Cyclopentene oxide 2-Cyclopentenone

The domino carbonylation and Diels-Alder reaction proceed only as an intramolecular version. Attempted carbonylation and intermolecular Diels-Alder reaction of conjugated 2-yne-4-enyl carbonates 101 in the presence of various alkenes as dienophiles give entirely different carbocyclization products without undergoing the intermolecular Diels-Alder reaction. The 5-alkylidene-2-cyclopenten-4-onecarboxy-lates 102 were obtained unexpectedly by the incorporation of two molecules of CO in 82% yield from 101 at 50 °C under 1 atm [25], The use of bidentate ligands such as DPPP or DPPE is important. The following mechanism of the carbocyclization of 103 has been proposed. The formation of palladacyclopentene 105 from 104 (oxidative cyclization) is proposed as an intermediate of 108. Then CO insertion to the palladacycle 105 generates acylpalladium 106. Subsequent reductive elimination affords the cyclopentenone 107, which isomerizes to the cyclopentenone 108 as the final product. 

Two asymmetric synthesis approaches to chiral cyclopentenone derivatives can be envisaged. The first, reduced to practice by Noyori (43), involved reduction of cyclopentene-l,4-dione with lithium aluminum hydride chirally modified with binaphthol to give R-4-hydroxycyclopent-2-en-l-one in 94% e.e. Alternatively, manganese dioxide oxidation of allylic alcohol [40] (Fig. 7), in analogy to the cis isomer (54), would be expected to give the same enone. 

It is accepted that this reaction involves the formation of the alkynedicobalt hexacarbonyl complex from an alkyne and Co2(CO)s by the evolving of two CO ligands, followed by the alkene coordination at one of the two enantiotopic Co atoms with concomitant CO insertion, and final reductive elimination of the metal to an a,(3-unsaturated cyclopentenone. In the traditional protocol, the reaction mixture is heated in toluene at 110°C, or tertiary amine A-oxides are added to promote the reaction at ambient temperature. For the purpose of stereochemical control, many Pauson-Khand reactions are designed as intramolecular reactions P " or using cyclic alkenes, such as norbornene. It has been found that the reactivity of cyclic alkenes is in the order of cyclohexene < cyclopentene < norbornene. For the intermolecular Pauson-Khand reaction, alkene is positioned adjacent to the less bulky acetylenic substituent during coordination because of steric hindrance, and a subsequent C-C bond forms between an alkenic... 

Enantioselective ring opening of epoxides was attained with (salen)Cr(III) complex (191) [68]. Cyclopentene derivatives (190) were converted with Me3SiN3 to azide-alcohols (192) in 80-90% yields up to 98% ee (Scheme 16.56). Kinetic resolution of racemic styrene oxide was performed in 98% ee. This reaction was applied to practical synthesis of enantiopure cyclic 1,2-aminoalcohols by reduction of the azide products by Pt02-catalyzed hydrogenation [69], to synthesis of cyclopentenone derivatives [70] to formal synthesis of bioactive compound Balanol [71], and to solid-phase synthesis of peptides [72]. 

In the area of carbocyclic nucleoside antibiotics, hydrolysis of the racemic esters 40 (R= n-Bu or ii-CeHis) by the lipase from Candida rugosa proceeds with very high enantiomeric selectivity, and from the resolved materials both enantiomers of aristeromydn were made by an established route. The authors report that a previous similar method (Vol.21, p. 182) is not as enantioselective. In a new synthesis of neplanocin A (43), the alcohol 41, derived from D-ribose, was converted to the cyclopentene 42 using an intramolecular insertion reaction of an alkylidene carbene. The new stereocentre in 42 was mostly of the wrong P-configuration, but could be corrected by a process of desilylation, oxidation and borohydride reduction. The biosynthesis of neplanocin A (43) and aristero-mycin has been reinvestigated, and the cyclopentenone 44 has been proposed as an intermediate, which is converted to aristeromycin via neplanocin A without any bifurcation. The 3-deaza-analogue 45 of 5 - or-aristeromydn has been prepared, and the antiviral activity of it and of the 7-deaza-compound (Vol.27, p. 235) are reported. ... 

The isotope distribution in the cyclopentenone moiety was subsequently established. Tetrahydrodeoxoaflatoxin Bj was converted by a series of reactions into, e.g., the methycyclopentane 53 which on Kuhn-Roth oxidation gave cK-2-methylcyclopentanoic acid (54) and acetic acid, which established the origins of C(l), C(2), and C(7) (see Fig. 8). Oxidation of the methyl-cyclopentene 55 with osmic acid and potassium chlorate gave the diketone 56... 

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