ISSN 2319-7625 (Online) (An International Research Journal), www.chemistry-journal.org
Synthesis of Heterocyclic Compounds by the Application of
Radical Cyclization
Sanjay Nath
Department of Chemistry,
Krishnath College, Berhampore, Murshidabad, W.B., INDIA. email: [email protected].
(Received on: March 27, 2018)
ABSTRACT
The radical based cyclization method have been recognized as one of the simplest and useful tools for regio- as well as stereoselective syntheses of carbo- and heterocyclic compounds. In this review article I have summarized the various ways of constructing heterocyclic rings by the application radical cyclization.
Keywords: Radical cyclization, tributyltin hydride, tandem radical cyclization, oxidative radical cyclization, fused heterocycles, regioselectivity, stereoselectivity.
1. INTRODUCTION
The fact that the five-membered ring closure is kinetically favoured5 is further supported by the work of Crich et al. Similarly, a mixture of both 5-exo and 6-endo products are obtained by the 5-exo/6-endo cyclization of acyl radical and, accordingly, the 5-exo
cyclization of acyl radicals onto a C=C bond is a kinetically favoured process.6
A number of reviews have been reported1,7 regarding general methodologies in radical chemistry containing useful discussions for the synthesis of heterocyclic protocols. Spiro-cycles can also be effectively synthesized by radical cyclization employing an intramolecular radical attack onto a cyclic olefin8,intramolecular addition of tertiary cyclic radicals to an alkene9 or alkyne10, or cyclization of a radical species containing a preoccupied quaternary carbon center.11
Different reagents are available for radical cyclization. Among them organotin hydrides12 especially tri-n-butyltin hydride has been proven to be an excellent radical
generating reagent for the synthesis of heterocycles using radical cyclization. The alternative procedure involving the use of small amount of tri-n-butyltin chloride with sodium cyanoborohydride for in situ generation of tri-n-butyltin hydride is also known. However, it is not useful in pharmaceutical synthesis due to its high toxicity13. Additionally it is very difficult to remove the tributyltin residues from the reaction mixtures. Purification of products is, therefore, very difficult and various attempts have been made to overcome this problem. Due to this problem various efforts have been directed towards the tin free radical chemistry.14
Tributylgermanium hydride (Bu3GeH)15 is the superior alternatives to nBu3SnH, devoid of all
these problems. Use of tris(trimethylsilyl)silane [(TMS)3SiH or TTMSS]15 and
polymethylhydrosiloxanes16 have been extensively developed. Although in most cases AIBN is used as a radical initiator, substantial amount of use of other diazine initiators, e.g. AMBN [azobis(methylisobutyronitrile)] in radical reactions is also reported. It is more soluble and can be used in cyclohexane as well as toluene as solvent. Cyclohexane is found to be the preferred solvent for nBu
3SnH mediated reactions because toluene and benzene not only act as a solvent
but also may participate in the radical reaction.
Radical reaction can also be performed using hypophosphorous acid.17 A novel radical
reduction in aqueous isopropyl alcohol using the combination of VA-061, hypophosphorous acid and triethyl amine was reported by Kita et al.18 Triethylborane (Et
3B) is a powerful reagent
for radical cyclization of oxime ethers and hydrazones intramolecularly concerted with the
,-unsaturated carbonyl group.19 Diethyl phosphite, (EtO)2P(O)H, has proved to be an useful
alternative and more versatile reagent for radical cyclization.20 Recently, Ce (IV) reagents, Ceric ammonium nitrate (CAN) and ceric-tetra-n-butylammonium nitrate (CTAN) are widely applied for the generation of radicals and radical cations for the construction of carbon-carbon bonds.21 The search for various heterocycles and many new methodologies has been a central goal for free radical chemists in recent years. In this review we will summarize the recent examples of the synthesis of heterocycles by radical cyclizations.
2. SYNTHESIS OF HETEROCYCLIC COMPOUNDS
Many natural products and pharmaceuticals contain oxygen-containing heterocycles and synthesis of these compounds remains an important challenge in total synthesis. A direct method for their synthesis is the cyclization of an oxygen-centered radical onto an olefin as alkoxy radicals add rapidly and irreversibly to alkenes. An oxygen-centered radical cyclization onto silyl enol ethers has been developed22 and utilized for the synthesis of versatile siloxy-substituted tetrahydrofurans. The reactions display excellent chemoselectivity for cyclization when competing terminal alkene cyclization, 1,5- hydrogen abstraction and β-fragmentation pathways are present. The compound 1 on treatment with Bu3SnH, AIBN in benzene at about
80 ºC produce compound 2 in excellent yield and excellent diastereomeric ratio (Scheme 1).
PhThO R1
R2
R3 OTBS
R4
O R
4
R2 R1
R3
OTBS Bu3SnH, AIBN
C6H6, 80 oC
1
2, 51-86 %
Scheme 1
R1 = H, Me, CHMe2; R2 = H, Et, Ph
R3 = H, Et, Ph R4 = H, Ph
A competition experiment22 was carried out by the same group to investigate the degree of the chemoselectivity of cyclization onto a silyl enol ether relative to a simple alkene. Compound 3 displays complete chemoselectivity for addition to the silyl enol ether and exclusively afforded the product 4 (Scheme 2).
OThPh OTBS O
Bu3SnH, AIBN
C6H6, 80oC
3
4
OTBS
69:31 dr. 81%
Scheme 2
Monocerin isolated from several fungal sources, which exhibits antifungal, insecticidal and plant pathogenic properties. It is a polyketide natural product. Synthesis of monocerin 7 was reported23 by Lee et al. via radical cyclization of a vinylic ether intermediate 5 to produce oxolane 6. The oxolane was converted then in few steps to produce (+) monocerin 7 (Scheme 3).
OMe MeO MeO
O SePh OMOM
(TMS)3SiH, Et3B
Toluene, -20oC
MeO
MeO OMe
O
OMOM 74%
5 6
O O MeO
MeO OMe O (+) Monocerin
Scheme 3
7
Tanaka et al. recently reported24 the samarium(II)-mediated spirocyclization by intramolecular addition of aryl radicals onto an aromatic ring. N-(2-iodophenyl)-N -methylbenzamides 8a-h on aryl radical coupling reaction mediated by SmI2 with HMPA in
N O Me
I R1
R2
R3
N OR1
R2
R3 Me
N O Me
R1
R2
R3
H
H SmI2, HMPA
iPrOH
THF, -35oC +
8a-h
9a-h, 30-90 % 10a-h, 10-60%
Scheme 4
R1 = H, Me, OMe, Br, NO2;
R2 = H, Me; R3 = H, Me OMe
N O Me
I R1
R2
R3
N O Me
R1
R2
R3 SmI2, HMPA
THF, 0oC
8a-h
11a-h, 25-60%
Scheme 5
R1 = H, Me, OMe, Br, NO 2
R2 = H, Me; R3 = H, Me, OMe
The key intermediate ester 17 for the synthesis of defucogilvocarcin M 18 can be synthesized via a xanthate-based free radical addition-cyclization sequence25 developed by Zard.26 The synthesis starts with the preparation of the required radical precursor 15a from
commercially available 2-hydroxy acetophenone 12 in three steps. A mixture of 13a and vinyl pivalate 14 in refluxing 1,2-dichloroethane was treated with a substoichiometric amount (30 mol %) of dilauroyl peroxide (DLP), to afford adduct 15a in 80% yield. The latter was then allowed to react with 1.4 equiv of DLP and the expected ring closure took place to form the
desired α-tetralone 16a in 42% yield (Scheme 6). Although this moderate yield may appear to be disappointing, this transformation would be quite difficult to accomplish by other means. Interestingly, when they attempted the radical sequence with the iodine derivative 13b (X = I) under the same reaction conditions, they obtained nothing but starting material or reduction product.
OH O i) BnBr, K2CO3, DMF, r.t
ii) Br2, Et2O, 0 oC to r.t
iii) KSC(S)OEt, acetone 0 oC to r.t
OBn O
X 13a, X = SC(S)OEt 90%, 3 steps
OPiv 14 DLP (30 mol%) 1,2-DCE reflux
OBn O
OPivSC(S)OEt 15a
DLP (1.4 eq) 1,2-DCE
OBn O
OPiv 16a (42%)
16a
OBn OMe
O
O
I OMe
Me
O
O OH OMe
OMe
Me i) PdCl2(PPh3)2
AcONa, DMA (65%) ii) H2, Pd/C, THF
1 atm (80%)
Scheme 6
Defucogilvocarcin M 18
12
17
reflux
Recently, Haung and co-workers27 for the synthesis of multifunctional tetrahydrofurans, have
used Titanium (III)-mediated radical cyclization of epoxyallene ethers. They also used this approach for the preparation of pyrrolidine derivatives (Scheme 7).
O
O (i) Cp2TiCl, THF
(ii) H3O+ O
HO
O .
+
19 20 (84%) 21 (6%)
Shanmugam et al.28 demonstrated the stereoselective 6-endo and 6-exo-trig cyclization
of propargyl and phenyl homopropargyl derivatives of Baylis-Hilmann adducts by the use of nBu
3SnH-mediated vinyl radical cyclization to afford polysubstituted functionalized
tetrahydropyrans in good yields. The protected propargyl derivatives 22 when treated with nBu
3SnH and a catalytic amount of radical initiator, AIBN in refluxing benzene afforded the
trisubstituted tetrahydropyran derivatives 23 in good yields via a 6-endo cyclization (Scheme 8). A pronounced effect of substituents on the 6-endo-trig cyclization was observed.
O Ar
CO2Me i) nBu
3SnH, AIBN
benzene, reflux ii) PPTS, CH2Cl2
r.t, 24h O
CO2Me
Ar
22 23
Scheme 8
Majumdar et al.reported29 the radical cyclization of substrates containing pyran-2-one
moiety. The substrates 24a-e were refluxed in dry toluene with nBu
3SnH in the presence of
AIBN as an initiator for 3-4 h to give the cyclized products 25a-e in 68-79 % yields (Scheme 9).
O O
OCH3
O O O
OCH3
O
nBu
3SnH, AIBN,
toluene, reflux, 3-4 h. Br
R1
R2
R3
R1 R2
R3
R1 = H, Me; R2 = H, Me;
R3 = H, OCH 3, 70-80%
Scheme 9
24a-e 25a-e,
The same group also reported the regioselective synthesis of tetracyclic [6,6] pyranothiopyran 27 by aryl radical cyclization.30 The starting materials, 4-aryloxymethyl-7-methylthiopyrano [3,2-c]pyran-5-ones 26a-f when refluxed in dry benzene with tributyl tin-hydride in the presence of AIBN as an initiator for 4-5 h afforded the cyclized products 27a-f in 75-80 % yields (Scheme 10).
O S
O R1
R2
R3 O
nBu
3SnH, AIBN, C6H6 reflux, 4-5h
O S
O R1
R2
R3 O
H
H R1 = H, -Ar, Me; R3 = H, Me; R2 = H, -Ar, Me, OMe; 70-80%
Scheme 10
Br
26a-f 27a-f
Regioselective unusual formation of spirocyclic 4-{2'-benzo(2',3'-dihydro)furo}-9-methyl-2,3,9-trihydrothiopyrano[2,3-b]indoles 29 has been reported31 by Majumdar et. al.via 4
-exo-trig aryl radical cyclizations and rearrangements. The compounds 28 were treated with nBu
3SnH (1.1 equiv.) in toluene at 80 oC in the presence of a radical initiator AIBN (0.5 equiv.)
for 3-4 h to give the the unusual products 29 in 75-80 % yields (Scheme 11).
N CH3
S O
Br
28
R1 R2
R3 R4
N CH3
S O Bu3SnH, AIBN,
dry toluene, 80 oC, 3-4 h, 75-80 %.
29
R4
R1 R2 R3
The formation of the products 29 is explicable by a 4-exo-trig cyclization of the initially generated aryl radical 30 at the enol ether part of the diene to give the more stable radical 31. The stability is due to the overlapping of the p-orbital of the radical center of 31 with the neighboring -system of the indole moiety. The highly strained oxetene ring may undergo ring opening leading to the formation of a resonance stabilized aryloxy radical 32. The aryloxy radical 32 may then undergo a 5-endo-trig cyclization to give the stable benzylic radical 33 followed by abstraction of a hydrogen radical to afford the unusual product 29 (Scheme 12).
N CH3 S O N CH3 S O R R O N CH3 S N CH3 S O R 32 . . AIBN 30 4-exo-trig .
. 5-endo-trig
33
.
32
ring opening Bu3SnH
+H N CH3 S O R 31 . N CH3 S O Br 28 R1 R2 R3 R4 Scheme 12 R N CH3 S O 29 R4 R1 R2 R3
Gharpure et. al. describes32 the synthesis of oxa- and aza-cage compounds based on
tandem radical cyclization. To a refluxing solution of the iodide 34 and AIBN in benzene a solution of n-Bu3SnH and AIBN in benzene was added dropwise, the oxa-cage compound 36
was obtained in good yields via a 5-exo-trig, 5-exo-dig tandem radical cyclization. Here primarily radical generated at C3 after first the 5-exo-trig cyclization i.e., after C1-C2 bond formation to form 35, which then undergo another tandem radical cyclization on the acceptor followed by subsequent reduction to generate a new type of cage compounds 36 (Scheme 13). The reaction was also carried out on the iodide 37, which contains an olefin as acceptor for second radical cyclization step. The reaction was found to work equally well leading to the formation of oxa-cage 38via two 5-exo-trig tandem radical cyclizations in good yield (Scheme 14). In a similar manner, the method could be used for the synthesis of the aza-cages 36d,e (entry 4,5 Scheme 13) with comparable efficiency.
X
O I
T n-Bu3SnH
AIBN, C6H6
reflux X O T X O T
entry Substrate X T product yield (%) 34a 34b 34c 34d 34e CH2
(CH2)
C(CH2)2
CH2
C(CH2)2
O O O N-Ts N-Ts 70 69 65 68 62
34 35 36
Scheme 13 36a 36b 36c 36d 36e 1 2 3 4 5 O I
O n-Bu3SnH
AIBN, C6H6
reflux O O
37 38 H
Scheme 14 84%
with Fe(NO3)3.9H2O in the presence of several chloride salts (LiCl, MgCl2, CuCl2, CoCl2,
ZnCl2, FeCl3) in boiling THF gave the nitrated cyclized products 40 along with a small quantity
of alkene (Scheme 15). A plausible mechanism for this conversion is shown in Scheme16.
X
Fe(NO3)3.9H2O
(1.2 equiv.)
FeCl3 (1.5 equiv.)
THF, reflux R2
R1
X O2N
R1 R2 Cl 1 2 3 4 5 6 7 8
C(CO2Me)2
C(SO2Ph)2
O NTs C(CO2Me)2
C(SO2Ph)2
O NTs Me Me Me Me H H H H Me Me Me Me H H H H 2.5 2 3.5 3 5 2 4 4 65% (78:22) 84% (55:45) 56% (50:50) 53% (55:45) 59% (85:15) 92% (73: 27) 46% (70:30) 91% (70:30) entry X R1 R2 time(h) yield %
(trans:cis)
Scheme 15
39 40
Thermal decomposition
NO2 + [Fe] complex
X R2 R1 1) Radical addition X R2 R1
O2N
2) Cyclization X O2N
R1
R2 LnFeIII
Cl LnFeII
3) Radical trap
X O2N
R1 R2 Cl
Scheme 16: Plausible mechanism Fe(NO3)3.9H2O
39 41 42 40
Curran et al.34introduced a mild and convenient radical approach for the synthesis of spirocyclohexadienones. The amide precursors 43 when treated with tristrimethylsilyl hydride in the presence of triethylborane (Et3B) in benzene at room temperature underwent ipso
cyclization of aryl radical to oxygen substituted aromatic ring to furnish the desired spiro-oxyindole derivatives 44. Ortho-cyclization leading to phenanthridones 45 is also obtained along with the spiro-products 44 (Scheme 17). However, the ratio of spiro-cyclic to phenanthridone products were different depending upon the groups attached to the phenol.
Further they also reported that, the compounds 46 when stirred with tristrimethylsilyl hydride and triethyl borane in benzene in the presence of air gave the desired spiro-dihydroquinolones 47 exclusively (Scheme 18).34
N O I OR R2 R1
(Me3Si)3SiH
Et3B, C6H6, air N
R2 R1 O O N OR R2
R1 O
43 44 45
Scheme 17 N O R1 I OR R3 R2
(Me3Si)3SiH
Et3B, C6H6, air
N R3 R2 R1 O O 46 47 Scheme 18 +
A cascade radical cyclization starting from an amidyl radical 48 has been used by Zard and co-workers35 for the preparation of tricycle 49, whose framework is found in the stemona alkaloids. Compound 49 has also been used for the construction of (±)-aspidospermidine 50 (Scheme 19).
N O
O MeO2C
OBz nBu 3SnH ACCN N O O
MeO2C H N
Recently, we reported36 the regioselective synthesis of six membered sulphur
heterocycles by the tin hydride–mediated radical cyclization of a number of sulfides and sulfones under mild and neutral conditions. The sulfides 53a,b were derived from 3(2H) benzothiofuranone 51 with either 2-bromobenzyl bromide 52a, or 2-bromo-5-methoxybenzyl bromide 52b by phasetransfer-catalyzed reaction, and the corresponding sulfones 54a,b were prepared by treatment of the corresponding sulfides 53a,b with 2 equiv. of m-CPBA in dry dichloromethane solution at room temperature (Scheme 20). The sulfides 53a,b and sulfones 54a,b were then reacted with nBu3SnH in presence of AIBN under nitrogen atmosphere at
80OC to afford regioselectively benzofuran-annulated six-membered sulfur heterocycles 55a,b
and 56a,b (Scheme 21).
O S
+ Br
Br
R
O S
Br
R O
S
Br R
O O
51 52a, R = H 52b, R = OMe
53a, R = H
53b, R = OMe 54a, R = H54b, R = OMe
Scheme 20
BTEAC 1% NaOH (aq) CH2Cl2, 30 min.
m-CPBA CH2Cl2
stirring, rt, 3-4 h
80-95% 90-95%
O S
Br
R O
S
R nBu3SnH
AIBN N2 atm
80OC, 4h O
S
Br
R O
S
R nBu3SnH
AIBN N2 atm
80OC, 4h
O O O O
Scheme 21 53a, R = H 53b, R = OMe
55a, R = H 55b, R = OMe
54a, R = H
54b, R = OMe 56a, R = H56b, R = OMe
90-95% 90-95%
Indolo [2,3-a] quinolizidines framework found in numerous natural products of varying structures and biological activities, can be prepared37 based on radical cyclization of 2-acyl-1-phenylthiotetrahydro-β-carboline 57 bearing a pendent α,β-unsaturated ester. Substrate 57 was subjected to standard radical cyclization condition [n-Bu3SnH (1.5 equiv),
ACCN (0.1 equiv), degassed PhMe (0.01 M), reflux] to afford the desired cis-lactum 58 (81%), along with a minor amounts of trans isomer 59 (12%) and radical reduction product 60 (6%).
Cis-lactum 58 can be utilized for the synthesis of natural products eburnaminal 61 and larutensine 62, which contain the indolo [2,3-a] quinolizidines framework (Scheme 22).
N N O
SPh
CO2Et
Boc
ACCN
PhMe, reflux N
N O
Boc
CO2Et
H
H
N N O
Boc
CO2Et
H
H N
N O
Boc
CO2Et
58, 81% 59, 12% 60, 6%
+
57
+
Scheme 22
N N O
Boc
CO2Et
H
H 58
N N
H
HO
OH
N N
H
O
eburnaminol, 61 larutensine, 62
n-Bu3SnH
allylamine and α-phenylselenenyl ketones 63 in the presence of TiCl4. The crude reaction
mixture on treatment with nBu3SnH and AIBN afforded cyclic imine 65 (18%) together with
reduced, hydrolyzed product 66 (57%). Slow addition of tris(tri-methylsilyl)silane (TTMSS) and AIBN over a period 2-3 h, improve the yield of the cyclic imine 65 to 46% together with reduced product 66 (15%) and trace amount of group transfer product 67 (Scheme 23).
O
allylamine TiCl4, Et2O
overnight
N SePh
AIBN, TTMSS C6H6, reflux, 8h
O N
N SePh
63
64
65, 46% 66, 15% 67, trace Scheme 23
+ +
-78 oC to rt SePh
2, 3 disubstituted indole derivativess can be synthesized39 by treatment of β-aryl-β -(benzotriazol-1-yl)-α-primary alkyl (or aryl)-α,β-unsaturated ketones 68 with n-Bu3SnH and a
catalytic amount of AIBN in PhH at reflux. Treatment of 68c with n-Bu3SnH (1.0 equiv) in
the presence of azobisisobutyronitrile (AIBN) (0.5 equiv) in benzene for 2h at reflux gave 2-(3-methylphenyl)-3-phenylindole 69c and 2-(3-methylphenyl)-3-benzoyl-3-phenyl-3H-indole 70c in 38% and 44% yields, respectively (Scheme 24). Upon increasing the concentration of
n-Bu3SnH (2.5 equiv) under the same conditions, the yield of 69c increased to 79% at the
expense of that of 70c (10%). A further increase of n-Bu3SnH (4.0 equiv) resulted in a yield
of 69c to 86% and no 70c was detected. Further when R1 = t-Bu, 3-acyl (or aroyl)-2-arylindoles
71 together with a significant amount of phenanthridines 72 were isolated.
NN N
Ar R1
O R2
n-Bu3SnH
AIBN PhH 80 oC, 2h
R1 = Pri-alkyl
aryl
R1 = tert-butyl N H R1
Ar
N Ar
N H
Ar
N R1O R2
O R2
t-Bu O
R2 +
+
n-Bu3SnH, AIBN
PhH, 80 oC, 2h
69 70
71 72
Scheme 24 68
Entry Substrate Ar R1 R2 Yield (%) 69 71 72
68a 68b 68c 68d 68e 68f 68g 68h
Ph Ph 3-MeC6H4
3-MeC6H4
Ph Ph 2-MeC6H4
4-MeOC6H4
Ph Ph Ph 2-Naphthyl
t-Bu
t-Bu
t-Bu
t-Bu
Me Ph Ph Ph Me Ph Ph Me
85 75 86 87
73 67 69 75
16 27 20 13 1
2 3 4 5 6 7 8
3. CONCLUSION
The results and the examples discussed above it show that radical-based cyclization is an efficient synthetic method for the preparation of different heterocyclic compounds with varying complexity. The literature on the synthesis of heterocycles by radical cyclization is vast and it is beyond the scope of this review to include all related aspects of the radical cyclization. Therefore, discussion is limited only to the radical cyclization leading to the formation of five-, six- membered and a few seven- and eight- membered heterocyclic compounds.
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