3- Cyclopentene-l,2-diol

The photooxidation of cyclopentadiene by singlet oxygen is one step of an industrial process to make 2-cyclopentene-l, 4-diol [40]. Hence the driver is a commercial one, namely to develop a continuous synthesis of this molecule. 

This reachon, of industrial interest, utilizes singlet oxygen generated by irradiation in the presence of Rose Bengal [40]. An endoperoxide is formed as intermediate which is converted to 2-cyclopentene-l,4-diol by reduchon with thiourea. 

GL 24] [R 1] [P 26] The feasibility of safely carrying out the oxidation of cyclopentadiene by singlet oxygen to 2-cyclopentene-l, 4-diol was demonstrated [40]. The explosive intermediate endoperoxide was generated and without isolation used onsite for a subsequent hydration reaction. By reduction with thiourea the pharmaceutically important product 2-cyclopentene-l,4-diol was so obtained. 

The prochiral meso form of 2-cyclopenten-l,4-diol (101) reacts with the (Z)-alkenyl iodide 102 to give the 3-substituted cyclopentanone 103 with nearly complete diastereoselectivity (98 2)[92], The reaction is used for the synthesis of prostaglandin. The alkenyl iodide 102 must be in the Z form in order to obtain the high diastereoselectivity. The selectivity is low when the corresponding ( >alkenyl iodide is used[93]. 

The photooxygenation of cyclopentadiene with Rose Bengal as photosensitizer was performed using a falling-film microreactor in methanol [317]. The endoperoxide is first generated and then reduced to 2-cyclopenten-l,4-diol, which is used as an intermediate in pharmaceutical drug synthesis. This route is not easily possible by batch processing because the explosive endoperoxide intermediate is formed in substantial amounts. 

Hydroxylation by hydrogen peroxide in t-butyl alcohol solution is catalyzed by osmium tetroxide. The catalyst is volatile and dangerous to handle but is conveniently used in a solution of the tertiary alcohol. The yields of diols are usually low (30-60%), and the process has not been adapted to large-scale preparations. In contrast to hydroxylation by performic acid, this procedure leads to cis addition of the two hydroxyl groups to the double bond. An extensive study of other catalysts has been made. Some catalysts, e.g., selenium dioxide and pertungstic acid, catalyze addition in the trans direction. Hydroxylation of cyclopentadiene takes place in the 1,4-positions to give 2-cyclopenten-l,4-diol. ... 

The dihydroxycyclopentene mixture was prepared from cyclopentene and peracetic acid. The mixture contains approximately 70% of 2-cyclopentene-l,4-diol. 

Starting with pure 2-cyclopentene-l,4-diol, the submitters obtained the dione in 67-79% yield. 

Cyclopentene-l,4-dione has been prepared by oxidation of 2-cyclopentene-l,4-diol with chromium trioxide in aqueous acetic acid or in aqueous acetone, and with silver chromate. The present method eliminates the tedious removal of large amounts of acetic acid and gives a higher yield. 

An application of the meso trick that does not, in principle, require recycling, has been provided (Fig. 7) by a Japanese group (25). Reaction of cis-2-cyclopenten-l,4-diol [36] with W-mesyl-S-phenylalanyl chloride gave, in addition to diester and recovered starting material, the diastereo-meric esters [37] and [38]. Separation was effected by either chromatography or crystallization. Conversion of the free alcohol of [37] to its tetra-hydropyranyl ether and saponification gave alcohol [39], Transfer of the... 

Reduction of epidioxides. Schenck and Dunlap used thiourea for reduction of epidioxides, obtained by 1,4-addition of singlet oxygen to conjugated dienes at low temperatures, to the corresponding diols. Kaneko et al. report that the diols can be obtained in one step by irradiation of the diene with singlet oxygen in the presence of thiourea. Under these conditions irradiation can be conducted conveniently at room temperature. This procedure was used to prepare cw-2-cyclopentene-l,4-diol and cw-2-cyclohexene-l,4-diol. 

Chemical resolution of a meso-elio/. Acylation of the symmetrical meyo-diol ciy-2-cyclopentene-l,4-diol (2) with 1 equiv. of the chiral reagent 1 in pyridine affords a mixture of the monoesters 3 and 4 in 517 yield, together with some of the diester. The mono esters were separated readily by fractional crystallization to give pure, optically active 3 and 4. These esters were converted into the (+ )-and ( —)-5 hydroxy ethers, respectively, and then into the optically active lactones ( + )- and (—)-6 by Claisen rearrangement and lactonization (6, 608-609). These lactones have been converted into both natural and unnatural prostaglandins. See scheme (I) at top of page 321. 

A cooled methanolic soln. of 6.4 g. cyclopentadiene containing rose Bengal and thiourea irradiated 2.5 hrs. at ca. 15° with the Pyrex-filtered light of a 450 w. high-pressure Hg-lamp in a O -stream, then stirred 12 hrs. at room temp, in the dark -> 5.7 g. ci5 -2-cyclopentene-l,4-diol. F. e. s. C. Kaneko, A. Sugimoto, and S. Tanaka, Synthesis 1974, 876. 

With a similar microreactor, a photoinduced [4 + 2]-cycloaddition was carried out by Jahnisch [12]. 2-Cyclopentene-l,4-diol is formed through the reaction of... 

In 1985, Duhamel reported 0-acylquinidines 86 (Fig. 7.9) as catalysts for the kinetic benzoylation of 2-cyclopenten-l,4-diol with 47% enantioselectivity, albeit in low yield (8%) [108]. 

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