The invention relates to a process for the preparation of enantiomerically enriched (1 S,4/t5-2-oxa-3-azabicyclo[2.2.1]hept-5-enes of fomriula ^V^ ll ^'^ wherein PG^ is an amino-protective group. It further relates to novel 5-O-protected (15,4/t)-3-(1-C-halo-a-D-ribofuranosyl)-2-oxa- 3-azabicyclo[2.2.1]hept-5-enes of formula yj "N^ (III) PG3 "^*^ wherein X Is a halogen atom selected from fluorine, chlorine, bromine and iodine, PG^ is a hydroxyl-protective group and PG^ is a 1,2-diol-protective group. N-Protected 2-oxa-3-azabicyclo[2.2.1]hept-5-enes are valuable intermediates in the synthesis of various pharmaceutically active ingredients. See e.g. EP-A-0 322 242 and EP-A-0 658 539 for the N-benzyloxycarbonyl derivative. While some racemic com¬pounds are relatively easily obtainable by hetero-Diels-Alder cycloaddition of nitroso compounds such as benzyl nitrosoformate (obtainable from benzyl AAhydroxycarba-mate by oxidation, e.g. with periodate) with cyclopentadiene, a commercially feasible method for the production of the enantiopure or enantiomerically enriched compounds with a wide variety of possible protective groups has not been available. It is therefore an objective of the present invention to provide a method for the pro¬duction of enantiomerically enriched N-protected (15,4/v)-2-oxa-3-azabicyclo[2.2.1]-hept-5-enes that uses commercially available or at least easily accessible starting materials and allows the synthesis of compounds with various protective groups. It has been found that enantiomerically enriched (1S,4/t)-2-oxa-3-azabicyclo[2.2.1]-hept-5-enes of formula wherein PG^ is an amino-protective group, can be prepared by a method comprising the steps of (i) reacting a protected 1-C-nltroso-p-D-ribofuranosyl hallde of formula \ 1 'NO / \ (II) PG3 wherein X is a halogen atom selected from fluorine, chlorine, bromine and iodine, PG2 is a hydroxyl-protective group, and PG3 is a 1,2-diol-protective group, with cyclopentadiene to obtain a (15,4/t)-3-(1-C-halo-a-D-ribofuranosyl)-2-oxa- 3-azabicyclo[2.2.1]hept-5-ene of formula PG2-o^''''^ y\ ^^ d b 0^% J PQ3 ^<^ wherein X, PG2 and PG^ are as defined above; (ii) hydrolyzing the compound obtained in step (i) to obtain free (1 S,4/5)-2-oxa-3-aza- bicyclo[2.2.1]hept-5-ene (I; PG^ = H) or the corresponding hydrohalide and the corresponding protected D-ribonolactone; and (iii) introducing the amino-protective group PG^. 2 This finding is quite surprising since it iiad been found that a structurally related xylose-derived a-chloronitroso compound underwent hetero-Diels-Alder cycloadditions with both 1,3-cyclohexadiene and 1,3-cycloheptadiene, but failed to give any cycloaddition product with cyclopentadiene (A. Hall et al., Chem. Commun. 1998, 2251-2252). Suitable amino-protective groups PG^ are in particular groups fomning a carbamate moiety with the amino nitrogen, such as simple all o^\ J PQ3 ^-^ wherein X, PG2 and PG^ are as defined above are novel and also an object of the invention. In a preferred embodiment of the (1S,4/^-3-(1-C-halo-a-D-ribofuranosyl)-2-oxa-3-a2a-bicyclo[2.2.1]hept-5-ene of formula III, X is chlorine. In another prefen-ed embodiment of the (1S,4/t)-3-(1-C-halo-a-D-ribofuranosyl)-2-oxa-3-azabicycloI2.2.1]hept-5-ene of formula III, PG2 is a triphenylmethyl group. In still another preferred embodiment of the (15,4/t)-3-(1-C-halo-a-D-ribofuranosyl)-2-oxa-3-azabicyclo[2.2.1]hept-5-ene of formula III, PG^ is an isopropylidene group According to the process of the invention it is possible to obtain the desired enantio-merically enriched (15,4/^-2-oxa-3-azabicyclo[2.2.1]hept-5-enes (I) in an enantiomeric excess (ee) of 80% or more, preferably 90% or more and particularly preferably 95% or more. The following non-limiting examples will illustrate the process of the invention and the preparation of the novel intermediates. Example 1 2,3-O-lsopropylidene-D-ribofLiranose Concentrated sulfuric acid (0.3 mL) was added to a suspension of D-ribose (12.5 g, 83 mmol) in acetone (125 mL). The reaction mixture was stirred at room temperature for 90 min to obtain a clear solution which was then neutralized with saturated aqueous sodium carbonate. The mixture was filtered over Celite® and concentrated in vacuo. Yield: 15.7 g(«100%) Example 2 2,3-C'-lsopropylidene-5-0-trityl-D-ribofuranose 2,3-(>lsopropylidene-D-ribofuranose (15.7 g, 83.1 mmol) was dissolved in pyridine (100 mL) and trityl chloride (27.8 g, 0.1 mol) was added. The mixture was stirred at room temperature for 24 h. The solvent was evaporated and the residue purified by column chromatography on silicagel using hexanes/ethyl acetate {\r.v= 4:1) as eluant. Yield: 32.3 g (90%) Example 3 2,3-C>-lsopropylidene-5-0-trityl-D-ribofuranose 2,3-Olsopropylidene-D-ribofuranose (20 g, 105.2 mmol) was dissolved in dichloro-methane (200 mL) at 0 °C. Triethylamine (10.9 g, 107.5 mmol) and a catalytic, amount of pyridine were added to the reaction mixture, followed by the addition of trityl chloride 9 (27.8 g, 0.1 mol). The mixture was stirred at 0 "C for 3 h and furtfier 12 h at room temperature. To the reaction mixture was added saturated aqueous sodium bi¬carbonate (80 mL) and the phases were separated. The organic phase was dried over anhydrous sodium sulfate, filtered, and the solvents were removed in vacuo. The caide product was used without further purification in the next step. Yield: 38.5 g (85%) Example 4 2,3-0-lsopropylidene-5-0-trityl-D-ribofuranose oxime (IV; PG2 = trityl, PG^ = =C(CH3)2) Hydroxylamine hydrochloride (58 g, 0.83 mol) was added to a solution of 2,3-0-iso-propylidene-5-(>trityl-D-ribofuranose (30 g, 0.69 mol) in pyridine (200 mL). The mixture was stirred at room temperature for 3 h and then water (250 mL) and dichloromethane (250 mL) were added and the phases were separated. The organic phase was dried over anhydrous sodium sulfate and filtered and the solvent was evaporated. The residue was purified by column chromatography on silicagel using hexanes/ethyl acetate (^: i' = 7:3) as eluant. Yield: 25.5 g (82%) Example 5 2,3-aisopropylidene-5-Otrityl-D-ribofuranose oxime (IV; PG2 = trityl. PG3 = =C(CH3)2) To hydroxylamine hydrochloride (10.9 g, 0.16 mol) in ethanol (150 mL) was added sodium bicarbonate (13.11 g, 0.16 mol). The reaction mixture was stin-ed at room temperature until the evolution of carbon dioxide ceased. Then 2,3-Oisopropylidene-5-O-trityl-D-ribofuranose (15 g, 0.34 mol), dissolved in ethanol (50 mL), was added and stirring was continued for 2 h. The reaction mixture was then filtered over a plug of silica and ethyl acetate (200 mL) and water (200 mL) were added. The organic phase was dried over anhydrous sodium sulfate and filtered and the solvent was evaporated. The crude product was used without further purification in the next step. Yield: 13.4 g (86%). Example 6 2,3-0-lsopropylidene-1--protected D-ribonolactone; and (iii) introducing the amino-protective group PG^. }2- 2. The process of claim 1 wherein the amino-protective group PG^ is a benzyloxy-carbonyl group and is introduced by reacting the (1 S,4/?)-2-oxa-3-azabicyclo-[2.2.1]hept-5-ene with benzyl chloroformate. 3. The process of claim 1 or 2 wherein X is chlorine. 4. The process of any of claims 1 to 4 wherein the 1,2-diol-protective group PG^ is an isopropylidene group. 5. The process of any of claims 1 to 5 wherein the steps (i) to (ill) are carried out without isolating the intermediate of formula ill or the free (1 S,4/?)-2-oxa-3-aza-bicyclo[2.2.1]hept-5-ene (I; PG^ = H) or its hydrohalide. 6. The process of any of claims 1 to 6 wherein the protected 1-C-nitroso-p-D-ribo-furanosyl halide of fonmula II has been prepared by reacting the corresponding protected D-ribofuranose oxime of formula \ 1 CV) PG3 wherein PG2 is a hydroxyl-protective group and PG^ is a 1,2-diol-protective group, with two equivalents of a hypohalite of formula M"^(OX)„- wherein X is chlorine, bromine or iodine, n is 1 or 2 and M is selected from the group consisting of alkali metals and alkaline earth metals. 7. A process for the preparation of a protected 1-C-nitroso-p-D-ribofuranosyl halide of formula \ I 'NO / \ (II) PG3 wherein X is a halogen atom selected from chlorine, bromine and iodine, PG2 is a hydroxyl-protective group and PG^ is a 1,2-diol-protective group, wherein a protected D-ribofuranose oxime of formula PG2_o^ \ / \ i (IV) cf PG3 wherein PG^ and PG^ are as defined above, is reacted with two equivalents of a hypohalite of formula M"^(OX)- wherein X is as defined above, n is 1 or 2 and M is selected from the group consisting of alkali metals and alkaline earth metals. 8. The process of claim 6 or 7 wherein the hypohalite is sodium hypochlorite. 9. The process of any of claims 1 to 8 wherein the protected D-ribonolactone obtained in step (ii) is recovered and reconverted into the protected 1-C-nitroso-p-D-ribofuranosyl halide (II). 10. The process of any of claims 1 to 9 wherein the hydroxyl-protective group PG^ is an optionally substituted triphenylmethyl group. 11. The process of any of claims 1 to 10 wherein the 1,2-cliol-protective group PG^ is an isopropylidene group. 12. A (15,4/;)-3-(1-C^halo-a-D-ribofuranosyl)-2-oxa-3-azabicyclo[2.2.1]hept-5-ene of formula 0 / \ / 'N^fX (III) PQ3 ^^<^ wherein X is a halogen atom selected from fluorine, chlorine, bromine and iodine, PG2 is a hydroxyl-protective group, and PG3 is a 1,2-diol-protective group. 13. The (1 S,4/?)-3-(1-(?-halo-a-D-ribofuranosyl)-2-oxa-3-azabicyclo[2.2.1]hept-5-ene of claim 12 wherein X is chlorine. 14. The (1 S,4/<)-3-(1 -C-halo-a-D-ribofuranosyl)-2-oxa-3-azabicyclo[2.2.1 ]hept-5-ene of claim 12 or 13 wherein PG^ is a triphenylmethyl group. 15. The (1 S,4/?)-3-(1-C-halo-a-D-ribofuranosyl)-2-oxa-3-azabicyclo[2.2.1]hept-5-ene of any of claims 12 to 14 wherein PG^ is an isopropylidene group. Dated this the 1st Day of March, 2012 MANISHA SINGH NAIR Agent for the Applicant [IN/PA-740] LEX ORBIS IP PRACTICE 1^