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As part of a comprehensive investigation of electronic effects on the stereochemistry of base-catalyzed 1,2-elimination reactions, we observed a new syn intramolecular pathway in the elimination of acetic acid from beta-acetoxy esters and thioesters. 1H and 2H NMR investigation of reactions using stereospecifically labeled tert-butyl (2R*,3R*)-3-acetoxy-2,3-2H2-butanoate (1) and its (2R*,3S*) diastereomer (2) shows that 23 +/- 2% syn elimination occurs. The elimination reactions were catalyzed with KOH or (CH3)4NOH in ethanol/water under rigorously non-ion-pairing conditions. By contrast, the more sterically hindered beta-trimethylacetoxy ester produces only 6 +/- 1% syn elimination. These data strongly support an intramolecular (Ei) syn path for elimination of acetic acid, most likely through the oxyanion produced by nucleophilic attack at the carbonyl carbon of the beta-acetoxy group. The analogous thioesters, S-tert-butyl (2R*,3R*)-3-acetoxy-2,3-2H2-butanethioate (3) and its (2R*,3S*) diastereomer (4), showed 18 +/- 2% syn elimination, whereas the beta-trimethylacetoxy substrate gave 5 +/- 1% syn elimination. The more acidic thioester substrates do not produce an increased amount of syn stereoselectivity even though their elimination reactions are at the E1cb interface.

Substituted pentynylcyclopentene precursors for the synthesis of pentalenene, pentalenic acid, and silphinene by intramolecular Pauson-Khand cycloaddition reaction have been prepared from 2-methylcyclopentanone via 5-methylcyclopentenyllithium. Conjugate addition of the latter to BHT methacrylate followed by methylation were the key step in enyne syntheais. Reaction of 4,4dimethyl-5-(5-methylcyclopentenyl)-l-pentyne with @(CO)8 produces two diastereomeric triquinane enones in an overall yield of 51%, with the ero-9-methyl isomer predominating by ratio of 81. This material was converted into pentalenene in two steps. Pauson-Khand reaction of the TBDMS ether of 4,4-dimethyl-5-(5-methylcyclopentenyl)-l-pentyn-3-01 proceeds in 33% yield. Three of the four possible stereoisomeric products are formed, with two of them, making up ca. 80% of the product mixture, poesessing the neceseary exo-methyl stereochemistry at C-9 for further elaboration into pentalenic acid. A formal synthesis of the latter was completed by reduction of one of the enone isomers into a ketone which had previously been carried on to the natural product. Pd(0)-catalyzed coupling of 1-iodo-5-methylcyclopentene to l-(trimethylsilyl)-1,4-pentadiyne and reduction over Lindlar's catalyst allowed efficient access to (2)-1-(5-methylcyclopentenyl)-l-penten-4-yne, but the latter could not be induced to undergo Pauson-Khand cyclization, thus foiling a planned approach to silphinene. IntroductionThe angularly fused triquinanes have proved to be challenging testing grounds for many cyclopentane syntheses. Several years ago we demonstrated the use of the Pauson-Khand cycloaddition reaction in the preparation of this ring system.'I2 That initial approach used symmetrical precursors and was not applicable to most of the natural products in the compound class. In order to direct our efforts toward triquinane natural products we began an evaluation of methods for the synthesis of cycloaddition substrates derived from l-(rt-pentynyl)-b methyl~yclopentene.~ We herein describe applications of

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