Cycloviracin B1

The first total synthesis of the antiviral agent cycloviracin B1 (1) is described which provisionally establishes the hitherto unknown configuration of the chiral centers on the lateral fatty acid chains as (3R,19S,25R,3'R,17'S,23'R). Key steps en route to this glycolipid include a highly efficient template-directed macrodilactonization step for the formation of the lactide core followed by a two-directional synthesis strategy for the completion of the fatty acid annexes. The latter comprises a modified Julia-Kocienski olefination carried out in the presence of the base-labile beta-hydroxyester moieties of the macrodiolide, as well as a titanium-catalyzed asymmetric addition of the dialkylzinc reagent 11 controlled by cyclohexane-1,2-diamine bistriflate 12 for the formation of the chiral center at C-17' which is selectively glycosylated by taking recourse to the trichloroacetimidate method.

The absolute stereochemistry at the site of attachment of the fatty acid residues to the lactide core of the glycolipids cycloviracin B1 (1) and glucolipsin A (13) has been elucidated as (3R,3'R) by comparison of their 13C NMR data with those of the three possible, differently configured core structures 9, 12, and 14. Moreover, a careful analysis of this set of NMR data allows us to conclude that the structures previously proposed for a seemingly closely related class of antivirally active compounds, i.e., the fattiviracin family, need revision. The key step en route to the symmetrical dilactones 9 and 12 consists of a highly efficient cyclodimerization process which exploits the template effect exerted by potassium cations on the hydroxy acid cyclization precursor. The latter is obtained in excellent overall yield by a sequence involving ring-opening Claisen condensation of pentadecanolide to form the functionalized beta-ketoester 4, asymmetric hydrogenation catalyzed by [(BINAP)RuCl2]2.NEt3, and a beta-selective glycosylation reaction using trichloroacetimidate 6. The unsymmetrical dilactone 14, in contrast, is prepared by a stepwise approach based on a Yamaguchi lactonization as the means to close the macrocyclic ring.

The first total synthesis of the antivirally active glycolipid cycloviracin B(1) (1) is described. The approach is based on a two-directional synthesis strategy which constructs the C(2)()-symmetrical macrodiolide core of the target by an efficient template-directed macrodilactonization reaction promoted by 2-chloro-1,3-dimethylimidazolinium chloride 14 as the activating agent. Attachment of the lateral fatty acid chains to the lactide core thus formed features not only one of the most advanced ligand-controlled addition reactions of a functionalized dialkyl zinc reagent to a polyfunctional aldehyde, but also a highly demanding Julia-Kocienski olefination of a tetrazolyl sulfone bearing electrophilic and base-labile beta-hydroxy ester motifs. By virtue of the flexibility of this synthesis plan, it was possible to prepare a series of macrodiolide cores differing only in the absolute stereochemistry at the branching points as well as a host of model compounds for the fatty acid appendices of cycloviracin. Comparison of these derivatives with the natural product allowed us to establish the as yet unknown absolute stereochemistry of 6 chiral centers of 1 as (3R,19S,25R,3'R,17'S,23'R). Thereby, the (13)C NMR shifts of the anomeric position of the beta-glycosides residing at those positions turned out to be excellent probes for the absolute configuration of the attached aglycones. The concise set of data thus obtained also makes clear that the proposed structure of the fattiviracins, a seemingly closely related family of glycoconjugates, is not matched by the published data. Finally, the biological activity of synthetic 1 and some of the key intermediates obtained en route to this natural product was investigated, showing that the entire construct is necessary for appreciable and selective antiviral activity.