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The Nitro group in organic sysnthesis - Feuer

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					10.2 SYNTHESIS OF INDOLES		339
	CH R1	H+
	2	(Fisher)
N	R2	(Fisher)		R1
N	R2			R1
N
H					R2
R1	O				R2
R1	O			N
		PhNH2, H+		H
N	R2	PhNH2, H+
N	R2	(Bishler)
H		(Bishler)
H
	MeSCH2CO2R		SMe		R
	MeSCH2CO2R				R	H2/Ni
	Et3N		R		O	H2/Ni	R
	(Gassman)		R		O		N
NHCl	(Gassman)		N		NO2		N
NHCl			N		NO2		H
			H
CH2R			R		R
CH2R	LDA				R	∆	R
	LDA					∆
	(Saegusa)					(Sundberg)
NC	(Saegusa)		N		N3	(Sundberg)	N
NC			N		N3		H
			H
			Scheme 10.5
OCH2Ph			OCH2Ph			OCH2Ph
OCH2Ph				N
Me	Me2NCH(OMe)2			N		Ra (Ni)
Me	Me2NCH(OMe)2					Ra (Ni)
			NO2			N2H4	N
NO2	N		NO2				H
NO2	N		95%				95%
	H		95%				95%

(10.49)

This method has been applied to a large-scale preparation of 6-bromoindole, which reacts with various arylboronic acids via the Suzuki reaction to afford 6-arylindoles (Eq. 10.50).64 6-Bromo-5-methoxyindole for use in the synthesis of marine bromoindole65 and 5-amino-7- ethoxycarbonylindole for use in synthesis of 1H-pyrrolo[3,2-g]quinazoline ring system (Eq. 10.51)66 have been prepared from the appropriate o-nitrotoluene.

	Me 1) (MeO)2CHNMe2			1) Pd(PPh3)4
	pyrrolidine, DMF			1) Pd(PPh3)4				N
	2) H2, Raney Ni		N	2)				N
Br	2) H2, Raney Ni	Br	N	2)				H
Br	NO2	Br	H	F	B(OH)2	F
			H	F	B(OH)2	F	90%	(10.50)
			74%				90%	(10.50)
Me	CO2Et			CO2Et				O
Me	CO2Et
	1) (MeO)2CHNMe2							NH
	2) H2, Pd/C		N	NO2				NH
	2) H2, Pd/C		N	NO2
O2N	NO2		H				N
			96%				H
							H

(10.51)

Batcho indole synthesis is a useful tool for synthesis of natural products. As outlined in Scheme 10.6, the Batcho indole synthesis is used for total synthesis of the slime mold alkaloid arcyriacyanin.67 Such indolocarbazole alkaloids represent a growing number of natural products isolated from soil organism, slime molds, and marine sources. They are important as antitumor compounds and protein kinase C and topoisomerase inhibitors.

Recently, synthesis of the 4-arylindole portion of the antitumor agent diazonamide has been achieved starting from 3-bromo-2-methylnitrobenzene via Suzuki coupling and the Batcho reaction.67b

340 SYNTHESIS OF HETEROCYCLIC COMPOUNDS

									Me
NH2			OTHP	1) (MeO)2CHNMe2OTHP				O	N	O OTHP
NH2			OTHP	1) (MeO)2CHNMe2OTHP				O		O OTHP
CH3 1) KNO2, H2SO4			CH3			1) EtMg Br
	2)			2) H2, pd/C	N	2)	Me
NO2	2)					O	N
	O					O	O
			NO2		H		Br	N		N
		1) 80%					Br	N		N
		1) 80%		60%		N		Boc		H
		2) 82%		60%		N			57%	H
		2) 82%				Boc			57%
								R = THP
			Me				Me	R = H
	O		N	O OTf		O	N	O
			N			O		O

PhN(SO2 CF3)2				Pd(OAc)2
								NH
		N		N		N
		N		N		H
		Boc		H		H
				H
						81%
			92%			81%

Scheme 10.6.

An important extension of this indole synthesis is the functionalization of the intermediate of indole. For example, acylation of the intermediate is possible (Scheme 10.7).68

The 2-aminophenethyl alcohols resulting from condensation of ortho-nitrotoluenes are good precursors for preparation of indoles. Watanabe and coworkers have developed ruthenium-cata- lyzed dehydrogenative N-heterocyclization for synthesis of indoles and other heterocycles from 2-aminophenethyl alcohols or 2-nitrophenylethyl alcohols (Eq. 10.52). 69a The oxidative cyclization of 2-aminophenethyl alcohols are also catalyzed by Pd-based catalysts.69

OH
Pd/C	N
RuCl2(PPh3)3	N	(10.52)
NO2	H
	96%

A neat synthesis of 4-nitroindole depends on an acylation-deacylation sequence from 2-methyl-3-nitroaniline, as shown in Eq. 10.53.70 On the other hand, treatment of N-protected indoles with acetyl nitrate generated in situ at low temperature gives the corresponding

					OEt
	Me	1) (MeO)2CHNMe2,		O	dioxane-H2O
		pyrrolidine, DMF			dioxane-H2O
Br	NO2	2) EtOC6H4COCl,		NMe2	reflux
Br	NO2	Et3N, PhH, reflux
			Br	NO2
		OEt
			Na2S2O4	Br	N
		O	H2O, EtOH, THF		H
	Br	NO2	reflux		OEt
		50% (overall)			63%

Scheme 10.7.

10.2 SYNTHESIS OF INDOLES 341

3-nitroindoles in good yields.71 The regioselective synthesis of nitroindoles is important for functionalization of indoles.72

NO2		NO2			NO2
NO2	HC(OEt)3	Me		(CO2Et)2, KOEt
Me	HC(OEt)3			(CO2Et)2, KOEt
	TsOH	N	OEt	DMF, DMSO	N
NH2		N	OEt		H
					H
		88%			71%
		88%			71%

(10.53)

As discussed in Chapter 9, various nucleophiles can be introduced at the ortho position of nitroarenes via the VNS process. This provides a useful strategy for the synthesis of indoles. One of the most attractive and general methods of indoles and indolinones would be the reductive cyclization of α-nitroaryl carbonyl compounds (Eq. 10.54). The VNS and related reactions afford α-nitroaryl carbonyl compounds by a simple procedure. For example, alkylation of 4-fluoronitrobenzene with a lactone silyl enol ether followed by reductive cyclization leads to tryptophols (Eq. 10.55).73

	R´			R´		R´
		R = alkyl		R	R = O-alkyl
X	R	H2, Pd/C	X		X		O
	N		X	O	H2, Pd/C	N
				O	H2, Pd/C		(10.54)
				NO2
	H			NO2		H
NO2				NO2			OH
	OSiMe3			O	F		OH
	+	1) F–		O	1) DIBAL
	+	2) DDQ		O	2) H2, Pd/C	N	(10.55)
	O	2) DDQ			2) H2, Pd/C	N	(10.55)
F				F		H
				F
				79%		98%
				79%		98%

The cyanomethylation of nitroarenes followed by alkylation and reductive cyclization yields indoles (see Chapter 9, which discusses the VNS reactions) (Eq. 10.56).74

CN			CN		CH2Ph
MeO	Ph3P-DEAD	MeO			MeO
MeO	Ph3P-DEAD	MeO	CH2Ph	H2, Pd/C
	PhCH2OH		CH2Ph		N
	PhCH2OH				N
NO2	DEAD		NO2		H
	O	O	70%	56%
	= EtOC–N=N–C–OEt			(10.56)

Improvement was reported in the synthesis of 7-alkoxyindoles by reaction of alkoxynitrobenzenes with vinylmagnesium bromide (Eq. 10.57).75 This reaction proceeds via the addition of the Grignard reagent ortho to the nitro group and subsequent [3,3]sigmatropic rearrangement (Bartori indole synthesis).76 This indole synthesis is applied to a short synthesis of the pyrrolophenanthridone alkaloid hippadine, as shown in Scheme 10.8.77

	+	MgBr	THF	N
NO2			–78 ºC		(10.57)
				H

OCHPh2 Ph2CHO

57%

342 SYNTHESIS OF HETEROCYCLIC COMPOUNDS
NO2	MgBr		KOH, DMSO		Br	N Br
	THF, –70 ºC, 3 h	N	O	CH2Br	Br
Br	THF, –70 ºC, 3 h	N	O	CH2Br
Br		H
		Br	O	Br		72%	O
		53%		RT, 2 h			O
		53%		RT, 2 h
						O
2 BuLi, THF, –78 ºC		O		X = H2 (49%)
2 CuI, P(OEt)3, 3 h		O	N	X = O (79%)
to RT, 21 h			N
to RT, 21 h

Scheme 10.8.

The intramolecular cyclization of nitrenes obtained either from deoxygenation of o-ni- trostyrene by trialkyl phosphites78 or from thermal or photochemical decomposition of o-azi- dostyrenes79 provides a useful method for the construction of indole nucleus (Cadogan-Sundberg indole synthesis). Holzapfel has used this method to synthesize several carbazoles and norharman from the appropriate 2-nitrobiphenyls and also several 2-methoxy- carbonylindoles from methyl o-nitrocinnamates.80 The novel generation of nitrenes from o-nitrostilbenes using CO and Se leads to an efficient synthesis of 2-arylindoles (Eq. 10.58).81 A new synthetic approach to the natural product arcyriaflavin-A, based on nitrene insertion, has been reported.81b

R
Se, Et3N	N
+ CO	N	(10.58)
DMF, 100 ºC	H	(10.58)
DMF, 100 ºC	H
NO2	65–78%	R
R = Me, OMe, CF3	65–78%

The metal carbonyls Fe(CO)5, Ru3(CO)12, and Rh6(Co)16 are catalysts for the deoxygenation of o-nitrostyrenes under carbon monoxide pressure to give indole derivatives (Eq. 10.59).82

CO2Me			CO2Me
+ 2CO	Fe(CO)3	N
	220 ºC		(10.59)

		H
NO2
		75%

The Sundberg indole synthesis using aromatic azides as precursors of nitrenes has been used in synthesis of various indoles. Some kinds of aryl azides are readily prepared by SNAr reaction of aromatic nitro compounds with an azide ion. For example, 2,4,6-trinitrotoluene (TNT) can be converted into 2-aryl-4,6-dinitroindole, as shown in Eq. 10.60.83

Me	OHC	O2N	NO2	O2N	NO2
O2N	OHC	CHO
O2N	NO2
	(piperidine)		NO2		NO2
		-H2O	NO2		NO2
		-H2O

NO2	67%

		10.2	SYNTHESIS OF INDOLES 343
O2N	N3	N3	NO2
NaN3, DMF
- NaNO2	NO		NO
	2		2
		73%
	O2N		NO2
HOCH2CH2OH		N	(10.60)
		N	N
160 ºC		H	H
160 ºC		NO2	NO2
		71%

The previously unknown 2-nitroindoles have been conveniently prepared from o-nitroben- zaldehyde via the Sundberg indole synthesis (Eq. 10.61).84

CHO			CHO 1) MeNO2, KOH,		NO2
	NaN3, DMF			EtOH	N3
	60 ºC			2) Ac2O, pyridine	N3
NO2			N3		81%
					81%
		Xylene		NO2	(10.61)
				N	(10.61)
		140 ºC		N
		140 ºC		H
			54%

Soderberg and coworkers have developed a palladium-phosphine-catalyzed reductive N-het- eroannulation of 2-nitrostyrenes forming indoles in good yields.85 For example, reaction of 6-bromo-2-nitrostyrene with carbon monoxide in the presence of a catalytic amount of palladium diacetate (6 mol%) and triphenylphosphine (24 mol%) in acetonitrile at 70 °C, gives 4-bromoindole in 86% yield (Eq. 10.62). Several functional groups, such as esters, ethers, bromides, triflates, and additional nitro groups, have been shown to be compatible with the reaction conditions.

	Br	Br
	Br
	Pd(OAc)2	(10.62)
	Ph3P, CO	(10.62)
	Ph3P, CO	N
	NO2	H
		86%

With the use of this methodology, 2,4-dimethylindole, 4-(hydroxymethyl)-2-methylindole, and 4-(methoxymethyl)-2-methylindole are readily obtained, as shown in Eq. 10.63.86 These indoles have been recently isolated from European Basidmycetes.87 Watanabe and coworkers have used a catalytic amount of PdCl2(PPh3)2-SnCl2 under carbon monoxide for reductive N-heterocyclization of o-nitrostyrenes.88

Me	Me
Me
Pd(OAc)2	Me	(10.63)
Ph3P, CO	Me	(10.63)
Ph3P, CO	N
NO2	H

66%

344 SYNTHESIS OF HETEROCYCLIC COMPOUNDS

Fused indoles, as shown in Eq. 10.64, are also prepared by this method.85a

MeO2C CO2Me

Pd(OAc)2

(10.64)

	dppp, DMF
	CO (60 psi)	N
NO2	120 ºC	N
NO2	120 ºC	H
		61%

Similar to the Fisher indole synthesis, reductive cyclization of nitro aromatics offers a powerful means of forming indoles. Reductive cyclization of ortho, 2′-dinitrostyrenes has occurred in many ways, by TiCl3, NaBH4-Pd/C, H2-Pd/C, and other reductive methods.89 Corey and coworkers have used the Borchardt modification (Fe-AcOH, silica gel, toluene at reflux for the reductive cyclization of o-β-dinitrostyrenes) to prepare 6,7-dimethoxyindole (Eq. 10.65) in a total synthesis of aspidophytine (see Schemes 3.3 and 3.4 in Section 3.2.1).89d

		NO2
		Fe, AcOH	N
MeO	NO2	MeO	N	(10.65)
MeO	NO2	silica gel	H
	OMe	toluene, ∆	OMe
	OMe		71%
			71%

Tin-mediated-radical cyclization of isonitriles provides a useful strategy for the preparation of indoles (Fukuyama reaction).90 This radical cyclization is used for synthesis of 6-hydroxy- indole-3-acetic acid, which is the aromatic subunit of Nephilatoxin. The requisite isonitriles are prepared from nitroarenes via amines (Eq. 10.66).91

			CO2Me						CO2Me
			CO2Me	Zn. AcOH
				Zn. AcOH
PhCH2O		NO2				PhCH2O		NH2
PhCH2O		NO2						96%
					CO2Me
	AcOCHO						POCl3
	pyridine		PhCH2O	NHCHO			Et3N, CH2Cl2
				96%			Et3N, CH2Cl2
				96%					CH2CO2Me
			CO2Me						CH2CO2Me
			CO2Me
				Bu3SnH				(10.66)
PhCH2O		NC		AIBN		PhCH2O			N
PhCH2O		NC		AIBN		PhCH2O		83%	H
92%								83%

The classic Reissert indole synthesis, involving the reductive cyclization of o-ni- trophenylpyruvic acid, has been used for synthesis of 2-ethoxycarbonyl-4-alkoxymethylindo- les.92 The modified Reissert reaction, involving the reductive cyclization of an o-nitrophenyl acetoaldehyde, has been adapted to solid-phase synthesis.93

Kraus has reported the synthesis of a tricyclic indole related to the pyrroloiminoquinone marine natural products (Scheme 10.9), in which an intramolecular SNAr and the reductive cyclization of a nitro aldehyde are involved as key steps.94 Related target compounds have been prepared by Joule and coworkers via a similar strategy.95

				10.2 SYNTHESIS OF INDOLES		345
OMe		OMe			OMe
OMe		O2N	NO2		OMe
1)		O2N	NO2		O2N	NO2
	O O O		OMe		O2N	NO2
	O O O			1) t-BuOK, CAN
	2) HNO3-H2SO4		O	2) BH3		CHO
	3) CH2N2	HN		3) DIBAL
NH2		O			HN
		O		OMe H
				OMe H
			O2N	N
		H2, Pd/C

42%

Scheme 10.9.

Rawal and Kozmin have utilized a Reissert type reaction in the total synthesis of tabersonine. The requisite nitro ketone is prepared by SNAr reaction of o-nitrophenylphenyliodonium fluoride with ketone silyl enol ether (Scheme 10.10).96

The reductive cyclization of o-nitrophenylacetic acids or esters gives oxyindoles, which has been applied to preparation of 6-hydroxy-7-methoxyoxyindole in a synthesis of (+)-paraher- quamide B (Scheme 10.11).42

The pyrrolo[2,3-d]pyrimidine anticancer agent is prepared utilizing, as a key sequence, Michael condensation of 2,6-diamino-4(3H)-pyrimidinone with nitroalkenes, followed by the Nef reaction that leads to the annulated pyrrole ring (Eq. 10.67).98

					NO2
			O		O
			O		CO2Et
NO2				EtOAc-H2O	CO2Et
NO2	+	HN		EtOAc-H2O	HN
	+	HN		50 ºC
				50 ºC	H2N N NH2
CO2Et		H2N N NH2			H2N N NH2
CO2Et					91%
				O	CO2H
				O
	1) NaOH		HN		(10.67)
	2) H2SO4				(10.67)
	2) H2SO4		H2N	N N
			H2N	N N
				H
				57%

Annulation of pyridine to indole is accomplished by a tandem aza-Wittig/electrocyclization strategy as shown in Eq. 10.68.99

		MeO2C	MeO	C
MeO2C		N	2	N
MeO2C	I	N		N
N	I
NO2	F–		TiCl3, NH4OAc
DMSO, THF		NO2	aq THF
		NO2	N
TBSO		O	H
		94%	89%

Scheme 10.10.

346 SYNTHESIS OF HETEROCYCLIC COMPOUNDS

(–)-paraherquamide A, R

= OH, R

= H2, X = N

= Me, R

O Me

(–)-paraherquamide B, R1 = H, R2 = H, R3 = H2, X = N

(–)-paraherquamide C, R1 = R2 = CH2, R3 = H2, X = N

O Me

(–)-paraherquamide D, R1 = O, R2 = CH2, R3 = H2, X = N

(–)-paraherquamide E, R1 = H, R2 = Me, R3 = H2, X = N

Scheme 10.11.

NO2

PPh3

R-NCO

N N

(10.68)

toluene

N N

reflux

79–97%

A novel indole ring formation involving DBU nucleophilic addition to 1,3,5-trinitrobenzene

has appeared as shown in Eq. 10.69.100

NO2

DBU

(10.69)

O2N

NO2

O2N

11%

10.3 SYNTHESIS OF OTHER NITROGEN HETEROCYCLES

10.3.1 Three-Membered Ring

The direct aziridination of nitroalkanes has been reported for the first time. Treatment of nitroalkene with an excess of CaO and NsONHCO2Et (Ns = 4-nitrobenzenesulfonyl) gives the α-nitroaziridine in good yields (Eq. 10.70).101 The reaction proceeds via aza-Michael reaction followed by a ring closure.

				R1		R2
	R2		CaO			NO2
R1	+	NsONHCO2Et	CaO		N	NO2
R1	+	NsONHCO2Et	CH2Cl2		N		(10.70)
	NO2		CH2Cl2		CO2Et		(10.70)
	NO2

				62–84%

10.3.2 Fiveand Six-Membered Saturated Rings

Pyrrolidines are structural subunits found in many natural and unnatural products, which have important biological activity.102 Depending on the substitution pattern and functionalization, pyrrolidines have been shown to be effective antibacterials,103 neuroexcitatory agents,104 potent venom,105 and glyosidase inhibitors.106

Nitro compounds have been extensively used for synthesis of pyrrolidines as discussed in Chapter 4 on the Michael addition and Chapter 8 on cycloaddition. Tandem [2 + 4]/[2 + 3]

10.3 SYNTHESIS OF OTHER NITROGEN HETEROCYCLES 347

cycloaddition using nitroalkenes provides an excellent route to stereoselective synthesis of pyrrolidines. This section describes the routes to the synthesis of pyrrolidines based on the Michael addition. The Michael addition of nitro compounds to α,β-unsaturated carbonyl compounds or esters followed by reduction gives pyrrolidines or related compounds. cis-2,3- Disubstituted pyrrolidines are available from nitro ketones, which are prepared by the Michael addition of nitromethane to enones. Reduction of the nitro ketones with Raney Ni gives the pyrrolines, which, on NaBH3CN reduction, give quantitative yields of cis-2,3-disubstituted pyrrolidines (Eq. 10.71).107

R	1	NO2 H2, Raney Ni	R2		R2
R		NO2 H2, Raney Ni		NaBH3CN

R2 O	R1	N	R1	N	(10.71)
				H

cis

Ruthenium complex catalyzes reductive N-heterocyclization of γ-nitroketones to give pyrroline derivatives (Eq. 10.72).108

	Me Me		Me
Ph	Ru(CO)12, CO	Ph
	NO2		N Me	(10.72)

O	50%
	50%
	N N

cis-2,5-Disubstituted pyrrolidines are prepared by the Michael addition of nitro compounds to enones followed by reduction with H2-Pd/C. This strategy has been widely used for synthesis of various alkaloids. Stevens and Lee have reported stereoselective total synthesis of the tail pheromone of the Pharaoh ant (Scheme 10.12)109 and gephyrotoxin 223, neurotoxic alkaloids

Cl		Mg, THF	O	O	MnO2		O	O
O O		H	O	O	OH		O	O		O
		H			OH					O
		O
								H
CH3(CH2)4NO2		O O	O		H2, Pd/C	O	O	H	N	H
TMG			O2N	(CH2)3CH3				–2		H
			64%				CO2			(CH2)3CH3
			64%				CO2H			(CH2)3CH3
							CO2H
			H			H
H	O+				pH 3.8–5.4
3		N				N
	–	N				N
HO				H			H
				H			H
		Me	(CH2)3CH3			Me	(CH2)3CH3
					H

NaCNBH3

Me (CH2)3CH3

68%

Scheme 10.12.

348 SYNTHESIS OF HETEROCYCLIC COMPOUNDS

SOPh

O Br

70%

CH3(CH2)4NO2

TMG

(CH2)3CH3

O2N

H O+

, Raney Ni

–

CO2

H2N

CO2H

(CH2)3CH3

64%

KCN

R = CN

R = CHO

(CH2)3CH3

96%

Scheme 10.13.

(Scheme 10.13).110 New alkaloids from ants, (3R, 5S, 9R)-3-butyl-5-(1-oxopropyl)indolizidine and (3R, 5R, 9R)-3-butyl-5-(1-oxopropyl)indolizidine, are identified and synthesized as outlined in Scheme 10.14.111

Battersby and coworkers have developed selective methods for total synthesis of chlorins on a model system, as shown in Scheme 10.15, in which the Michael addition of 5-(2-nitroethyl) pyrrole to enone and reductive cyclization are used as key steps.112

EtONa

EtO		+	EtOH
EtO		O


		NO2
	O			O	H
	O

EtO			H2, Pd/C	EtO
EtO			EtOH		HN
		O2N	EtOH	90%	HN	H
		O2N		90%		H
		O (CH2)3CH3			(CH2)3CH3
		80%			(CH2)3CH3
		H		H
	LDA				R = OEt
		N	+	N	R = OH
	I2	(CH2)3CH3		(CH2)3CH3	R = Et
		(CH2)3CH3	R	(CH2)3CH3
		R O	R	O
		16%		49%
				49%

Scheme 10.14.

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