Pyridinium 1,2,3,6-tetrahydro

Pyridinium-Salze werden mit Natrium- und Kaliumboranat in protonenhaltiger Losung nach folgendem Mechanismus zu den 1,2- und 1,4-Dihydro- sowie 1,2,3,6- und 1,2,3,4-Tetrahydro-pyridinen bzw. zu den Piperidinen reduziert1 ... 

Mit Lithiumalanat werden aus quartaren Pyridinium-Salzen Di-7 und Tetrahydro-pyridine , mit Aluminiumhydrid3, Natriumalanat4 9 10, Natrium-bis- 2-methoxy-athoxy]-di-hydrido-aluminat,0und Pyridin-Boran11 in unterschiedlichen Mengen auch Piperidine er-halten. 

Omission of the phenolic group from cyclazocine results in a molecule which retains analgesic activity. In a classical application of the Grewe synthesis,15 the methylated pyridinium salt 54 is condensed with benzylmagnesium bromide. There is thus obtained the dihydropyridine 55. Treatment of that intermediate with sodium borohydride results in reduction of the iminium function to afford the tetrahydro derivative 56. Cyclization of 56 on treatment with acid leads to the desired benzomorphan nucleus. The cis compound (57) is separated from the mixture of isomers and demethylated by the cyanogen bromide procedure (58,... 

A recent development is the transfer hydrogenation of heterocyclic systems such as pyrrole, pyridinium and quinoline systems. Whilst at present the yields and enantioselectivities are modest, further development may improve this situation. For example, 1-methyl-isoquinoline has been reduced to the tetrahydro species and 1-picoline has been reduced to 1-methylpiperidine [86]. Interestingly, these reductions involve alkene as well as imine reduction. 

It is quite difficult to reduce benzene or pyridine, because these are aromatic stmctures. However, partial reduction of the pyridine ring is possible by using complex metal hydrides on pyridinium salts. Hydride transfer from lithium aluminium hydride gives the 1,2-dihydro derivative, as predictable from the above comments. Sodium borohydride under aqueous conditions achieves a double reduction, giving the 1,2,5,6-tetrahydro derivative, because protonation through the unsaturated system is possible. The final reduction step requires catalytic hydrogenation (see Section 9.4.3). The reduction of pyridinium salts is of considerable biological importance (see Box 11.2). 

Pyridine is difficult to reduce (as is benzene ), but pyridinium salts, e.g. alkylpyridinium halides, are partly reduced by hydride transfer reagents such as lithium aluminium hydride (LiAlH ) and sodium borohydride (NaBH4). LiAlH, which must be used in anhydrous conditions, only gives the 1,2-dihydro derivative, but the less vigorous reductant NaBH in aqueous ethanol yields the 1,2,5,6-tetrahydro derivative (Scheme 2.30)1... 

Die waBrig-alkalische Ringspaltung von 1,1,2-Trimethyl- bzw. l,l-Dimethyl-2-ethyl-1,2,3,6-tetrahydro-pyridinium-jodid fuhrt in guten Ausbeuten zu 1 -Dimethylamino-(2Z,4E)-hexadien (65%) bzw. l-Dimethylamino-(2Z,4E)-heptadien (64%)2 ... 

BASF AG CRBPII dba DBN DBU DIBAL-H DMAP DMF DMF-DMA DMPU HMDS HMPA HMPT H-LR LDA LDE LRAT MCPBA MOM NMO NMP PCC PhH = Badische Anilin- Soda Fabrik AG = cellular retinol-binding protein type II r dibenzylideneacetone = 1,5-diazabicyclo[4.3.0]non-5-ene = l,8-diazabicyclo[5.4.0]undec-7-ene = diisobutylaluminium hydride = 4-dimethylaminopyridine = A V-dimethylformamide = A,V-dimethylformamide, dimethylacetal = 1,3 -dimethyl-3,4,5,6-tetrahydro-2( 1H)-pyrimidone = hexamethyldisilazane = hexamethylphosphoramide = hexamethylphosphorous triamide = Hoffmann-La Roche = lithium diisopropylamide = lithium diethylamide = lecithin retinol acyltransferase = m-chloroperbenzoic acid = methoxymethyl = iV-methylmorpholine oxide = l-methyl-2-pyrrolidinone = pyridinium chlorochromate = benzene... 

The pyridine ring in l,3-dimethylimidazo[l,2-a]pyridinium salts (196) is reduced in two two-electron steps in aqueous media to a dihydro- and then to a tetrahydro compound 197 [Eq. (116)],... 

Tetrahydropyrido[2,l-6][l,3]oxazine (29) afforded pyridinium bromide (30) on the action of aqueous hydrogen bromide, and no reaction occurred on treatment with NaBEL) in methanol [77JCS(P2)759]. Treatment of 3-oxo-2,3,7,8,9,10-hexahydro-l//-[l,3]oxazino[3,2-a]quinolinium chloride with ethanol yielded l-(2-carboxyethyl)-5,6,7,8-tetrahydro-2(l//)-quinolinone (69MI1). 

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