2-Cyclopentenyl acetate

The irradiation of 2-methoxytropone (A) leads to methyl 4-oxo-2-cyclopentenyl-acetate (D). The reaction can be followed by analytical gas chromatography and two intermediates are observed. These have the structures B and C. Indicate a mechanism by which each of the three successive reactions might occur. The first two steps are photochemical, while the third is probably an acid-catalyzed reaction which occurs under the photolysis conditions. 

ENANTIOSELECTIVE HYDROLYSIS OF cis-3,5-Dl ACETOXYCYCLOPENTENE (1 R,4S)-(+)-4-H YDROXY-2-CYCLOPENTENYL ACETATE... 

HPLC grade ethyl acetate was purchased from J. T. Baker Inc. and distilled from phosphorus pentoxide prior to use. Although the product, (1R,4S)-(+)-4-hydroxy-2-cyclopentenyl acetate [(+)-1], is more soluble in ethyl acetate, emulsion concerns dictated the use of pure ether for the first three extractions and then a 1 1 ether/ethyl acetate mixture for all subsequent extractions. When emulsions did occur during the extraction process, the unemulsified portion of the aqueous phase was drained and a few mL of brine solution was added to the emulsified phase. Two or three quick shakes alleviated the problem. 

R,4S)-(+)-4-Hydroxy-2-cyclopentenyl acetate [(+)-1] is an important synthetic precursor. It provides optically active starting material via the versatile intermediate 4-oxo-2-cyclopentenyl acetate,7 for important cyclopentanoids such as the prostaglandins8 and carbocyclic nucleosides.9 Because of the medicinal significance of these compounds more efficient routes, with better enantioselectivities have been devised to nonracemic 1. Enzymatic catalysis has become the dominant methodology for induction of this optical activity. 

In addition to the title procedure,10 other en2ymatic preparations of nearly optically pure (1R,4S)-(+)-4-hydroxy-2-cyclopentenyl acetate [(+)-1 ]s-11 and its optical antipode (-)-i 12,13.14,15.16 are known. These en2yme-catalyzed procedures are derivatives of two basic strategies (1) the enantioselective hydrolysis of the meso-diacetate 2,5.10 11 12 13 or (2) the enantioselective transacetylation of the parent meso-diol.14 15-16 Although (-)-1 has been successfully prepared by either route, the (+)-enantiomer is available only via the hydrolytic approach. 

A. (4R)-(+)-Acetoxy-2-cyclopenten-l-one. A flame-dried, 2-L, three-necked, round-bcttomed flask, equipped with a Teflon-coated magnetic stirring bar, is purged with nitrogen and charged with 1.0 L of dry dichloromethane (Note 1), 1.5 g of anhydrous sodium acetate (Note 2), 70 g of 4 A molecular sieves (Note 3), and 23 g (162 mmol) of (1R,4S)-(+)-4-hydroxy-2-cyclopentenyl acetate (Note 4). Finely powdered pyridinium chlorochromate (50 g, 240 mmol) is added portionwise over a period of 5 min, and the mixture is stirred at room temperature for 3 hr (Note 5), then filtered through a pad of Florisil. The filtrate is concentrated on a rotary evaporator,... 

High purity (>99% ee) (1 R,4S)-4-hydroxy-2-cyclopentenyl acetate exhibiting 23... 

Although (1 R,4S)-(+)- and (lS,4R)-(-)-4-hydroxy-2-cyclopentenyl acetate are both available by enzyme-promoted enantioselective hydrolysis,8 9 different enzymes are, of course, required to achieve this stereochemical divergence. Economy would be realized if one of these enantiomeric products could serve as the starting point for the preparation of both antipodal forms of structurally more advanced intermediates. The importance of (4R)-(+)-10>11 and (4S)-(-)-tert-butyldimethylsiloxy-2-cyclopenten-1-one12 to prostaglandin synthesis is well established. The latent potential of these highly functionalized building blocks for the enantiospecific synthesis of other natural... 

This collection begins with a series of three procedures illustrating important new methods for preparation of enantiomerically pure substances via asymmetric catalysis. The preparation of 3-[(1S)-1,2-DIHYDROXYETHYL]-1,5-DIHYDRO-3H-2.4-BENZODIOXEPINE describes, in detail, the use of dihydroquinidine 9-0-(9 -phenanthryl) ether as a chiral ligand in the asymmetric dihydroxylation reaction which is broadly applicable for the preparation of chiral dlols from monosubstituted olefins. The product, an acetal of (S)-glyceralcfehyde, is itself a potentially valuable synthetic intermediate. The assembly of a chiral rhodium catalyst from methyl 2-pyrrolidone 5(R)-carboxylate and its use in the intramolecular asymmetric cyclopropanation of an allyl diazoacetate is illustrated in the preparation of (1R.5S)-()-6,6-DIMETHYL-3-OXABICYCLO[3.1. OJHEXAN-2-ONE. Another important general method for asymmetric synthesis involves the desymmetrization of bifunctional meso compounds as is described for the enantioselective enzymatic hydrolysis of cis-3,5-diacetoxycyclopentene to (1R,4S)-(+)-4-HYDROXY-2-CYCLOPENTENYL ACETATE. This intermediate is especially valuable as a precursor of both antipodes (4R) (+)- and (4S)-(-)-tert-BUTYLDIMETHYLSILOXY-2-CYCLOPENTEN-1-ONE, important intermediates in the synthesis of enantiomerically pure prostanoid derivatives and other classes of natural substances, whose preparation is detailed in accompanying procedures. 

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