ASSIGNMENT OF STEREOCHEMISTRY IN THE OLIGOMYCIN/RUTAMYCIN/CYTOVARICIN FAMILY OF ANTIBIOTICS - ASYMMETRIC-SYNTHESIS OF THE RUTAMYCIN SPIROKETAL SYNTHON

The absolute stereochemistry of the rutamycin antibiotics 2a,b has been established through asymmetric synthesis of the known degradation product 4. One of the key steps in the assemblage process involves acylation of the metalated hydrazone 6 with the N-methoxy-N-methyl amide 5. Both of these enantiomerically pure intermediates have been prepared in good overall yield and high diastereoselectivity (de > 94%). All absolute stereochemical relationships were established through alkylation and aldol bond constructions using N-acyloxazolidinone chiral auxiliaries. Subjection of 17 to acid hydrolysis/deprotection resulted in loss of protecting groups and subsequent spiroketalization to 19 (80%). Silylation of the secondary alcohol in 19 was followed by a samarium-catalyzed Meerwein-Ponndorf-Verley reduction to provide the equatorial alcohol 20 in excellent yield and stereoselectivity (de = 97%). Control experiments indicate that this surprisingly stereoselective reaction operates under kinetic control and that the observed stereochemical outcome may be the result of coordination of the reactive reducing agent to the axial spiroketal oxygen. Conversion of 20 to triol 4 afforded material that is identical with the rutamycin degradation product in all respects. These results establish that the absolute stereochemistry of the rutamycins is as shown (2a,b).