Synthetic utility of 1-hydroxycarbazole-2-carbaldehydes - Syntheses of furo-, oxazino- and pyranocarbazoles

Article abstract of DOI:10.1135/cccc20071579

Reaction of 1-hydroxycarbazole-2-carbaldehydes 1 with phenacyl bromide yielded 2-benzoyl-10H-furo[2,3-a]carbazoles 2, whereas the reaction with ethyl bromoacetate yielded 1-oxo-1,2-dihydro[1,4]oxazino[2,3,4-jk]carbazole-4- carbaldehyde (4). The reaction o

Full text of DOI:10.1135/cccc20071579

Syntheses of Furo-, Oxazino- and Pyranocarbazoles
1579
SYNTHETIC UTILITY OF 1-HYDROXYCARBAZOLE-2-CARBALDEHYDES –
SYNTHESES OF FURO-, OXAZINO- AND PYRANOCARBAZOLES
1
2,
Arputharaj E. MARTIN and Karnam J. RAJENDRA PRASAD *
Department of Chemistry, Bharathiar University, Coimbatore 641 046, Tamil Nadu, India;
1
2
e-mail: ebenezermartin@yahoo.com, prasad_125@yahoo.com
Received July 24, 2007
Accepted Novem ber 14, 2007
Reaction of 1-h ydroxycarbazole-2-carbaldeh ydes
1 with ph en acyl brom ide yielded
2-ben zoyl-10H-furo[2,3-a]carbazoles 2, wh ereas th e reaction with eth yl brom oacetate yielded
1-oxo-1,2-dih ydro[1,4]oxazin o[2,3,4-jk]carbazole-4-carbaldeh yde (4). Th e reaction of 1 with
eth yl acetoacetate an d also with dieth yl m alon ate yielded th e pyran ocarbazoles 6 an d 7, re-
spectively. All th e products were ch aracterized by spectral an d an alytical m ean s. Plausible
m ech an ism s of product form ation are proposed.
Keyw ord s: Carbazoles; Pyran es; Fused h eterocycles; Aldeh ydes; Cyclization s; Furocarbazoles.
Murraya Koenigii1 is prolific for th e source of various carbazole alkaloids like
pyran o-, furo-, oxazin o- an d pyridocarbazoles. Th e presen ce of carbazole
n ucleus in th e fram e work of various ph arm acologically active com poun ds
with an ti-tum or2–4 an d an ti-HIV5,6 activities con tin ue to prom ote th eir
syn th etic efforts. Am on g th e carbazoles, som e facile precursors such as
2-acetylcarbazol-1-ol7, 2-ben zyliden e-2,3,4,9-tetrah ydrocarbazol-2-on e8,
an d reagen ts like tricarbon yl[(1-5-η)-cycloh exadien ylium ]iron tetrafluoro-
borate9, are available wh ich could be tran sform ed in to desired carbazole
alkaloids usin g various reaction s. For in stan ce, 2-acetylcarbazol-1-ol7 is an
im portan t syn th etic precursor of carbazole alkaloids for obtain in g bio-
gen etically possible carbazole alkaloids, Th ese classical syn th etic protocols
for th e syn th esis of carbazole alkaloids suffer from som e disadvan tages such
as low yield7 an d lack of easy availability/preparation of th e reagen t9,
m ultisteps an d h arsh reaction con dition s10. In th is con n ection , we felt th at
th e easily accessible in term ediate, 1-h ydroxycarbazol-2-carbaldeh yde11–13
reported from our laboratory would m ake th e task m uch easier to obtain
various an n ulated carbazole alkaloids like furo-, oxazin o- an d pyran o-
carbazoles. Earlier workers from our laboratory h ave reported th e syn th esis
of 2-acetyl-10H-furo[2,3-a]carbazoles14. Keepin g th is fact in m in d, in our
presen t study, we focused our atten tion on th e syn th esis of som e m ore oxy-
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© 2007 Institute of Organic Chemistry and Biochemistry
doi:10.1135/cccc20071579

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Martin, Rajendra Prasad:
gen ated fused carbazoles like furo-, oxazin o- an d pyran ocarbazoles with
som e substituen ts.
In an an ticipation to get 2-ben zoyl-10H-furo[2,3-a]carbazoles 2 from
1-h ydroxycarbazole-2-carbaldeh ydes11–13 1, we treated 1a with ph en acyl
brom ide15 an d calcin ed K2CO3 in aceton e an d obtain ed a crude solid m ass,
wh ich on purification by colum n ch rom atograph y over silica gel usin g
petroleum eth er/eth yl acetate (95:5) gave a yellow powder.
From th e spectral an d an alytical results, th e product was ch aracterized as
2-ben zoyl-7-m eth yl-10H-furo[2,3-a]carbazole (2a). Th e reaction was gen er-
alized for oth er carbazole derivatives 1b–1e (Sch em e 1).
SCHEME 1
Mech an istically it m ay be con sidered worth wh ile th at first th e
O-alkylation took place at th e C1-OH group of carbazole. Th en th e abstrac-
tion of an acid proton of C1-OCH2COPh by base followed by th e
carban ion ic attack on th e aldeh yde carbon yl carbon an d water elim in ation
yielded th e com poun d 2 (Sch em e 2).
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SCHEME 2
Earlier it was reported th at th e reaction of 1-h ydroxycarbazole-2-carb-
aldeh yde (1e) with eth yl ch loroacetate an d calcin ed K2CO3 in DMF yielded
eth yl 7-m eth yl-10H-furo[2,3-a]carbazole-2-carboxylate12,16 (3e). So we h ave
plan n ed to derive th e sam e alon g with its m eth yl an d ch loro substituted
derivatives usin g th e reaction of 1 with eth yl brom oacetate. Th erefore in
an oth er experim en t, we treated 1-h ydroxycarbazole-2-carbaldeh yde deriva-
tives 1 with eth yl brom oacetate an d calcin ed K2CO3 in aceton e. Th e reac-
tion of 1a yielded a crude product, wh ich on purification by colum n ch ro-
m atograph y over silica gel usin g petroleum eth er/eth yl acetate (98:2) gave a
pale yellow powder an d ch aracterized as 8-m eth yl-1-oxo-1,2-dih ydro[1,4]-
oxazin o[2,3,4-jk]carbazole-4-carbaldeh yde (4a). Here th e reaction en ded up
with th e form ation of th e un expected un kn own carbazole rath er th an th e
expected eth yl 7-m eth yl-10H-furo[2,3-a]carbazole-2-carboxylate (3a). Th e
reaction was gen eralized for oth er carbazole derivatives 1b, 1d an d 1e
except 1c (Sch em e 1). Th e support in favour of th ese un expected products,
1-oxo-1,2-dih ydro[1,4]oxazin o[2,3,4-jk]carbazole-4-carbaldeh ydes 4a, 4b
an d 4d , 4e, is also obtain ed from th e crystal structure (Fig. 1) of 1-oxo-
1,2-dih ydro[1,4]oxazin o[2,3,4-jk]carbazole-4-carbaldeh yde17 (4e).
Th e reaction of 1-h ydroxy-8-m eth ylcarbazole-2-carbaldeh yde (1c) with
eth yl brom oacetate un der th e sam e con dition yielded a wh ite spon gy m ass
after purification by colum n ch rom atograph y. From th e spectral results we
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Martin, Rajendra Prasad:
h ave con cluded th at th e product realized was 1-[(eth oxycarbon yl)m eth oxy]-
8-m eth ylcarbazole-2-carbaldeh yde (5c), wh ich is th e possible in term ediate
in th e proposed m ech an ism of th e con version of 1 in to 4 (Sch em e 3). In
order to cyclize 1-[(eth oxycarbon yl)m eth oxy]-8-m eth ylcarbazole-2-carb-
aldeh yde (5c), we carried out th e reaction of 1-h ydroxy-8-m eth ylcarbazole-
2-carbaldeh yde (1c) with eth yl acetoacetate in DMF/K2CO3. Here we h ave
also realized th e sam e com poun d 1-[(eth oxycarbon yl)m eth oxy]-8-m eth yl-
carbazole-2-carbaldeh yde (5c), rath er th an th e expected cyclized product.
Th e plausible m ech an ism for th e form ation of com poun d 4 h as been pro-
posed (Sch em e 3). In th e form ation of com poun ds 4a, 4b an d 4d , 4e, th e
in itial attack of a lon e electron pair on th e oxygen of th e C1-OH at th e
m eth ylen e carbon of th e BrCH2 group of eth yl brom oacetate is assum ed,
followed by HBr elim in ation producin g O-alkylated product 5. Th en th e
lon e electron pair on th e carbazole NH group again attacks th e ester car-
bon yl group of C1-OCH2COOCH2CH3 to give in term ediate I as we observed
in th e previous case. After th at th e in term ediate I on prototropic sh ift fol-
lowed by elim in ation of eth an ol produces th e respective products. Here we
can con clude th at in th e previous case th e elim in ation of water m olecule is
effected due to exten ded con jugation with ph en yl group wh ereas h ere
th ere is n o exten ded con jugation available to prom ote th e elim in ation of
water m olecule rath er th an th e eth an ol m olecule. In th e case of 1c, th e
com poun d 5c h as been a product, in stead of eith er of th e cyclized prod-
ucts (expected/realized). It h as been con cluded th at th e m eth yl group at
C8 position restricted th e approach of C1-OCH2COOCH2CH3 to th e carb-
azole NH group an d furth er cyclization was preven ted. Also th e K2CO3 in
FIG. 1
ORTEP drawin g of crystal structure of 4a
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Syntheses of Furo-, Oxazino- and Pyranocarbazoles
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aceton e/DMF is n ot basic en ough to abstract th e acid proton at C1
(C1-OCH2COOCH2CH3), th us stoppin g th e reaction at th is stage, to give 5c.
We h ave also attem pted th e syn th esis of pyran ocarbazoles. For th at we
treated 1-h ydroxycarbazole-2-carbaldeh ydes 1 with eth yl acetoacetate an d
dieth yl m alon ate. Wh en 1-h ydroxy-6-m eth ylcarbazole-2-carbaldeh yde (1a)
reacted with eth yl acetoacetate in pyridin e an d a catalytic am oun t of
piperidin e, yielded a yellow solid m ass. From th e spectral an d an alytical
results, its structure was assign ed as 3-acetyl-8-m eth yl-2,11-dih ydropyran o-
[2,3-a]carbazol-2-on e (6a) .
SCHEME 3
Wh en th e sam e type of th e reaction was exten ded to dieth yl m alon ate,
1a yielded a yellow solid m ass. All th e spectral an d an alytical details con -
firm th e product as eth yl 8-m eth yl-2-oxo-2,11-dih ydropyran o[2,3-a]carb-
azole-3-carboxylate (7a).
A plausible m ech an ism for th e form ation of com poun ds 6 an d 7 is pro-
posed. In itially th e lon e electron pair on th e h ydroxy group of 1 attacks th e
carbon yl carbon of th e ester group of both th e active m eth ylen e com -
poun ds, eth yl acetoacetate an d dieth yl m alon ate to give th e respective
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Martin, Rajendra Prasad:
O-alkylated products by elim in ation of eth an ol m olecule. Th en th e base ab-
stracts th e h igh ly acid proton at C-1 followed by th e in tram olecular attack
of th e carban ion on th e aldeh yde carbon yl carbon at C-2 an d water elim i-
n ation yielded th e respective products 6 an d 7.
In con clusion , th e reaction of 1-h ydroxycarbazole-2-carbaldeh yde (1e)
an d its derivatives substituted at C-6, C-7 an d C-8 with ph en acyl brom ide
led to th e form ation of n ovel 2-ben zoyl-10H-furo[2,3-a]carbazoles 2. Th e re-
action of h ydroxy aldeh ydes 1 with eth yl brom oacetate yielded th e n ovel
1-oxo-1,2-dih ydro[1,4]oxazin o[2,3,4-jk]carbazole-4-carbaldeh ydes 4 pro-
vided th at th e rin g carbon atom C-8 was n ot substituted. Th e attem pts to
syn th esize pyran ocarbazoles were successful yieldin g th e pyran o[2,3-a]carb-
azoles 6 an d 7.
EXPERIMENTAL
Meltin g poin ts were determ in ed with a Mettler FP 51 apparatus (Mettler In strum en ts,
Switzerlan d) an d were un corrected. IR spectra (νm ax, cm –1) were recorded usin g KBr on a
Sh im adzu FTIR-8201PC spectroph otom eter (Sh im adzu, Japan ). NMR spectra (δ, ppm ; J, Hz)
were recorded in CDCl3 or DMSO-d6 on a Varian AMX 400 FT-NMR (Varian Australia,
Australia) usin g TMS as in tern al stan dard. Mass spectra were recorded on a Jeol-JMS-D-300
m ass spectrom eter (70 eV; Jeol, Japan ). Microan alyses were don e on a Vario EL III Model
CHNS an alyzer (Vario, Germ an y). Th e purity of th e products was tested by TLC usin g glass
plates coated with silica gel G (Hi Media Laboratories, In dia) an d petroleum eth er/eth yl
acetate (85:15). Ph en acyl brom ide h as been prepared by brom in ation 15 of acetoph en on e (Hi
Media Laboratories, In dia). Eth yl brom oacetate was purch ased from Loba Ch em ie In dia Ltd.
2-Ben zoyl-10H-furo[2,3-a]carbazoles 2a–2e. Gen eral Procedure
A m ixture of th e respective 1-h ydroxycarbazole-2-carbaldeh yde 1 (0.001 m ol), ph en acyl bro-
m ide (200 m g, 0.001 m ol) an d calcin ed potassium carbon ate (276 m g, 0.002 m ol) in dry
aceton e (15 m l) was refluxed on a steam bath for 3 h . Th e reaction was m on itored by TLC.
After com pletion of th e reaction th e solven t was rem oved an d th e reaction m ixture was
poured in to ice water. Th e solid was filtered off, wash ed with water an d dried. Th en it was
purified by colum n ch rom atograph y on silica gel usin g petroleum eth er/eth yl acetate (95:5)
as eluan t, after rem oval of th e solven t th e respective 2-ben zoyl-10H-furo[2,3-a]carbazole 3
was obtain ed.
2-Benzoyl-7-methyl-10H-furo[2,3-a]carbazole (2a): Yellow am orph ous powder, m .p. 170–172 °C.
Yield 62%. For C22H15NO2 (325.4) calculated: 81.21% C, 4.65% H, 4.30% N; foun d: 81.25% C,
4.62% H, 4.34% N. IR: 2922, 2856, 1688, 1630, 968. 1H NMR (400 MHz, DMSO-d6): 12.21 b s,
1 H (H-10, deuterium -exch an geable); 8.13–7.98 m , 3 H (H-4, H-5, H-2′); 7.99 s, 1 H (H-3);
7.96 s, 1 H (H-6); 7.78–7.60 m , 3 H (H-3′, H-5′, H-6′); 7.57–7.52 d, 1 H, J(8,9) = 8.30 (H-8);
7.50–7.45 d, 1 H, J(9,8) = 8.30 (H-9); 7.34–7.24 m , 1 H (H-4′); 2.45 s, 3 H (CH3). 13C NMR
(100 MHz, DMSO-d6): 189.7 (COPh ); 136.2 (C-2); 127.5 (C-3); 139.9 (C-10b); 130.8 (C-10a);
130.1 (C-9a); 129.4 (C-5a); 128.1 (C-5b); 126.9 (C-1′); 126.8 (C-2′, C-6′); 123.2 (C-3′, C-5′);
120.9 (C-4′); 119.9 (C-7); 118.7 (C-8); 118.1 (C-6); 117.1 (C-9); 116.3 (C-4); 113.7 (C-5);
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110.4 (C-3a); 19.3 (CH3). MS (EI, 70 eV), m/z (rel.%): 325 (64) [M+]; 324 (100); 311 (28); 296
(60); 270 (22); 198 (25).
2-Benzoyl-8-methyl-10H-furo[2,3-a]carbazole (2b): Yellow am orph ous powder, m .p. 164–166 °C.
Yield 58%. For C22H15NO2 (325.4) calculated: 81.21% C, 4.65% H, 4.30% N; foun d: 81.18% C,
4.61% H, 4.37% N. IR: 2921, 2850, 1692, 1636, 969. 1H NMR (400 MHz, DMSO-d6): 12.22 b s,
1 H (H-10, deuterium -exch an geable); 8.20–7.07 m , 10 H (H-4, H-5, H-6, H-7, H-9, H-2′, H-3′,
H-4′, H-5′, H-6′); 7.93 s, 1 H (H-3); 2.52 s, 3 H (CH3). 13C NMR (100 MHz, DMSO-d6): 190.0
(COPh ); 137.2 (C-2); 127.2 (C-3); 140.4 (C-10b); 131.1 (C-10a); 130.2 (C-9a); 129.8 (C-5a);
128.0 (C-5b); 126.4 (C-1′); 125.0 (C-2′, C-6′); 123.2 (C-8); 121.0 (C-4′); 119.8 (C-3′, C-5′);
118.2 (C-7); 116.7 (C-6); 116.1 (C-9); 114.3 (C-4); 112.4 (C-5); 109.7 (C-3a); 19.3 (CH3).
MS (EI, 70 eV), m/z (rel.%): 325 (62) [M+]; 324 (100); 311 (34); 296 (56); 270 (28); 198 (20).
2-Benzoyl-9-methyl-10H-furo[2,3-a]carbazole (2c): Yellow am orph ous powder, m .p. 174–176 °C.
Yield 66%. For C22H15NO2 (325.4) calculated: 81.21% C, 4.65% H, 4.30% N; foun d: 81.16% C,
4.66% H, 4.30% N. IR: 2922, 2856, 1696, 1622, 1331, 964. 1H NMR (400 MHz, DMSO-d6):
12.23 b s, 1 H (H-10, deuterium -exch an geable); 8.15–8.10 d, 1 H, J(5,4) = 8.30 (H-5);
8.09–7.94 m , 3 H (H-2′, H-5′, H-6′); 7.97 s, 1 H (H-3); 7.79–7.61 m , 3 H (H-6, H-7, H-8);
7.60–7.54 d, 1 H, J(4,5) = 8.30 (H-4); 7.28–7.22 m , 1 H (H-3′); 7.19–7.13 t, 1 H, J(4′,3′,5′) =
7.4 (H-4′); 2.64 s, 3 H (CH3). 13C NMR (100 MHz, DMSO-d6): 190.4 (COPh ); 136.4 (C-2);
126.0 (C-3); 140.4 (C-10b); 132.0 (C-10a); 130.6 (C-9a); 129.9 (C-5a); 129.0 (C-5b); 127.7
(C-1′); 125.4 (C-2′, C-6′); 122.9 (C-3′, C-5′); 121.4 (C-4′); 120.0 (C-9); 118.9 (C-7); 117.2
(C-8); 116.9 (C-6); 115.2 (C-4); 113.4 (C-5); 110.0 (C-3a); 19.9 (CH3). MS (EI, 70 eV), m/z
(rel.%): 325 (60) [M+]; 324 (100); 311 (32); 296 (50); 270 (18); 199 (12); 198 (22).
2-Benzoyl-7-chloro-10H-furo[2,3-a]carbazole (2d): Yellow am orph ous powder, m .p. 145–147 °C.
Yield 54%. For C21H12ClNO2 (345.8) calculated: 72.97% C, 3.50% H, 4.05% N; foun d:
72.77% C, 3.53% H, 4.03% N. IR: 2924, 2855, 1688, 1629, 1577, 976. 1H NMR (400 MHz,
DMSO-d6): 12.54 b s, 1 H (H-10, deuterium -exch an geable); 8.35–8.31 d, 1 H, J(6,8) = 1.96
(H-6); 8.21–8.17 d, 1 H, J(5,4) = 8.40 (H-5); 8.09–8.03 m , 2 H (H-4, H-9); 7.98 s, 1 H (H-3);
7.78–7.56 m , 5 H (H-2′, H-3′, H-4′, H-5′, H-6′); 7.48–7.43 dd, 1 H J(8,9) = 8.6, J(8,6) = 1.96
(H-8). 13C NMR (100 MHz, DMSO-d6): 190.1 (COPh ); 136.4 (C-2); 126.1 (C-3); 141.0 (C-10b);
138.7 (C-10a); 136.0 (C-7); 130.4 (C-9a); 129.4 (C-5a); 127.7 (C-5b); 124.6 (C-1′); 121.9 (C-2′,
C-6′); 120.7 (C-3a); 119.0 (C-3′, C-5′); 117.0 (C-4′); 116.1 (C-8); 115.1 (C-6); 114.7 (C-9);
113.1 (C-4); 109.4 (C-5). MS (EI, 70 eV), m/z (rel.%): 345 (62) [M+]; 347 (28) [M2+]; 316 (50);
309 (27).
2-Benzoyl-10H-furo[2,3-a]carbazole (2e): Yellow am orph ous powder, m .p. 164–166 °C. Yield
64%. For C21H13NO2 (311.3) calculated: 81.01% C, 4.21% H, 4.50% N; foun d: 81.07% C,
4.18% H, 4.47% N. IR: 2924, 2850, 1626, 1332, 970. 1H NMR (400 MHz, DMSO-d6): 12.35 b s,
1 H (H-10, deuterium -exch an geable); 8.25–7.20 m , 11 H (H-4, H-5, H-6, H-7, H-8, H-9, H-2′,
H-3′, H-4′, H-5′, H-6′); 7.97 s, 1 H (H-3). 13C NMR (100 MHz, DMSO-d6): 190.4 (COPh );
135.6 (C-2); 126.1 (C-3); 139.0 (C-10b); 132.1 (C-10a); 130.1 (C-9a); 129.4 (C-5a); 126.9
(C-5b); 124.6 (C-1′); 121.7 (C-2′, C-6′); 120.4 (C-3′, C-5′); 118.4 (C-4′); 117.9 (C-7); 117.1
(C-8); 116.9 (C-6); 116.0 (C-9); 114.4 (C-4); 113.2 (C-5); 110.1 (C-3a). MS (EI, 70 eV), m/z
(rel.%): 311 (70) [M+]; 310 (100); 296 (27); 282 (62); 256 (22); 185 (20); 184 (20).
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Reaction of 1-Hydroxycarbazole-2-carbaldeh ydes 1a–1e with Eth yl Brom oacetate.
Gen eral Procedure
A m ixture of th e respective 1-h ydroxycarbazole-2-carbaldeh yde 1 (0.001 m ol), eth yl brom o-
acetate (166 m g, 0.001 m ol) an d calcin ed potassium carbon ate (276 m g, 0.002 m ol) in dry
aceton e (15 m l) was refluxed in a steam bath for 3 h . Th e reaction was m on itored by TLC.
After com pletion of th e reaction th e solven t was rem oved an d th e m ixture was poured in to
ice water. Th e solid was filtered off, wash ed with water an d dried. It was purified by colum n
ch rom atograph y on silica gel usin g petroleum eth er/eth yl acetate (95:5) as eluan t to give th e
respective product 4. In th e case of 1c, th e product 5c is obtain ed in stead of 4c.
8-Methyl-1-oxo-1,2-dihydro[1,4]oxazino[2,3,4-jk]carbazole-4-carbaldehyde (4a): Pale yellow
am orph ous powder, m .p. 145–147 °C. Yield 52%. For C16H11NO3 (265.3) calculated: 72.45% C,
4.18% H, 5.28% N; foun d: 72.40% C, 4.20% H, 5.23% N. IR: 2921, 2850, 1712, 1665, 1453.
1H NMR (400 MHz, CDCl3): 10.29 s, 1 H (CHO); 8.12–8.11 d, 1 H, J(6,5) = 8.2 (H-6); 8.06 s,
1 H (H-7); 7.78–7.72 d, 1 H, J(10,9) = 8.1 (H-10); 7.69–7.63 d, 1 H, J(9,10) = 8.1 (H-9);
7.51–7.47 d, 1 H, J(5,4) = 8.2 (H-5); 5.24 s, 2 H (H-2); 2.49 s, 3 H (CH3). 13C NMR (100 MHz,
CDCl3): 195.7 (CHO); 173.4 (C-1); 139.4, 137.4, 136.4, 128.6, 122.1, 120.5, 119.2, 118.1,
117.4, 116.6, 113.4, 110.1 (12 arom . C); 67.3 (C-2); 19.3 (CH3). MS (EI, 70 eV), m/z (rel.%):
265 (43) [M+]; 264 (20); 236 (100); 194 (23); 180 (21); 164 (32).
9-Methyl-1-oxo-1,2-dihydro[1,4]oxazino[2,3,4-jk]carbazole-4-carbaldehyde (4b ): Pale yellow
am orph ous powder, m .p. 152–154 °C. Yield 48%. For C16H11NO3 (265.3) calculated: 72.45% C,
4.18% H, 5.28% N; foun d: 72.41% C, 4.16% H, 5.26% N. IR: 2922, 2856, 1708, 1668, 1642,
1451. 1H NMR (400 MHz, CDCl3): 10.40 s, 1 H (CHO); 8.24 s, 1 H (H-10); 7.92–7.86 d, 1 H,
J(6,5) = 7.9 (H-6); 7.81–7.76 d, 1 H, J(7,8) = 8.1 (H-7); 7.58–7.52 d, 1 H, J(8,7) = 8.1 (H-8);
7.36–7.30 d, 1 H, J(5,6) = 7.9 (H-5); 5.16 s, 2 H (H-2); 2.58 s, 3 H (CH3). 13C NMR (100 MHz,
CDCl3): 196.4 (CHO); 171.7 (C-1); 140.3, 137.9, 137.2, 128.7, 122.1, 120.3, 119.5, 118.7,
118.1, 116.7, 113.3, 110.4 (12 arom . C); 67.6 (C-2); 19.2 (CH3). MS (EI, 70 eV), m/z (rel.%):
265 (48) [M+]; 264 (29); 236 (100); 194 (16); 180 (18); 164 (40).
1-[(Ethoxycarbonyl)methoxy]-8-methylcarbazole-2-carbaldehyde (5c): Wh ite spon gy m ass, m .p.
166–168 °C. Yield 68%. For C18H17NO4 (311.3) calculated: 69.44% C, 5.50% H, 4.50% N;
foun d: 69.58% C, 5.47% H, 4.52% N. IR: 3285, 2921, 2850, 1764, 1658, 1450. 1H NMR
(400 MHz, CDCl3): 10.40 s, 1 H (CHO); 10.03 b s, 1 H (H-9); 7.98–7.90 m , 2 H (H-4, H-5);
7.68–7.62 d, 1 H, J(3,4) = 8.1 (H-3); 7.34–7.30 d, 1 H, J(7,6) = 8.1 (H-7); 7.21–7.15 m , 1 H
(H-6); 4.85 s, 2 H (OCH2COOCH2CH3); 4.35–4.26 q, 2 H, J = 7.1 (OCH2COOCH2CH3);
2.65 s, 3 H (CH3); 1.36–1.27 t, 3 H, J = 7.1 (OCH2COOCH2CH3). 13C NMR (100 MHz,
CDCl3): 197.5 (CHO); 173.2 (OCH2COOCH2CH3); 140.0, 138.1, 137.0, 128.0, 121.1, 120.2,
119.1, 118.0, 117.4, 116.4, 112.9, 111.2 (12 arom . C); 68.4 (OCH2COOCH2CH3); 64.3
(OCH2COOCH2CH3); 20.1 (CH3); 19.5 (OCH2COOCH2CH3). MS (EI, 70 eV), m/z (rel.%): 311
(36) [M+]; 282 (44); 266 (100); 264 (12); 236 (98); 198 (32).
8-Chloro-1-oxo-1,2-dihydro[1,4]oxazino[2,3,4-jk]carbazole-4-carbaldehyde (4d ): Pale yellow
am orph ous powder, m .p. 163–165 °C. Yield 50%. For C15H8ClNO3 (285.7) calculated:
63.06% C, 2.82% H, 4.90% N; foun d: 62.89% C, 2.84% H, 4.90% N. IR: 2923, 2885, 1716,
1682, 1643, 1445. 1H NMR (400 MHz, CDCl3): 10.42 s, 1 H (CHO); 8.36–8.32 d, 1 H, J(6,5) =
8.2 (H-6); 8.01–7.98 d, 1 H, J(7,9) = 1.9 (H-7); 7.84–7.79 d, 1 H, J(10,9) = 8.2 (H-10);
7.61–7.55 m , 2 H (H-5, H-9); 5.18 s, 2 H (H-2). 13C NMR (100 MHz, CDCl3): 198.3 (CHO);
174.1 (C-1); 140.7, 139.4, 137.8, 129.9, 121.8, 120.5, 119.7, 118.8, 117.8, 114.7, 112.4, 110.6
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(12 arom . C); 67.4 (C-2). MS (EI, 70 eV), m/z (rel.%): 285 (40) [M+]; 287 (12) [M2+]; 284 (20);
256 (100); 251 (22); 214 (26).
1-Oxo-1,2-dihydro[1,4]oxazino[2,3,4-jk]carbazole-4-carbaldehyde (4e): Pale yellow crystals,
m .p. 160–162 °C. Yield 50%. For C15H9NO3 (251.2) calculated: 71.71% C, 3.61% H, 5.58% N;
foun d: 71.81% C, 3.63% H, 5.56% N. IR: 2924, 2857, 1693, 1642, 1445. 1H NMR (400 MHz,
CDCl3): 10.41 s, 1 H (CHO); 8.44–8.37 d, 1 H, J(6,5) = 8.2 (H-6); 8.05–8.01 d, 1 H, H-10,
J(10,9) = 7.8 (H-10); 7.84–7.79 d, 1 H, J(5,6) = 8.2 (H-5); 7.67–7.63 m , 1 H (H-9); 7.63–7.58 d,
1 H, J(7,8) = 8.2 (H-7); 7.53–7.49 m , 1 H (H-8); 5.17 s, 2 H (H-2). 13C NMR (100 MHz,
CDCl3): 198.1 (CHO); 169.4 (C-1); 139.9, 137.4, 130.1, 129.7, 121.3, 120.6, 119.2, 118.6,
118.0, 114.4, 112.8, 109.9 (12 arom . C); 67.1 (C-2). MS (EI, 70 eV), m/z (rel.%): 251 (37)
[M+]; 250 (26); 222 (100); 180 (20); 179 (11).
3-Acetyl-2,11-dih ydropyran o[2,3-a]carbazol-2-on es 6a–6e.
Gen eral Procedure
A m ixture of th e respective 1-h ydroxycarbazole-2-carbaldeh yde 1 (0.001 m ol), eth yl aceto-
acetate (0.2 m l, 0.001 m ol) in dry pyridin e (20 m l) was h eated to 50–60 °C on an oil bath .
Th en piperidin e (3–4 drops) was added dropwise an d th e m ixture was refluxed for 2 h . Th e
reaction was m on itored by TLC. After com pletion of th e reaction th e reaction m ixture was
n eutralized with cold aqueous HCl an d poured in to ice water. Th e solid was filtered off,
wash ed with water an d dried. Th en it was purified by colum n ch rom atograph y on silica gel
usin g petroleum eth er/eth yl acetate (80:20) as eluan t to give th e respective product.
3-Acetyl-8-methyl-2,11-dihydropyrano[2,3-a]carbazol-2-one (6a): Yellow am orph ous powder,
m .p. 210–212 °C. Yield 52%. For C18H13NO3 (291.3) calculated: 74.22% C, 4.50% H, 4.81% N;
foun d: 74.12% C, 4.54% H, 4.77% N. IR: 3302, 2920, 1709, 1670, 1586. 1H NMR (400 MHz,
DMSO-d6): 12.27 b s, 1 H (H-11, deuterium -exch an geable); 8.84 s, 1 H (H-4); 8.13–8.07 d,
1 H, J(6,5) = 8.2 (H-6); 8.02 s, 1 H (H-7); 7.65–7.59 d, 1 H, J(5,6) = 8.2 (H-5); 7.50–7.44 d,
1 H, J(10,9) = 8.40 (H-10); 7.38–7.30 d, 1 H, J(9,10) = 8.40 (H-9); 2.62 s, 3 H (COCH3);
2.48 s, 3 H (CH3). 13C NMR (100 MHz, DMSO-d6): 198.7 (COCH3); 166.2 (C-2); 137.4 (C-4);
134.4 (C-3); 136.9, 136.1, 131.3, 130.5, 130.0, 129.1, 122.5, 121.7, 117.8, 114.7, 112.4, 110.6
(12 arom . C); 25.9 (COCH3); 19.0 (CH3). MS (EI, 70 eV), m/z (rel.%): 291 (64) [M+]; 276 (45);
262 (100); 248 (32); 244 (22); 217 (39); 196 (23).
3-Acetyl-9-methyl-2,11-dihydropyrano[2,3-a]carbazol-2-one (6b): Yellow am orph ous powder,
m .p. 198–199 °C. Yield 55%. For C18H13NO3 (291.3) calculated: 74.22% C, 4.50% H, 4.81% N;
foun d: 74.29% C, 4.57% H, 4.83% N. IR: 3323, 2923, 1717, 1673, 1587. 1H NMR (400 MHz,
DMSO-d6): 12.24 b s, 1 H (H-11, deuterium -exch an geable); 8.86 s, 1 H (H-4); 8.10–8.04 d,
1 H, J(6,5) = 8.0 (H-6); 7.64–7.59 d, 1 H, J(5,6) = 8.0 (H-5); 7.42 s, 1 H (H-10); 7.14–7.01 m ,
2 H (H-7, H-8); 2.83 s, 3 H (COCH3); 2.63 s, 3 H (CH3). 13C NMR (100 MHz, DMSO-d6):
197.2 (COCH3); 162.9 (C-2); 139.2 (C-4); 135.1 (C-3); 138.6, 137.9, 134.3, 131.8, 129.8,
129.1, 124.5, 121.9, 119.8, 118.2, 112.9, 111.7 (12 arom . C); 26.6 (COCH3); 21.0 (CH3).
MS (EI, 70 eV), m/z (rel.%): 291 (55) [M+]; 276 (31); 262 (100); 248 (38); 244 (22); 224 (20);
217 (45); 196 (18).
3-Acetyl-10-methyl-2,11-dihydropyrano[2,3-a]carbazol-2-one (6c): Yellow am orph ous powder,
m .p. 215–217 °C. Yield 52%. For C18H13NO3 (291.3) calculated: 74.22% C, 4.50% H, 4.81% N;
foun d: 74.31% C, 4.44% H, 4.73% N. IR: 3332, 2923, 1720, 1670, 1585. 1H NMR (400 MHz,
DMSO-d6): 12.29 b s, 1 H (H-11, deuterium -exch an geable); 8.75 s, 1 H (H-4); 8.01–7.93 m ,
2 H (H-5, H-6); 7.47–7.43 d, 1 H, J(7,8) = 8.2 (H-7); 7.39–7.35 d, 1 H, J(9,8) = 8.0 (H-9);
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Martin, Rajendra Prasad:
7.29–7.20 m , 1 H (H-8); 2.78 s, 3 H (COCH3); 2.64 s, 3 H (CH3). 13C NMR (100 MHz,
DMSO-d6): 199.7 (COCH3); 161.0 (C-2); 140.2 (C-4); 135.4 (C-3); 138.1, 137.1, 133.8, 132.0,
129.1, 126.5, 124.4, 120.9, 118.8, 118.0, 114.3, 106.4 (12 arom . C); 23.9 (COCH3); 21.7
(CH3). MS (EI, 70 eV), m/z (rel.%): 291 (72) [M+]; 276 (40); 262 (100); 248 (25); 244 (20);
217 (45).
3-Acetyl-8-chloro-2,11-dihydropyrano[2,3-a]carbazol-2-one (6d ): Yellow am orph ous powder,
m .p. 222–224 °C. Yield 60%. For C17H10ClNO3 (311.7) calculated: 65.50% C, 3.23% H,
4.49% N; foun d: 65.84% C, 3.20% H, 4.55% N. IR: 3322, 2925, 1728, 1653, 1586. 1H NMR
(400 MHz, DMSO-d6): 12.34 b s, 1 H (H-11, deuterium -exch an geable); 8.80 s, 1 H (H-4);
8.32 s, 1 H (H-7); 8.22–8.16 d, 1 H, J(6,5) = 8.5 (H-6); 7.63–7.57 d, 1 H, J(5,6) = 8.5 (H-5);
7.44–7.32 m , 2 H (H-9, H-10); 2.63 s, 3 H (COCH3). 13C NMR (100 MHz, DMSO-d6): 199.8
(COCH3); 162.2 (C-2); 139.4 (C-4); 133.7 (C-3); 141.9, 137.8, 134.8, 132.6, 131.1, 128.3,
124.9, 120.7, 116.8, 113.9, 111.4, 107.4 (12 arom . C); 27.9 (COCH3). MS (EI, 70 eV), m/z
(rel.%): 311 (100) [M+]; 313 (38); 282 (78); 268 (45); 264 (32); 237 (20); 216 (18).
3-Acetyl-2,11-dihydropyrano[2,3-a]carbazol-2-one (6e): Yellow am orph ous powder, m .p.
187–189 °C. Yield 58%. For C17H11NO3 (277.3) calculated: 73.64% C, 4.00% H, 5.05% N;
foun d: 73.91% C, 3.97% H, 5.10% N. IR: 3295, 2920, 1715, 1674, 1588. 1H NMR (400 MHz,
DMSO-d6): 12.37 b s, 1 H (H-11, deuterium -exch an geable); 8.84 s, 1 H (H-4); 8.27–8.21 d,
1 H, J(10,9) = 7.8 (H-10); 8.17–8.12 d, 1 H, J(6,5) = 8.1 (H-6); 7.69–7.64 d, 1 H, J(7,6) = 8.2
(H-5); 7.61–7.56 d, 1 H, J(5,6) = 8.2 (H-5); 7.55–7.49 m , 1 H (H-8); 7.30–7.23 m , 1 H (H-9);
2.68 s, 3 H (COCH3). 13C NMR (100 MHz, DMSO-d6): 200.7 (COCH3); 167.0 (C-2); 139.4
(C-4); 138.4 (C-3); 136.7, 134.3, 131.3, 130.5, 129.9, 127.5, 122.2, 121.0, 118.8, 114.3, 111.7,
109.0 (12 arom . C); 25.9 (COCH3). MS (EI, 70 eV), m/z (rel.%): 277 (60) [M+]; 262 (45); 248
(100); 230 (35); 203 (27); 182 (30).
Eth yl 2-Oxo-2,11-dih ydropyran o[2,3-a]carbazole-3-carboxylates 7a–7e.
Gen eral Procedure
A m ixture of respective 1-h ydroxycarbazole-2-carbaldeh yde 1 (0.001 m ol), dieth yl m alon ate
(0.2 m l, 0.001 m ol) in dry pyridin e (20 m l) was h eated to 50–60 °C in an oil bath . Th en
piperidin e (3–4 drops) was added dropwise an d th e m ixture was refluxed for 2 h . Th e reac-
tion was m on itored by TLC. After com pletion of th e reaction th e reaction m ixture was n eu-
tralized usin g cold aqueous HCl an d poured in to ice water. Th e solid was filtered off,
wash ed with water an d dried. Th en it was purified by colum n ch rom atograph y on silica gel
usin g petroleum eth er/eth yl acetate (80:20) as eluan t.
Ethyl 8-methyl-2-oxo-2,11-dihydropyrano[2,3-a]carbazole-3-carboxylate (7a): Yellow am or-
ph ous powder, m .p. 246–248 °C. Yield 64%. For C19H15NO4 (321.3) calculated: 71.02% C,
4.71% H, 4.36% N; foun d: 71.12% C, 4.64% H, 4.30% N. IR: 3289, 2921, 1731, 1645, 1294.
1H NMR (400 MHz, DMSO-d6): 12.34 b s, 1 H (H-11, deuterium -exch an geable); 8.90 s, 1 H
(H-4); 8.08 s, 1 H (H-7); 7.95–7.90 d, 1 H, J(6,5) = 8.1 (H-6); 7.50–7.45 d, 1 H, J(10,9) = 8.1
(H-10); 7.40–7.34 d, 1 H, J(9,10) = 8.4 (H-9); 7.33–7.27 d, 1 H, J(5,6) = 8.1 (H-5); 4.26–4.16 q,
2 H, J = 7.2 (COOCH2CH3); 2.55 s, 3 H (CH3); 1.34–1.22 t, 3 H, J = 7.2 (COOCH2CH3).
13C NMR (100 MHz, DMSO-d6): 170.3 (COOEt); 157.7 (C-2); 140.4 (C-4); 117.3 (C-3); 135.3,
133.9, 132.7, 127.4, 124.0, 120.9, 120.0, 118.7, 115.8, 114.1, 110.4, 107.6 (12 arom . C); 59.3
(COOCH2CH3); 21.9 (CH3); 15.4 (COOCH2CH3). MS (EI, 70 eV), m/z (rel.%): 321 (100) [M+];
276 (45); 262 (98); 248 (32); 244 (22); 217 (39); 196 (23).
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Ethyl 9-methyl-2-oxo-2,11-dihydropyrano[2,3-a]carbazole-3-carboxylate (7b ): Yellow am or-
ph ous powder, m .p. 218–220 °C. Yield 60%. For C19H15NO4 (321.3) calculated: 71.02% C,
4.71% H, 4.36% N; foun d: 71.18% C, 4.68% H, 4.39% N. IR: 3354, 2923, 1730, 1652, 1285.
1H NMR (400 MHz, DMSO-d6): 12.21 b s, 1 H (H-11, deuterium -exch an geable); 8.92 s, 1 H
(H-4); 8.14–8.03 m , 2 H (H-5, H-6); 7.67–7.56 m , 2 H (H-7, H-8); 7.42 s, 1 H (H-10);
4.35–4.27 q, 2 H, J = 7.1 (COOCH2CH3); 2.83 s, 3 H (CH3); 1.37–1.29 t, 3 H, J = 7.1
(COOCH2CH3). 13C NMR (100 MHz, DMSO-d6): 168.0 (COOEt); 154.1 (C-2); 141.2 (C-4);
116.9 (C-3); 134.7, 132.1, 130.8, 129.1, 125.6, 119.9, 1118.7, 116.4, 115.8, 114.4, 108.9,
104.3 (12 arom . C); 60.1 (COOCH2CH3); 19.7 (CH3); 16.1 (COOCH2CH3). MS (EI, 70 eV),
m/z (rel.%): 321 (100) [M+]; 276 (40); 262 (90); 248 (50); 244 (21); 217 (22).
Ethyl 10-methyl-2-oxo-2,11-dihydropyrano[2,3-a]carbazole-3-carboxylate (7c): Yellow am or-
ph ous powder, m .p. 197–199 °C. Yield 52%. For C19H15NO4 (321.3) calculated: 71.02% C,
4.71% H, 4.36% N; foun d: 70.87% C, 4.76% H, 4.31% N. IR: 3231, 2928, 1717, 1611, 1266.
1H NMR (400 MHz, DMSO-d6): 12.24 b s, 1 H (H-11, deuterium -exch an geable); 8.79 s, 1 H
(H-4); 8.01–7.90 m , 2 H (H-6, H-7); 7.58–7.55 m , 1 H (H-8); 7.44–7.40 d, 1 H, J(5,6) = 8.2
(H-5); 7.38–7.34 d, 1 H, J(9,10) = 8.0 (H-9); 4.50–4.43 q, 2 H, J = 7.1 (COOCH2CH3); 2.63 s,
3 H (CH3); 1.49–1.43 t, 3 H, J = 7.1 (COOCH2CH3). 13C NMR (100 MHz, DMSO-d6): 169.3
(COOEt); 154.7 (C-2); 139.9 (C-4); 118.3 (C-3); 136.8, 134.0, 130.9, 130.0, 126.2, 122.4,
121.0, 119.1, 116.0, 114.1, 109.7, 104.9 (12 arom . C); 59.7 (COOCH2CH3); 23.1 (CH3); 16.1
(COOCH2CH3). MS (EI, 70 eV), m/z (rel.%): 321 (100) [M+]; 276 (45); 262 (97); 248 (32); 244
(27); 217 (41); 196 (23).
Ethyl 8-chloro-2-oxo-2,11-dihydropyrano[2,3-a]carbazole-3-carboxylate (7d ): Yellow am or-
ph ous powder, m .p. 256–258 °C. Yield 50%. For C18H12ClNO4 (341.8) calculated: 63.26% C,
3.54% H, 4.10% N; foun d: 63.38% C, 3.50% H, 4.03% N. IR: 3270, 2925, 1729, 1654, 1306.
1H NMR (400 MHz, DMSO-d6): 12.26 b s, 1 H (H-11, deuterium -exch an geable); 8.87 s, 1 H
(H-4); 8.18 s, 1 H (H-7); 7.95–7.90 d, 1 H, J(6,5) = 8.2 (H-6); 7.50–7.45 m , 3 H (H-5, H-9,
H-10); 4.32–4.26 q, 2 H, J = 7.1 (COOCH2CH3); 1.36–1.29 t, 3 H, J = 7.1 (COOCH2CH3).
13C NMR (100 MHz, DMSO-d6): 168.8 (COOEt); 155.9 (C-2); 140.6 (C-4); 116.7 (C-3); 139.5,
137.9, 132.1, 129.4, 126.1, 121.8, 120.1, 118.7, 115.7, 113.6, 111.6, 109.8 (12 arom . C); 59.7
(COOCH2CH3); 14.9 (COOCH2CH3). MS (EI, 70 eV), m/z (rel.%): 341 (100) [M+]; 343 (28);
296 (47); 282 (95); 268 (32); 237 (39).
Ethyl 2-oxo-2,11-dihydropyrano[2,3-a]carbazole-3-carboxylate (7e): Yellow am orph ous pow-
der, m .p. 222–224 °C. Yield 68%. For C18H13NO4 (307.3) calculated: 70.35% C, 4.26% H,
4.56% N; foun d: 70.21% C, 4.30% H, 4.60% N. IR: 3352, 2924, 1746, 1636, 1249. 1H NMR
(400 MHz, DMSO-d6): 12.34 b s, 1 H (H-11, deuterium -exch an geable); 8.94 s, 1 H (H-4);
8.27–8.22 d, 1 H, J(6,5) = 8.3 (H-6); 8.17–8.12 d, 1 H, J(5,6) = 8.3 (H-5); 7.65–7.61 d, 1 H,
J(10,9) = 8.2 (H-10); 7.60–7.47 m ,
3 H (H-7, H-8, H-9); 4.36–4.27 q, 2 H, J = 7.1
(COOCH2CH3); 1.36–1.30 t, 3 H, J = 7.1 (COOCH2CH3). 13C NMR (100 MHz, DMSO-d6):
171.0 (COOEt); 154.1 (C-2); 138.4 (C-4); 116.3 (C-3); 137.9, 135.1, 134.4, 130.3, 127.8,
124.5, 121.3, 119.3, 116.3, 110.6, 107.4, 106.7 (12 arom . C); 59.1 (COOCH2CH3); 16.4
(COOCH2CH3). MS (EI, 70 eV), m/z (rel.%): 307 (100) [M+]; 262 (40); 248 (94); 215 (16); 182
(12).
The authors are grateful to prof. J. S. Yadav, the Director, ISO quality assurance cell, IICT,
Hyderabad and prof. K. V. Ramanathan, the Chairman, NMR research centre, IISc, Bangalore for
providing mass spectral data and NMR spectra, respectively.
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