Preparation of (2 R )-2-acetoxy- D -forosamine for the total synthesis of spinosyns

A synthesis of 2-acetoxy-D -forosamine ( 4 ) starting from 1,2-O -propylidene- α - D -abequose (3,6-dideoxy-1,2-O -propylidene- α - D -xylo-hexopyranose) ( 5 ) was developed by introduction of an azide moiety at C-4 with inversion of configuration. The 2-acetoxy group in 4 allows a β -selective glycosidation of a secondary alcohol moiety which is necessary in the total synthesis of spinosyns.


Introduction
The spinosyns represent a group of chemical related metabolites which were extracted from the soil organism Saccharopolyspora spinosa in 1986 and which reveal a strong insecticidal activity (Figure 1).The compounds contain a macrocyclic lactone connected to a tricyclic backbone. 1oth the structure and the mode of action of these insecticides are unique; they bind to the γ-amino butyric acid (GABA) receptor and in addition they interact with the nicotineacetylcholine receptor (n-AChR) localized in the postsynaptic cells.This leads to an ion influx and therefore to a generally increased muscle activity which causes to the death of the insect. 2At the moment the drug is produced fermentatorically from cell cultures and is being marketed e.g.under the brands Spinosad®, Tracer® and Success® which contain a mixture of spinosyn A (1) and D (2) in a ratio of 85:15.Since first signs of resistance in Thailand and Hawaii have occurred 3 , new analogues of the drug have to be developed for a conscientious resistance management.Several syntheses of spinosyns have already been described. 4One of the main problems in these syntheses is the stereoselective introduction of the forosamine moiety, which is crucial for the bioactivity of the spinosyns.Forosamine (3) is a 2-deoxysugar which makes it difficult to control the stereochemistry of the glycosidation to give the desired β-glycoside (Scheme 1).Very recently, W. R. Roush described an efficient synthesis and highly β-selective glycosidation of a 4-azido-2-acetoxy analogue of forosamine.4d Besides the removal of the acetoxy group this strategy furthermore implies two additional steps: the reduction of the azide group and a dimethylation to form the forosaminyl glycoside which could be problematic with easily reducible glycosyl acceptors.It was therefore our strategy to use a 2-acetoxyforosamine (4) for a β-selective glycosidation with the benefit of the neighbouring group effect of the acetoxy group including the 4-dimethylamino group.Herein, we describe the synthesis of 2-acetoxyforosamine (4) from 1,2-O-propylidene-α-D-abequose (3,6-dideoxy-1,2-O-propylidene-α-D-xylo-hexopyranose) 5 (5) and its use in a β-selective glycosidation.

Results and Discussion
Starting from literature known 1,2-O-propylidene-α-D-abequose (5) the introduction of the equatorial amino group at C-4 called for a substitution with an N-nucleophile.Therefore, the axial hydroxy group at C-4 was converted into the methane-6 and the p-toluenesulphonate 7 6 and 7, respectively; the corresponding trifluoromethanesulphonate was also prepared 8 , but proved to be too unstable in the following reactions using lithium dimethylamide or sodium azide in DMF at 0 °C, respectively.Under the latter conditions however, 6 and 7 could easily be transformed into the corresponding azide 8. 9 A comparison of the two substrates 6 and 7 revealed that the mesylate 7 was not only easier to generate but also gave the better yields in the substitution with 91 % for two steps.The configuration of C-4 in 8 could be revealed by 1 H-NMR spectroscopy showing a doublet at δ = 3.17 with a coupling constant of J 4,5 = 9.0 Hz, which proves an equatorial orientation of the azide group.
8 was reduced to the corresponding amino compound 9, 10 which was transformed into the described dimethylamino compound 10 by a reductive amination. 11A one-pot process in which the azide 8 was treated directly with formaldehyde and palladium on charcoal under a hydrogen atmosphere in methanol did not lead to the desired compound although the reaction conditions of the single steps were comparable to the two-step process.
The following acid catalyzed opening of the 1,2-propylidene acetal moiety in 10 with simultaneous peracetylation turned out to be more difficult than expected.Thus, neither trifluoroacetic acid nor p-toluenesulphonic acid were strong enough to allow an opening of the acetal; triflic acid led to a decomposition of the substrate.The best result was obtained with perchloric acid at -12 °C in acetic anhydride which gave the peracetylated sugar 11 in 76 % yield. 12he final step of the synthesis of 2-acetoxyforosamine (4) was a selective deprotection of the anomeric hydroxyl group.The common procedure using hydrazinium acetate 13 gave the product only in a modest yield of 33 %.The purification also proved to be rather difficult as the products and the acetylated hydrazine species showed similar polarities and solubilities.Reaction of 11 with benzylamine, 14 methanolic ammonia and porcine pancreas lipase 15 , respectively did not show any conversion.On the other hand the use of sodium methoxide or potassium carbonate led to an unwanted solvolysis of both acetate moieties in 11 even at 0 °C.The best result was obtained with ethanolamine in ethyl acetate at ambient temperature which gave the target compound 4 in 73 % yield. 16o prove the strategy for a β-selective glycosidation with the help of the neighbouring group effect of the 2-acetoxy group the trichloroacetimidate 12 was formed as the glycosyl donor using 5 equiv. of trichloroacetonitrile and 0.5 equiv. of polymer supported DBU 17 at r.t. in 75 % yield with a β:α-ratio of >15:1 (Scheme 3).For the glycosidation with 12, the secondary alcohol isopropanol was used to allow a good comparison with the spinosyn aglycon.Reaction of 12 with isopropanol in the presence of BF 3 •OEt 2 and acetonitrile gave the glycosides 13 and 14 as an anomeric 3:1 mixture in favour of the desired β-anomer 14 in 78 %.

Experimental Section
General Procedures.Melting points were measured with a Mettler FO61 melting point apparatus and are uncorrected.Optical rotations were taken with a Perkin-Elmer 241 spectrometer. 1H-and 13 C-NMR spectra were recorded with Mercury-200, VXR-200, Unity 300, Inova-500, Unity Inova-600 (Varian) or AMX 300 (Bruker) spectrometers.Chemical shifts are reported in δ ppm referenced to TMS ( 1 H-NMR) or to CDCl 3 ( 13 C NMR) as internal standard.IR spectra were taken with a Bruker Vector 22 and UV spectra with a Perkin-Elmer Lambda 2 spectrometer.Mass spectra were measured with a Varian MAT 311A (low resolution) and with a MAT 731 (high resolution).Microanalyses were performed on a CHN 2000 from LECO with the combustion unit MIKRO U/D from Heraeus.Precoated silica gel SIL G/UV254 (Macherey-Nagel GmbH & Co KG) was used for TLC, and silica gel 60 (0.040-0.063 mm) (Merck KGaA) was used for flash chromatography.All reactions were performed under argon in oven-dried glassware.Solvents were dried and distilled prior to use by the usual laboratory methods, commercially available chemicals were used without further purification.