SYNTHETIC APPROACHES TO LANKAMYCIN AND ROSARAMICIN

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Lankamycin and rosaramicin are respectively 14 and 16 membered macrolides which belong to antibiotic families widely used in clinical and veterinary practice. This factor, added to their complex, polyfunctional skeleta have proved alluring to synthetic organic chemists and many advances in methodology have resulted from efforts directed towards their syntheses. Sugar-based approaches to macrolide synthesis have become popular within the past five years, but the attractiveness of the concept suffers from the limitation that (most) sugars are only five or six carbon units in length, and provisions for connecting two sugar subunits together invariably produce rich mixtures of diastereomers. Thus, any advantages gained by stereoselective syntheses of various sub-units are negated by the formation of such diastereomeric mixtures at advanced stages of the synthesis. In order to overcome these shortcomings, an approach based on "folded glycoside" concept is being developed which features considerable chiral economy in that asymmetric centers on the sugar moieties are not "wasted", and conversely, asymmetric centers on the target molecule are made to coincide with asymmetric centers on the sugar portion.