Chapter 6.1 Six-membered ring systems: Pyridines and benzo derivatives

230

Chapter 6.1 Six-Membered Ring Systems: Pyridines and Benzo Derivatives

Robert D. Larsen and Ian W. Davies Department of Process Research, Merck Research Laboratories, Merck & Co., lnc. Rahway, N J, USA

6.1.1

INTRODUCTION

Many new applications of organometallics to heterocyclic synthesis were disclosed in the 1998 literature. Because of the tremendous efficiency of these reactions, as well as their tolerance for other functional groups these transformations will be covered more thoroughly. The hetero-Diels-Alder reaction also continues to produce new variants. Improvements in the condensation and alkylation approaches to the synthesis of the pyridine class have particularly been evidenced in the asymmetric syntheses of the reduced forms. Numerous applications to the combinatorial synthesis of the pyridine class of compounds were reported this year. Although these methods are often very ingenious, they generally use existing methodology to prepare the heterocycle. Due to this and the specialized nature of this work these accounts will not be covered here.

6.1.2 PYRIDINES 6.1.2.1 Preparation of Pyridines The application of organometaUics in pyridine synthesis continues to expand. Palladiumcatalyzed hetero-annelation reactions of internal alkynes were reported <98TL627>. A wide variety of aryl acetylenes undergo the process in excellent yield and high regioselectivity to give pyridines and isoquinolines <98JOC5306>. A unique two-step process couples tungsten alkynols 1, aldehydes and nitriles to the fused pyridines 2 (Scheme 1) <98JA4520>. Bipyridines have been prepared by cobalt catalyzed [2+2+2] cyclization of 5-hexenylnitrile and 1,3-diynes, as an alternative to biaryl couplings <98JA12147>.

231

W ~ "'1 R'CH2CHO W O 1 OH BF3"Et20 W=CpW(CO)3

PhCN Ph--,~N~O Me3NO R , ~

R'

2

Scheme I

Diels-Alder reactions of oximino sulfonates 3 and a variety of 1,3-dienes proceed with regiocontrol opposite to that observed with conventional imino dienophiles providing expeditious routes to substituted pyridines 4 <98JOC7840> (Scheme 2). Hetero Diels-Alder reaction between 2,3-dimethyl-l,3-butadiene and per-fluorooctanonitrile at 1500 bar gives the dihydropyridine which is slowly oxidized to the pyridine <98T11027>. The reaction of ethynyltributyltin with 1,2,4triazines in 1,2-dichlorobenzene gives predominantly the 4-stannyl pyfidine <98TL2549>. Synthesis of pyridines, branched oligopyridines and super-branched oligopyridines has been achieved via 1,2,4 triazines as precursors <98TL6687, 98TL6691, 98TL8817, 98TL8821, 98TL8825>. The 'LEGO' system of building these molecules relies on the inverse electron demand Diels-Alder reaction of the triazine and a variety of electron-rich, strained dienophiles, e.g., norbomadiene as an acetylene surrogate.

N/OTs

O~O

Me2AICI,diene i ' ~ I O OTs NaOMe-NCS..._ -78 oc, , O~ rt, 15h

~CO2Me

3

Scheme2 1,1,1-Ethanetriacetonitrile, readily prepared from 3-methylpentenedienoate, reacts with Grignard reagents to give 2-aminopyridine derivatives 5 <98TL383>.

C~C CN N

RMgCl R= Meor Ph

H2N

R 5

Iminophosphoranes couple with acetylenic esters to give a mixture of regioisomeric pyridines <98H(48)2551>. A simple procedure for the preparation of 4(1H)-pyridones 7 involves the addition of the 13-enaminophosphonates 6 to an acylketene <98H(47)517>. O O ii //0

/[~P<-OEt OEt + "~~0 H2N R B

Toluene ,, reflux

R H

7

Hydrophilic 3-pyridinols are prepared in six steps from fructose and isomaltulose <98T10703>. The syntheses proceed via furfuraldehydes, which are converted to the intermediate furfuryl amines. A one-pot synthesis of 6-trifluoromethylated pyridines has been reported. Formation of the dihydropyridine takes place in the presence of POC13/pyridine-SiO2 in refluxing acetonitrile <98H(48)779>. Oxidation is completed by the addition of manganese dioxide. 4Trifluoromethylpyridines were prepared from trifluoromethyl 13-diketones and 13-amino-13-aryl-

232 acrylonitrile derivatives <98JHC805>. 1,2-Dihydropyfimidines and pyridines are also available from ~,[5-bis(trifluoroacetyl)vinylamine, ketones and ammonia <98H(48)2347>. New methods for the oxidation of dihydropyridines to pyridines have been disclosed. Aqueous tert-butyl hypochlorite, bismuth nitrate, ruthenium trichloride-molecular oxygen or silica gelsupported ferric nitrate successfully oxidize dihydropyridines to pyridines <98SC2789, 98S713, 98TL4895, 98SC207>. Anodic two-electron oxidation of 1,4-dihydropyridines produces pyridines. Advantages of this electrochemical approach include low cost and low environmental impact <98SC3163>. A novel photo-induced aromatization of Hantsch dihydropyridines in carbon tetrachloride gives pyfidines in essentially quantitative yield <98CC2451 >.

6.1.2.2 Reactions of Pyridines

A modified Sonogoshira reaction has been developed for the synthesis of alkynyl pyridines <98JOC1109>. 3-Pyridinyl and quinolinyl methyl ketones have been prepared by palladiumcatalyzed olefination of 3-bromopyridines with enol ethers <98JHC717>. The role of base and cation effects on the Suzuki cross-coupling of arylboronic acids with halopyridines was examined <98JOC6886>. Palladium-catalyzed couplings of halopyridines to the bipyridines are a catalytic alternative to the Ulmann reaction <98T13793>. Cross couplings of the 4-triflate of 2-pyridones with aryl- and heteroaryl-boronic acids afford the biaryl systems at room temperature under nonaqueous conditions <98SL1408>. Electrochemical couplings of 2-chloropyridine and arylbromides at a magnesium or zinc anode takes place in the presence of 5 mol% nickel(H) bromide-bipyridine complex at room temperature to give 2-arylpyridines <98T1289>. Electron donating groups facilitate the coupling reaction. The nitrogen-directed C-H activation of pyridine-containing substrates with metal catalysts is highly selective. A rhodium-catalyzed directed ortho-arylation of 2-arylpyridine with arylstannes was reported. <98CC2439>. N-benzylaminopyridine derivatives 8 undergo chelation-assisted alkylation with olefins using a Ru(0) catalyst <98CC1405> (Scheme 3). Acylation of the olefin 9 is effected with ethylene and CO using Ru3(CO)l2 <98JOC5129>. Dinuclear pyridine-l-imine rhodium (III) complexes undergo intramolecular C-H activation at the benzylic position of the pyridine ligand <98CL1217>. Rhodium(II)-catalyzed decomposition of vinyl diazomethanes in the presence of dihydropyridines leads to the formation of the azabicyclo[3.2.2]nonane nucleus <98TL2707>.

~N 8

H Ru3(00)12~ ~ N L.ph 1-pentene ~ ~ u

H Ph

Ru3(CO)I2 N ethylene,CO

Et NO

9

Scheme 3 3-Amino-dialkylamino pyridines are readily prepared by the direct reaction of the secondary amine with the 3-amino-2-chloropyridines using the amine as solvent <98OPP709>. Amination of nitro-pyridines with methoxyamine in the presence of stoichiometric zinc(II) chloride gives adjacently aminated nitropyridines <98CC1519>. The ortho-selectivity is presumed to arise via neighboring group participation. 2-Isopropylpyridine has been successfully metalated and functionalized using potassium diisoproylamide <98TL8771>. Ortho-directed lithiation of 4-chloropicolinamide with LDA gives exclusively the products of C-3 substitution <98H(47)811>. Unsymmetrical bis-heteroaromatic pyridines are prepared by C-6 lithiation of 2-methoxy or 2-thiomethyl pyridine and treatment with various heterocyclic electrophiles <98JCS(P1)3515>. The coupled products undergo aromatization during aqueous workup. This method obviates the need for the commonly employed organozinc or

233 organotin reagents. 2,6-Bis(triisopropylsilyl)pyridine, prepared by metal-halogen exchange of the dibromide, is an example of the effect of strong steric screening on basicity, since it does not dissolve in 6M hydrochloric acid <98TL6151 >. Nitration of 6-hydroxy-l-methylpyridone leads to a dimerization product rather than the previously reported 3-nitropyridone <98H(48)2643>.

6.1.2.4 Pyridine N-oxides and Pyridinium Salts Pyridine N-oxides are prepared simply and efficiently with a rhenium catalyst and bis(trimethylsilyl)peroxide <98TL761>. A similar rhenium system uses MTO/H202 <98JOC 1740>, but the former system is optimal. Various pyridine derivatives are oxidized with perfluoro cis-2,3dialkoxyaziridines <98T7831>. The selectivity for nitrogen atom oxidation versus carbon-carbon bond oxidation is solvent dependent with aprotic solvents favoring the formation of N-oxides. An unusual deoxygenation of pyridine N-oxides was reported using methanesulfonyl chloride and triethylamine in dichloromethane <98CL829>. Alkyl-substituted pyridine-N-oxides are obtained in higher yield than pyridine N-oxide. This reaction is useful in the synthesis of Manzamine intermediates. 4-Silylsubstituted methyl nicotinates 11 are prepared by the reaction of a silylcuprate with an Nacylpyridinium salt 10 to give the intermediate dihydropyridine <98H(48)2653, 98H(48)1313>. Oxidation using p-chloroanil gives the pyridine (Scheme 4). N-Silylpyridinium salts react with PhMgBr to give the corresponding N-silyl-l,4-dihydropyridines <98TL9275>. Ri R2 ,,t R2 \ S'i~R3 if" \ S'i~R3

~C02Me]

(RiR2R3SiLi)2CuCN

O/~OMe10]

=~.N~/CO2Me . ~,~C02Me p-chloranil

O.~OMe 4

11

Scheme

Aminocyclopentinols can be prepared by photolysis of N-alkylpyridinium salts 12 to provide the intermediate r aziridines <98HCA1095>. Photolysis of pyridinium salts can also lead to aminodiols, which were converted to the Mannostatin A <98JOC6072>. l~ N ~ O H

-__

12

6.1.3 QUINOLINES

6.1.3.1 Preparation of Quinolines Palladium-catalyzed couplings (Scheme 5) of o-allyl or o-isopropenyl-N-tosylanilides 13 with vinyl halides (triflates) produce the dihydroquinolines 14 <98TL1885, 98TL2515, 98T9961, 98TL2965, 98JOC4554>. With the o-vinylic anilide a mixture of indoles and extended chain byproducts are also obtained. The chiral quinolinone 16 is prepared by palladium-catalyzed aryl-nitrogen coupling between the beta-aminoacid 15 and aryl bromides, followed by acid-catalyzed intramolecular acylation <98TA1137>. o-Nitrophenyltriflates are

234 useful precursors to quinolines through a sequential Heck reaction, followed by reductive cyclization <98H(48)215>.

Pd(OAc)2,Na2CO3 y n-Bu4NCI/ DMF

~NHTs + B r ~ 13

[~

+

Br

Pd/CulK2CO3.~ .~,/CO2Me 1)TEBA, H2N H.....

0

2) PPA

......

15

H 16

Scheme 5

The hetero-Diels-Alder reaction has been used in a number of cases to prepare the quinoline ring (Scheme 6). Methacrolein dimethylhydrazone undergoes cycloaddition with naphthoquinones to afford the quinoline ring of aza-anthroquinones <98T8421, 98H(48)1867>. The generation of an o-azaxylylene 17 produces tetrahydroquinolines upon cycloaddition with dienophiles <98H(48)1103, 98JHC467>. When reacted with C-60 a quinoline-fused fullerene results <98JOC8074>. Oxidative rearrangement of 1-aminobenzotriazole (18) forms the transient 1,2,3-benzotriazine, which undergoes the hetero-Diels-Alder with dienophiles to produce quinolines <98CPB332>. The cycloaddition of arylaldimines with olefins produces tetrahydroquinolines <98TL5765, 98T875>.

~ [~NN H~~~:~t ~ 3 l ~ I {~NCO2Et 17

{~ NN'IN I

18

NH2

OMe

DCB,180~

N

~~

OMe

. N2

Scheme 6

An electrocyclization reaction ofN-mesitylketenimines affords quinolinones <98JOC5779>. Condensation reactions are a traditional approach for preparing the quinoline ring (Scheme 7). A DBU/silane or DBU/Lewis acid-mediated double condensation of nitroarenes with substituted cinnamyl phenyl sulfones 19 provides the 2-aryl-4-phenylsulfonylquinolines 20 <98T2607>. This procedure employs the opposite polarity generally seen in the coupling of anilines with electrophilic three-carbon units. 3-Cyanoquinolines are prepared by condensation of anilines with 3,3-dimethoxy-2-formylpropanitrile <98TL4013>. Similar acid-catalyzed ring

235 closures at the 4-position form the quinoline ring system <98SC463, 98S186, 98JHC183>. 2Isopropenylaniline 21 is condensed with ketones using Lewis acid catalysis to afford the 2,2disubstituted-l,2-dihydroquinoline system <98JPR309, 98TL5139>. Rather than the condensation occurring at the C-N bond or at the 4-position, ring formation via carbon-carbon bond condensation between the 2,3- or 3,4 positions is an alternative <98H(48)641>.

$O2Ph L~ ~..

$O2Ph DBU,BTMSA

Ph

NO2 +

CH3CN

Ph

19

20

+ 21

Scheme 7

Intramolecular nucleophilic displacement of an aryl halide by the nitrogen of a beta-enamino ketone side chain affords the quinolinone ring system under basic conditions <98T83, 98JHC541, 98MI1>. A variety of quinoline syntheses proceed through radical intermediates (Scheme 8). Intramolecular radical cyclization of an aryl bromide onto the silylated acetylene affords the 4methyl-l,2-dihyroquinoline 22 <98TL2965>. In another example, cyclization onto a pyfidine ring leads to the synthesis of toddaquinoline <98TL5875>. The enyne-ketenimines 23 form the biradical intermediates 24 upon cycloaromatization to the quinolines <98AG(E)1562, 98JOC3517, 98AG(E)2371>. SiMe3

~

Bu3SnH,AIBN 22

Ph 23

R

1,4-CHD 24 Scheme 8

1

Ph ~ Ph

R

Ph Ph

Photocyclization is used successfully to prepare quinolines through ring closure of the nitrogen atom onto the aromatic ring. Irradiation of alpha-dehydronaphthylalanine affords the quinolinone <98TL4083>. Similarly, 3-(naphthylamino)-2-alkenimines cyclize to the benzoquinoline system <98T6929, 98T14113>. In order to oxidize the sulfonamide 25 for ring closure to the tetrahydroquinoline 26 irradiation is required <98JOC5193>. No reaction takes place in the dark.

236

PhI(OAc)2,12,hv y

I

I

25

SO2CF3

26

SO20F3

Reductive ring closures are prevalent in the synthesis of quinolines. Incorporating a 1,3dipolar cycloaddition and reductive ring closure a quinoline analogue of phenylalanine is prepared <98JCR(S)796>. Using a modification of the reductive amination Borch method delayed addition of NaBH3CN- affords 1,2,3,4-tetrahydroquinolines <98H(48)171>. An intramolecular reductive coupling of the nitrile and nitro groups with low-valent titanium produces the aminoquinoline 27 <98JCR(S)398, 98SC3249, 98JCS(P1)2899>. The BaylisHillman reaction of a vinyl ketone with o-nitrobenzaldehyde, followed by reduction is used in the synthesis quinolines <98CC2563>.

N

1) TiCI4,Sm, THF

~

N

2) 10% K2CO3

H2 27

Oximes are useful precursors to quinolines (Scheme 9). A reductive, base-induced cyclization of the O-aryl oxime 28 affords the tetrahydroquinoline <98CL437>. The reduction prevented the normal dihydro-cyclization product from disproportionating to the quinoline and tetrahydroquinoline <98BCJ2945>. By adding DDQ atter the cyclization the quinoline product is isolated selectively.

~ OH

_.1) Nail 2) AcOH, DDQ

OH 28

O~~L~ O2N

NO2

Scheme 9

The Friedl~inder synthesis can be used to prepare a variety of quinolines from pivaloylanilines <98S1176>. A series of 4-perfluoroalkylquinolines was synthesized by reaction of the 2perfluoroalkyl-substituted anilines with the lithium enolate of acetaldehyde <98T7947>. Using the Skraup reaction anilines and acroleins are converted to quinoline-5,8-diones in a three-step one-pot process <98H(48)2647>. 2-Azidoarylketones are converted to 2-aminoquinolines by cyclization under Vilsmeier conditions <98T14327, 98TL3837>. The Wittig intermediate 29 affords an entry into 4-aminoquinolines (Scheme 10) <98T1647>. A rearrangement of 1-oxa5-azabicyclo[5.5]undec-2-en-4-one provides the 5,6,7,8-tetrahydroquinolinone ring system <98S265>. o-Ethynyl-N-malonanilide is a useful building block for the synthesis of 3,4disubstituted quinolin-2-ones <98S[A46>. The quinolinones can be converted further to the corresponding quinolines or 2-substituted derivatives via the triflate.

237

{~

~~ ).~~CN f P P+h 3 Br"

ON , ~ ~ " ~ PPh3Br" ~-~

NH2

~ 29

Nail

-N~x H

Scheme 10

Methods for the preparation of chiral 2-substituted or 2,3-disubstituted 1,2,3,4tetrahydroquinolines from chiral piperidines <98H(47)747> or the epoxide of 2'aminochalcones <98TL8495>, respectively, were reported.

6.1.3.2 Reactions of Quinolines

New methods for substitution of the quinoline ring were reported (Scheme 11). 4Quinolinones are converted to 2,3-trans disubstituted tetrahydroquinolines 30 by addition of organometallics followed by trapping of the enolate with N-halosuccinimides <98SL649>. Cyanomethyl or ethoxycarbonylmethyl are introduced at the 2-positions of 1methylquinolinium salt through cesium fluoride activation of trimethylsilylacetonitrile or acetate <98JCR(S)660>. Quinoline is activated with phenylchloroformate and silver triflate for substitution at the 2-position preparing the alkynylquinoline 31 <98CL547>. Similarly, allylstannane is added by acylation of the quinoline <98JHC871>. A diastereoselective addition of acetylides at the 2-position is carried out with differentially protected chiral 2(quinolyl)propanol <98TA3923>. Rather than acylate the quinoline nitrogen to add at the 2position the N-oxide can be used to substitute the ring with indole <98TIA749>. Zeolites are used for preparation of 2-methylquinoline from quinoline <98MI2>. Sulfur or selenium is introduced at the 4-position of methyl quinolinium salts <98S99>.

O

1) BF 3

2) PhMgCI~-- ~ ~ C I

TMS - -

3) NCS

CICO2Ph,AgOTf

I

CO2Et

I

30

CO2Et

Ph Ph31

Scheme 11

Lithiation of 2-chloroquinoline at the 3-position provided a handle for preparation of the 3hydroxyquinoline alkaloid, jineol <98H(48)2379>. Directed ortho metalation of 8-fluoro-6(methoxymethoxy)quinoline at the 7-position, overcoming the formation of the 1,2-addition product, is achieved by using methyllithium as base <98HCA1088>. Methods for converting quinolines to benzocarbacephems <98JCS(P1)1203>, phenanthridines <98T10167>, and indol-2-ones <98H(48)2309> were reported. Substitution of the quinoline ring by displacement reactions was explored. The amine displacement of the halide of 7-haloquinolin-4-one-3-carboxylic acids is enhanced by boron chelation of the ketoacid <98H(48)1111>. 4-Chloroquinolines are effective substrates for preparing the 4-amino derivatives <98H(48)71>. Acid hydrolysis of 4-amino-3quinolinesulfonamides, in turn, provide the 4-oxo-derivatives <98H(48)1249>. The 4-position of 1-alkyl-l,4-dihydro-4-oxoquinoline-3-carboxylic acid can be activated with thionyl chloride <98JCS(P1)3137>. A halogen dance reaction of 3-fluoro-4-iodoquinoline to the 2-iodo species with substitution at the 4-position was discovered <98TL6465>.

238 Base-catalyzed aldol reactions of 2-quinolinecarboxaldehyde, as well as pyridinecarboxaldehyde exhibit unusual reactivity <98TL3903>. By using pyridine as the reaction medium the bis-adduct is avoided in favor of the aldol product. Oxidation of the pyridine ring can be carried out enzymatically to a variety of oxygenated derivatives <98CC683>. Hypervalent iodine reagents effectively convert 2-aryl-l,2,3,4tetrahydro-4-oxoquinolines to the 4-alkoxy-2-arylquinolines <98TL9113>. Quinolines are reduced to the 1,2,3,4-tetrahydroquinolines with zinc borohydride in the presence of catalytic dimethylaniline under sonication <98SC485> or with indium metal in ethanol <98SL1029>. Vinylquinolines can be electroreductively coupled via radical anion-substrate cycloaddition affording the trans-diheteroarylcyclobutane <98CC539>.

6.1.4

ISOQUINOLINES

6.1.4.1 Preparation of Isoquinolines Palladium-catalyzed cyclizations of substrates, such as ortho-iodobenzyl enamines <98SL1231, 98SL1227> and N-ortho-iodobenzoyl propargyl amines <98T2595> are useful for the formation of the isoquinoline ring (Scheme 12). Benzo[a]phenanthridine alkaloids are synthesized using an aryl-aryl coupling with a novel Pd reagent <98H(48)1989>. The tricyclic benzo[a]quinolizine ring system 32 is prepared by an intramolecular Heck reaction <98JOC4936>. The intermolecular coupling of the ortho-iodobenzaldimine 33 and alkynes leads to isoquinolines <98JOC5306>. N-(2'-Phenylphenyl)benzenesulfonamides react with acrylates through aryl C-H activation with a palladium-copper catalyst system in the air to afford the phenanthridine ring system <98JOC5211, 98TL4111 >. h MeO

Pd2(dba)a'Ph3 P

~

0

33

N-t-Bu

Ph ~

+

/Ph

MeO 32

Ph

Pd(OAc)2_Ph3P ~ Na2CO3.DMF

0IJ Ph Ph

Scheme 12

An ortho-allylbenzylamines attached to an in situ-generated isobenzofuran undergoes an intramolecular Diels-Alder reaction to form the benzo[c]phenanthridine system <98TL9785>. Cyclization of the nitrile ylide 34 produces the cyclopropa[c]isoquinoline <98JCS(P1)807>. Nitriles react with ortho-quinodimethanes to yield the corresponding 3-substitutedisoquinolines <98SL 1381 >.

~~'N~___ph 34

-

-

4-

~

[~~N

239 The Pummerer reaction is effective for the closure of tetrahydroisoquinolines at the C-4/aryl bond <98CPB430, 98H(48)981>. In one study the effect of the ring methoxyl groups was explored <98CPB918>. Whereas, the Pummerer reaction (Scheme 13) is mostly used for the closure of the C-4/aryl bond, recent examples give ring closure at the C-1/aryl bond via an Nacyliminium intermediate 35 <98JOC6778, 98JOC1144>.

Ph Ar = 3,4-dimethoxyphenyl

ph~,-

35

"SEt

Scheme ]3 A two-step procedure for preparing the protoberberine ring involves a radical cyclization of an aryl bromide onto an alkyne, followed by an acid-catalyzed transannular cyclization of the 10-membered lactam <98TL6551>. Radical cyclization of the xanthate 36, prepared by radical-induced intermolecular xanthate-transfer addition to an N-allylbenzamide, is carried out with lauroyl peroxide to provide the 4-alkyl tetrahydroisoquinoline <98TL7295>. Intramolecular radical cyclizaton of an ortho-bromobenzylamine onto a sulfolene moiety forms the isoquinolino-3-sulfolene, which upon heating generates the isoquinolino-o-quinodimethane <98T12609>. A radical cyclization of an o-bromo-N-allylbenzylamine can be used to prepare a protoberberine analogue <98H(47)639>.

~ ~

S...j~OEt

NEt

II O

~

2Me

S

lauroyl peroxide ~--

C02Me

~,,,,,~~~ N Et

36

The preparation of tetrahydroisoquinolines through the cyclization of N-acyliminium intermediates (Scheme l4) remains the best improvement upon this traditional approach to these compounds <98T7395, 98T12361>. New applications of the method include an oxidative Pictet-Spengler reaction of the TMSmethyl-phenethylamine 37 <98JOC860>. Treatment of the thioamide 38 with bromoacetylchloride produces the S,N-acetal 39 upon cyclization. Reductive removal affords the isoquinoline <98TL4761>. The synthesis of Nacyltetrahydroisoquinolines is achieved through intramolecular alpha-amidoalkylation between N-acylphenethylamines and aldehydes <98SC2137>. The cyclization of the carbamates of phenethylamines is a useful method for preparing isoquinolinones <98JOC7795, 98TL6609>. The Ritter reaction has been applied to the generation of activated iminium intermediates for cyclization to isoquinolines <98MC17, 98MC153>. Other examples of acid-catalyzed iminium ion cyclizations appeared in the literature <98H(48) 1623, 98EJOC2101 >. The synthesis of 5-substituted tetrahydroisoquinolines via any of three methods - PictetSpengler, Pomeranz-Fritsch, or lithiation/electrophilic trapping- was explored <98T9023>. Synthesis of 3-carboxy-l,2-dihydro-l-oxoisoquinoline via a glycine anion equivalent was carried out <98SC3769>. Lewis acid-induced reaction of homophthalic acid with imines

240 provides isoquinolinic acid <98TL829> and imine addition-cyclization tolylbenzonitrile affords the 1-amino-dihydroisoquinoline <98TL 1227>.

MeO~

MeO~

CAN

MeO/~L~J (NCOPh 37 TMS

H MeO 38

MeO'//J.L,,,.,~.J.~NCOPh Ra-Ni

.eOy

"~==:S

,v

MeO.,~~ 39

/

onto ortho-

M e O ~

M e O / ~ . ~ NCOCH

N,~= 0 S '--J

Scheme 14

The asymmetric synthesis of isoquinolines remains an active area of research (Scheme 15). The Pictet-Spengler reaction of the beta-iminosulfoxide 40 affords the chiral 1,1-dialkylsubstituted tetrahydroisoquionoline <98EJOC435>. The use of a chiral sulfimine provides an asymmetric synthesis of 3-arylisoquinolines <98TL3099>. The cyclization of the orthovinylphenethylamine 41 induced with the chiral selenium reagent 42 affords the chiral 2methyltetrahydroisoquinoline after removal of the selenide <98S162>. Chiral 1phenylisoquinolines were prepared by addition of benzaldimines to chiral dioxolane-chelated aryllithium intermediates <98EJOC711>. Deracemization of 4-aryltetrahydroisoquinolines using BuLi-sparteine and reprotonation leads to chiral 4-arylisoquinolines <98TA2509>. The asymmetric synthesis of 4-hydroxylisoquinolines is carried out using the phenylglycinol derivatives of the benzaldimines <98TA1809, 98TA151 >.

MeO~

TFA

MeO~

NH~--cF3

41

~

MeO./~-'~,,, NH "CF3

o,%.~-~~

,0

MeO~

MeO~

II

NHBoc

42

o-S.f-To.

eOTf .v

MeO/~'~NBoc 90%e.e. SeAr*

Scheme 15

6.1.4.2 Reactions of Isoquinolines

The reactions of isoquinolinium N-arylimides 43 with strained cycloalkenes <98JHC643>, electron-deficient/electrophilic ethylene derivatives <98EJOC387, 98T3735>, heterocumulenes <98EJOC379>, enamines <98JHC637>, and acetylenes <98T8451 > were studied.

241

[~~/N +"N"Ar

MeO2C"'~/CO2Me ~~,/N.N ~ Ar

43

K,...~

r,""

C02Me

'v'~v2~-'

Diastereoselective alkylations of isoquinolines continue to be studied through the preparation of chiral amine derivatives (Scheme 16). The enantioselective 1- and 3- alkylation and 1,3-dialkylation reactions are achieved using any of three chiral isoquinolinium derivatives. The procedure is used in the synthesis of (+)-salsolidine 44 <98JOC1767>. The 4,4'-dialkylation of the chiral tetrahydroisoquinolin-3-one 45 provides an asymmetric entry into crinine-type alkaloids <98T10363>. The chiral reduction of N-(S)-methylbenzyl-l-benzyl3,4-dihydroisoquinolinium intermediates has been studied <98H(48)1623>.

1)MeMgl

MeO~

..._ M e O ~

MeO'/"'JL'"~"J~N'~O'PhH3)2)H2NaBH4' AoOH"v ~ O

OH 1)LDA,Mel

45 Ph

M e O ~ NH44

~ O

:,,BuLi,,,rCH CO2 t

OH Ph

Scheme

16

Addition reactions of 3,4-dihydroisoquinolines are used actively as a means to prepare more complex isoquinolines (Scheme 17). The addition reactions of phthalide derivatives are applied to the synthesis of protoberberine analogues <98T7485, 98SL387>. Annelation reactions of 3,4-dihydroisoquinolines with an acetylcyclohexanone is used in the synthesis of 8-azasteroids <98MC183>. Enantioselective hydrogenation of a 1-benzyloxymethyl-3,4dihydroisoquinoline 46 with an iridium (I)-(R)-BINAP-phthalimide complex sets the chirality at the 1-postion in the synthesis of (S)-calycotomine <98TA183>. Allylation or benzylation at the 1-position through N-acylation and addition of allylstannane or benzylstannane, respectively, is effective at preparing the corresponding 1-substituted-l,2-dihydroisoquinolines <98H(47)125, 98TL1721>. Isoquinoline undergoes diastereoselective addition at the 1position with organomagnesium or zinc reagents when activated as the chiral acylated intermediate 47 <98EJOC2019>.

MeOl 11~ "1 MeO~ 46

MeO MeO'/..~.....~-....~/__ NH 86%e.e. OBn

Ir, (R)-BINAP,H2 N

~OBn o ....N/ 4'7 ~

o

RMgBr

NH

v

R Scheme

17

242 Ring rearrangements can offer efficient means to obtain more complicated compounds (Scheme 18). The Dakin-West reaction of the N-acyl-l,2,3,4-tetrahydroisoquinoline-1carboxylic acid 48 occurs unexpectedly when treated with TFAA <98H(48)285>, providing an effective synthesis of the azepine 49 <98H(48)555>. Another example of a ring expansion of an isoquinoline to the azepine 50 proceeds through an ammonium ylide/Stevens [1,2]rearrangement <98TL4159>.

MeO~ II

1)LAH

MeO I

MoO

I

MeO..-~A...~ NC(O)-t-Bu pyridine MeO"~"..~~ N 3)H2 48 CO2H F3C)NOCO. _ _ t.Bu ~ N

O~oEt

Cu(acac)2~I ~ . / ~

N2

MEOW._ 49

~t'3

~

EtO2C \\0

50

0

Scheme 18

Introduction of oxygen on the isoquinoline framework is an important reaction in the synthesis of pharmacologically interesting molecules. A carbonyl transposition is used to prepare the 4-oxo-tetrahydroisoquinoline 51 <98TL3409>. Lead tetraacetate oxidizes the 4position of noraporphines and aporphines <98H(47)911>. The angular position of 4-aryldecahydroisoquinolines is hydroxylated with MCPBA <98H(48)949>.

~

OH

O OMe

Ph3P'12~

[~N

Ts

Ts 51

The intermolecular hydride shif~ of 4-hydroxy-4-aryl-tetrahydroisoquinolines is induced with acid <98H(48)2473>. A reductive fomaylation of isoquinolines with formamide provides the N-formyl derivatives <98SC1433>. 4-Arylidene-isoquinolinediones are prepared using KF-alumina under dry reaction conditions <98SC3195>. Oxidative transformation of indenoisoquinolines with OsO4/MMO affords the isoquinoline-3-spiro-3'-phthalides <98JOC5736>. The cycloaddition of 1,2-dihydroisoquinolin(on)es with substituted nitrosoalkenes or alkenes produces the oxazinoisoquinoline or cyclobutane-adduct of the isoquinoline, respectively <98T65, 98JCR(S)678>.

6.1.5 A C R I D I N E S

A key aspect of preparing the acridinc ring system is the site of ring juncture. Two applications of the Ullmann-Goldberg condensation were reported using the coupling of ohalobenzoic acid and an aniline derivative <98JHC471, 98SC147>. This approach is also used in the preparation of 4,5-difluoroacridine <98TL813>. A more modem approach to forming the aryl-nitrogen bond involves the Buchwald-Hartwig palladium-catalyzed C-N bond formation. The diaryl amine intermediate is closed to the acridine 52 by directed orthometalation <98SL419>.

243

" ~ O

O

1) PdCI2(dppf) dppf, Nat-OBu

+ H2N

2) Methylation 3) LDA

J 52 Me

2-Vinyl-4-oxoquinoline is converted to an acridine derivative by acidic rearrangement <98H(48)687>. The reactions of substituents at the 9-position was explored. 9-Azidoacridines undergo cycloaddition with alkynes to the triazoles followed by thermolysis to produce pyrido-fused acridines <98JCS(P 1)915, 98JCS(P 1) 1677>. The 9-carboxy group is converted to acridinone with horseradish peroxidase and peroxide to provide chemiluminescent compounds <98JOC930>. Generation of a 9-isothiocyanate leads to a series of spirodihydroacridinethiazolines <98SC4171>. The reduction of 9-substituted acridines with Ni-A1 alloy gives the octohydroacridines and 9,10-dihydroacridines depending on the substituent <98H(48) 1663>. Proflavine derivatives are reacted with formaldehyde to provide (3,4dihydro[1,3]oxazino)acridines <98H(48)755> and dibromobenzene under palladium-catalyzed conditions to afford photochemically active polyamines containing the acridine group <98CL709>. The reaction of 4,5-difluoroacridine with potassium diphenyphosphine provides a new type of tridentate phosphorus-nitrogen ligand <98TL813>. The reaction of 9-methylacridine and its quaternary salts with pyrylium salts affords ring transformation to the spiro[cyclohexadiene-dihydroacridines] 53 <98JHC787>. The sulfopropylation of acridines with microwave irradiation increases the rate of the reaction <98TL9587>. The method is also effective with pyridines and quinolines. Alkylation with neopentyl 3-trifluoropropanesulfonate produces chemiluminescent labels <98JOC5636>. AF

Ar

A

Ar CIO4"

v I

53

Me

6.1.6 PIPERIDINES 6.1.6.1 Synthesis of Piperidines Ruthenium-catalyzed ring-closing metathesis of a diene has become a predominant means to construct piperidines <98TL9563, 98TL6711, 98EJOC2583, 98TL6175, 98CC2053, 98JOC3158>. Simply by including ethene the ring closing of an enyne 54 is now a practical process, as well <98JOC6082>. Without the olefin the catalytic cycle stops to yield only 19% of 55. Zirconium-mediated cyclizations of an enyne <98S552> and diene <98S557> are also feasible.

244

CI2(Cy3P)2Ru ~ ethylene, CH2CI2

"N 54

q's

q's 55

Palladium-catalyzed intramolecular aminations onto an allylic alcohol <98TL5971> or allene <98TL5421> are effective for the preparation of piperidines. Similarly, an intermolecular version produces the spiropiperidine 56 <98JOC2154>. Organolanthanides are also used in the intramolecular hydroamination of allenic systems <98JA4871>. A regioselective rhodium-catalyzed cyclohydrocarbonylation of an amido-co, co'-diene provides fimctionalized piperidines <98TL4599>. O

O

I~1NHPhI ~

~

P Na2C dO (O+Ac)2-P3,h3DMF P 56

"

v

An interesting nickel-catalyzed cyclization between a diene and aldehyde affords the piperidine 57 <98T1153>.

OHC L"N +s

Ni(c~ ~_Et3SiO Et3SiH,THF 57

+s

Titanium reagents are capable of nitrogen fixation and subsequent incorporation of the nitrogen into a 1,5-ketoacid. This process is very effective in producing the piperidine ring system <98JOC4832, 98AG(E)636>. The hetero-Diels-Alder reaction is capable of preparing complex systems in few steps. The use of imino dienophiles <98JOC4500> is extended to the catalytic enantioselective aza-Diels Alder <98AG(E)979, 98AG(E)3121>. The umpulong reaction of chiral a, fl-unsaturated sulfimines with olefins affords tetrahydropyridines in high yield and moderate diastereoselectivity <98EJOC1629>. The cycloaddition of a nitrone with an olefin <98JCS(P1)893> or nitroso compound with a diene <98HCA1417> affords piperidines after reductive ring opening. The preparation of the piperidine ring by intramolecular cyclization of an allylsilane onto an iminium or acyliminium ion is a continually explored area of synthesis <98T10309, 98SL921, 98H(48)507, 98EJOC2461>. In one case the iminium species is generated by oxidation of the trimethylsilyl group 58, in contrast to the usual approach of condensation of an amine with an aldehyde <98JOC841>. Phenylylycinol is effectively employed as a chiral auxiliary in the cyclization of piperidines. Through iminium intermediates a variety of chiral piperidine analogues can be prepared <08AG(E)104, 98T8783, 98H(47)263>. These 1-aza-4oxabicyclo[4.3.0]nonane derivatives undergo a variety of substitution reactions to prepare complex piperidines.

245

...... (nBu4N)2Ce(NO3)6 ~NB'n TMS r~NBn '

58 TMS Enzymatic resolution of prochiral esters, such as 59, is an effective means for obtaining chiral piperidines <98JOC3492, 98JOC9548>. A review on the use of enzymes to prepare enantiopure 1,4-dihydropyridines appeared this year <98H(48)1943>.

BnHN ..,.,....,.,.,~ C02 Et I C02Et 59 Ph

1) PLE 2) TFA 3) DCC' HOBT

Bn (N ~.O ~["'C02Et ~Ph

Pipecolic acid synthesis remains an area of active interest. A variety of the methods described above have been used for preparing pipecolic acid or its derivatives. Using the Rucatalyzed metathesis reaction N-allyl-substituted amino acids are cyclized to the piperidine. A chiral hetero-Diels-Alder reaction of N-silylaldimines is followed by oxidation of the aryl group to the acid <98JOC3918>. Pipecolic acids are prepared by reaction of N-acylpyridinium species with a silylmethyl Grignard, which acts as a carboxy equivalent <98SL1337>. The enantioselective synthesis of an allenyl derivative of pipecolic acid was reported <98EJOC2461>. (S)-2-Pipecolic acid can be prepared through enzymatic resolution of 6bromo-2-hydroxyhexanenitrile and cyclization to the chiral 2-cyanopiperidine <98TA1597>. A review of the enzymatic cyclization of aminoadipic acid to 6-oxo-pipecolic acid was published <98MI3>. Using a chiral phase-transfer catalysis to prepare the amino acid derivative 60 (Scheme 19), chiral pipecolic acid is prepared <98TL5347>. O

ph,.,l~N.~o.t_Bu

Ph

PTC* CsOH, RX

~ , -.J1..O.t_Bu ,O Ph...~N. -~ 601~h I~iCH2)4Cl Scheme19

H ,.-[J..O.t.Bu H

Ring enlargement of the pyrrolidine 61 (Scheme 20) derived from (S)-methylpyroglutamate affords an enantioselective synthesis of 2,3-disubstituted piperidines <98TL9447>. The oxazinone 62 is converted to the piperidine ring via the conformationally restricted Claisen rearrangement as part of the synthesis of (-)-methyl palustamate <98JOC7490>.

c. ~ P h

ph.,.J

MeS02CI

~'"Ph

>....co.Fo H O ~ " ~ ' " N/

OH 61

Bn ph "/j

TIPSOTf, Et3N~ TIPSO.,-'-~-'~ "Nn "CO2TIPS

62 Scheme 20

Interesting approaches to the preparation of piperidines involve ring closure at the nitrogen bond (Scheme 21). Polyalkyl-substituted piperidines are prepared by stereocontrolled double nucleophilic addition of primary amines to the 7-oxo-enimide 63 <98SL652>. Aspartic acid is an effective building block for 4-aminopiperidine-2-one derivatives <98SL885>. Unsaturated aldimines carrying a y-chloropropyl unit at the y-position are converted to 3-vinyl, 3-ethylidinepiperidines or 5-functionalized-l,2,3,4-piperidines depending on the nucleophile and base

246 <98T2563>. Similarly, a ~/-chloroalkanamide undergoes ring closure to the piperidinone with potassium t-butoxide <98H(48)481>. Diastereoselective nucleophilic addition onto chiral sulfinimines, followed by ring closure is an avenue to the asymmetric synthesis of piperidines <98TA2201, 98TL5951>. 2,3,4,5-Tetrahydropyridines are formed by the ring closure of an active methine at the 6-position onto the nitrogen of an O-methylsulfonyloxime group using DBU <98CL607>. A reverse-Cope cyclization of the beta-hydroxyhydroxylamine 64, prepared by hydroxylamine opening of an epoxide, fomas the functionalized piperidine N-oxides <98TL9089>. The aza-annelation of enaminones with itaconic anhydride was explored in the synthesis piperidinones <98JCS(P1)3437>. Two sequential AD reactions are followed by aminocyclization to yield the chiral 2,6-disubstituted piperidines <98T13505, 98JOC2224>. A practical asymmetric synthesis of both enantiomers of 6-(hydroxymethyl)piperidinone is accomplished through an AD reaction of an omega-acid followed by azide displacement of the internal alcohol and reductive cyclization <98S 1141>.

OHC

COXc 9

1) BnNH2, MgSO4 2) Pd/C, H2

,l

Bn

63

'~COXc

Xc = (S)-5-benzyloxazolidinone HO

Bn~

64 Scheme 21

Formation of the zinc enolate of 65 allows ring closure onto the olefin <98JOC566>.

•

NB/__~nCO2Me 65

2) ZnBr2 1)LDA 3) H20

~ C "--

Bn

O2Me

6.1.6.2 Reactions of Piperidines

Activation of piperidinones to palladium coupling as the ketene aminal triflates <98JOC3810> or phosphates <98CC1757> can be followed by carbonylation to afford the cyclic amino acid or vinylation to afford the diene 66 (Scheme 22). The latter can undergo Diels-Alder reactions to provide the octahyroquinoline. Conversion of a piperidinone to the chromium carbene 67 is also used in the synthesis of cyclic amino acids <98TL3683>. The utility of the 1-aza-4-oxabicyclo[4.3.0]nonane synthon 68 in the asymmetric synthesis of piperidines continues to be exploited. The preparation of an a-cyanoenamine allowed the tandem asymmetric alkylations to provide chiral carbocyles and heterocycles <98JOC1619>. The incorporation of the benzotriazole moiety of 68 provides an entry into chiral 2,6disubstituted piperidines via sequential Grignard additions <98JOC6699>. Other examples for the preparation of chiral 2,6-disubstituted piperidines via addition of Grignard reagents were reported <98T13955, 98JOC2711, 98TA2419, 98T9357>. (-)-Sedamine is prepared with this methodology <98H(47)263>.

247

Ph ~OTf (~bz MeO~~

PhN--L'-/O

I

Cbz

(~bz 66 K2Cr(CO)5 M e O ~ ""-

I

O

MeOH

N ~Cr(CO)5 I 67

~N / I

""~CO2Me

Scheme 22

Ph~o

1) PhMgBr 2) MeMgBr

H

3) H2,Pd/C 68 The enantioselective synthesis of the piperidine alkaloids was carried out by asymmetric imine hydrosilylation of 69 setting the chirality at the 2-postion <98JOC6344>.

C3H7 69

chiral titanocene PhSiH3 ~

H

....C3H7

The oxidation of dihydropyridines provides a handle for introducing functionality into the piperidine ring. The electrophilic oxidative addition of 1,4-dihydropyridines is carried out with halogenating agents to afford the 3-halo-2-substituted dihydropiperidines <98JOC2728>. With properly attached substituents on the heterocycle iodocyclization of 70 affords polycyclic ring systems <98TL5089, 98T12379>. Similarly, the oxidation of a tetrahydropyridine affords the 3-hydroxy-2-substituted piperidines <98TL3413>. A stepwise, cascade oxidation of 4phenyltetrahydropyridine with potassium permanganate produces ring cleavage to the 1formylamino-3-arylpropan-3-ones <98MC137>. 3-Acetoxylation of N-acyl-2,3-dihydro-4pyridinones is carded out stereoselectively with lead tetraacetate <98TL5693>. I

"~1~ MeO2c

12'NaHCO3~ 70

~ N ~ HH MeO2c- v

~'1

Photocycloaddition of dihydropyridinones with olefins <98HCA303, 98CC2509> forms the bicyclo[4.2.0] ring system. Tethered dihydropyridinones undergo photodimerization selectively to the head-to-head adduct <98TL 187>. Ring transformation of an hydroxymethyl dihydropyridinone to a pyranone is carried out with acetic acid treatment <98T915>. The 2-chloromethyl group of a piperidine is used as a

248 means for ring expansion to the 3-azidoazepine via formation of the aziridinium intermediate followed by azide ring opening <98H(48)427>. Ring transformation of a 2,6-diaryltetrahydropyridine to the 1-aminopiperidine is effected by hydrazinolysis with loss of one aryl group <98JCR(S)491 >. Biocatalysis has been applied to the preparation of piperidine building blocks. Baker's yeast stereoselectively reduces ketopiperidinecarboxylates to the fl-hydroxypiperidine-carboxylates <98JCS(P1)3673, 98ACS461>. Enzymatic resolution of piperidine building blocks with esterases is applied to the asymmetric synthesis of piperidine natural products <98TA1951, 98TA2133, 98TA1519>. A cleaner biohydroxylation of a piperidine is possible by protection as the Cbz rather the N-benzoyl <98.ICS(P 1)3365>. The intermediacy of N-acyliminium ions of piperidines (Scheme 23) provides a handle for further functionalization of the 2-position of 71 <98SL206>. The intramolecular cyclization provides polycyclic ring systems <98JOC6914, 98TA4361>. The analogous nitrone is also used for the preparation of 2,2,6,6-tetrasubstituted piperidines <98TL2565>. Radical cyclization onto an N-acylenamide 72 provides fused nitrogen heterocycles <98T10349>. TiCI4 MeO

o/2---o

~

~TMS

71 CHO

C,HO

Bu3SnH AIBN

I

r

Scheme 23 Piperidin-2-ones undergo asymnaetric alkylations as a chiral oxazolidine derivative <98TL1025> or with the addition o f a chiral lithium amide base <98TL9723>. The unexpected enolization and alkylation of a 3-piperidinone was reported <98H(48)2157>. An Sul' reaction of the 1,4-bis(arylsulfonyl)-tetrahydropyridine 73 provides highly regio-and stereoselective alkylations of the piperidine <98SL55, 98SL58>.

Et3AICI ~ Et. . . . . . .

Fs

6.1.7

73

Bn

$

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98H(48)1111 98H(48)1249 98H(48)1313 98H(48)1623 98H(48)1663 98H(48)1867 98H(48)1943 98H(48)1989 98H(48)2157 98H(48)2309 98H(48)2347 98H(48)2379 98H(48)2473 98H(48)2551 98H(48)2643 98H(48)2647 98H(48)2653 98HCA303 98HCA1088 98HCA1095 98HCA1417 98JA4520 98JA4871 98JA12147 98JCR(S)398 98JCR(S)491 98JCR(S)660 98JCR(S)678 98JCR(S)796 98JCS(P1)807 98JCS(P1)893 98JCS(P1)915 98JCS(P1)1203 98JCS(P1)1677 98JCS(P1)2899 98JCS(P1)3137 98JCS(P1)3365 98JCS(P1)3437 98JCS(P1)3515 98JCS(P1)3673 98JHC183

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