Tetrahydropyridines preparation

Direct addition of Grignard reagents to Zincke-derived chiral pyridinium salts such as 99, meanwhile, allowed subsequent reduction to 1,2,3,6-tetrahydropyridines (e.g., 100, Scheme 8.4.32). This strategy provided entry to asymmetric syntheses of (-)-lupetidin and (+)-solenopsin. Tetrahydropyridines prepared by reduction of chiral... 

Partial hydrogenation of pyrrole derivatives and partial dehydrogenation of pyrrolidines afford /I -pyrrolines (80-82). However, because of the complex nature of the reaction, it is of little preparative value. The same is true for isomerization of /) -pyrrolines to /) -pyrrolines (83). A photodehydrogenation of 2,6-dimethylpiperidine (26) has been observed recently, affording 2,6-dimethyl-3,4,5,6-tetrahydropyridine (27) in a good yield (84)-... 

Eda and Kurth applied a similar solid-phase combinatorial strategy for synthesis of pyridinium, tetrahydropyridine, and piperidine frameworks as potential inhibitors of vesicular acetylcholine transporter. One member of the small library produced was prepared from amino-functionalized trityl resin reacting with a 4-phenyl Zincke salt to give resin-bound product 62 (Scheme 8.4.21). After ion exchange and cleavage from the resin, pyridinium 63 was isolated. Alternatively, borohydride reduction of 62 led to the 1,2,3,6-tetrahydropyridine 64, which could be hydrogenated to the corresponding piperidine 65. 

Marazano and co-workers have used the Zincke reaction extensively to prepare chiral templates for elaboration to substituted piperidine and tetrahydropyridine natural products and medicinal agents. For example, 3-picoline was converted to Zincke salt 40 by reaction with 2,4-dinitrochlorobenzene in refluxing acetone, and treatment with R- -)-phenylglycinol in refluxing n-butanol generated the chiral pyridinium 77. Reduction to... 

Two polymorphic forms of 3- 2-[4-(6-fluorobenzisoxazol-3-yl)-l,2,3,6-tetrahydropyridin-l-yl]ethyl -2-methyl-6,7,8,9-tetrahydro-4//-pyrido[l,2-n] pyrimidin-4-one (137 R = H) were prepared (99MIP1). Racemic 9-hydroxy-2-methyl-3- 2-[4-(6-fluorobenzo[r/ isoxazol-3-yl)-l,2,3,6-tetrahydro-l-pyridyl] ethyl -6,7,8,9-tetrahydro-4//-pyrido[l, 2-n]pyrimidin-4-one was resolved into its (R)- and (5)-isomers (OOMIPIO). 

Reaction of tetrahydropyridin-4-one 119 and l,r-carbonyldiimidazole furnished l,3,4,4n,5,6-hexahydropyrido[l,2-c][l,3]oxazine-l,6-dione 120 (99JA2651). Similarly, pyrido[l,2-c][l,3]oxazine-l-one 121 and [1,3] oxazino[4,3-n]isoquinoline-4-one 122 were prepared from the respective 2-(2-hydroxypropyl)piperidine and l-(2-hydroxypropyl)-1,2,3,4-tetrahy-droisoquinoline (99JOC3790). Reaction of a 2 1 diastereomeric mixture of l-(l,2-dihydroxyethyl)-6,7-dihydroxy-l,2,3,4-dihydroisoquinolines 123 and 124 with l,l -carbonyldiimidazole gave a 2.7 1 mixture of 1,9,10-trihy-droxy-l,6,7,ll/)-tetrahydro-2//,4//-[l,3]oxazino[4,3-n]isoquinoline-4-ones 125 and 126, which were separated on preparative TLC plate (99BMC2525). 

The intramolecular cycloaddition has proven to be the method of choice for the preparation of steroids. A diastereomeric mixture of 204, prepared from 191 and tosylate 203 has been cleanly converted to dl-estra-1,3,5(10)-trien-17-one (205) in 85% yield (equation 130). A second example of the intramolecular cycloaddition reaction is the formation of the cycloadduct (209), the key intermediate in a synthesis of the As-pidosperma alkaloid aspidospermine, upon heating 208 at 600 °C (equation 131)124. The sulfone 208 can be prepared by reaction of 3-ethyl-3,4,5,6-tetrahydropyridine (206) with the acid chloride 207. 

A catch and release synthesis of tetrazoles and cyclic amidines has been reported making use of solid-supported oximes [94]. When bound sulpho-nyloximes, obtained by reacting polymer supported sulfonyl chloride with oximes, were reacted with nucleophiles, tetrazoles or cychc amidines were obtained (Scheme 19). Alternatively, the use of TMS-CN affords imino nitriles, which have been used as intermediates for the preparation of indoles, 1,2,3,4-tetrahydropyridines, quinoxalines and benzimidazoles. 

Oxa-tetrahydropyridines are interesting intermediates for the preparation of pharmaceuticals and natural product based alkaloid systems. A modified Hantzsch reaction was developed under microwave irradiation for the preparation of 2-oxa-tetrahydropyridines 173 by reaction of Meldrum s acid, a /3-ketoester and an aldehyde, using NH4OAC as the source of ammonia (Scheme 62). Yields ranged from 81 to 91% at temperatures of 100-130 °C depending on the substrate (the aldehyde) employed. All the products obtained have the same structure except for the aromatic substituent in position 4 [109]. 

Scheme 62 A modified Hantzsch reaction for the preparation of 2-oxa-tetrahydropyridines...

Phenyl-l,2,3,6-tetrahydropyrido[2,l- ][l,3]thiazino[3,2- ]quinolin-6-ones were prepared by the reaction of 2-mercapto-5-phenyl-l,4-dihydroquinolin-4-ones with 1,3-dihalopropane <1997JAK97/278780>. 7-Acetyl-2-aryl-9-cyano-6-methyl-8-phenyl-3,4-dihydro-277,877-pyrido[2,l- ][l,3]thiazin-4-ones were obtained from 5-acetyl-3-cyano-6-methyl-4-phenyl-l,2,3,4-tetrahydropyridine-2-thione with 3-aryl-2-propenoyl chloride <2002CHE761>. Reaction... 

Michael addition of (benzotriazol-l-yl)acetonitrile 557 to a,[)-unsatu rated ketones followed by heterocyclization provides new means for preparation of 2,4,5-trisubstituted pyridines. The reaction is catalyzed by bases. In the presence of secondary amines, a nucleophilic attack of amine on the CN group in adduct 556 initiates the cyclization to tetrahydropyridine 558 that subsequently eliminates water and benzotriazole to give pyridine 559. Analogously, in the presence of NaOH, pyridone 560 forms, via intermediate 561 (Scheme 88) <1997JOC6210>. 

The direct preparation of pyridinium salts (35) from acyclic starting materials 33 and 34 was reported by Adamczyk and co-workers <00TA2289>. Pyridinium salts have also been used to form other heterocyclic ring systems such as imidazolo[l,2-a]pyridines as reported by Katritzky <00JOC9201>. A final report worthy of comment is the preparation of chiral 2,6-disubstituted tetrahydropyridines via chiral pyridinium salts . 

Using trimethylsilyl triflate, a one-pot reaction of acetoxyallylation and O-silylation of nitrones, gave silylated hydroxylamines (673). Enantiomers of the naturally occurring alkaloid dihydropinidine, potential antifeedants against the pine weevil Hylobius abietis, were prepared by diastereoselective, dimethylzinc mediated addition of pinacolyl 2-propenylboronate to (/ )- and to (S )-2-methyl tetrahydropyridine-A-oxide, obtained from D-alanine and L-alanine, respectively (Scheme 2.190) (674). 

A stereoselective total synthesis of ( )-hirsutine has been developed by Brown et al. (179). Catalytic hydrogenation of hydroxycyclopentenone 327, prepared previously (180), afforded a mixture of isomeric diols 328, which were quantitatively cleaved by sodium periodate to supply 329. Reductive amination of 329 with tryptamine resulted in tetrahydropyridine 330, which upon treatment with aqueous methanol in the presence of hydrochloric acid gave indolo-[2,3-a]quinolizine 321 with pseudo stereochemistry. Conversion of 321 to ( )-hirsutine was accomplished in a similar manner by Wenkert et al. (161) via selective reduction with diisobutylaluminum hydride and methylation with methanol (179). 

In 2008, Lhommet and co-workers, by extrapolation of a previously described polycyclic version [144], proposed the three-component condensation of acrolein, (5)-2-phenylglycinol, and various acyclic 1,3-dicarbonyls in toluene in the presence of 4 A molecular sieves for the preparation of chiral, bicyclic functionalized tetrahydropyridines (Scheme 50) [145]. These heterocycles may be used as chiral building blocks for the synthesis of alkaloids, as illustrated by the total enantiose-lective synthesis of (—)-lupinine in five steps and 29% overall yield. 

Alternative methods for preparing l-(4-methoxyphenyl)-l,2,5,6-tetrahydropyridine have not been reported. 

A variety of 1,2,5,6-tetrahydropyrldines can be prepared by the reaction of (Z)-4-(trimethy1si1yl)-3-butenam1nes with aldehydes. Representative examples are summarized in Table I. Cycllzatlons with paraformaldehyde occur readily in refluxing acetonitrile, while cycllzatlons with other aldehydes require higher temperatures. Tetrahydropyridines with substituents at atoms -1, -2, -3, and -4 have been regloselectively prepared in this way. In no case was any trace of a regioisomeric tetrahydropyridine detected. 

W-Acyliminium ions are useful intermediates for preparing heterocycles in general. This methodology was applied to the synthesis of functionalized pipecolic acids <99EJOC1127>. A -But-3-enyl-Af-styrylformamides undergo cyclization to the tetrahydropyridine when treated with 9-BBN triflate <99T4481>. 

The allylically activated chiral methanimidamides are more reactive and can be prepared from 2,5-dihydro-l//-pyrrole or 1,2,5,6-tetrahydropyridine by heating with a chiral auxiliary substituted methanimidamide in toluene. Deprotonation of the more acidic 1 -iminomethy 1-2,5-dihydro-1//-pyrrole with butyllithium was complete after a few minutes, even at — 100 °C41. Alkylation afforded a mixture of regioisomers, 2-alkylated 2,5-dihydro-l //-pyrrole 1 (n = 1) and 3-alkylated 2,3-dihydro-l//-pyrrole 2 (n = 1), the former strongly predominating (about 92 8). During hydrazinolysis of the 2-substituted 2,5-dihydro-1 //-pyrrole 1 (n = 1) the minor product decomposed, thus separation of the regioisomers was unnecessary. About 80-85% of the chiral auxiliary (S)-l- m-butoxy-3-methyl-2-butanamine was recovered after hydrazinolysis. 

In sharp contrast to the development of the [4 + 2] cycloaddition reactions of electron-rich 2-azadienes, reports dealing with the chemistry of electron-poor 2-azadienes remained unknown until a few years ago. In fact, the first cycloaddition of an electron-withdrawing substituted 2-azadiene was observed in 1986 by Wulff and Bohnke [86AG(E)90] while they were preparing dehydroaminoacid derivatives (Scheme 48). They isolated AL(arylmethylene)dehydroalanine methyl esters 208 by dehydration of the Schiff base of the serine methyl ester 207 and found that it dimerized through a [4 + 2] cycloaddition to give tetrahydropyridine derivative 209 in 56% yield as a sole diasteroisomer. 

Macrocyclic 2-pyrones (113) and (114) are prepared from enamine (115) and ketene (75HCA2409). Reduction of cyano ketone (116) with LAH affords an amino ketone which spontaneously cyclizes to generate a tetrahydropyridine (117) dehydrogenation by palladium on charcoal produces the aromatized phane (118) (71TL671). 

A mixture of silver(I) fluoride and 2,2 -bipyridine replaces chlorine with fluorine in 1-chloro-octane in 86% yield after 45 minutes at 130°C.28 Some heterocyclic polychloro derivatives are also fluorinatcd by silver(I) fluoride. Thus, perfluoro(2,3,4,5-tetrahydropyridine) (1) and per-fluoro(3,4-dihydro-2//-pyrrole) are prepared from 2,2,6-trichloro-3,3,4,4,5,5-hexafluoro-2,3.4,5-tetrahydropyridine and 2,2,5-trichloro-3,3,4.4-tetrafluoro-3,4-dihydro-2//-pyrrole by reaction with silver(l) fluoride at room temperature.31 Since the fluorination of 2,2,5-trichloro-3,3,4,4-tetrafluoro-3,4-dihydro-2//-pyrrole with silver(l) fluoride is slow, recycling is necessary.31... 

Intramolecular [4 + 2] cycloaddition reactions of enamides have provided a route to hydroindole and hydroquinoline ring systems (80JA3294,5274). In this work, the diene portion was initially masked as a 2-substituted 2,5-dihydrothiophene 1,1-dioxide. Thus, reaction of the acid chloride (312) with 3,4,5,6-tetrahydropyridine (311) afforded the masked enamido diene (313), which was converted to the enamido diene (314) upon brief refluxing in xylene. Thermolysis of (314) afforded the hydrolulolidine (315) in 45-55% yield. Additionally, (313) could be transformed to (315) directly by passage of a 1% solution in toluene through a vertical tube (600 °C oven temperature) (Scheme 67). The method was used to prepare a known precursor to aspidospermine. 

The preparation of conjugated dienes from pyridines is exemplified by the transformation of 2-picoline into the sex pheromone (669) of Lobesia botrana, a major pest of vineyards (Scheme 154) (80TL67). Thus, the lithio salt of 2-picoline was alkylated by 2-(5-chloropentyl-oxy)tetrahydropyran, the resulting pyridine (665) N-methylated, and the pyridinium salt reduced by sodium borohydride. Quaternization of the 1,2,3,6-tetrahydropyridine (666) and Hofmann elimination gave the (7 , 9Z)-undecadien-l-ol (667) as the sole isomer. Protection of the alcohol and treatment of the corresponding ammonium salt (668) of the amine with lithium dimethylcuprate gave pure (669) after hydrolysis, acetylation and HPLC purification. 

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