STEREOSELECTIVE SYNTHESES OF THE NONACTATE ESTERS VIA INTRAMOLECULAR OXYMERCURATIONS OF ALLENES

The nactin antibiotic subunits (±)-nonactic acid, (±)-homononactic acid, and (±)-bishomononactic acid were synthesized as their methyl esters (14, 15, 16) by a route which forms the cis-2,5-disubstituted tetrahydrofuran ring moiety by a one-pot oxymercuration-transmetalation (with palladium)-methoxycarbonylation, which converts the γ-silyloxy alienes 27-29 to the 2(10)- or 2(11)-dehydrononactate methyl esters 31-33 with excellent (>98:<2) cis:trans stereoselectivity. The γ-silyloxy alienes were synthesized from the corresponding anti-1,3-diols 18-20, which in turn were prepared by the reduction of the β-hydroxy ketone aldol products 23-25 using tetramethylammonium triacetoxyborohydride. Catalytic hydrogenation of the dehydrononactate intermediates yielded 50:50 mixtures of the title products and their C2 epimers. A remarkable “chelation-controlled” reduction of the dehydrononactate 32 to primarily the 1“2-epi” product, using magnesium in methanol, was observed. The versatility of this synthetic route was demonstrated by the synthesis of the unnatural nonactate homologue (±)-methyl “trishomononactate” 17. © 1993, IEEE. All rights reserved. © 1990, American Chemical Society. All rights reserved.