3-Deoxy-6′-hydroxy-13-norurdamycinone B (32a) and
3-deoxy-13-norurdamycinone B (32b) have been synthesized in eight steps from
5,8-dimethoxynaphthalen-1-ol (9) both in 28% overall yield. The key
step in this approach is the boron trifluoride diethyl etherate promoted
β-C-glycosylations of (9) and
1,3,4,6-tetra-O-acetyl-2-deoxy- and
1,3,4-tri-O-acetyl-2-deoxy-D-arabino-hexopyranoses
(13) and (19). The solvent, acetonitrile, was essential for the success of
these reactions. Acetylation of the
C-glycosyl-5,8-dimethoxynaphthalen-1-ols (16) and (20)
followed by oxidation with ceric ammonium nitrate gave
C-glycosyl-5-acetoxy-1,4-naphthoquinone derivatives (21)
and (23) in 63 and 49% overall yields from (9). Selective deacetylation
of the C 5 acetoxy groups of (21) and (23) was achieved by treatment with
boron trifluoride etherate in dichloromethane to give
3,4,6-tri-O-acetyl- 2-deoxy- and
3,4-di-O-acetyl-2,6-dideoxy-β-D-arabino-hexopyranosyl-5-hydroxy-1,4-naphthoquinones
(24) and (25) respectively. The tetra-O-acetyl diborate
promoted Diels–Alder reactions of (24) and (25) with
(±)-(E)-1-acetoxy-3-(2-methoxyvinyl)cyclohex-2-ene
(8) each gave a 1 : 1 mixture of diastereoisomeric cycloadducts which, upon
treatment with 1,8-diazabicyclo[5.4.0]undec-7-ene gave
(1R*)-1-acetoxy-9-(3,4,6-tri-O-acetyl-2-deoxy-β-D-arabino-hexopyranosyl)-
and
(1R*)-1-acetoxy-9-(3,4-di-O-acetyl-2,6-dideoxy-β-D-arabino-hexopyranosyl)-8-hydroxy-1,2,3,4-tetrahydrobenz[a]anthracene-7,12-dione
(30a) and (30b) respectively. Sequential deacetylation and photochemical
oxidation of (30a) and (30b) gave the targets (32a) and (32b) respectively.
Synthesis of 4,7-indolequinones. The oxidative demethylation of 4,7-dimethoxyindoles with ceric ammonium nitrate.