Takai methylation

I. Takai, A. Yamamoto, Y. Ishido, T. Sakakibara, and E. Yagi, Reactions of methyl 4,6-0-bcnzylidcnc-2,3-dideoxy-2-C- >-tolylsulfonyl-/S-D-mv/ o-hex-2-enopyranosidc and its phenyl analog with several nucleophiles stereoselective preparation of 3-O-acyl-D-arabino- and -D-rifco-hex- 1-emtol derivatives, Carbohydr. Res., 220 (1991) 195-207. 

For the first example, we chose to acylate olefin alcohol la. This was readily accomplished using acetic anhydride and 4-DMAP in pyridine to provide ester 17. Methylenation, using Takai s (10) protocol, yielded the acyclic enol ether 18 which was subsequently cyclized with 15 mol % of the Schrock catalyst 6 in hot toluene to afford the glycal 19 in good yield. Hydroboration and oxidative work-up led to the methyl-C-glycoside 20 (Scheme 4). With this proof of principle in hand, we then set out to prepare a number of additional examples as shown in Table 1 (11). 

Farnsworth, C.C., Kawata, M., Yoshida, Y., Takai, Y., Gelb, M.H., and Glomset, J.A. (1991). C terminus of the small GTP-binding protein smg p25A contains two geranylger-anylated cysteine residues aud a methyl ester. Proc Natl Acad Sci USA 88 6196-6200. 

T. Nakata et al. developed a simple and efficient synthetic approach to prepare (+)-methyl-7-benzoylpederate, a key intermediate toward the synthesis of mycalamides. The key steps were the Evans asymmetric aldol reaction, stereoselective Claisen condensation and the Takai-Nozaki olefination. The diastereoselective Claisen condensation took place between a 5-lactone and the lithium enolate of a glycolate ester. 

Jones PA, Takai D. The role of DNA methylation in mammalian epigenetics. Science 2001 293 1068-70. 

Nevertheless, in some cases the [2+2]-cycloaddition reaction under ultrasonic irradiation does not proceed or the yield is low. Different zinc powders were tested for the [2+2]-cycloaddition of dichloroketene with 1-methyl-l-cyclohexene. In line with the results of Takai et who reported the importance of catalytic amouts of lead on zinc reactions, we observed that zinc dust free of lead impurities was indeed poorly reactive, affording only ca. 10% of the cycloadduct. However, the reactions carried out under the same conditions, but with zinc powders containing lead as an impurity (from Union Mini re, Belgium), gave the cycloaddition product in 65-75% yield. The addition of a catalytic amount (0.5 mol%) of PbCl2 afforded highly reproducible results (80% yield after distillation). 

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