中文名称 | 英文名称 | CAS号 | 化学式 | 分子量 |
---|---|---|---|---|
—— | methyl 2-(4-acetyl-7-bromo-5-hydroxy-6-methoxy-2,3-dihydronaphtho[1,2-b]furan-2-yl)acetate | 269057-95-6 | C18H17BrO6 | 409.233 |
The synthesis of a dimeric pyranonaphthoquinone (8) was investigated focusing on a late-stage biaryl coupling of suitably functionalized bromonaphthofurans by using Suzuki–Miyaura methodology. Bromonaphthofuran (16) underwent reaction with bis(pinacolato)diboron in the presence of PdCl2(dppf) to afford boronate ester (21) and furonaphthofuran (22). ‘In situ’coupling of the boronate ester (21) with aryl bromide (16) to the desired dimer (11) was not realized. Bromonaphthofuran (17), prepared by Diels–Alder/retro-Claisen reaction of bromonaphthoquinone (24) with diene (25), underwent Suzuki–Miyaura coupling to naphthofuran (27) and boronate ester (28). Numerous attempts to alter the reaction conditions to effect homocoupling of bromide (17) to biaryl (19) were unsuccessful. Bromopyranonaphthoquinone (18) prepared by oxidative rearrangement of (17) failed to undergo Suzuki–Miyaura coupling.
The preparation of 8-bromokalafungin (20) which is a key intermediate for the synthesis of C-glycoside containing pyranonaphthoquinone antibiotics related to medermycin (6) is described. Although attempts to selectively monobrominate kalafungin (1) at C8 were unsuccessful, 8,10-dibromokalafungin (21) was prepared by using excess N-bromosuccinimide in chloroform. Selective bromination at C6 on a naphthalene ring was achieved upon treatment of naphthol (9) with N-bromosuccinimide (1 equiv.). Conversion of naphthol (9) into naphthoquinone (15) was effected by methylation, Baeyer–Villiger oxidation, acetylation via a Fries rearrangement and oxidation with ceric ammonium nitrate. Conversion of the 7-bromo quinone (15) into 8-bromokalafungin (20) proceeded through subsequent addition of 2-trimethylsilyloxyfuran (16) followed by oxidative rearrangement of the resultant furonaphthofuran (17) to furonaphthopyran (18). After reduction of the lactol (18) to cis ether (19), demethylation and epimerization at C5 with boron tribromide afforded 8-bromokalafungin (20). 8-Bromokalafungin (20) failed to undergo Pd(0)-mediated cross-coupling reactions with the stannyl glucal (22).