Structure of the Sugar Components of the Quinocycline Complex

The sugar components of quinocycline A and isoquinocycline A and of quinocycline B and isoquinocycline B could be obtained as methylglycosides by methanolysis of the antibiotics with methanol‐trifluoroacetic acid. The NMR spectrum of the α‐ and β‐methylglycosides of the sugar from quinocycline A and isoquinocycline A is fully consistent with the structure 2,6‐dideoxy‐4‐ C ‐[1′‐hydroxyethyl]‐ l ‐ xylo ‐hexopyranose (Fig, 1, compound III, R = H, α‐anomer) deduced from previous chemical studies and from an X‐ray analysis of isoquinocycline A. The absolute configuration at C‐5 of compound III was confirmed as l by isolation of l ‐(+)‐lactic acid after periodate oxidation. The α‐ and β‐methylglycosides of the sugar from quinocycline B and isoquinocycline B show in the mass spectrum a molecular ion (C 9 H 16 O 5 ) possessing 2 hydrogen atoms less than that of the methylglycosides of compound III (C 9 H 18 O 5 ). For one of the two anomeric methylglycosides from quinocycline B and isoquinocycline B the structure α‐ O ‐methyl–2,6‐dideoxy‐4‐ C ‐[1′‐oxoethyl]‐ l ‐ xylo ‐ or l ‐ ribo ‐hexopyranoside (Fig.3, compound IVa or IVb) was deduced mainly from the NMR spectrum and by isolation of l ‐(+)‐lactic acid after periodate oxidation. The relative configuration of the hydroxyl groups at C‐3 and C‐4 as trans (compound IVa) was established by the fact that the methylglycoside does not form a borate complex. The carbonyl function of compound IVa gives a sharp band at 1740 cm −1 in the infrared. The structure of compound IVa was confirmed by the formation of the α‐methylglycoside of compound III (R = CH 3 ) as one product of sodium borohydride reduction of the α‐methylglycoside, compound IVa. The sugar of quinocycline B and isoquinocycline B is therefore 2,6‐dideoxy‐4‐ C ‐[1′‐oxoethyl]‐ l ‐ xylo ‐hexopyranose.