Vicarious Nucleophilic Substitution of hydrogen (VNS) offers a great potential for nucleophilic substitution of aromatic hydrogen with carbon, oxygen and nitrogen groups. Typical VNS consists of a reaction of nitroarenes with nucleophiles bearing a good leaving group at the nucleophilic center to furnish the corresponding ortho-or para-substituted nitroarenes. In this reaction, the leaving group connected with the nucleophile is eliminated from σH-adduct instead of unstable hydride anion. Thus, the leaving group acts as a vicarious leaving group; hence, this reaction is called “vicarious nucleophilic substitution of hydrogen” (VNS). Among the VNS reactions, VNS amination is of great importance as a direct introduction of an amino group (NH2) into an aromatic ring to give aminonitroarenes. Sulfenamides, 4-amino-1, 2, 4-triazole, 1, 1, 1-trimethylhydrazinium iodide, hydroxylamine and O-alkylhydroxylamine have been reported as VNS aminating agents so far. This review summarizes the VNS aminations by classifying into each aminating agents.
Samarium diiodide (SmI2) is widely employed as a useful single-electron reducing agent in organic synthesis because of its adequate reducing ability and suitable solubility in organic solvent such as THF. Although samarium diiodide itself can reduce aldehydes, ketones, alkyl bromides, and iodides, the reduction requires long period of heating and stirring. To enhance the reducing ability of SmI2, therefore, several unique methods have been developed recently, which include the addition of ligands, the combination with other metals, and photoactivation. This review deals with the recent advance in low-valent rare earth metal-assisted new reduction systems.
A classical route to chiral ferrocenes is optical resolution of the racemate and limited compounds have been prepared by this method. Recent successes in the synthesis of a variety of chiral ferrocenes without optical resolution now have attracted considerable attention in asymmetric synthesis. The chiral ferrocenylalcohols, -amines, -acetals, and-oxazolines can be readily prepared from achiral acyl-, formylferrocenes, and ferrocenecarboxylic acids. These substituents attached to cyclopentadienyl group of ferrocenes are strong ortho directing groups which lead to planar chiral ferrocenes with highly regioselection (>99% de). The planar chiral ferrocene can be also prepared by enantioselective lithiation of the achiral ferrocenylamide and amine with the chiral lithium reagent. C2 symmetrical 1, 1'-ferrocenediol is obtained from asymmetric reduction of 1, 1'-diacyl ferrocene and can be converted to the corresponding ferrocenediamine and-diphosphine derivatives. New planar chiral ferrocenyl phosphines and alcohols are examined in the benchmark asymmetric allylic substitution and dialkylzinc addition to an aldehyde, respectively.
syn-Dihydroxymetacyclophanes were converted to bridged calix  arene analogs by base-cat-alyzed condensation with formaldehyde. A remarkable template effect was observed on this condensation. Enlarged calix  arene analogs were synthesized by two different methods. All calix  arene analogs took the cone-type conformation and kept it even at high temperatures. From them, the unique chiral calixarenes were successfully synthesized by the following two methods : one is Williamson ether synthesis and the other is a stepwise ether cleavage with a mild Lewis acid. The high regioselectivity is recognized by the latter stepwise method. Also from them, the ionophores having oligoethylene glycol unit were obtained, which efficiently extracted larger alkali metal ions like K+, Rb+, and Cs+ than smaller ones like Li+ and Na+. Their ion selectivity apparently changed by the chain length of crown ether. All racemates obtained were resolved by the chiral HPLC column. The planar chiral calixarenes recognized the chirality of guest molecules. Thus, the (-) -receptor resolved strongly forms 1 : 1 complex with (R) - (+) - α-phenylethylammonium picrate.
A practical synthesis of methyl (2R, 3S) -3- (4-methoxyphenyl) glycidate (-)-2, a key intermediate for diltiazem (1), was accomplished through chiral dioxirane-mediated asymmetric epoxidation of methyl (E) -4-methoxycinnamate 3 with C2-symmetric binaphthyl ketone 4 (Yang catalyst) used as chiral catalyst. An efficient synthesis of 4 was developed, which involves novel optical resolution combined with efficient racemization of unwanted enantiomer and subsequent non-high diluted macrolactonization. The use of inexpensive sodium metal and a catalytic amount of methanol in the Claisen-Schmidt condensation of p-anisaldehyde with methyl acetate allowed an economical access to 3. Treatment of 3 with chiral dioxirane, generated in situ from chiral ketone 4, provided (-) -2 in 78% ee and 87% yield. The crude mixture of (-) -2 and 4 was efficiently separated by the use of a novel and simple equipment performing lipase-catalyzed transesterification and continuous dissolution and crystallization to furnish desired optically pure (-) -2 and recovery of 4 both in excellent yield [74% (based on 3) and 91%, respectively].