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

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).