| Summary: | The methyl group in (R)-3,5-O-benzylidene-L-rhamnono-1,4-lactone 2, prepared from L-rhamnose 1 in 41% yield without need for chromatography, is axial and provides a good example of Mills' rule that the 'O-inside' conformations are, in general, more stable than alternative 'H-inside' conformations. The lactone 2 may be converted in two steps in an overall 57% yield to the rhamnono-oxetane 3, which should be useful in generating oxetane dipeptide isosteres and oxetane β-amino acids and in determining the value of the oxetane ring in inducing secondary structure in small peptidomimetics. Methyl 2,4-anhydro-(S)-3,5-O-benzylidene-L-rhamnonate 11, in which both the phenyl and methyl groups are equatorial, is slightly more thermodynamically stable than 3, providing a rare exception to Mills' rule. X-ray crystal structures of 2 and 11 are reported. Copyright (C) 2000 Elsevier Science Ltd.
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