S-Benzyl group

Remarkably, the presence of water showed beneficial effects on both reaction rates and selectivities, while facilitating the iminium ion hydrolysis step in the catalytic cycle. Computational studies suggest an asynchronous mechanism for the reaction [23-25], where the attack of the diene to the / -carbon atom of the iminium ion is the rate-limiting step [23], and the 7r,7r-interaction between the phenyl ring of the catalyst s benzyl group and the olefinic 7r-system of the iminium ion accounts for the selectivity of the reaction [6, 24]. 

EtiSiH, cat. EtiN, cat. PdCl2, reflux, 3 h, 80% yield. If the reaction is performed in the presence of f-BuMe2SiH, the f-butyldimethylsilyl carbamate can be isolated because of its greater stability. S-Benzyl groups are stable to these conditions, but benzyl esters and benzyl ethers are cleaved. A similar procedure has been published, but in this case the benzyl ether was stable to the cleavage conditions. Alkenes are stable to these conditions. ... 

Of course, it is possible to decrease the stability of the S-benzyl group toward acids by substitution with electron-releasing groups. Thus, in the S-p-methoxybenzyl group the benzyl cation is sufficiently stabilized to allow cleavage of the S —C bond with liquid hydrogen fluoride under fairly mild conditions while the S-2,4,6-trimethylbenzyl group can be removed with trifluoromethanesulfonic acid in trifluoroacetic acid in the persence of w-cresol. 

A number of oxytocin analogs have been prepared using the S-benzyl group a summary may be found in the volumes by E. Schroder and K. Liibke, The Peptides , Academic Press, New York, N.Y. 1965. 

Nesvadba and Roth [66] have provided a description of an apparatus and have developed standardized conditions for the sodium-ammonia reduction. The reduction is relatively unselec-tive and removes iV-benzylcarbonyl, iV-tosyl, JV-trityl, as well as N- and O-benzyl groups. Recently Ives [67] has proposed that S-benzyl groups could be removed by electrolysis in liquid ammonia. 

The (-l-)-piperazic acid trifluoroacetate salt 208 was obtained from the cycloaddition of 4-phenyltriazoline-3,5-dione 204 with chiral diene 205 by Hamada and co-workers (Scheme 41.44). The reaction between the diene and the dienophile from —78°C to room temperature in dichloro-methane without additives afforded a 1 1 mixture of diaster-eomers. The use of 0.5 equivalents of titanium tetrachloride pleasingly resulted in an increase of the ratio to 84 16 in favor of 206a. After optimization, it was found that with 3 equivalents of the Lewis acid, the diastereoselectivity reached a 97 3 ratio in 95% yield. T vo equivalents of TiCLi are chelated by the two carbonyls of the dienophile, whereas 1 equivalent serves as the chelator between the two carbonyl functions in the chiral diene. The major conformer 207a that precluded the A strain is then engaged in a e cycloaddition with the azo compound opposite to the face of the chiral auxiliary s benzyl group. 

Some triethylamine and a catalytic amount of PdClg added to N-carbobenzoxy-glycyl-DL-alanine and 4 moles of triethylsilane, refluxed 3 hrs., the catalyst removed, and methanol added glycyl-DL-alanine. Y ca. 80%.— Benzyl esters are also cleaved whereas S-benzyl groups are not affected. F. e. s. L. Birkofer, E. Bierwirth, and A. Ritter, B. 94, 821 (1961). 

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