Iminium ethers

Scheme 78 <1999JOC4381>). The reaction afforded lactams and lactones in a ratio dependent upon the base used in the hydrolysis of iminium ether intermediates 187 (Scheme 79). When the hydrolysis is performed in the presence of NaHC03, lactone derivative 185 was predominant while in the presence of KOH, azocane 184 was the sole reaction product (Scheme 78). 

Optically active imidazolines are generally obtained from enantiopure 1,2-diamines. For example, chiral ferro-cenylimidazolines are prepared from ferrocenyl carboxylic acid 1241 (Scheme 310). Amide 1242 is activated by 0-alkylation with Et30 BF4, generating iminium ether tetrafluoroborate salt 1243. The formation of the imidazoline ring in 1244 is accomplished by condensation of 1243 with the chiral diamine 1245 at room temperature without the need to isolate the intermediate imidate 1243 <20050L4137>. 

As described earlier, quinone monoketals (e.g., 139) are not only good substrates for the DPM reaction, but they also participate in acid-catalyzed [5+2]cycloaddition reactions with styrene derivatives (e.g., p-methylstyrene) to givebicyclo[3.2.1]octenediones (e.g., 140) that can themselves participate in ODPM processes to give multiftinctional, donor-acceptor cyclopropanes (e.g., 141). The last type of conpound can participate in fiirther reactions generating several additional and novel scaffolds that may be useful in drug discovery settings. Meanwhile, iminium ethers that are readily derived from [5+2] cycloadducts of type 140 participate in ADPM rearrangements. 

Mechanistically, these reactions are related to the oxazoUne synthesis shown in Scheme 7.3. Both involve the initial formation of an oxonium ion followed by the now-intramolecular addition of the appended azide to this cation (Scheme 7.11). In the present case, migration with loss of N2 occurs to afford an iminium ether salt (e.g. 16), which can be isolated by precipitation with cold THE. Treatment of the iminium salt with aqueous NaHCOs then delivers an A-hydroxyalkyl lactam. 

It has been shown that in addition to iminium ether hydrolysis, other heteroatom and C-based nucleophiles may be incorporated into the lactam products. Through the addition of nucleophiles in a solution of DMF at ambient temperature, a range of lactams... 

Scheme 7.12 Effect of pH on mode of iminium ether hydrolysis...

Schmidt Rearrangement Reactions with Aikyi Azides 203 Table 7.4 NucieophiUc additions to iminium ethers... 

More recently the acylation of aldehyde enamines has been reinvestigated (75) and shown to proceed normally when the enamine is added to the acid chloride. The morpholine enamine of isobutyraldehyde (98), on being added to an ether solution of acetyl chloride, afforded the iminium salt (99), from which the ketoaldehyde (100) was obtained in 66% yield by hydrolysis (75). 

Iminium salts are readily available from C protonation of the corresponding enamines (7). Experimentally the procedure is very simple The enamine dissolved in ether or some other solvent is treated with an appropriate acid such as anhydrous hydrogen chloride or 70% perchloric acid. The iminium salt usually separates and is then collected. Protonation at low temperatures... 

The 2,7-naphthyridine system 53 (Scheme 8.4.18) was combined with 2,4-dinitrochlorobenzene and 2-amino glycerol for in situ reaction of the resulting Zincke salt. The resulting naphthyridinium 54 was trapped by Bradsher cycloaddition with (Z)-vinyl ether 55, providing tetracycle 56 (X-ray) upon internal addition of one of the diastereotopic hydroxymethyl groups to the resulting iminium. This approach was also extended to the use of chiral 2,7-naphthyridinium salts, prepared via the analogous Zincke process. ... 

The chiral (V-camphanoyl iminium ion 7, prepared by hydride abstraction from 2-camphanoyl-l,2,3,4-tetrahydro-6,7-dimethoxyisoquinoline 6 (see Appendix) with triphenylcarbenium te-trafluoroborate, reacts with silyl enol ethers to give 1-substituted tetrahydroisoquinoline derivatives with reasonable diastereoselectivity, 0°. On addition of titanium(IV) chloride, prior to the addition of the silyl enol ether, the diastereoselectivity gradually rises to an optimum at 2.5 equivalents of the Lewis acid, but the yield drops by 20%. 

This method fails, however, with bicyclic ketones such as 1-tetralones even in the presence of TsOH, affording only enol trimethylsilyl ethers such as 107 a [114, 115]. A subsequent investigation revealed that cyclohexanone reacts with equivalent amounts of N-trimethylsilyldimefhylamine 463 in the presence of TMSOTf 20 at -30 °C to give the enol silyl ether 107 a, whereas reaction of cyclohexanone, benzaldehyde, and chlorodimethyl ether with 463 and TMSOTf 20 or TCS 14 at 1-20 °C afforded the iminium salts 547, 548, and 549 in high yield [116-118]. Analogously, N-trimethylsilylpyrrolidine 550 and N-trimethylsilylmorphoHne 294 convert aldehydes such as benzaldehyde, at ambient temperature in the presence... 

With trimethylsilyl iodide 17 the 0,N-acetal 457 gives the iminium iodide as reactive intermediate this converts the enol silyl ether 107 a in situ into the Man-nich-base 669, in 81% yield, and hexamethyldisiloxane 7 [195]. On treatment of the 0,N-acetal 473 (or the N-silylated Schiff base 489) with TMSOTf 20 (or Zny, the intermediate iminium triflate adds to the ketene acetal 663 to give mefhoxytri-methylsilane 13 a and silylated / -amino esters such as 670, which are readily transsilylated by methanol to give the free / -aminoester [70, 196] (Scheme 5.61). 

Scheme 2.12 shows some representative Mannich reactions. Entries 1 and 2 show the preparation of typical Mannich bases from a ketone, formaldehyde, and a dialkylamine following the classical procedure. Alternatively, formaldehyde equivalents may be used, such as l>is-(di methyl ami no)methane in Entry 3. On treatment with trifluoroacetic acid, this aminal generates the iminium trifluoroacetate as a reactive electrophile. lV,A-(Dimethyl)methylene ammonium iodide is commercially available and is known as Eschenmoser s salt.192 This compound is sufficiently electrophilic to react directly with silyl enol ethers in neutral solution.183 The reagent can be added to a solution of an enolate or enolate precursor, which permits the reaction to be carried out under nonacidic conditions. Entries 4 and 5 illustrate the preparation of Mannich bases using Eschenmoser s salt in reactions with preformed enolates. 

Clerici and Porta reported that phenyl, acetyl and methyl radicals add to the Ca atom of the iminium ion, PhN+Me=CHMe, formed in situ by the titanium-catalyzed condensation of /V-methylanilinc with acetaldehyde to give PhNMeCHMePh, PhNMeCHMeAc, and PhNMeCHMe2 in 80% overall yield.83 Recently, Miyabe and co-workers studied the addition of various alkyl radicals to imine derivatives. Alkyl radicals generated from alkyl iodide and triethylborane were added to imine derivatives such as oxime ethers, hydrazones, and nitrones in an aqueous medium.84 The reaction also proceeds on solid support.85 A-sulfonylimines are also effective under such reaction conditions.86 Indium is also effective as the mediator (Eq. 11.49).87 A tandem radical addition-cyclization reaction of oxime ether and hydrazone was also developed (Eq. 11.50).88 Li and co-workers reported the synthesis of a-amino acid derivatives and amines via the addition of simple alkyl halides to imines and enamides mediated by zinc in water (Eq. 11.51).89 The zinc-mediated radical reaction of the hydrazone bearing a chiral camphorsultam provided the corresponding alkylated products with good diastereoselectivities that can be converted into enantiomerically pure a-amino acids (Eq. 11.52).90... 

Several lupin alkaloids have been derived from the unsaturated quinalozidine 433, that was obtained in the treatment of amine 431 with ortho-quinone 432. This quinone behaves as a model of topaquinone, the cofactor of copper-containing amine oxidases. The cyclization step involved a nucleophilic attack of the piperidine nitrogen of 431 onto a side-chain aldehyde function that is unmasked by the oxidative deamination. Quinolizine 433, when treated with dehydropiperidine, gave the oxime ether 434 that, on ozonolysis followed by reduction, afforded sparteine 10, presumably via the bis(iminium) system 435 (Scheme 102) <1996JOC5581>. 

Different rate-determining steps are observed for the acid-catalyzed hydration of vinyl ethers (alkene protonation, ks kp) and hydration of enamines (addition of solvent to an iminium ion intermediate, ks increasing stabilization of a-CH substituted carbocations by 71-electron donation from an adjacent electronegative atom results in a larger decrease in ks for nucleophile addition of solvent than in kp for deprotonation of the carbocation by solvent. 

A very elegant expansion of the synthetic utility of this intramolecular amination was the insertion reactions into ethereal G-H bonds. Du Bois and co-workers have exploited this reactivity to prepare cyclic sulfamates that are then used as iminium ion equivalents. Upon treatment with a suitable Lewis acid, nucleophilic addition reactions... 

Besides the allylation reactions, imines can also undergo enol silyl ether addition as with carbonyl compounds. Carbon-carbon bond formation involving the addition of resonance-stabilized nucleophiles such as enols and enolates or enol ethers to iminium salt or imine can be referred to as a Mannich reaction, and this is one of the most important classes of reactions in organic synthesis.104... 

In the presence of alcohols, the corresponding ethers are formed and added nucleophiles such as chloride ion40 or azide ion41 lead to the chloro- and azido-amine products, respectively. Rate constants are independent of the concentration of added nucleophile. Labelled 180 from the solvent is incorporated in the product42. All the evidence points to a reaction mechanism where water is lost from the O-protonated reactant to give a nitrenium ion-iminium ion intermediate which is rapidly trapped by a nucleophile (H2O in this case) to give the final product. This is shown in Scheme 7. Protonation at N- is likely to be more extensive, but there is no pathway to products from the N-protonated intermediate. 

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