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A Remarkable Effect of C−O−C Bond Angle Strain on the Regioselective Double Nucleophilic Substitution of the Acetal Group of Tetraacetal Tetraoxa-Cages and a Novel Hydride Rearrangement of Tetraoxa-Cages

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journal contribution
posted on 16.05.1997, 00:00 by Hsien-Jen Wu, Jyh-Haur Chern
A remarkable effect of C−O−C bond angle strain on the regioselective double nucleophilic substitution of the acetal group of tetraacetal tetraoxa-cages and a novel regioselective and stereoselective hydride rearrangement of tetraoxa-cages are reported. Reaction of the tetraacetal tetraoxa-cages 1 with 3 equiv of triethylsilane (at −78 °C), cyanotrimethylsilane (at 25 °C), and allyltrimethylsilane (at −78 °C) in dichloromethane in the presence of TiCl4 gave the double nucleophilic substitution products 2, 6, and 7 in 85−90% yields, respectively. No detectable amount of other regioisomers was obtained. Reaction of 1a with (methylthio)trimethylsilane and (phenylthio)trimethylsilane in dichloromethane in the presence of TiCl4 at −78 °C gave the symmetric products 10a,b and the unsymmetric products 11a,b in ratios of 8−10:1. The stereochemistry of the symmetric substitution products was proven by X-ray analysis of the crystalline compound 10a. The mechanism of the double nucleophilic substitution of the tetraoxa-cages 1 are discussed. Treatment of the tetraoxa-cages 1a,c and 22a−c with 2 equiv of TiCl4 or MeSO3H in dichloromethane at 25 °C for 3 h regioselectively and stereoselectively gave the novel hydride rearrangement products 16a,b and 23a−c respectively. No detectable amount of other regioisomers was observed. The stereochemistry of the hydride rearrangement was proven by DIBAL-H reduction of 16 and 23 and X-ray analysis of the reduction product 24a. We attribute the high regioselectivity of the double nucleophilic substitution and the hydride rearrangement of the tetraoxa-cages 1 to the bond angle strain of the unusually large bond angle of C(3)−O(4)−C(5) of the tetraoxa-cages.

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