Studies with Altrose Derivatives – A Convenient Approach to α‐Linked Digitoxosides

The 2‐ O ‐(cyclohexylcarbamoyl)‐3,4‐ O ‐(2,2,2‐trichloroethylidene)‐protected α‐altropyranosyl fluorides 1 , 7 , and 11 were used as starting materials for intra‐ and intermolecular glycosylations in the presence of BF 3 . Because of the effective neighbouring group participation of the carbamoyl moieties, the corresponding trans ‐glycosides 2 , 4 , 6 , 12 , and 14 can be stereoselectively prepared from 1 and 11 , respectively, in good yields. In the case of the 6‐ O ‐formyl derivative 7 the intramolecular glycosylation to 1,6‐anhydro‐2‐ O ‐(cyclohexylcarbamoyl)‐3,4‐ O ‐(2,2,2‐trichloroethylidene)‐β‐ D ‐altropyranoside ( 8 ) is possible, when no other competing nucleophiles are present. Furthermore, the deoxygenation of the altrose moiety of 6‐ O ‐[6‐deoxy‐3,4‐ O ‐(2,2,2‐trichloroethylidene)‐α‐ l ‐altropyranosyl]‐1,2: 3,4‐di‐ O ‐isopropylidene‐α‐ D ‐galactopyranose ( 15 ) in 2‐position and the hydrogenolysis of the trichloroethylidene acetal protecting group were simultaneously realised. 6‐ O ‐(2,6‐Dideoxy‐3,4‐ O ‐ethylidene‐α‐ l ‐ ribo ‐hexopyranosyl)‐1,2:3,4‐di‐ O ‐isopropylidene‐α‐ D ‐galactopyranose ( 16 ) was obtained by reduction of the intermediately prepared 2‐xanthate of 15 with Bu 3 SnH/AIBN.