J=10.8 Hz, 1H, 7-OH), 1.68 (d, J=12.3 Hz, 1H, 8-H), 1.30 (d, J=7.2 Hz, 3H, 16-CH3), 1.13 (s, 9H, tBu); 13C-NMR (75 MHz, CD3OD, T=320K) δ 9.42, 27.31, 29.63, 33.17, 42.59, 51.35, 55.86, 66.20, 68.12, 73.68, 76.12, 77.03, 77.21, 81.36, 85.44, 94.34, 102.23, 110.68, 114.84, 114.94, 130.42, 156.56, 161.06, 172.52, 172.80, 177.90; HRMS (FAB) calcd for C30H34O13N 616.2030, found 616.2028.
1-tert-Butylcarbamoyloxy-10-(4-methoxy-benzyloxy)-GC (50). To a solution of 24 (9.3 mg, 16.6 mmol) in CH2Cl2 (110 mL) and iPr2EtN (17.4 mL, 99 mmol, 6 equiv), was added solution of p-nitrophenyl chloroformate (10.0 mg, 50 mmol, 3 equiv) in CH2Cl2 (40 mL). The mixture was stirred for 15 min and tBuNH2 (17.4 mL, 166 mmol, 10 equiv) and THF (100 mL) were added. The mixture was then stirred for additional 12 h and then quenched with aq. HCl (1 mL, 1M), extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (40-100% EtOAc/hexanes) to obtain 50 as a white solid (6.6 mg, 60%). 1H-NMR (400 MHz, CDCl3) δ 7.23 (d, J=8.6 Hz, 2HAR), 6.86 (d, J=8.6 Hz, 2HAR), 5.91 (s, 1H, 12-H), 5.55 (d, J=4.6 Hz, 1H, 1-H), 5.21 (d, J=4.3 Hz, 1H, 6-H), 5.15 (d, J=11.8 Hz, 1H, benzyl), 4.82 (bd, J=11.8 Hz, 1H, benzyl), 4.75 (s, 1H, 10-H), 4.67 (d, J=5.0 Hz, 1H, 2-H), 4.62 (bs, 1H, —NH—), 4.30-4.19 (m, 1H, 7-H), 3.80 (s, 3H, —OMe), 3.23 (bs, 1H, 3-OH), 3.20 (q, J=7.3 Hz, 1H, 14-H), 2.17 (d, J=11.8 Hz, 1H, 7-OH), 1.63 (d, J=12.3 Hz, 1H, 8-H), 1.32 (d, J=7.3 Hz, 3H, 16-CH3), 1.19 (s, 9H, tBu), 1.01 (s, 9H, tBu); 13C-NMR (75 MHz, CDCl3) δ 8.68, 28.72, 29.02, 32.20, 41.06, 50.51, 50.92, 55.31, 65.29, 66.83, 72.16, 73.96, 75.27, 80.41, 84.41, 92.26, 100.34, 108.79, 113.88, 127.77, 130.14, 151.70, 159.72, 170.25, 171.01, 174.94; HRMS (FAB) calcd for C33H40O13N 658.2500, found 658.2516.
1-(Morpholine-4-carbonyloxy)-10-(4-methoxy-benzyloxy)-GC (51). To a solution of 24 (10.8 mg, 19.3 mmol) in CH2Cl2 (118 mL) and iPr2EtN (20.3 mL, 116 mmol, 6 equiv), was added solution of p-nitrophenyl chloroformate (11.7 mg, 58 mmol, 3 equiv) in CH2Cl2 (55 mL). The mixture was stirred for 15 min and morpholine (13.5 mL, 154 mmol, 8 equiv) and THF (100 mL) were added. The mixture was then stirred for additional 35 min and then quenched with aq. HCl (1 mL, 1M), extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (50-100% EtOAc/hexanes) to obtain 51 as a white solid (9.8 mg, 75%). 1H-NMR (400 MHz, CDCl3) δ 7.20 (d, J=8.5 Hz, 2HAR), 6.88 (d, J=8.5 Hz, 2HAR), 5.97 (s, 1H, 12-H), 5.57 (d, J=5.3 Hz, 1H, 1-H), 5.23 (d, J=11.0 Hz, 1H, benzyl), 5.11 (d, J=4.3 Hz, 1H, 6-H), 4.82 (s, 1H, 10-H), 4.71 (d, J=5.3 Hz, 1H, 2-H), 4.62 (d, J=11.0 Hz, 1H, benzyl), 4.32-4.21 (m, 1H, 7-H), 3.82 (s, 3H, —OMe), 3.57 (s, 1H, 3-OH), 3.54-3.33 (m, 5H, morph.), 3.31-3.18 (m, 1H, morph.), 3.21 (q, J=7.2 Hz, 1H, 14-H), 2.89-2.66 (m, 2H, morph.), 2.39 (d, J=11.7 Hz, 1H, 7-OH), 1.69 (d, J=12.3 Hz, 1H, 8-H), 1.32 (d, J=7.2 Hz, 3H, 16-CH3), 1.15 (s, 9H, tBu); 13C-NMR (75 MHz, CDCl3) δ 8.49, 29.14, 32.33, 41.16, 43.84 (morph.), 50.41, 55.41, 65.12, 65.94 (morph.), 66.21 (morph.), 66.68, 72.83, 74.97, 75.54, 76.28, 80.20, 84.25, 92.15, 100.08, 108.99, 113.97, 128.01, 129.64, 152.38, 159.75, 170.21, 170.66, 175.06; HRMS (FAB) calcd for C33H38O14N 672.2292, found 672.2307.
1-(Piperidine-carbonyloxy)-10-(4-methoxy-benzyloxy)-GC (52). To a solution of 24 (10.9 mg, 19.4 mmol) in CH2Cl2 (119 mL) and iPr2EtN (20.3 mL, 116 mmol, 6 equiv), was added solution of p-nitrophenyl chloroformate (11.7 mg, 58 mmol, 3 equiv) in CH2Cl2 (55 mL). The mixture was stirred for 15 min and piperidine (15.4 mL, 155 mmol, 8 equiv) and THF (100 ml) were added. The mixture was then stirred for additional 30 min and then quenched with aq. HCl (1 mL, 1M), extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (40-70% EtOAc/hexanes) to obtain 52 as a white solid (6.9 mg, 53%). 1H-NMR (400 MHz, CDCl3) δ 7.20 (d, J=8.6 Hz, 2HAR), 6.87 (d, J=8.6 Hz, 2HAR), 5.95 (s, 1H, 12-H), 5.54 (d, J=5.0 Hz, 1H, 1-H), 5.22 (d, J=11.0 Hz, 1H, benzyl), 5.10 (d, J=4.4 Hz, 1H, 6-H), 4.80 (s, 1H, 10-H), 4.72 (d, J=5.0 Hz, 1H, 2-H), 4.64 (d, J=11.0 Hz, 1H, benzyl), 4.32-4.22 (m, 1H, 7-H), 3.81 (s, 3H, —OMe), 3.55 (s, 1H, 3-OH), 3.52-3.40 (m, 1H, piper.), 3.32-3.19 (m, 1H, piper.), 3.22 (q, J=7.3 Hz, 1H, 14-H), 2.77-2.56 (m, 2H, piper.), 2.25 (d, J=12.0 Hz, 1H, 7-OH), 1.68 (d, J=12.3 Hz, 1H, 8-H), 1.63-1.50 (m, 1H, piper.), 1.49-1.29 (m, 5H, piper.), 1.32 (d, J=7.3 Hz, 3H, 16-CH3), 1.14 (s, 9H, tBu); 13C-NMR (75 MHz, CDCl3) δ 8.68, 23.98 (piper.), 25.09 (piper.), 25.56 (piper.), 29.15, 32.31, 41.11, 44.62 (piper.), 44.84 (piper.), 50.53, 55.36, 65.25, 66.95, 72.68, 74.77, 75.56, 76.45, 80.38, 84.36, 92.18, 100.36, 108.86, 113.88, 128.07, 129.89, 152.37, 159.63, 170.20, 170.81, 174.88; HRMS (FAB) calcd for C34H40O13N 670.2500, found 670.2510.
7-Methylcarbamoyloxy-GC (53). To a solution of 44 (5.3 mg, 8.7 mmol) in acetonitrile (108 mL) and CHCl3 (35 mL) was added aq. solution of (NH4)2Ce(NO3)6 (10.6 mL, 17.3 mmol, 2 equiv, 1.63M). The mixture was stirred for 1.5 h, aq. solution of (NH4)2Ce(NO3)6 (5.3 mL, 8.7 mmol, 1 equiv, 1.63M) was added and mixture was further stirred for 12 h. Volatiles were removed under reduced pressure and residue was purified by flash chromatography (40-99% EtOAc/1% AcOH/hexanes) to obtain 53 as a white powder (3.0 mg, 69%). 1H-NMR (300 MHz, 315K, CD3OD) δ 6.11 (s, 1H, 12-H), 5.32 (d, J=4.3 Hz, 1H, 6-H), 5.18 (dd, J=12.8, 4.3 Hz, 1H, 7-H), 5.13 (s, 1H, 10-H), 4.54 (d, J=7.6 Hz, 1H, 2-H), 4.18 (d, J=7.6 Hz, 1H, 1-H), 3.00 (q, J=7.0 Hz, 1H, 14-H), 2.74 (s, 3H, CH3), 2.00 (d, J=12.9 Hz, 1H, 8-H), 1.22 (d, J=7.0 Hz, 3H, 16-CH3), 1.14 (s, 9H, tBu); HSQC correlation data in the Supporting Information; HRMS (FAB) calcd for C22H28O12N 498.1612, found 498.1633.
7-tert-Butylcarbamoyloxy-GC (54). To a solution of 45 (5.7 mg, 8.6 mmol) in acetonitrile (108 mL) and CHCl3 (35 mL) was added aq. solution of (NH4)2Ce(NO3)6 (10.6 mL, 17.3 mmol, 2 equiv, 1.63M). The mixture was stirred for 1.5 h, aq. solution of (NH4)2Ce(NO3)6 (5.3 mL, 8.7 mmol, 1 equiv, 1.63M) was added and mixture was further stirred for 12 h. Volatiles were removed under reduced pressure and residue was purified by flash chromatography (40-70% EtOAc/1% AcOH/hexanes) to obtain 54 as a white powder (3.7 mg, 80%). 1H-NMR (400 MHz, CDCl3) δ 6.01 (bs, 1H, OH), 5.98 (s, 1H, 12-H), 5.42 (m, 1H, 6-H), 5.16 (dd, J=12.8, 4.1 Hz, 1H, 7-H), 5:14 (s, 1H, 10-H), 4.96 (s, 1H, —NH—), 4.69 (d, J=7.9 Hz, 1H, 2-H), 4.32 (bd, J=7.9 Hz, 1H, 1-H), 4.23 (bs, 2H, OH), 3.14 (bq, J=7.1 Hz, 1H, 14-H), 1.98 (d, J=12.8 Hz, 1H, 8-H), 1.31 (s, 9H, tBu), 1.27 (d, J=7.1 Hz, 3H, 16-CH3), 1.10 (s, 9H, tBu); HSQC correlation data in the Supporting Information; HRMS (FAB) calcd for C25H34O12N 540.2081, found 540.2098.
7-(Morpholine-4-carbonyloxy)-GC (55). To a solution of 46 (5.8 mg, 8.7 mmol) in acetonitrile (108 mL) and CHCl3 (35 mL) was added aq. solution of (NH4)2Ce(NO3)6 (10.6 mL, 17.3 mmol, 2 equiv, 1.63M). The mixture was stirred for 1.5 h, aq. solution of (NH4)2Ce(NO3)6 (5.3 mL, 8.7 mmol, 1 equiv, 1.63M) was added and mixture was further stirred for 12 h. Volatiles were removed under reduced pressure and residue was purified by flash chromatography (50-99% EtOAc/1% AcOH/hexanes) to obtain 55 as a white powder (2.9 mg, 60%). 1H-NMR (400 MHz, CD3OD) δ 6.14 (s, 1H, 12-H), 5.34 (d, J=4.4 Hz, 1H, 6-H), 5.24 (dd, J=12.8, 4.4 Hz, 1H, 7-H), 5.15 (s, 1H, 10-H), 4.54 (d, J=7.6 Hz, 1H, 2-H), 4.18 (d, J=7.6 Hz, 1H; 1-H), 3.73 -3.37 (m, 8H, morph.), 3.00 (q, J=7.1 Hz, 1H, 14-H), 2.06 (d, J=12.8 Hz, 1H, 8-H), 1.22 (d, J=7.1 Hz, 3H, 16-CH3), 1.15 (s, 9H, tBu); HSQC correlation data in the Supporting Information; HRMS (FAB) calcd for C25H32O13N 554.1874, found 554.1874.
7-(Piperidine-carbonyloxy)-GC (56). To a solution of 47 (5.8 mg, 8.6 mmol) in acetonitrile (108 mL) and CHCl3 (35 mL) was added aq. solution of (NH4)2Ce(NO3)6 (10.6 mL, 17.3 mmol, 2 equiv, 1.63M). The mixture was stirred for 1.5 h, aq. solution of (NH4)2Ce(NO3)6 (5.3 mL, 8.7 mmol, 1 equiv, 1.63M) was added and mixture was further stirred for 12 h. Volatiles were removed under reduced pressure and residue was purified by flash chromatography (40-70% EtOAc/1% AcOH/hexanes) to obtain 56 as a white powder (3.0 mg, 64%). 1H-NMR (400 MHz, CDCl3) δ 6.21 (bs, 1H, OH), 5.98 (s, 1H, 12-H), 5.41 (m, 1H, 6-H), 5.25 (dd, J=12.8, 4.3 Hz, 1H, 7-H), 5.14 (s, 1H, 10-H), 4.68 (d, J=7.9 Hz, 1H, 2-H), 4.36 (bs, 1H, OH), 4.32 (bd, J=7.9 Hz, 1H, 1-H), 3.55-3.26 (m, 4H, piper.), 3.15 (m, 1H, 14-H), 2.04 (d, J=12.8 Hz, 1H, 8-H), 1.75-1.40 (m, 6H, piper.), 1.26 (d, J=7.0 Hz, 3H, 16-CH3), 1.11 (s, 9H, tBu); HSQC HSQC correlation data in the Supporting Information; HRMS (FAB) calcd for C26H34O12N 552.2081, found 552.2067.
1-Methylcarbamoyloxy-GC (57). To a solution of 49 (3.5 mg, 5.6 mmol) in acetonitrile (70 mL) and CHCl3 (23 mL) was added aq. solution of (NH4)2Ce(NO3)6 (6.9 mL, 11.2 mmol, 2 equiv, 1.63M). The mixture was stirred for 6.5 h, aq. solution of (NH4)2Ce(NO3)6 (6.9 mL, 11.2 mmol, 2 equiv, 1.63M) was added and the mixture was further stirred for 12 h. Volatiles were removed under reduced pressure and the residue was purified by flash chromatography (70-99% EtOAc/1% AcOH/hexanes) to obtain 57 as a white powder (2.4 mg, 86%). 1H-NMR (400 MHz, CD3OD) δ 6.06 (s, 1H, 12-H), 5.46 (d, J=5.3 Hz, 1H, 1-H), 5.22 (d, J=4.1 Hz, 1H, 6-H), 5.07 (s, 1H, 10-H), 4.66 (d, J=5.5 Hz, 1H, 2-H), 4.31 (dd, J=12.6, 4.1 Hz, 1H, 7-H), 3.10 (q, J=7.2 Hz, 1H, 14-H), 2.83-2.68 (m, 3H, —CH3), 1.78 (d, J=12.5 Hz, 1H, 8-H), 1.27 (d, J=7.2 Hz, 3H, 16-CH3), 1.22 (s, 9H, tBu); HSQC correlation data in the Supporting Information; HRMS (FAB) calcd for C22H28O12N 498.1612, found 498.1618.
1-tert-Butylcarbamoyloxy-GC (58). To a solution of 50 (3.7 mg, 5.6 mmol) in acetonitrile (70 mL) and CHCl3 (23 mL) was added aq. solution of (NH4)2Ce(NO3)6 (6.9 mL, 11.2 mmol, 2 equiv, 1.63M). The mixture was stirred for 6 h, aq. solution of (NH4)2Ce(NO3)6 (6.9 mL, 11.2 mmol, 2 equiv, 1.63M) was added and the mixture was further stirred for 12 h. Volatiles were removed under reduced pressure and the residue was purified by flash chromatography (40-70% EtOAc/hexanes) to obtain 58 as a white powder (2.5 mg, 84%). 1H-NMR (400 MHz, CD3OD) δ 6.05 (s, 1H, 12-H), 5.45 (d, J=5.1 Hz, 1H, 1-H), 5.28 (d, J=4.2 Hz, 1H, 6-H), 5.06 (s, 1H, 10-H), 4.64 (d, J=5.1 Hz, 1H, 2-H), 4.33 (dd, J=12.4, 4.2 Hz, 1H, 7-H), 3.10 (q, J=7.3 Hz, 1H, 14-H), 1.77 (d, J=12.4 Hz, 1H, 8-H), 1.34 (s, 9H, tBu), 1.27 (d, J=7.3 Hz, 3H, 16-CH3), 1.22 (s, 9H, tBu); HSQC correlation data in the Supporting Information; HRMS (FAB) calcd for C25H34O12N 540.2081, found 540.2075.
1-(Morpholine-4-carbonyloxy)-GC (59). To a solution of 51 (6.5 mg, 9.6 mmol) in acetonitrile (120 mL) and CHCl3 (39 mL) was added aq. solution of (NH4)2Ce(NO3)6 (11.8 mL, 19.2 mmol, 2 equiv, 1.63M). The mixture was stirred for 6.5 h, aq. solution of (NH4)2Ce(NO3)6 (11.8 mL, 19.2 mmol, 2 equiv, 1.63M) was added and the mixture was further stirred for 12 h. Volatiles were removed under reduced pressure and the residue was purified by flash chromatography (70-99% EtOAc/1% AcOH/hexanes) to obtain 59 as a white powder (4.3 mg, 80%). 1H-NMR (400 MHz, CD3OD) δ 6.10 (s, 1H, 12-H), 5.39 (d, J=6.6 Hz, 1H, 1-H), 5.22 (d, J=4.2 Hz, 1H, 6-H), 5.09 (s, 1H, 10-H), 4.74 (d, J=6.6 Hz, 1H, 2-H), 4.28 (dd, J=12.4, 4.2 Hz, 1H, 7-H), 3.76-3.62 (m, 4H, morph.), 3.58-3.38 (m, 4H, morph.), 3.12 (q, J=7.1 Hz, 1H, 14-H), 1.79 (d, J=12.4 Hz, 1H, 8-H), 1.28 (d, J=7.1 Hz, 3H, 16-CH3), 1.22 (s, 9H, tBu); HSQC correlation data in the Supporting Information; HRMS (FAB) calcd for C25H32O13N 554.1874, found 554.1874.
1-(Piperidine-carbonyloxy)-GC (60). To a solution of 52 (3.8 mg, 5.6 mmol) in acetonitrile (70 mL) and CHCl3 (23 mL) was added aq. solution of (NH4)2Ce(NO3)6 (6.9 mL, 11.2 mmol, 2 equiv, 1.63M). The mixture was stirred for 6.5 h, aq. solution of (NH4)2Ce(NO3)6 (6.9 mL, 11.2 mmol, 2 equiv, 1.63M) was added and the mixture was further stirred for 12 h. Volatiles were removed under reduced pressure and the residue was purified by flash chromatography (50-100% EtOAc/hexanes) to obtain 60 as a white powder (2.5 mg, 79%). 1H-NMR (400 MHz, CD3OD) δ 6.09 (s, 1H, 12-H), 5.40 (d, J=6.4 Hz, 1H, 1-H), 5.22 (d, J=4.2 Hz, 1H, 6-H), 5.08 (s, 1H, 10-H), 4.71 (d, J=6.4 Hz, 1H, 2-H), 4.32 (dd, J=12.4, 4.2 Hz, 1H, 7-H), 3.68-3.50 (m, 2H, piper.), 3.46-3.20 (m, 2H, piper.), 3.12 (q, J=7.2 Hz, 1H, 14-H), 1.79 (d, J=12.4 Hz, 1H, 8-H), 1.75-1.52 (m, 6H, piper.), 1.28 (d, J=7.2 Hz, 3H, 16-CH3), 1.22 (s, 9H, tBu); HSQC correlation data in the Supporting Information; HRMS (FAB) calcd for C26H34O12N 552.2081, found 552.2061.
10-benzyloxy-isoGC-1,6-carbonate (61): To a solution of 16 (9.6 mg, 18.2 mol) in CH2Cl2 (113 □L) and iPr2EtN (19.1 L, 0.109 mmol, 6 equiv), was added solution of p-nitrophenyl chloroformate (11.0 mg, 54.6 □mol, 3 equiv) in CH2Cl2 (50 L). The mixture was stirred for 15 min and then quenched with aq. HCl (1 mL, 1M), extracted with EtOAc (3×) and dried with MgSO4. The crude product was purified by flash chromatography (30-50% EtOAc/hexanes) to obtain 61 as a white solid (9.3 mg, 92% yield, 82% purity). 1H NMR (300 MHz, CDCl3): δ=7.45-7.28 (m, 5HAR), 5.82 (s, 1H), 5.63 (s, 1H), 5.49 (d, J=10.6 Hz, 1H), 5.05 (s, 1H), 4.88 (d, J=4.0 Hz, 1H), 4.79 (d, J=10.6 Hz, 1H), 4.76 (d, J=4.0 Hz, 1H), 4.73 (s, 1H), 3.26 (q, J=7.8 Hz, 1H), 2.96 (s, 1H), 2.28 (s, 1H), 1.35 (d, J=7.8 Hz, 3H), 1.11 (s, 9H, tBu). MS (FAB) calcd for C28H29O12 557.17, found 557.56.
Biological Testing
Functional glycine receptor assay. Pharmacological activities of the compounds were evaluated in a FLIPR® Membrane Potential Assay (Molecular Deives, Crawley, UK) using a HEK293 cell line stably expressing the human homomeric α1 GlyR.53 Briefly, α1 GlyR-HEK293 cells were split into poly-D-lysine-coated 96-well black Opti-plates (Packard). 16-24 h later, the medium was aspirated, and 100 mL KREBS Buffer [140 mM NaCl/4.7 mM KCl/2.5 mM CaCl2/1.2 mM MgCl2/10 mM HEPES/11 mM D-Glucose, pH 7.4] supplemented with FLIPR® Membrane Potential Assay loading dye and various concentrations of the test compounds was added to the wells. The plate was incubated at 37° C. in a humidified 5% CO2 incubator for 30 min and assayed in a NOVOstarTM (BMG Labtechnologies, Offenburg, Germany) measuring emission at 560 nm caused by a 530 nm excitation. Fluorescence measurements were performed immediately before and up to 1 min. after addition of glycine (final concentration 100 mM) to the wells. The experiments were performed in duplicate at least three times for each compound. Percentage inhibition was calculated as: (Responseglycine−Responsetest cmpd+glycine)/Responseglycine. Inhibition below 20% is characterized as no inhibition (NI).
Results
Synthesis
Ginkgolide C (GC, 3, FIG. 1) was chosen as the starting material as it carries the most hydroxyl groups of the ginkgolides, thus providing possibilities for a more diverse library. Secondly, GC (3), together with GB (2), is the most potent ginkgolide GlyR antagonists (Stromgaard, 2003).
Solid-phase synthesis. The use of solid-phase synthesis (SPS) for the preparation of ginkgolide derivatives appeared to be particularly attractive, as the lack of chromophores renders them difficult to handle in solution. Moreover, the presence of multiple hydroxyl groups provides several possibilities of solid-phase attachment. GC (3) has four different hydroxyl groups, three secondary and one tertiary, which exhibit different reactivities and can therefore be functionalized selectively. In general, 10-OH is the most reactive for alkylation and esterification (Corey, 1992). However, acid catalyzed esterification (Jaracz, 2002), as well as reaction of GC (3) with alkyl and aryl sulfonyl halides takes place at 7-OH (Cazaux, 1995; Vogensen, 2003) a selective silylation at 1-OH has also been described (Weingers, 1991).
Initially, attempts were made to use a silicon linker, which could be attached selectively to 1-OH of GC (3), followed by selective derivatizations of 10-OH, 7-OH, and if possible 3-OH. In a previous example of selective derivatization of 1-OH, the bulky tert-butyldiphenylsilyl was used as a protection group, which however prevented further derivatization at 10-OH (Weinges, 1991). Hence, it was envisaged that decreasing the bulk of the silicon linker could allow further derivatization of 10-OH. Three silicon linkers with variation in the steric bulk at silicon were synthesized according to known procedures (Chan, 1985; Randolph, 1995). Interestingly, isopropyl substituents on the silicon linker as in entry 1 (Table 1) reacted selectively at 1-OH, while decreasing the size of the substituents led to preferential reaction at 7-OH. These linkers, however, were unstable as evidenced by leaching from the resin, and no further studies were pursued.







TABLE 1







Attachment of GC to silyl resins; mode of attachment.













 












loadinga
mode of


entry
linker
(mmol/g)
attachmentb













1




0.85
1-OH


 


2




0.52
1-OH/7-OH (2:1)


 


3




0.35
7-OH


 


4c




1.5
—





aLoading determined by attachment of Disperse Red, cleavage and spectrophotometric measurements.


bMode of attachment determined by acetylation, cleavage and 1H-NMR spectra of cleaved compounds. Leaching detected with linkers in entries 2-4.


cPrepared from commercial DES resin.






Alternatively, the use of a sulfonyl linkage was studied by anchoring GC (3) through 7-OH, followed by selective derivatization at 10-OH and/or 1-OH and nucleophilic cleavage to provide derivatives of GB (2), and GC (3) with an inverted C-7 configuration. Therefore, a pipsyl moiety was introduced at 7-OH of GC (3) to form 7-O-pipsyl-GC (6) (Scheme 1), which was then attached to a TentaGel-NH2 resin using Pd-catalyzed carbonylation, similar to that described by Takahashi et al (Takahashi, 2001) to give 7 (Scheme 1).




Scheme 1. Attachment of ginkgolide C to solid-phase via sulfonyl linker.
However, treatment of 7 with nucleophiles such as tetrabutyl ammonium fluoride (TBAF) or tetrabutyl ammonium hydride (TBAH) did not release any ginkgolide derivatives. In addition, the solution-phase version of the same displacement of 7-O-pipsyl-GC (6) gave none of the desired products either, thus suggesting that the 7-O-sulfonyl group is not a sufficiently good leaving group as opposed to a trifoliate group (Vogensen, 2003). Attachment of GC through 1-OH or 3-OH using other methods did not lead to success probably due to steric hindrance. In addition, substituents at 3-OH readily undergo elimination leading to a thermodynamically favored a,b unsaturated lactone F (Weinges, 1987).
Instead of direct attachment of GC (3) to the solid-phase, attachment via a building block was considered. Ideally, such a building block should function as a multidetachable linker, so that depending on cleavage conditions, ginkgolide derivatives with or without the first building block could be generated. Since GC (3) can be selectively and efficiently benzylated at 10-OH with benzyl bromide under mild conditions in solution (Vogensen, 2003), a benzyl group could serve as the first building block, followed by derivatization of 7-OH, translactonization to form C-6 iso-GC derivatives, removal of 7-OH to reroute into GB derivatives, and if possible functionalize 1-OH and 3-OH, as well. Immobilization of benzyl bromide was investigated in various manners. Initially 4-(bromomethyl)-phenylacetic acid was anchored to the Wang resin and Rink amide resin to give 8 and 9, respectively (Scheme 2). The immobilized benzyl bromides were then reacted with GC (3) to give resins 10 and 11. Cleavage with TFA produced GC derivatives 12 and 13, respectively, which revealed that GC (3) was attached at 10-OH with a high selectivity. The purity of the cleaved products was >90% as determined by 1H-NMR. Benzyl bromide can also be immobilized to a silicon linker (Plunkett, 1995) representing a traceless linkage, which was also investigated. Resin 14 was synthesized in three steps from bromopolystyrene and 4-(allyldimethylsilyl)benzyl alcohol (Lee, 2001; Iglesias, 2001) followed by reaction with GC (3) to provide resin-bound 10-0-benzyl-GC 15 (Scheme 2). Cleavage with TFA provided 16 in 85% purity.







Scheme 2. Attachment of ginkgolide C to a solid-phase via 10 -O-benzyl using either Wang resin, Rink amide resin, or traceless silicon linker.
The benzylated Wang resin was selected for further investigation as it provided sufficiently pure products. Selective hydroxyl group functionalizations were studied and compared with similar reactions in solution. Treatment of resin-bound ginkgolide 10 with benzoic anhydride and Hünig's base resulted in a mixture of iso-GC benzoates and unreacted starting material (Scheme 3). On the other hand, solution-phase reaction of 10-O-benzyl-GC (16) under similar conditions produced iso-GC monobenzoate 17 as the major product.
Acetylation of resin 10 with Ac2O in the presence of Hünig's base provided iso-GC-bisacetate 18 in a purity of only ca. 60%, while the same reaction in solution provided iso-GC-bisacetate 19 in 90% purity (FIG. 2). Similarly, several other reactions on solid-phase such as transformation of GC into GB (Cazaux, 1995; Weinges, 1991) either failed entirely, or proceeded with very low conversion and purity, as compared to the same reaction in solution. It became clear that a solid-phase strategy is not suited for the preparation of a ginkgolide derivative library.




Scheme 3. Comparison of ginkgolide C benzoylation using solid-phase or solution-phase chemistry.
Solution-Phase Synthesis. Because of the limitations in solid-phase strategy, further derivatizations were performed by solution-phase chemistry. 10-O-benzyl-GC (16) turned out to be a suitable precursor for a wide variety of transformations (Scheme 4). GC-7-benzoate (21) was prepared from 16 by reaction with benzoyl chloride and pyridine in CH2Cl2 in 79% yield. Iso-GC-6-benzoate 17 was efficiently formed (77% yield) by exchanging pyridine with Hünig's base. This remarkable difference in reaction pattern is associated with stabilization of a translactonized anion intermediate by hydrogen bonding with 3-OH as reported previously (Jaracz, 2002). Iso-GC-1,6-bisbenzoate 22 was also prepared in 64% yield by further increasing the amount of benzoylation reagent and prolonging the reaction time. Heating of benzoate 17 with Hünig's base in DMF to 100° C. for 1.5 h lead to a remarkable migration of benzoate from C-6 to C-1 with subsequent translactonization of the iso-GC skeleton back to the GC skeleton to form 23 (78% yield, Scheme 4). The driving force for this rearrangement is the greater thermodynamic stability of the original ginkgolide core. Namely, the bulky tert-butyl group is axial in iso-GC while it is pseudo-equatorial in GC and the 6-membered ring E in iso-GC adopts a boat conformation.




Scheme 4. Synthesis of ginkolide C benzoates. PMB =p-methoxybenzyl.
According to previously established methods23,43 10-(4-methoxy-benzyloxy)-GC (24) and 10-(2-pyridinyl-methoxy)-GC (25) were prepared from GC in 94% and 97% yields, respectively (Scheme 4). Benzoate derivatives 26, 27, 28 and 29 were prepared from 24 and 25 using the same conditions as described above for 16 (Scheme 4). The ester derivatives prepared from 24 were consequently subjected to oxidative removal of the p-methoxybenzyl (PMB) group by cerium(IV) ammonium nitrate (CAN) to obtain derivatives 30, 31 and 32 (60-81% yield, FIG. 3). Milder oxidation reagents, such as dichlorodicyano quinone (DDQ) did not cleave the PMB group.
Introduction of carbamate moieties into the various hydroxyl groups of ginkgolides was next investigated since such derivatives would yield important information for SAR studies. Carbamates at C-7 were efficiently prepared from 16 in two steps (Scheme 5) by reaction with p-nitrophenylchloroformate and pyridine in CH2Cl2 to form 33 in 61% yield; carbonate 33 was further reacted with various aliphatic amines in THF to yield carbamates 34-37 (81-92% yield, Table 2).




Scheme 5. Synthesis of C-7 carbamates from 10-alkyl substituted GC.PMB =p-methoxybenzyl.
Again, a remarkable difference was observed when Hünig's base was used instead of pyridine (Scheme 6).




Scheme 6. Synthesis of C-1 carbamates from 10-alkyl substituted GC, Showing migratory mechanism. PMB=p-methoxybenzyl.
It is plausible that a highly reactive iso-GC 6-(p-nitrophenyl)carbonate is formed first, which reacts further with 1-OH as in the case of the migration of benzoate in 23. The product of this reaction, however, is cyclic carbonate 38 with a bridge between C-1 and C-6. This compound is unstable and hydrolyzes easily even on silica gel to form 16. Treatment of 38 with various aliphatic amines in THF provided facile opening of the carbonate bridge to form carbamates at C-1 with consequent translactonization of ring E into the native form, 39-42. Even bulky amines such as tert-butylamine reacted to completion in both cases forming 35 and 40, i.e., ginkgolides with two tert-butyl groups. All three reactions are performed in one pot to obtain the desired C-1 products in 50-78% yield from 16 after a single chromatography (Table 2). The corresponding sets of carbamates 44-47 (C-7) and 49-52 (C-1) were prepared from 24 (Schemes 5 and 6, Table 2), while removal of the PMB group afforded the nonaromatic ginkgolide carbamates 53-60 (Table 3).
Biological Activity
The ginkgolide derivatives were investigated for their abilities to antagonize glycine-induced responses from homomeric al GlyRs. In brief, homomeric al GlyR was stably expressed in HEK293 cells and a fluorescence-based membrane-potential kit was used to detect receptor activity. 53 The decrease in glycine response was measured by pre-incubation of the test compound prior to addition of 100 mM glycine. GC (3) was used as the reference compound. At a concentration of 10 mM GC (3) the response induced by glycine was inhibited by 97%. Due to the relatively low potency of all derivatives, they were investigated at a concentration of 100 mM for determination of the %-inhibition (Tables 2-5). However, none of the ginkgolide derivatives tested were more potent than the parent compound GC (3), and it follows that any modification of the hydroxyl groups leads to a significant decrease in biological activity. Thus, in the discussion below it should be noted that the relative differences between these derivatives are only minor since all compounds are only weak antagonists of the GlyR.







TABLE 2







Chemical yields and biological activities of C-7 and C-1 carbamates


of 10-alkyl-GC.


 








 

















%


compound
R
R1
R2
yield (%)
inhibitiona





34
Bn
H
CONHMe
88b
NI


35
Bn
H
CONHtBu
88b
29


36
Bn
H
CONC4H8O
92b
NI


37
Bn
H
CONC5H10
81b
NI


39
Bn
CONHMe
H
64c
NI


40
Bn
CONHtBu
H
59c
42


41
Bn
CONC4H8O
H
78c
NI


42
Bn
CONC5H10
H
50c
31


44
PMB
H
CONHMe
74d
NI


45
PMB
H
CONHtBu
77d
39


46
PMB
H
CONC4H8O
90d
NI


47
PMB
H
CONC5H10
86d
29


49
PMB
CONHMe
H
67e
NI


50
PMB
CONHtBu
H
60e
33


51
PMB
CONC4H8O
H
75e
NI


52
PMB
CONC5H10
H
53e
NI





aInhibition of 100 μM glycine-induced reponse by 100 μM of test compound; %-inhibition was calculated as: (Responseglycine − Responsetest cmpd+glycine)/Responseglycine. Values are means of three independent experiments preformed in duplicate.


bfrom 33.


cfrom 16.


dfrom 43.


efrom 24. NI = no inhibition, i.e., inhibition below 20%. PMB = p-methoxybenzyl.






Ginkgolide derivatives with carbamates in position 1 or 7, and a benzyl or PMB group in the 10-position were either inactive at the concentrations tested, or only moderately active (Table 2). Interestingly, all derivatives with moderate activity contained a tert-butyl carbamate, and the presence of this group, rather than the position seems to be of importance for biological activity in this set of derivatives tested. For the corresponding carbamates without the benzyl or PMB group it appears as if both the position and the nature of the substituent are of importance (Table 3): Compounds 57 and 58 show a ca. 70% inhibition of the glycine responses, whereas other derivatives are either inactive or only moderately active. It is interesting to compare the activities of 57 with those of 39 and 49, the only difference being the benzyl substitution at 10-OH, thus indicating that the latter is detrimental for activity at GlyRs. Moreover, the activities of 57 and 58 compared to those of 53 and 54 show that substitution at 1-OH is preferred over 7-OH, while comparison with 59 and 60 indicates that there might be limitation to the size of the substituent that is tolerated.







TABLE 3







Chemical yields and biological activities of C-7 and C-1 carbamates


of ginkgolide C.


 













 













 















%


compound
R1
R2
yield (%)
inhibitionb





53
H
CONHMe
69
NI


54
H
CONHtBu
80
NI


55
H
CONC4H8O
60
44


56
H
CONC5H10
64
34


57
CONHMe
H
86
76


58
CONHtBu
H
84
71


59
CONC4H8O
H
80
NI


60
CONC5H10
H
79
33





a (NH4)2Ce(NO3)6, MeCN/H2O/CHCl3. PMB = p-methoxybenzyl


aInhibition of 100 μM glycine-induced response by 100 μM of test compound; %-inhibition was calculated as: (Responseglycine − Responsetest cmpd+glycine)/Responseglycine. Values are means of three independent experiments performed in duplicate. NI = no inhibition, i.e., inhibition below 20%. PMB = p-methoxybenzyl.






The benzylated or benzoylated derivatives in Table 4 show a certain degree of variation in activities. The most potent derivatives are those with only one benzyl substituent in 10-OH, such as compounds 16, 24 and 25, and particularly 25 with a 2-picolyl substituent exhibits a 75% inhibition. Addition of benzoyl groups do not improve activity, while the presence of only benzoyl group, such as 30 and 32 leads to a complete loss in activity (Table 4).







TABLE 4







Biological activities of ginkgolide derivatives.


 








 











compound
R
R1
R2
% inhibitiona





12
MeO2CCH2C6H4CH2
H
H
NT


13
NH2COCH2C6H4CH2
H
H
NT


16
Bn
H
H
59


21
Bn
H
Bz
45


23
Bn
Bz
H
60


24
PMB
H
H
52


25
2-picolyl
H
H
75


26
PMB
H
Bz
35


28
PMB
Bz
H
NI


29
2-picolyl
H
Bz
51


30
H
H
Bz
NI


32
H
Bz
H
NI





aInhibition of 100 μM glycine-induced response by 100 μM of test compound; %-inhibition was calculated as: (Responseglycine − Responsetest cmpd+glycine)/Responseglycine. Values are means of three independent experiments performed in duplicate. NT = no tested. NI = no inhibition, i.e., inhibition below 20%. PMB = p-methoxybenzyl.






Finally a range of iso-ginkgolide derivatives were investigated, with some of them showing reasonable potencies (Table 5). Particularly compound 17, with a 90% inhibition in glycine responses is the most potent of the 43 derivatives, although still considerably less potent than GC (3), a 30 uM concentration of 17 giving only a 39% inhibition. However, rearrangement of the ginkgolide skeleton, as in the iso-ginkgolide derivatives, seems to be an acceptable modification for GlyR inhibition.







TABLE 5







Biological activities of iso-ginkgolide derivatives.


 








 











compound
R
R1
R2
% inhibitiona





17
Bn
H
Bz
90


18
MeO2CCH2C6H4CH2
Ac
Ac
NT


19
Bn
Ac
Ac
21


20
PMB
Ac
Ac
36


22
Bn
Bz
Bz
42


27
PMB
H
Bz
49


31
H
H
Bz
63





aInhibition of response induced by 100 μM glycine by 100 μM test compound; %-inhibition was calculated as: (Responseglycine − Responsetest cmpd+glycine)/Responseglycine. Values are means of three independent experiments performed in duplicate. NT = not tested. PMB = p-methoxybenzyl.






Discussion
We have shown that the solid-phase reactions of ginkgolides differ from those in solution; particularly, selectivity is greatly diminished. Although the preparation of linear peptides, oligosaccharides and nucleotides are performed routinely by the solid-phase method, the branched sugar analogs have to be prepared in solution using extensive protection/deprotection strategy (Plante, 2001; Tanaka, 2002; Ning, 2002). We have shown that ginkgolide C in solution can be selectively transformed into various derivatives via rearrangement and migration. Since hydroxyl groups that are unchanged in the product, e.g. 3-OH and 7-OH, participate during the reaction process their protection would presumably hamper the entire process. Thus the role of 7-OH in reactions described here is fundamental.
The synthesized ginkgolide derivatives were investigated as GlyR antagonists using a high-throughput assay, hence providing the first SAR study of ginkgolides and GlyR. In general, the ginkgolide derivatives prepared were less potent than the parent compound, GC (3) and in many cases the derivatives were totally devoid of activity. These results indicate that substitutions of the hydroxyl groups in ginkgolides are not beneficial for antagonistic activity at GlyR, and clearly other means of improving the activity of the parent compounds will be required in the future. In addition, these results show that the structure-activity relationships differ significantly from those observed with PAFR, as best exemplified by the iso-ginkgolide derivatives. When tested against PAFR, iso-GC acetates were devoid of activity;41 however, in the present studies, although much less potent than the parent GC 3, the iso-ginkgolide derivative 17 was the most potent of the compounds tested.
Previous studies have indicated that ginkgolides might antagonize GlyRs by binding to an intra-ion channel site (Stromgaard, 2002; Stromgaard, 2003), thus acting as a plug in the ion channel of the GlyR. Moreover, it was suggested that the conformation of the ginkgolide was very important for antagonistic activity (Stromgaard, 2002). This study has shown that any modification of the hydroxyl groups leads to a significant loss in activity, which could be attributed to a change in the overall structure of the ginkgolides or by a reduction in the hydrogen bonding capacity of the hydroxyl groups. Further studies are needed to elucidate the binding of ginkgolides to the GlyRs.
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## Claims

1. A compound having the structure:



wherein R is H or -A-Ar,
where A is an alkyl group; and
Ar is an aryl group, which may contain heteroatoms and may be unsubstituted or substituted by one to five substituents each selected from the group consisting of alkoxy, —CH2CO2R4, and —CH2CONR5R6;
where R4 is an alkyl group; and
R5 and R6 are each, independently, hydrogen or a branched or unbranched alkyl group;


wherein R1 is H, —CONC5H10, —CONC4H8O or —COR7,
where R7 is alkyl, aryl or —NR5′R6′;
where R5′ and R6′ are each, independently, hydrogen, methyl, or t-butyl;

wherein R2 is present or absent, and when present is benzoyl, —CO2C6H4NO2, —CONHMe, —CONHtBu, —CONC5H10, —CONC4H8O, —CONR5R6 or —CO-phenyl which phenyl is substituted,
where R5 and R6 are each, independently, hydrogen, methyl, t-butyl,
or —CO—Z-R8 where R8 is alkyl, or aryl; and Z is oxygen;
or R2 is H and R1 is H, and R is p-methoxybenzyl, or picolyl,
or R2 is H and R1 is —C(O)NHt-Bu, or —C(O)NHMe, and R is H,

wherein R3 is present or absent, and when present is —COR9;
where R9 is alkyl or aryl;

wherein only one of R2 or R3 is present in the compound;
wherein only two of R, R1, R2 and R3 are H; and
wherein each of a and b designates a single covalent bond which is present or absent,
where bond a is present when R3 is absent and bond b is present when R2 is absent;

or an optically pure enantiomer or a pharmaceutically acceptable salt of the compound.

2. The compound of claim 1, having the structure



wherein R, R1, and R2 are as defined in claim 1.

3. The compound of claim 1, having the structure



wherein R, R1, and R3 are as defined in claim 1.

4. The compound of claim 2, wherein R is H or -A-Ar-;
where A is —CH2—,
Ar is a phenyl group or a picolyl group, either of which may be substituted or unsubstituted by a substituent selected from the group consisting of methoxy, —CH2CO2Me, and —CH2CONH2.

5. The compound of claim 4, wherein R is H, para-methoxybenzyl or picolyl.

6. The compound of claim 2, wherein R1 is H or —COR7;
where R7 is methyl, phenyl or -NR5R6;
where R5 and R6 are each, independently, hydrogen, methyl, or t-butyl, or wherein R5 and R6 together with the nitrogen to which they are attached form a morpholine or piperidine.

7. The compound of claim 6, wherein R1 is H, benzoyl, —COMe, —CONHMe, —CONHtBu, —CONC5H10, or —CONC4H8O.

8. The compound of claim 2, wherein R2 is H, —CONR5R6 or —CO-phenyl where the phenyl is unsubstituted or substituted by one to five substituents which are each, independently, halogen or —NO2.

9. The compound of claim 1, wherein R2 is H, benzoyl, —CO2C6H4NO2, —CONHMe, —CONHtBu, CONC5H10, or CONC4H8O.

10. The compound of claim 3, wherein R is H or -A-Ar-;
where A is —CH2—,
Ar is a phenyl group or a picolyl group, either of which may be substituted or unsubstituted by a substituent selected from the group consisting of methoxy, —CH2CO2Me, and —CH2CONH2.

11. The compound of claim 10, wherein R is H, p-methoxybenzyl or picolyl.

12. The compound of claim 3, wherein R1 is H or —COR7,
where R7 is methyl, phenyl or —NR5R6;
where R5 and R6 are each, independently, hydrogen, methyl, or t-butyl, or wherein R5 and R6 together with the nitrogen to which they are attached together form a morpholine or piperidine.

13. The compound of claim 12, wherein R1 is H, phenyl, —COMe, —CONHMe, —CONHtBu, —CONC5H10 or CONC4H8O.

14. The compound of claim 3, wherein R3 is benzoyl, —COMe or —CO2C6H4NO2.

15. The compound of claim 1, having the formula

16. The compound of claim 1, having the structure

17. The compound of claim 1, having the structure

18. The compound of claim 1, having the structure

19. The compound of claim 1, having the structure

20. The compound of claim 1, having the structure

21. A composition comprising an amount of the compound of claim 15, 16, 17, 18 or 19 and a pharmaceutical carrier.

22. A process of preparing the compound of claim 1, comprising the step of reacting a compound having the structure




with an electrophile to form the compound.

23. A process of preparing the compound of claim 2, comprising the steps of:
i) reacting a compound having the structure





with either 4-methoxybenzyl chloride or benzyl chloride, and a base to form a compound having the structure




wherein R is benzyl or p-methoxybenzyl; and
ii) reacting the product of step i) with an electrophilic reagents to form the compound of claim 2.

24. A process of preparing the compound of claim 3, comprising the steps of:
i) reacting a compound having the structure





with 4-methoxybenzyl chloride or benzyl chloride, and a base to form a compound having the structure




wherein R is benzyl or p-methoxybenzyl; and
ii) reacting the product of step i) with electrophilic reagents to form the compound of claim 3.

25. A process of preparing the compound of claim 15, comprising the steps of:
i) reacting a compound having the structure





in dimethylformamide with a resin and a base to form a compound having the structure:



ii) cleaving the product of step i) from the resin by mixing the product of step i) with trifluoracetic acid in a polar solvent to form a compound having the structure





wherein R10 is —H; and

iii) dissolving the product of step ii) and an amine base in a solvent and reacting with benzoic anhydride, thereby forming the compound of claim 15.

26. A process of preparing the compound of claim 16, comprising the step of reacting a compound having the structure




with 2-picolyl chloride in a polar solvent with a base, thereby forming the compound of claim 16.

27. A process of preparing the compound of claim 17, comprising the steps of:
i) reacting a compound having the structure





with 4-methoxybenzyl chloride in a polar solvent with a base, providing a compound having the structure




and
ii) dissolving the product of step i) and a amine base in a polar solvent and reacting with benzoic anhydride,
iii) and contacting the product of step ii) with cerium (IV) ammonium nitrate, thereby forming the compound of claim 17.

28. A process of preparing the compound of claim 18, comprising the steps of:
i) reacting a compound having the structure





with 4-methoxybenzyl chloride in a polar solvent with a base, providing a compound having the structure




and
ii) dissolving the product of step i) and iPr2EtN base in a solvent and reacting with p-nitrophenyl chloroformate and then with MeNH2, forming a compound having the structure





and
iii) dissolving the product of step ii) in a solvent and reacting with (NH4)2Ce(NO3)6, thereby forming the compound of claim 18.

29. A process of preparing the compound of claim 19, comprising the steps of:
i) reacting a compound having the structure





with 4-methoxybenzyl chloride in an a polar solvent with a base, providing a compound having the structure




and
ii) dissolving the product of step i) and iPr2EtN base in a solvent and reacting with p-nitrophenyl chloroformate and then with tBuNH2, thereby forming the product





and
iii) dissolving the product of step ii) in a solvent and reacting with (NH4)2Ce(NO3)6, thereby forming the compound of claim 19.