Dihydropyridine, imine reactions with

A high density of electrons associated with atoms C(3) and C(5) of 1,4-dihydropyridines and 1,4-dihydropyrimidines is also observed when these heterocycles undergo electrophilic substitutions such as Friedel-Crafts [315, 316, 317, 318, 319, 320] and Vilsmeier [297, 321] reactions (Scheme 3.99). In [315] it was shown that treatment of dihydropyridines 371 with aroyl or acyl chlorides 372 in the presence of SnCl4 leads to acylation of the heterocycle at position 3 (compounds 373). Dihydropyridines 374 and dihydroazolopyrimidines 376 undergo Vilsmeier reaction with the formation of the corresponding derivatives 375 and 377. It is interesting that imine heterocycle 376 after Vilsmeier reaction exists in the enamine tautomeric form. The tautomerism of dihydroazines and factors influencing it will be discussed in detail in Sect. 3.8. 

Inspired by the recent observation that imines are reduced with Hantzsch esters in the presence of achiral Lewis or Brpnsted acid catalysts (Itoh et al. 2004), we envisioned a catalytic cycle for the reductive amination of ketones which is initiated by protonation of the in situ generated ketimine 10 from a chiral Brdnsted acid catalyst (Scheme 13). The resulting iminium ion pair, which may be stabilized by hydrogen bonding, is chiral and its reaction with the Hantzsch dihydropyridine 11 could give an enantiomerically enriched amine 12 and pyridine 13. 

Imine Alkynes Cyclization. Bu2MeP is the ligand of choice for the formation of 1,2-dihydropyridines by reaction of a sul-fonylimine with some alkynes. In stoichiometric conditions, the isolation of azanickelacycles intermediates was possible, but the reaction also proceeded catalytically giving directly the final dihydropyridine (eq... 

Heteroatom Wittig chemistry also includes reactions of N-sulfonyl imines. It was demostrated that these compounds underwent olefination reactions with nonstabilized phosphonium ylides under mild conditions to afford an array of both Z- and E-isomers of 1,2-disubstituted alkenes, allylic alcohols, and allylic amines.Additionally, studies of the reactions of 5-bromo-4,6-dimethyl-2-thioxo-l,2-dihydropyridine-3-carboni-trile and thiazolidinone with phosphorus ylides have proved the formation of new phosphonium ylides. Annulations via P-ylides are a common occurrence in the literature. For example, on photochemical irradiation, phosphonium-iodonium ylides were shown to undergo 1,3-dipolar cycloaddition reactions with triple bonds, via a carbene intermediate, to yield furans. " Even more common are the reactions of Morita-Baylis-Hillman (MBH) acetates and carbonates. Zhou et al. demostrated that these substrates were able to generate very reactive 1,3-dipoles in the presence of tertiary phosphines the dipoles then underwent cycloaddition reactions to yield annulation products (Scheme 16). ... 

Step, aldol condensation to form the benzylidene derivative (12-3). Conjugate addition of a second mole of acetoacetate would then afford the 1,5-diketone (12-4). Reaction of the carbonyl groups with ammonia will lead to the formation of the dihydropyridine ring. Alternatively, acetoacetate may go on to form the imine (12-5) reaction of this with the aldol product (13-3) will give the same dihydropyridine. The product, nifedipine (12-6) [13], has been used extensively for the treatment of angina and hypertension. 

The reaction of ketocarbenoids with pyrroles leads to either substitution or cyclopropanation products, depending on the functionality on nitrogen. With N-acylated pyrrole (209) reaction of ethyl diazoacetate in the presence of copper(I) bromide generated the 2-azabicyclo[3.1.0]hex-3-ene system (210) and some of the diadduct (211 Scheme 44).162163 On attempted distillation of (210) in the presence of copper(I) bromide rearrangement to the 2-pyrrolylacetate (212) occurred, which was considered to proceed through the dipolar intermediate (213). In contrast, on flash vacuum pyrolysis (210) was transformed to the dihydropyridine (214). A plausible mechanism for the formation of (214) involved rearrangement of (210) to the acyclic imine (215), which then underwent a 6ir-electrocyclization. 

MISCELLANEOUS REACTIONS OF DIHYDROPYRIDINES Additional tests for net hydride transfers initiated by single-electron transfer include the use of substrates in which such pathways would necessarily involve readily ring-opened cyclopropylmethyl or readily cyclized 5-hexenyl radicals. Products from these radical reactions are not formed in NAD+/ NADH dependent enzymic reductions or oxidations (Maclnnes et al., 1982, 1983 Laurie et al., 1986 Chung and Park, 1982). Such tests have also been applied in non-enzymic reductions. Thus cyclopropane rings in cyclopropyl 2-pyridyl ketones, or imines of formylcyclopropane (van Niel and Pandit, 1983, 1985 Meijer et al., 1984) survive Mg+2 catalysed reduction by BNAH or Hantzsch esters but are opened by treatment with tributylin hydride. 

Allyl and benzyl bromides react with a,/ -unsaturated nitriles in the presence of indium(i) iodide under sonication to produce the corresponding allylated and benzylated imines, involving exclusive addition of the allyl/benzyl group to the nitrile moiety (Equation (63)).273 The reaction of allylindium reagents with methyl cyanoacetates affords the corresponding allylation-enamination products (Equation (64)).27 l-Acyl-l,2-dihydropyridines are prepared by indium-mediated allylation of 1-acylpyridinium salts (Equation (65)).275 Quinoline and isoquinoline activated by... 

Rhodium-catalyzed chelation-assisted C—H bond functionalization reactions (enantioselective annulation of aryl imines, dihydropyridine synthesis from imines and ahcynes, one-pot synthesis of pyridines from imines and alkynes, 2-arylpyridine alkylation with imines) 12ACR814. Synthesis of pyridine and dihydropyridine derivatives by regjo- and stereoselective addition to N-activated pyridines 12CRV2642. 

Based on previous studies where the imines were reduced with Hantzsch dihydropyridines in the presence of achiral Lewis [43] or Brpnsted acid catalysts, [44] joined to the capacity of phosphoric acids to activate imines (for reviews about chiral phosphoric acid catalysis, see [45-58]), the authors proposed a reasonable catalytic cycle to explain the course of the reaction (Scheme 3) [41]. A first protonation of the ketimine with the chiral Brpnsted acid catalyst would initiate the cycle. The resulting chiral iminium ion pair A would react with the Hantzsch ester lb giving an enantiomerically enriched amine product and the protonated pyridine salt B (Scheme 3). The catalyst is finally recovered and the byproduct 11 is obtained in the last step. Later, other research groups also supported this mechanism (for mechanistic studies of this reaction, see [59-61]). 

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