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The mixed carbonate ester intermediate (X) has been obtained as follows: The reaction of dihydrofuran (I) with propargyl alcohol (II) and N-iodosuccinimide (NIS) gives the propargyl ether (III), which is cyclized by means of tributyltin hydride and AIBN in refluxing toluene to yield the perhydrofuro[2,3-b]furan (IV). The oxidation of the methylene group of (IV) with ozone in methanol/dichloromethane affords the bicyclic ketone (V), which is reduced with NaBH4 in ethanol to provide racemic (VI). The digestion of (rac)-(VI) with immobilized lipase 30 and acetic anhydride in DME provides a mixture of the (3R)-alcohol (VII) and the (3S)-acetoxy derivative (VIII) that is separated by chromatography. Finally, the (3R)-(VII) alcohol is condensed with disuccinimidyl carbonate (IX) and TEA in acetonitrile to give the target mixed carbonate ester intermediate (X).

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The reaction of butadiene monooxide (XI) with phenylmagnesium bromide (XII) and CuCN in THF gives trans-4-phenyl-2-buten-1-ol (XIII), which is enantioselectively epoxidated with Ti(O-iPr)4, diethyl D-tartrate and tBu-OOH to yield the (2R,3R)-epoxide (XIV). The reaction of (XIV) with Ti(O-iPr)4 and N3-SiMe3 in refluxing benzene affords the chiral azidodiol (XV), which is epoxidated by means of 2-acetoxyisobutyryl chloride (AcBCl) and NaOMe in THF to provide the chiral azidoepoxide (XVI) (1). Opening of the epoxide ring of (XVI) with isobutylamine (XVII) in hot isopropanol gives the secondary amine (XVIII), which is acylated with 4-aminophenylsulfonyl chloride (XIX) and pyridine in dichloromethane to yield the corresponding sulfonamide (XX). The reduction of the azido group of (XX) with H2 over Pd/C in THF/methanol/HOAc affords the expected primary amine (XXI), which is finally condensed with the already reported intermediate (X) in dichloromethane to provide the target carbamate.