Oxime ethers 0-benzyl

These intermediates with benzyhnagnesinm bromides gave 2-benzyl-2/7-azirines 8 in 50-80% yields, bnt potassinm phthahmide and sodium salt of benzenethiol converted them into the corresponding azirines 9 and 10 in good yields (equation 4). Similar Neber-type cyclization of a-hthio oxime ethers to highly reactive azirines was recently reported. ... 

Intermolecular reactions between 0-benzyl oximes and ketones (or aldehydes) are limited to those with formaldehyde O-benzyl oxime. However, intramolecular coupling proceeds with carbonyl-tethered oxime ethers (Equation (69)). 

Silylation of benzyl C-H bonds using hydrosilanes can also be performed with the aid of Ru3(CO)12-catalyst (Table 1) [9]. This silylation occurs only at benzylic CH3 groups. Pyridine, pyrazole, and hydrazones function as good directing groups. Benzylamines, oxime ethers, dimethylanilides, and aryl pyridyl ethers have no activity in this silylation. 

It is noteworthy that this Cl reagent can be applied to a wide range of organic radicals involving rather stable radicals such as a-keto radical, a-alkoxyalkyl radical, and the benzyl radical for which the aforementioned radical formylation system with CO cannot be applied. The high reactivity of phenyl sulfonyl oxime ether is supported by kinetic studies [29]. The approximate rate constants for the addition of a primary alkyl radical to this phenyloxime ether was determined to be k=9.6xl0 M s at 25 °C, which is 1.8 times faster than the addition to aery-... 

Synergists do not have to contain the MDP grouping. Other compounds showing similar activity include aryl propynyl ethers, propynyl oxime ethers, propynyl phosphorate esters and benzyl thiocyanates. 

A useful application of an enolate-oxime ether condensation, described by Weeks, Volkmann and co-woricers, is found in the synthesis of 6-aminomethylpenicillin derivative (218), a potent -lactamase inhibitor. As shown in Scheme 45, the sensitive penicillin Grignard (216) is condensed with ethyl formaldoxime at -80 C in the presence of Bp3-OEt2 to afford adduct (217). The use of Bp3-OEt2 is critical because it allows the reaction to proceed at the low temperature required for the stability of enolate (216). Hydrogenolysis of (217) simultaneously results in removal of the ethoxy, bromo and benzyl groups, affording (218). 

Table 24 Condensation Reactions of Silyl Ketene Acetals (219) and O-Benzyl Oxime Ethers (220)...

Table 23 -Lactam-forming Reactions of O-Benzyl Oxime Ethers (214) and Enolates of Ester (213)IN...

Samarium diiodide has also been used for the intramolecular coupling of aldehydes and ketones with O-benzyl formaldoxime [83], for the corresponding intermolecular coupling with diphenylhydrazone [84,85] and for the intramolecular coupling of an a,p-unsaturated ester with an oxime ether [86] (Scheme 39). In all these cases the addition of HMPA was found to be essential for a successful reaction. 

Cyclization reactions. 3-Cyano-V-alkoxyindoles are formed when a-cyanobenzyl oxime ethers are oxidized with FeCls (a one-electron oxidant). Cyclization follows the generation of the benzylic radicals. ... 

Similarly, in the synthesis of (+)-7-deoxypancratistatin, a 6-exo cyclization of a benzylic radical onto the oxime ether was employed as a key step [8],... 

This free-radical acylation approach is extended for the synthesis of a-keto esters and ketones using phenylsulfonyl methoxycarbonyl oxime ether 5 [23] and bis-methanesulfonyl oxime ether 6, respectively (Scheme 6) [24], 5 is more reactive and effective than 2b. For instance, radical reaction of tert-butyl iodide with 5 gave tert-butyl oxime ester in 65% yield, whereas the use of 2b gave the corresponding tert-butyl oxime ether in 15% yield. In free-radical-mediated ketone synthesis via a sequential radical acylation approach, 6 is used as a carbonyl equivalent geminal radical acceptor. This method works well with primary alkyl iodides but somewhat less efficiently with secondary iodides and can be applied to prepare unsymmetrical acyclic ketones as well as cyclic ketones. It is noteworthy that stable allylic and benzylic radicals react smoothly with 6. 

Oxime ethers have been synthesized (as mixtures of E- and Z-isomers) similarly to oximes, by reacting acylphosphonates with the respective 0-methyl- or 0-benzylhy-droxylamines Alternatively, treatment of diethyl a-hydroxyiminophosphonates with benzyl bromide in the presence of sodium methoxide in boiling methanol gave pure (E)-0-benzylhydroxyiminophosphonates in good yields ... 

In the asymmetric reduction of ketones, stereodifferentiation has been explained in terms of the steric recognition of two substituents on the prochiral carbon by chirally modified reducing agents40. Enantiomeric excesses for the reduction of dialkyl ketones, therefore, are low because of the little differences in the bulkiness of the two alkyl groups40. In the reduction of ketoxime ethers, however, the prochiral carbon atom does not play a central role for the stereoselectivity, and dialkyl ketoxime ethers are reduced in the same enantiomeric excess as are aryl alkyl ketoxime ethers. Reduction of the oxime benzyl ethers of (E)- and (Z)-2-octanone with borane in THF and the chiral auxiliary (1 R,2S) 26 gave (S)- and (R)-2-aminooctane in 80 and 79% ee, respectively39. 

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