The stereoselective synthesis of urdamycinone B (17) was achieved in a 21% overall yield from C-glycosyl-naphthoquinone 197. The key reaction was the Diels-Alder cycloaddition reaction of 197 and siloxydiene (�)-117 promoted by a chiral Lewis acid derived from (S)-3,3�-diphenyl-1,1�-binaphthalene-2,2�-diol (291), BH₃.THF and acetic acid. An effective kinetic resolution of (�)-117 occurred. Four cycloadducts 199a-d were formed in a ratio between 84:8:2:6 and 70:9:2:19. Aromatisation of the mixture by treatment with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) gave 200a and 200b in 4:1 ratio. A sequence of reactions involving deacetylation, conversion of a phenyldimethylsilyl group into a hydroxyl group and photooxidation gave a 4:1 mixture of urdamycinone B (17) and its C-3 epimer (154). Separation of these products was achieved by high performance liquid chromatography (HPLC).
The C-glycosyl donor, 1,3,4-tri-O-acetyl-2,6-dideoxy-D-glucopyranose (204), was synthesised from readily accessible tri-O-acetyl-D-glucal (237) using two approaches. The first involved a sequence of deacetylation, tosylation, lithium aluminium hydride (LiAlH₄) reduction and acetylation to give di-O-acetyl-6-deoxy-D-glucal (242). The triphenylphosphine hydrogen bromide (TPPHBr) catalysed addition of acetic acid to 242 gave 204 in overall yields ranging from 0 to 32%. The step involving the reduction of the tosylate intermediate was the cause of the variable yields.
The alternative synthesis started with the TPPHBr catalysed addition of benzyl alcohol to 237. Subsequent deacetylation, tosylation and reduction with LiAlH₄ gave benzyl 2,6-dideoxy-D-glucopyranoside (250). Acetylation and hydrogenolytic debenzylation gave 3,4-di-O-acetyl-2,6-dideoxy-D-glucopyranose (247). Acetylation gave 204 in 40% overall yield.
A third approach to 204 involved selective tosylation of methyl α-D-mannopyranoside (258) and subsequent treatment with 2,2-dimethoxypropane under acidic conditions to give acetonide 255. LiAlH₄ reduction of the tosylate gave methyl 6-deoxy-2,3-O-isopropylidene-α-D-mannopyranoside (256). Acidic hydrolysis of 256 and subsequent acetylation afforded 1,2,3,4-tetra-O-acetyl-6-deoxy-α-D-mannopyranoside (260). Treatment of 260 with hydrogen bromide in acetic acid and subsequent reductive elimination with a zinc-copper couple gave 242. The addition of acetic acid catalysed by TPPHBr afforded 204 in 18% overall yield.
The final synthesis of 204 started with thiophenyl 2,3,4,6-tetra-O-acetyl-α-D-mannopyranoside (269). A sequence of deacetylation, tosylation and LiAlH₄ reduction gave thiophenyl 2,3-O-isopropylidene-6-deoxy-α-D-mannopyranoside (274). The structure of 274 was confirmed from a single crystal X-ray diffraction study. Hydrolysis of the isopropylidene group of 274 and subsequent acetylation afforded thiophenyl 6-deoxy-2,3,4-tri-O-acetyl-α-D-mannopyrannoside (282). Treatment of 282 with iodine monobromide and subsequent reductive elimination with zinc-copper couple gave 242. The TPPHBr catalysed addition of acetic acid to 242 afforded 204 in 19% overall yield.
Differentially protected C-glycosyl donor, 1,3-di-O-acetyl-4-O-benzyl-2,6-dideoxy-D-mannopyranose (265), was synthesised from 274. The benzylation of 274 gave thiophenyl 6-deoxy-2,3-O-isopropylidene-4-O-benzyl-α-D-mannopyranoside (276). Acidic hydrolysis followed by acetylation afforded thiophenyl 6-deoxy-1,2-di-O-acetyl-4-O-benzyl-α-D-mannopyranoside (278) which, upon bromination by iodine monobromide, gave thiophenyl 6-deoxy-1,2-di-O-acetyl-4-O-benzyl-α-D-mannopyranosyl bromide (279). The reductive elimination of 279 with zinc-copper couple gave 3-O-acetyl-4-O-benzyl-6-deoxy-D-glycal (264). The TPPHBr catalysed addition of acetic acid to 264 afforded 1,3-di-O-acetyl-4-O-benzyl-2,6-dideoxy-D-mannopyranose (265) in 16% overall yield from 274. The instabillity of bromide 279 affected the yield of 265.
A C-glycosylation study of 2-naphthol 227 and 1,4-dimethoxy-5-hydroxynaphthalene (205) with 2-deoxy-glycosyl acetates was undertaken. Boron trifluoride diethyl etherate (BF₃.Et₂O) and scandium triflate [Sc(OTf)₃] proved effective promoters. For example, the glycosylation reaction of donor 265 and 227, promoted by 0.5 equivalents of Sc(OTf)₃, afforded C-glycoside 2-hydroxy-1-[3�-O-acetyl-4�-O-benzyl-2,6-dideoxy-β-D-manno-hexopyranosyl]-naphthalene (289) in 85% yield.
| Identifer | oai:union.ndltd.org:ADTP/217570 |
| Date | January 2005 |
| Creators | Osman, Hasnah, n/a |
| Publisher | University of Otago. Department of Chemistry |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Hasnah Osman |
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