Enynones in Organic Synthesis. 7. Substituent Effects on the .alpha.-Tocopherol-Catalyzed Cyclization of Enynones to Methylenecyclopentenones. Convenient Syntheses of Members of the Methylenomycin Class of Antibiotics
摘要:
Substituent effects on the a-tocopherol (vitamin E, 3b) catalyzed cyclization of a wide variety of enynones 1a-z to methylenecyclopentenones 7a-z have been examined, with particular emphasis given to electron-withdrawing and -donating groups at positions 2-4 and 6. In general, electron-withdrawing groups at positions 4 and 6 dramatically accelerate the cyclization process, while strong electron-donating groups at positions 3 and 4 completely inhibit reaction. Relatively little effect is exerted by groups at C-2, except for the methyl ester derivative 1i, which is totally unreactive. This methodology was employed in the syntheses of the methylenecyclopentenone antibiotics methylenomycin B (7a) and desepoxy-4,5-didehydromethylenomycin A (7z) and in formal syntheses of methylenomycin A (8) and xanthocidin (9).
Enynones in Organic Synthesis. 7. Substituent Effects on the .alpha.-Tocopherol-Catalyzed Cyclization of Enynones to Methylenecyclopentenones. Convenient Syntheses of Members of the Methylenomycin Class of Antibiotics
摘要:
Substituent effects on the a-tocopherol (vitamin E, 3b) catalyzed cyclization of a wide variety of enynones 1a-z to methylenecyclopentenones 7a-z have been examined, with particular emphasis given to electron-withdrawing and -donating groups at positions 2-4 and 6. In general, electron-withdrawing groups at positions 4 and 6 dramatically accelerate the cyclization process, while strong electron-donating groups at positions 3 and 4 completely inhibit reaction. Relatively little effect is exerted by groups at C-2, except for the methyl ester derivative 1i, which is totally unreactive. This methodology was employed in the syntheses of the methylenecyclopentenone antibiotics methylenomycin B (7a) and desepoxy-4,5-didehydromethylenomycin A (7z) and in formal syntheses of methylenomycin A (8) and xanthocidin (9).
A convenient synthesis of methylenomycin B. Further mechanistic studies on the SET catalyzed electrocyclization of enynones.
作者:Peter A. Jacobi、Reginald O. Cann、David F. Skibbie
DOI:10.1016/s0040-4039(00)74185-2
日期:1992.4
Methylenomycin B (3a) has been prepared in approximately 55% yield by SET catalyzed cyclization of enynone 1a. Mechanistic studies indicate that initial electron donation takes place at C-3 in enolized intermediates of type 2a.
Enynones in Organic Synthesis. 7. Substituent Effects on the .alpha.-Tocopherol-Catalyzed Cyclization of Enynones to Methylenecyclopentenones. Convenient Syntheses of Members of the Methylenomycin Class of Antibiotics
作者:Peter A. Jacobi、Harry L. Brielmann、Reginald O. Cann
DOI:10.1021/jo00097a036
日期:1994.9
Substituent effects on the a-tocopherol (vitamin E, 3b) catalyzed cyclization of a wide variety of enynones 1a-z to methylenecyclopentenones 7a-z have been examined, with particular emphasis given to electron-withdrawing and -donating groups at positions 2-4 and 6. In general, electron-withdrawing groups at positions 4 and 6 dramatically accelerate the cyclization process, while strong electron-donating groups at positions 3 and 4 completely inhibit reaction. Relatively little effect is exerted by groups at C-2, except for the methyl ester derivative 1i, which is totally unreactive. This methodology was employed in the syntheses of the methylenecyclopentenone antibiotics methylenomycin B (7a) and desepoxy-4,5-didehydromethylenomycin A (7z) and in formal syntheses of methylenomycin A (8) and xanthocidin (9).