Imines conversion

Although monodentate isocyanide-tantalum complexes are available by alternative routes,204 the majority of studies are concerned with the resultant 2-imines. Conversely, similar reactions with nitriles give monodentate imine moieties, which can undergo reactions while remaining attached to the metal.195,203-209... 

An elegant application of the intramolecular IDA cycloaddition of an indole derivative with an in situ generated ortho quinone methide imine (conversion of 30... 

Dopa quinone imine conversion factor M,sexta Pharate pupal cuticle 11,63... 

To summarize, dopa quinone imine conversion factors have been detected in cuticle and/or hemolymph from only three species of Lepidoptera. Other kinds of regulatory factors such as dihydroxy-indole blocking factor have not been detected in insect tissues. The precise physiological roles played by conversion factors that generate indoles is unknown. They may be modulators of reactions associated primarily with melanization. 

Decarboxylation of imines derived from a-hydroxycarbonyl compounds, having a free hydroxyl group in the fS-position relative to the nitrogen atom, yields the corresponding azomethine yhd. Another alternative of the imine conversion is the formation of imine betaine and intramolecular cycHsation of the product to an oxazoHdin-5-one derivative. Both of these compounds provide by decarboxylation azomethine yhd, which can exist in two forms stabihsed by resonance that differ in the position of the C=N bond. 

A later variation of the general method, which extends the scope to 20-ketones, involves reaction of the ketone with benzylamine to give the imine, followed by conversion to the A-acetyl derivative with acetic anhydride. Although the resulting compound also has a A -double bond, it does not react sufficiently fast with peracid, and a A -double bond can not be preserved. 

The formation of bicyclic imines (263,264) from piperidine enamines and y-bromopropyl amines may appear at first sight to be a simple extension of the reactions of enamines with alkyl halides. However, evidence has been found that the products are formed by an initial enamine exchange, followed by an intramolecular enamine alkylation. Thus y-bromodiethylamino-propane does not react with piperidinocyclohexene under conditions suitable for the corresponding primary amine. Furthermore, the enamine of cyclopentanone, but not that of cyclohexanone, requires a secondary rather than primary y-bromopropylamine, presumably because of the less favorable imine to enamine conversion in this instance. 

We have previously discussed that keto-aldehydes react with anilines first at the aldehyde carbon to form the aldimine. Subsequent condensation with another aniline formed a bis-imine or enamino-imine. The aniline of the ketimine normally cyclizes on the aldimine (24 —> 26). Conversely, cyclization of the aldimine could be forced with minimal aniline migration to the ketimine using PPA (30 —> 31). The use of unsymmetrical ketones has not been thoroughly explored a few examples are cited below. One-pot enamine formation and cyclization occurred when aniline 48 was reacted with dione 49 in the presence of catalytic p-TsOH and heat. Imine formation occurred at the less-hindered ketone, and cyclization with attack on the reactive carbonyl was preferred. ... 

Analogous to the selectivity observed for the conversion of 48 into 50, pyridyl 51 formed enamine 52 which underwent cyclization to give 4-pyridyl-substituted quinoline 53. Again, imine formation first occurs on the less hindered ketone and subsequent cyclization on the more reactive carbonyl occurred in high yield. ... 

Dihydro-2/7- 74 and -4//-l,2-oxazines and thiazines 75 are interrelated by prototropy, being enamines and imines, respectively. In the case of oxazines, the imine form 75 is favored, and there are several well established examples of this system, including the parent heterocycle 75 (X = O) [84MI2]. No tautomeric equilibrium between the 2H and 4H forms has been observed under normal conditions in solution or in the solid state. However, the formation of intermediate 2H isomers 77 was proposed both for the conversion of 3-phenyl-5,6-dihydro-4//-l,2-oxazine 76 (R = Ph, r = R = H) into 2-phenylpyrrole(89TL3471) under strong basic conditions and for thermal decomposition of cyclopentene-fused 1,2-oxazine 76... 

J -methylpyrrolidone by conversion to the imine ( 7) by sequential reaction with triethyloxonium tetraf1uoroborate and then anhydrous ammonia. When this is reacted with 2,6-dimethyl-phenyl i socyanate, the centrally acting muscle relaxant xilobam (8) is formed. ... 

Reduction Conversion of Nitriles into Amines Reduction of a nitrile with LiAIH4 gives a primary amine, RNH . The reaction occurs by nucleophilic addition of hydride ion to the polar C=N bond, yielding an imine anion, which still contains a C=N bond and therefore undergoes a second nucleophilic addition of hydride to give a dianion. Both monoanion and dianion intermediates are undoubtedly stabilized by Lewis acid-base complexafion to an aluminum species, facilitating the second addition that would otherwise be difficult Protonation of the dianion by addition of water in a subsequent step gives the amine. 

Fischer s original method for conversion of the nitrile into an aldehyde involved hydrolysis to a carboxylic acid, ring closure to a cyclic ester (lactone), and subsequent reduction. A modern improvement is to reduce the nitrile over a palladium catalyst, yielding an imine intermediate that is hydrolyzed to an aldehyde. Note that the cyanohydrin is formed as a mixture of stereoisomers at the new chirality center, so two new aldoses, differing only in their stereochemistry at C2, Tesult from Kiliani-Fischer synthesis. Chain extension of D-arabinose, for example, yields a mixture of D-glucose and o-mannose. 

Figure 29.15 Mechanism of steps 2-4 of amino acid transamination, the conversion of a PLP-amino acid imine to PMP and an a-keto acid.

The intramolecular Heck reaction presented in Scheme 8 is also interesting and worthy of comment. Rawal s potentially general strategy for the stereocontrolled synthesis of the Strychnos alkaloids is predicated on the palladium-mediated intramolecular Heck reaction. In a concise synthesis of ( )-dehydrotubifoline [( )-40],22 Rawal et al. accomplished the conversion of compound 36 to the natural product under the conditions of Jeffery.23 In this ring-forming reaction, the a-alkenylpalladium(n) complex formed in the initial oxidative addition step engages the proximate cyclohexene double bond in a Heck cyclization, affording enamine 39 after syn /2-hydride elimination. The latter substance is a participant in a tautomeric equilibrium with imine ( )-40, which happens to be shifted substantially in favor of ( )-40. 

Since these adducts undergo reductive desulfuration with Raney nickel, optically active aryl methyl sulfoxides are versatile reagents for the conversion of imines to optically active amines. 

The first step in either direction consists of addition of NaHS03 to one of the double bonds of the ring, which gives an enol (or enamine) that tautomerizes to the keto (or imine) form. The conversion of 12 to 13 (or vice versa) is an example of 16-12 (or... 

For the addition of an organometailic compound to an imine to give a primary amine, R in RCH=NR would have to be H, and such compounds are seldom stable. However, the conversion has been done, for R = aryl, by the use of the masked reagents (ArCH=N)2S02 [prepared from an aldehyde RCHO and sulfamide (NH2)2S02]. Addition of R"MgX or R"Li to these compounds gives ArCHR"NH2 after hydrolysis. ... 

In a series of three papers, Noguchi and co-workers have reported their continuing studies on the formation of heterocycle-fused azepine systems <96X13081, 96X13097, 96X13111>. A typical example is the conversion of the aldehyde 15 into the azepines 16 and 17 (Scheme 3). Xhe reaction also proceeds with imines when the dihydroazepine prior to bridging can be isolated. Mechanistic and stereochemical aspects of the reaction have been explored. 

The overall pathway for the conversion of the unsaturated azido ether 281 to 2,5-dihydrooxazoles 282 involves first formation of the dipolar cycloaddition product 287, which thermolyzes to oxazoline 282 or is converted by silica gel to oxazolinoaziridine 288. While thermolysis or acid-catalyzed decomposition of triazolines to a mixture of imine and aziridine is well-documented [71,73], this chemoselective decomposition, depending on whether thermolysis or exposure to silica gel is used, is unprecedented. It is postulated that acidic surface sites on silica catalyze the triazoline decomposition via an intermediate resembling 289, which prefers to close to an aziridine 288. On the other hand, thermolysis of 287 may proceed via 290 (or the corresponding diradical) in which hydrogen migration is favored over ring closure. 

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