Cyclopentane-1,2-diols, reaction

For further contributions on the dia-stereoselectivity in electropinacolizations, see Ref. [286-295]. Reduction in DMF at a Fig cathode can lead to improved yield and selectivity upon addition of catalytic amounts of tetraalkylammonium salts to the electrolyte. On the basis of preparative scale electrolyses and cyclic voltammetry for that behavior, a mechanism is proposed that involves an initial reduction of the tetraalkylammonium cation with the participation of the electrode material to form a catalyst that favors le reduction routes [296, 297]. Stoichiometric amounts of ytterbium(II), generated by reduction of Yb(III), support the stereospecific coupling of 1,3-dibenzoylpropane to cis-cyclopentane-l,2-diol. However, Yb(III) remains bounded to the pinacol and cannot be released to act as a catalyst. This leads to a loss of stereoselectivity in the course of the reaction [298]. Also, with the addition of a Ce( IV)-complex the stereochemical course of the reduction can be altered [299]. In a weakly acidic solution, the meso/rac ratio in the EHD (electrohy-drodimerization) of acetophenone could be influenced by ultrasonication [300]. Besides phenyl ketone compounds, examples with other aromatic groups have also been published [294, 295, 301, 302]. 

In the same publication, an enantioselective process was attempted wherein commercially available (2f ,3/ )-butane-2,3-diol was used to generate the chiral cyclopentene 57, which was cyclopropanated to afford gcm-dibromocyclopropane 58 (Scheme 4.20). Unfortunately, when this substrate was subjected to the reaction conditions outlined above, product 59 was obtained as a 1 1 mixture of diastereomers. This result implies that selectivity in these trapping processes is unaffected by the presence of a chiral auxiliary on the remote carbon of the cyclopentane framework. 

Several routes to cycloalkanes have been developed (Fig. 4). Bolton [29] described the synthesis of azabicyclo[4.3, 0]nonen-8-ones using an intramolecular Pauson-Khand cycliza-tion. The relative stereochemistry was controlled in this cyclization step which proceeded in good yield regardless of whether the nitrogen atom bore an allyl (shown) or propargyl (not shown) substituent. The ene reaction was employed in a route to trans-substituted cyclopentane and cyclohexanes [30], Reductive cleavage from the resin with LiBFL, provided the diol or, alternatively, cleavage with Ti(OEt)4 produced the diester. 

Reaction of the diol with p-toluenesulfonyl chloride in pyridine, however, produced the ditosylate in nearly quantitative yield. SN2 displacements by chloride on neopentyl tosylate, which bears certain structural similarities to the ditosylate precursor of CAMPHOS, have been shown to give good yields of neopentyl chloride. However, when l,2,2-trimethyl-l,3-bis(hydroxymethyl)-cyclopentane ditosylate was allowed to react with sodium chloride in hexa-methylphosphoramide, in an attempt to form the dichloride, only N, A -dimethyl-p-toluenesulfonamide was isolated. Reaction of the ditosylate with lithium chloride in ethoxyethanol was exothermic and HC1 was evolved but the dichloride was not isolated. The isolated product contained at least one oleflnic bond. Similarly, in N, TV-dimethylformamide, lithium chloride and the ditosylate gave a product that decomposed on distillation. Faced with such repeated failures, a dihalide route to CAMPHOS was abandoned in favor of a more direct approach reaction of the ditosylate with diphenylphosphide anion. 

A more versatile reducing agent is samarium diiodide, which promotes chemoselective cyclizations of functionalized keto aldehydes in a stereodefined manner to form 2,3-dihydrocyclopentane carboxylate derivatives in good yields and with diastereoselectivities of up to 200 1 (equation 38)7 The reaction proceeds via selective one-electron reduction of the aldehyde component and subsequent nucleophilic attack on the ketone moiety. Stereochemical control is established by chelation of the developing diol (19) with Sm " " which thereby selectively furnishes cis diols (equation 39). The stereoselective M/-cyclization of 1,5-diketones to cis cyclopentane-1,2-diols using TiCU/Zn has been used to prepare stereodefined sterically hindered acyclic 1,2-diols when a removable heteroatom, such as sulfur or selenium, is included in the linking chain (equation 40). 

An nnnsnal ring-contraction reaction occnrs on the acid-catalyzed interaction of trimeth-ylhydroqninone 144 with cycloalkane-1,2-diols (e.g. 145) to form the spiro componnds 146 (eqnation 67) . Besides two isomers of cyclohexane-1,2-diols 145, this rearrangement was also described for cyclopentane-, cycloheptane- and cyclooctane-1,2-diols . 

Lead tetraacetate is very frequently used for cleavage of 1,2-diols and preparation of fhe resulting carbonyl compounds. The rate of reaction is highly dependent on the stereochemistry of the substrate. There is usually correlation between the rate of oxidation and the spatial proximity of the hydroxy groups. For example, the rate of the oxidative cleavage of cis-cyclopentane-l,2-diol is much faster than that of trans isomer. It is, however, possible to oxidize trans-l,2-diol to diketone (Scheme 13.51) [72 a]. 

Figure 6.6 shows our synthetic plan for testudinariol A (149). Because the structural feature of target molecule 149 is its C2-symmetry, 149 can be obtained by dimerization or its equivalent operation of A. The intermediate A may be prepared from B by (Z)-selective installation of the two-carbon appendage. For the stereoselective construction of the cyclopentane portion of B, an intramolecular ene reaction is appropriate employing C as the substrate. The intramolecular oxy-Michael-type cyclization of D has been adopted to prepare the tetrahydropyran ring of C. The intermediate D can be synthesized from F [(R)-glycidol] via the known diol E. 

---