Takai reagent

Methylenation of 58 with an excess of the Takai reagent and subsequent RCM with the second generation Grubbs catalyst 7 (35 mol%) gave an intermediate (bis)-C-glycal which was directly subjected to hydroboration by BH3 THF and subsequent oxidative work-up to afford the C-trisaccharide 60 in 49% yield over three steps (26). Although double RCM reactions are known (27), this was the first time it has been used in the context of C-saccharide synthesis. 

Tebbe reagent, Grubbs reagent, Petasis reagent, Takeda reagent and Takai reagent are... 

A valuable feature of the Nin/Crn-mediated Nozaki-Takai-Hiyama-Kishi coupling of vinyl iodides and aldehydes is that the stereochemistry of the vinyl iodide partner is reflected in the allylic alcohol coupling product, at least when disubstituted or trans tri-substituted vinyl iodides are employed.68 It is, therefore, imperative that the trans vinyl iodide stereochemistry in 159 be rigorously defined. Of the various ways in which this objective could be achieved, a regioselective syn addition of the Zr-H bond of Schwartz s reagent (Cp2ZrHCl) to the alkyne function in 165, followed by exposure of the resulting vinylzirconium species to iodine, seemed to constitute a distinctly direct solution to this important problem. Alkyne 165 could conceivably be derived in short order from compound 166, the projected product of an asymmetric crotylboration of achiral aldehyde 168. 

K. Takai, K. Sakogawa, Y. Kataoka, K. Oshima, and K. Utimoto 180 PREPARATION AND REACTIONS OF ALKENYLCHROMIUM REAGENTS 2-HEXYL-5-PHENYL-1-PENTEN-3-OL... 

Another way of synthesis of ketencdithioacetals and of alkenyl sulfides makes use of an alkylidenation of dithiocarbonates or thiolesters with a reagent introduced by Takai [439] and prepared from R CHBr2, Zn, TiCl4 and TMEDA (Table 4.4). 

Apart from the Takai method and titanium reagents such as 15, silyl reagents 16 and 17 frequently find application in the synthesis of vinylic silanes from carbonyl compounds. Reagent 16 can be utilized with aldehydes and non-enolizable ketones in a reaction analogous to the Peterson olefination Reagent 17 also reacts successfully with enolizable ketones.6... 

The first progress was made by Takai and Lombardo, who developed an in situ entry to titanium-alkylidene chemistry starting from the reagent combinations 5 and 6 (Scheme 4) [9]. These reactions proceed via a gem-dizinc compound 7 (its formation is catalyzed by traces of lead or lead(II) salts), which is subsequently transmetalated with TiCl4 to the titanium-alkylidene species 8, the actual olefination reagent. To date, 8 has not been characterized in detail [10]. These in situ reagents exhibit chemoselectivities similar to those of the structurally defined methylenation reagents 1-3. 

The advantage of the Takai-Lombardo reagents is the possibility of transferring substituted alky-... 

Scheme 4. Takai-Lombardo reagents TMEDA= N.iV,Af, N -tetramethyl-ethylenediamine.

Scheme 5. General alkylidenation of esters to enol ethers 9 and synthesis of disubstituted ( )-alkenes 10 from aldehydes and chromium(II) reagents according to Takai and Utimoto.

Cahiez, G. Figadere, B. Tetrahedron Lett. 1986,27,4445. For other organometaUic reagents with high selectivity towards aldehyde functions, see Kauffmann, T. Hamsen, A. Beirich, C. Angew. Chem. Int Ed. 1982, 21, 144 Takai, K. Kimura, K. Kuroda, T. Hiyama, T. Nozaki, H. Tetrahedron Lett. 1983, 24, 5281 Soai, K. Watanabe, M. Koyano, M. Bull. Chem. Soc. Jpn. 1989, 62, 2124. 

The reagents of the third step are also known as the Takai olefination conditions. Which functionality of the modified molecule reacts ... 

DDQ is a standard reagent for /7 /ra-methoxybenzyl (PMB) deprotection. The resulting free hydroxyl group is then oxidized to the corresponding aldehyde, which undergoes a subsequent Takai olefination. 

The addition of y-alkoxy allylic chtDmium(III) reagents to aldehydes has been reported by Takai and coworkers. This work is reviewed in Section 1.6.3.6. 

Takai and coworkers have recently reported their fmdings on the addition of y-alkoxyallylic chromium compounds to aldehydes. The reagents, which were generated by the reduction of dialkyl acetals (149) with C1CI2 in the presence of trimethylsilyl iodide, added to aldehydes (150) to produce vicinal diols... 

X = OTf) may fail to accomplish metal exchange and consequently hinder the formation of the alke-nylchromium reagent. Few examples of the use of enol triflates as progenitors of vinyl carbanion equivalents have appeared. Consequently, the work from Takai and coworkers is particularly useful. The alkenylchromiums generated from enol triflates are functionally indistinguishable from those generated from iodoalkenes (equation 65 and table 10). This fact is disclosed by comparison of Table 9 with Table 10. 

The generation of alkynylchr(xnium reagents and their addition to carbonyl compounds has been studied by Takai and cow( ers. The chemoselectivity of these reagents resembles that of other organo-chromium(ni) reagents. Preparation of alkynylchromiums can be accomplished by treatment of haloalkynes with chromium(II) chloride in DMF. Representative addition reactions of alkynylchromiums are offered in equations (75) to (79). 

Diastereoselective addition of Aiophenylalkylchromium(III) reagents has been studied by Takai and coworkers. Treatment of aldehyde (260) with a-chloro sulfide (261), Lil and CrCh gives 3-phenylthio alcohols (262) and (263) (equation 87 and Table 14). The syn isomer (262) was generated dia-stereoselectively in all cases (262 263 4 1). The diastereoselectivity of the addition reactions of thio-phenylalkylchromiums is especially noteworthy as the lithium reagent prepared from phenyl ethyl sulfi and BuT i-RMPA adds to benzaldehyde in a stereorandtxn fashion (contrast entry 2, table 14). 

Takai and coworkers have reported that the formation of alkyl-gem-dichromium compounds (269) can be achieved by the CrCk reduction of gem-diiodoalkanes (268 equation 90). These reagents add to aldehydes and affcnd alkylidenation products (271) and (272) efficiently and with high levels of ( )-al-... 

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