HETEROCYCLES AND CARBOCYCLES FROM (0-14-DIPHENYL-2- BUTENE-1,4-DIONE (trans-DBE) 2.1 Introduction (E)-l,4-diphenyl-Z-butene-1.4-dione (tratu-DBE) 3 is an useful multifunctional molecule. It can be prepared by Friedel-Crafts acylation of fumaryl chloride 2 with benzene 1 (Scheme 2.1).' Similarly several derivatives of trans-DBE 3 can be prepared easily. trans-DBE 3 possess two a$-unsaturated carbonyl groups with two phenyl rings in 1A-position. The nucleophilic addition to a$-unsaturated carbonyl in 3 can be achieved either in 1.2- or 1,4- fashion. Conjugate addition, i.e., the 1,4-addition of a nucleophile is known as Michael addition. The resultant Michael adducts can be further utilized for the generation of carbocycles and heterocycles. We became interested in the utility of trans-DBE 3 for the synthesis of hitherto unknown tetrahydroquinoline derivatives via conjugate addition of the anion generated from cyclohexanone followed by oxidative amination-cyclization. While executing this task we isolated several carbocycles formed by following domino-pathways. We have also conducted the Robinson annulation reaction on trans-DBE 3 with methyl acetoacetate and utilized the product for further interesting transformations. Results from these studies are described in the present chapter. To place the results in right perspective, we now give a brief review of the literature on the utility of trans-DBE 3 in Synthetic Organic Chemistry. We have gathered the literature references from 1990 onwards as there is already a review on this topic for the previous period.2 The references are arranged in the chronological order. While presenting the literature, only examples, of the parent molecule is given. In all the publications selected for the review, details of the generality of the procedure are
68
Embed
HETEROCYCLES AND CARBOCYCLES - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/972/8/08_chapter 2.pdf · Several bioactive heterocycles incorporating two or more heteroatoms
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
HETEROCYCLES AND CARBOCYCLES FROM (0-14-DIPHENYL-2- BUTENE-1,4-DIONE (trans-DBE)
2.1 Introduction
(E)-l,4-diphenyl-Z-butene-1.4-dione (tratu-DBE) 3 is an useful multifunctional
molecule. It can be prepared by Friedel-Crafts acylation of fumaryl chloride 2 with
benzene 1 (Scheme 2.1).' Similarly several derivatives of trans-DBE 3 can be prepared
easily. trans-DBE 3 possess two a$-unsaturated carbonyl groups with two phenyl rings
in 1A-position. The nucleophilic addition to a$-unsaturated carbonyl in 3 can be
achieved either in 1.2- or 1,4- fashion. Conjugate addition, i.e., the 1,4-addition of a
nucleophile is known as Michael addition. The resultant Michael adducts can be further
utilized for the generation of carbocycles and heterocycles. We became interested in the
utility of trans-DBE 3 for the synthesis of hitherto unknown tetrahydroquinoline
derivatives via conjugate addition of the anion generated from cyclohexanone followed
by oxidative amination-cyclization. While executing this task we isolated several
carbocycles formed by following domino-pathways. We have also conducted the
Robinson annulation reaction on trans-DBE 3 with methyl acetoacetate and utilized the
product for further interesting transformations. Results from these studies are described in
the present chapter. To place the results in right perspective, we now give a brief review
of the literature on the utility of trans-DBE 3 in Synthetic Organic Chemistry. We have
gathered the literature references from 1990 onwards as there is already a review on this
topic for the previous period.2 The references are arranged in the chronological order.
While presenting the literature, only examples, of the parent molecule is given. In all the
publications selected for the review, details of the generality of the procedure are
1 2 3
Reagents and conditions: i. AICI,, rt, 2h.
Scheme 2.1
2.1.1 Synthesis of carbocycles from trans-DBE 3
Saba reported the synthesis of interesting carbocycle I-trans-2,3-cis-
tribenzoylcyclopropanes 4 by the reaction of the phenacyi bromide 5 with trans-DBE 3
(Scheme 2.2).' Thus, stereospecific cycloaddition of tt-ans-DBE 3 with the carbene
intermediate generated from phenacyl bromide 5 furnished l,2,3-tribenzoylcyclopropane
4 in good yield.
5 3
Reagents and conditions: i. KICO,, DMF, rt.
Scheme 2.2
The dienophiles such as trans-DBE 3 react with the dienes to form interesting
carbocycles. Parakka and coworkers reported the tandem [4t2] double cycloaddition
reaction of the diene, 5,6-di[(E)-l-bromomethylidene]-l.3-cyclohexadiene 7, generated
from l,2-di(dibromomethyl)benzene 6 with two moles of trans-DBE 3 to form adduct 8
(Scheme 2.3)', which sewed as an intermediate for the preparation of novel heterocycles
like pymoles and thiophenes.
6 7 3
Reagents and conditions: i. Nal, acetone, reflux.
Scheme 2.3
Br Br
Hatanaka and coworkers reponed the synthesis of cyclopentenone derivatives 13 with
defined stereochemistly from [runs-DBE 3 (Scheme 2.4).' The phosphorane 9 was
allowed to react with truns-DBE 3 in T H F at room temperature via [3+2] annulation to
give dienes 11 and 12 through intermediate 10. The treatment of the mixture of dienes 11
and 12 with aqueous acetic acid gave a single stereoisomer of cyclopentenone 13.
0 + p h ~ ~ ~ h ~ coph
0 9% COPh COPh
OEt 0
+ phl$m~t2- kc; Ph Ph
PhOC
11 12 13
Reagents and conditions: i. THF, -30 OC to rt, 48 h; ii. aq. AcOH, THF, rt
Scheme 2.4
Hatanaka and coworkers also reported the [3+2] annulation of winig salt, (3-
alkoxycarbony12-ox~pro~hyllidene)~hosphoranes 14 with trans-DBE 3 to give
cyclopenteno?e 15 in a single step in good yield (Scheme 2 . ~ 1 . ~
I Ph
3 14 15
Reagenfs and conditions: i. THF, rt, 12 h.
Scheme 2.5
Al-Arab and coworkers synthesized highly substiwted cyclohexanol derivative 17 in
a single pot reaction of acetopheneon? 16 and rrans-DBE 3 using sodium ethoxide as
base (Scheme 2.6).' Formation of this product is expected to go through Michael addition
followed by aldol condensation.
3 16 17
Reagents and conditions: i . NaOEt, anhyd. EtZO, rt, 30 mjn.
Scheme 2.6
Reagents and condilions: i. Sm12. THF, HMPA, rt, 5 min
Scheme 2.7
Cabera and coworkers synthesized highly functionalized cyclopentanol derivative 18
by the one-electron transfer reducing agent samarium (11) iodide on trans-DBE 3. The
byproduct from the reaction was 1.4-diphenyl-1,4-butanedione 19 (Scheme 2.7).'
2.1.2 Synthesis of heterocycles from trans-DBE
Atfah and coworkers reponed the synthesis of oxabicyclo[2.2.2]octanones 21 from a
facile one-pot reaction by the condensation of trans-DBE 3 and arylacetonitrile 20 in the
presence of sodium ethoxide (Scheme 2.8): Apparently, the reaction was going through
double Michael addition, ~ntramolecular aldol condensation followed by deamination-
cyclization.
0 1
ph" t Phi-m - 0
PhOC
3 20 2 1
Reagents und conditions: i. NaOEt, EtzO, rt.
Scheme 2.8
Kaupp and coworkers synthesized several polycyclic cage compounds 24 via the
intermediate 23 by the reaction of enamines 22 with truns-DBE 3. The reaction pathway
involved ene addition followed by [2+2+2] cycloaddition involving two carbonyl groups
and enamine double bond (Scheme 2.9).1°
22 3 23
Reagents and conditions: i. MeOH, n, 2-3 days.
Scheme 2.9
Rajakumar and Kannan synthesized 7-[(4-methylphenyl)sulfonyl]-5,6-di[(Z)-I-
phenylmethylidenel-7-azabicyclo[2.2.1]hept-2-ene 27 by the cycloaddition reaction o f 1-
[(4-methylphenyI)su~fonyl]-IN-pyrrole 25 with dienophile such as (runs-DBE 3 in the
presence of catalytic amount of BF,.Et20 (Scheme 2.10)." Reduction of the cycloadduct,
yl(pheny1)methanone 26 with NaBH,I NiC12 resulted in 27. Remarkably, the
cycloaddition did not go without BF3.Et20 in either benzene or decalin at reflux.
Reugents undcondrtions: i. BF,.Et>O, CaHa, 50 "C, 50 h; ii. a. NaBH4, EtOH b. NiCl,.
Scheme 2.10
Rao and Subbaraju reported the straight foward synthesis o f 3,4-diaroyl-2-
pyrazolines 28 through cycloaddition of diazomethane to truns-DBE 3 (Scheme 2.1 I)."
Reagents and conditions: i. CH~NI . 0 OC, 24 h.
Scheme 2.1 1
The C2 symmetric chiral amine 31 of high enantomerlc purity have been prepared
from trans-DBE 3 in four steps (Scheme 2.12)." Important reaction in the sequence
reaction sequence is the asymmetric reduction with Ipc2BCI of dione, I,4-diphenyl-l,4-
butancdione derived from trans-DBE 3. Addition of allylamine to crude dimaylate,
obtained from 29, gave a reasonable yield of (2S,SS)-l-allyl-2,S-diphenyltetrahydro-lH-
pyrrole 30. Chiral amines of the type 31 are useful in asymmetric synthesis.
Reagents and conditions: i. SnCl21 HCI; ii. Ipc2BC1, THF, -78 OC -i rt; iii. MsCI, Et;N, CH2C12, -20 'C; iv. CHI=CHCH~NH~, EtlN, 0 "C-i n; v. (Ph,P);RhCI. H20, CH;CN, reflux.
Scheme 2.12
Mataka and coworkers synthesized [(2S,3R,4R)-4-benzoyl-I-methyl-2-
phenyltetrahydro-1H-3-pyrrolyl](phenyl)methanone 34 by the three-component
condensation reaction of N-methyl glycine 33, benzaldehyde 32 with trarrs-DBE 3
(Scheme 2.13).14 The transformation was proceeding through dipolar cycloaddition
involving acyliminium ions intermediates.
3 32 33
Reagents and conditions: i. toluene, reflux, 27 h.
Scheme 2.13
The above strategy was extended to the cyclic amino acids 35 and benzaldehyde 32 to
furnish the col~esponding bicyclic derivatives 36 (Scheme 2.141.'~
phOYph PhOC N
LX 3 35 32 36
X = S, CH2, CH2CH2
Reagents and conditions: i. toluene, reflux. 27 h.
Scheme 2.14
Kumar and Boykin synthesized 3-alkoxy-. 3-aryloxy-. and several other 3-substituted
amino-2,5-diarylfurans 37 by reductive furanization of tr.arl.r-DBE 3 with phosphorous
trichloride (Scheme 2.15).Ii The procedure was useful for the synthesis of 2,3,5-
trisubstituted hrans.
0 ph,L.&y~h L dR
o phAy,J-ph
3 37
R = OMe, OPh, 4-Me-C,H,O, 4-Et00CC6H40, morphollno
Reagents and condrtrons: i PCII. 4h, rt.
Scheme 2.15
Singal had reported the 1.3-cycloaddition of nitrone 38 to trans-DBE 3 as a method
for the synthesis of densely substituted isoxazolidine derivatives of the type 39. Thus,
trans-DBE 3 reacted with C-(2-nitrostyry1)-N-phenyl nitrone 38 to give isoxazolidine 39
as a single isomer (Scheme 2.16).16
i Q ph&ph 0 + cia- % 0 0
PhPh 3 38 39
Reagents and conditions: i. dry ChHh. reflux, 48 h.
Scheme 2.16
Several bioactive heterocycles incorporating two or more heteroatoms have been
prepared from trans-DBE 3 using 1.3-dipolar additlon strategies. Duan and coworkers
reported the reaction of trans-DBE 3 with trithiacyl trichloride 40 to give 3,4-dibenzoyl-
1,2,5-thiadiazoles 41 (Scheme 2. 17).17
0 C1 0 0
N'/S%N i - P h w p h F ' ~ T ~ ~ ~ , , , 0 CI/s+NNIs~CI N,S/N
3 40 41
Reagents and conditions: i. CC4, Nz atm, reflux, 10 h.
Scheme 2.17
ph&Ph + M ~ O N H ,
0 NH2 0
3 42 43
Reagents andconditiuns: i. NaOEt, EtOH, rt, 4 h.
Scheme 2.18
~ e i h o x ~ a m i n e 42 was found to be an efficient aminating agent for a$-unsaturated-
14-dicarbonyl compounds. Seko and Miyake synthesized 2-amino-l,4-diphenyl-2-
butene-14-dione 43 by using methoxyamine 42 as the aminating agent for trans-DBE 3
(Scheme 2.18).18
Kaupp and coworkers reported the one-pot synthesis of the pyrroles 45 from the
primary or secondary enaminoketone 44 and trans-DBE 3. The reaction of the enamino
ketone 44 and trans-DBE 3 resulted in conjugate addition product, which underwent
dehydrative cyclization to furnish pyl~ole 45 (Scheme 2. 19).19
Reagents and conditions: i. MeOH, reflux, 3 h .
Scheme 2.19
Rao and Jothilingam synthesized 2,s-diphenyl-IH-pyrole 46 from trans-DBE 3 and
ammonium formate in a microwave assisted one-pot operation. The transformation goes
through domino pathways via palladium assisted transfer hydrogenation followed by a
Paal-Knorr reaction (Scheme 2.20).'~ Ammonium formate was used as a source of
hydrogen and nitrogen.
3 46
Reagents and conditions: I HCOONHI, PdlC (lo%), PEG-200, Pv, 200 W, 2 min.
Scheme 2.20
Abu-Orabi had reported the 1,3-dipolar cycloaddition reaction of trans-DBE 3
with azide.47; enamine 48 was formed which was stabilized in the form of enolimine 49.
Formation of the enamine 48 was expected to go through thermally unstable triazoline 50
(Scheme 2.21).*'
49 50
Reagents und conditions: i. EtOH, reflux.
Scheme 2.21
Earlier in our ~ahoratory,' rrari\-DBE 3 was used as a starting material for further
elaboration to the cyclopenla[h]pyridine derivative 53. Conjugate addition of
cyclopentanone 51 to trans-DBE 3 hmished triketone 52. Amination-cyclization of the
triketone 52 with ammonium acetate resulted in the unstable heterocycle 53. In the
transformation of 52 to 53, 1,s-diketone was involved in amination-cyclization sequence
and not 1,4-diketone which is also present in 52 (Scheme 2.22).22
Reagents and conditions: i. activated Ba(OH)l, EtOH, rt, 12 h; ii. CHICOONH,, MeOH, 18 h.
Scheme 2.22
Thus, it is clear from the literature survey that trans-DBE 3 served as a versatile
precursor towards the synthesis of both the carbocycles as well as the heterocycles. In the
following units, results from the reactions of trans-DBE 3 with different nucleophiles and
their subsauent transformation to heterocycles I carbocycles are described.
2.2 Results and discussion
2.2.1 Convenient synthesis of 4-aroyl-2-aryl-5,6,7,8-tenrahydroquinolines from 1,s
diketone precursors
The quinoline nucleus and its partially reduced forms can be readily recognized as
structural motifs in many physiologically active alkaloids.23 Therefore, synthesis and
characterization of various quinoline derivatives is of continuing interest. We have been
particularly interested in the synthesis and characterization of 5,6,7,8-tetrahydroquinoline
derivatives as possible precursors for aza-steroid type compounds. The 5,6,7,8-
tetrahydroquinoline moiety is found in alkalo~ds isolated from the plants belonging to
aizoaceae 5 4 - ~ 5 , ~ ~ arnaryllidaceae 5fi2$ and mtaceae 57-58'' families. In addition to the
n a ~ r a l products, a few 5,6.7.8-tetrahydroquinoline derivatives 59-61 also found
therapeutic use (Fig. 2.
H ~ N ~ S
59 60 Fig. 2.1
In an effort to develop convenient and environment friendly synthesis of 5,6,7,8-
tetrahydroquinoline derivatives, we attempted amination-cyclization reaction on the
triketones 63 derived from the conjugate addition of cyclohexanone 62 to ham-DBE 3.
The triketone 63 bas been transformed to the conesponding hitherto unknown phenyl(2-
phenyl-5,6,7,8-tetrahydro-4-quinolinyl)metham)ne 70 following one-pot amination-
cyclization-aromatization route. This multi-step one-pot transformation can be conducted
in a very short duration under environmentally benign microwave mediated reactivity
enhancement conditions.
Conjugate addition (Michael Reaction) of the anion generated from cyclohexanone 62
to tram-DBE 3 in the presence o f activated Ba(OH)> in ethanol medium furnished
inseparable mixture of diasterorneric triketones 63 in 60 % yield in 3:l ratio (Scheme
2.23).
0 0 p h y O o ph*ph + 6 -pha
0
3 62 63
Reagents and conditions: i. activated Ba(OH)2, EtOH, rt, 12 h.
Scheme 2.23
The structure of the triketone 63 was secured on the basis of IR, 'H NMR, l3C NMR
and mass spectra. The IR spectrum of the triketone 63 showed the cyclohexanone
carbonyl absorption at 1700 cm.' and the aromatic ketone absorption at 1670 ern.'. The
'H NMR spectrum (Fig. 2.2) revealed a multiplet for C-2-H at 6 4.66-4.72 ppm. The "C
NMR spectrum (Fig. 2.3) revealed seven signals in the aliphatic region, three for
carbonyl groups and eight for aromatic carbons for the major isomer.
Previously, we had isolated several carbocyclic products along with anticipated 1,5-
diketones from activated Ba(OH)2 mediated condensation reaction of some .a$-
unsaturated ketones, viz. chalcone or ~henyl vinyl ketone and cy~lopentanone.~~ In the
present case, the rnaction of rrans-DBE 3 with cyclohexanone 62, we did not detect the
formation of any such c&ocylic products. Similarly, we found only the formation of the
Fig. 2.22 300 MHz (CDC13!CC14, 1 : I ) '1-1 spectrum of (4-bromophenyl)[(lR.2R.3R,4S,5S)-4.5- di(4-hromohenzoyl)-2-(4-bromorophenyl)-3-ethoxy-2-hydoycycontyl]methanonc (76)
I 76 - (75 M H z , CDCII!CC~. I I) i; -
I
I I
- , y r - . , , . , . , i
m Ilp . Fig. 2.23 75 MHz (CDCl,ICC14, I : I ) "C spectrum of (4-bromophenyl)[(lR,2R.3R,4S,5~-4,5-di(4- btomobenzoy\)-2-(4-bromorophenyl)-3-ethoxy-2-hydoxycycotyl]mt (76)
Fig. 2.24 301 hlHz (CDCII CCI.. I : I I H NMR speclmm old~as~crcornenc rnlxture o f 13-ethox)- 2-h!drox! -4.5.d1(4-mcrh! lbenzo) l)-2-(4-mth) p h n I c c l o p e n ] 4 m e h Iphm) I)n~rthanonc
(77)
(75 MHz.CDClj (C',, 1.1)
Fig. 2.25 75 MHz (CDCI,/CCI,, 1:l) "C NMR spectrum of diastereomeric mixture of [3-ethoxy- 2-hydroxy-4,5-di(4-methylbenzoyl)-2-(4-methylphenyl)cyclopentyl](4-rnethylphenyl)methanone
(77)
73
Scheme 2.30
Next, the three-component dommo reaction was attempted w~th different nucleophiles
to assess the formation of the cylopentanol derivatives. The reactions of trans-DBE 3
with az~de, cyanide, n~tromethane and thiolate anions were conducted. There was no
reaction of rrans-DBE 3 w ~ t h the phenyl thiolate and the heptanethiolate anlon. While
n~tromethane and methoxy anlons added to rrans-DEE 3 in conjugate mode to furnish
known and 84;' the product from conjugate addition of azide anion underwent
reduction to yield the reported'8~3~-amino-l,4-diphenylbut-2-ene-~,4-dione 43 (Scheme
2.31). The reaction of rrans-DBE 3 with cyanide anion resulted in a complex mixture of
products. The NMR spectra of crude material indicated the presence of conjugated
addition product 82 along with several other polymeric species. However, there was no
lndicat~on for the formation of cyclopentanol derivatives of the t y e 73. In conclusion,
the present smdy revealed that cylopentanol formation I S resh~cted to ethoxy anion.
Ph+'h OMeO
/ iii
' h h
I
Ph . 0 0
L ph+~h ph+~h
O2N 0 NH, 0 83 3 43
Reagents and conditions: i. NaCN, activated Ba(OH)2, EtOH, rt, 12h; ii. CH,N02, activated Ba(OH)2, EtOH, rt, 12 h; iii. activated Ba(OH)z, MeOH, rt, 12 h; iv. NaN,, activated Ba(OH)z, EtOH, IT, 12 h.
Scheme 2.31
2.2.3 Condensation reactions of trans-DBE with methyl acetoacetate
Robinson annulation of a$-unsaturated carbonyl compounds with active methylene
ketones is a well-known method for generating six-member rings.36 In thls method, three
reactions namely Michael addition, aldol condensation and dehydration take place
consecutively in domino process. The 2-ene- 1,4-diones e.g. trans-DBE 3 are interesting
substrates to study Robinson annulation reaction, I~~II .? -DBE 3 is a bifunctional molecule
with two carbonyl groups conjugated to a common double bond. To the best of our
knowledge Robinson annulation on ene-diones are not known. Therefore, we conducted
Robinson annulation on trans-DBE 3 and its derivatives with methyl acetoacetate (MAA)
85. Initial reaction would be base mediated M~chael addition of the anion generated from
methyl acetoacetate 85 to trans-DEE 3 to furnish triketone 86. Subsequently, the reaction
may follow Robinson annulation course leading to the formation o f cyclohexenone
derivative 87. On the other hand, reaction may also result in cyclopentenone derivative
88 (Scheme 2.32).
O / Y C i 3 P ~ $ ~ ~
,TOCH, OCH, 0 H,COC
85 86. "+,
0
88
Scheme 2.32
When trans-DBE 3 was treated with methyl acetoacetate 85 in the presence o f
activated Ba(OH)* in methanol, the reaction furnished the cyclohexanone derivative 89 as
a single diastereomer in about 60% yield (Scheme 2.33). The product was formed by
Michael addition followed by aldol condensation.
OCH,
3 85 89
Reagents andconditions: i. activated Ba(OH)2, MeOH, rt, 12 h.
Scheme 2.33
2.2.3.1 Structure and stereochemistry of cyclohexanone derivative 89
Structure and stereochemistry of the product (white solid, mp 152-154 OC, C21H2005)
Condensation Reactions of (E)-1,4-diphenyl-2-butene-1,4-dione (3) with methyl
acetoacetate (85)
To the stirred suspension of activated Ba(OH)2 (145 mg, 0 3 5 mmol) in 10 mL dry
methanol, methyl acetoacetate 85 (540.6 mg, 4.66 mmol) was added and allowed to stir at
room temperature for 15 min. Then, truns-DBE 3 (1.0 g, 4.24 mmol) was added onion wise and continued the stirring for 12 h. The reaction mixture turned to reddish brown.
The reaction was monitored by TLC, which showed a product formation. Then, activated
Ba(OH)2 was filtered through celite pad, concentrated the reaction mixture under vacuo
to 3 m ~ , ~h~ reaction mixture was then dissolved in 40 mL dichloromethane and poured
over ice-cooled water. The organic layer was separated, washed again with water (3 x 30
mL) and collected the organic layer. The organic layer was further washed with brine
solution (2 x 10 mL) and finally dried over anhydrous Na2S04. The dichloromethane
from the reaction mixture was removed under reduced pressure. The resulted reddish
brown mixture was loaded into column (silica gel, 100-200 mesh) and eluted using 15%
EtOAc-hexanes as an eluent to furnish methyl (IS,2R,4R)-2-benzoyl-4-hydroxy-6-0~0-4-
phenylcyclohexane-I-carboxylate 89 (599 mg, 60%).
Methyl (1S,2R,4R)-2-benzoyl-l-hydroxy-6-0~0-4-
phenylcyclohexane-1-carboxylate (89): a colorless sol~d; mp
1. Conant, 1. B.; Lutz, R. E. J. Am. Chem. Soc., 1923, 45, 1303. (b) Campaigne, E.; Faye, W. 0.J. Org. Chem., 1952,17, 1405.
2. Jeyalakshmi, K. Ph. D. thesis submitted lo Pondicherr)~ Uni~~ersitj: Pondicherq! INDIA, 1999.
3. Saba, A. Guzz. Chim Itul., 1991,121,55; CAN. 1991,115:70954.
4. Parakka, J. P.; Sadanandan, E. V.; Cara, M. P. J. Org. Chem., 1994,59,4308.
5. Hatanaka, M.; Himeda, Y.; Tanaka, Y.; Ueda, I. Tetruhcdron Lett., 1995, 36, 3211.
6. Hatanaka, M.; Ishida, A.; Tanaka, Y.; Ueda, I. Tetrahedron Left., 1996, 37,401
7. Al-Arab, M. M.; Atfeh, M. A,; Al-Saleh, F. S. Tetrahedron, 1997,53, 1045.
8. Cabrera, A.; Lagadee, R. L.; Shanna, P.; Arias, J. L.; Toscano, R. A,; Velasco, L.; Gavino, R.; Alvarez, C.; Salman, M. J Chem. Soc. Perkin Trans I., 1998,3609.
9. Atfah, M. A,; Al-Arab, M. M. J Heterocycl. Chem., 1990,27, 599
10. Kaupp, G.; Poyodda, U.; Atfah, A,; Meier, H.; Vierengel, A. Angew. Chem. Int. Ed Engi., 1992,31,768.
l I. Rajakumar, P.; Kannan, A. Ind. J. Chem., 1993,32B, 1275.
12. Rao, K. S.; Subbaraju, G. V. Ind. J. Heterocycl. Chem., 1994,4,19.
13. Chong, J. M.; Clarke, I S.; Koch, 1.; Olbach, P. C.; Taylor N. J. Tetrahedron: Asj~mmetfy, 1995, 6,409.
14. Mataka, S.; Kitagawa, H.; Tsukinoki, T.; Tashiro, M.; Takahashi, K.; Kamata, K. BUN. Chem. Soc. Jpr~., 1995, 68, 1969.
15. Kumar, A,; Boykin, D. W. S)nth. Commun., 1995, 25, 2071
16. Singal, K. K. Synth. Commun., 1996,26,3571
17. Duan, X. G.; Duan, X. L.; Rees, C. W.; Yue, T. Y. J. Chem. Soc. Perkin Trans I , 1997,2527.
18. Seko, S.; Miyake, K. Synth. Commurz., 1999, 29,2487.
19. K ~ ~ P P , G.; Schmeyers, J.; Kuse, A.; Atfeh, A. Angew. Chem. In!. ed. Engl., 1999, 38, 2896.
20. Rao, H. S. P.; Jothilingam, S. Tetrahedror~ Lett., 2001, 42,6595
21. Abu-Orabi, S. T. Molecules, 2002, 7, 302.
22. Rao, H . S. P.; Senthilkumar, S. P.; Jeyalakshmi, K. Heterocyclic Commun., 2003, 9.65.
23. (a) Jones, G. In Comprcl?~rlsive Hcterocj~clic Chemistry; Katritzky, A. R.; Rees, C. W., Ed.; Pergamon Press: Oxford 1984; Vol. 2. Chap. 8, pp 395-510. (b) Jones, G. In Contprehensive Heter.ocylic Cherni.~tq' I / ; Katritzky, A. R.; Rees, C. W.; Scriven, E. F. V., Ed.; Pergamon Press: Oxford 1996; Vol. 5, Chap. 5.05, pp 167- 243.
24. (a) Yamada, 0.; Ogasawara, K. Tetrahedron Lett., 1998, 39, 7747. (b) Goehring. R. R. Tetrahedron LEN., 1994, 35, 8145. (c) Jeffs, P. W.; Capps, T. M.; Johnson, D. D.; Redfearn, R. J. Org. Chem.. 1982, 47, 3611. (d) Koyama, J.; Sugita, T.; Tagahara, K.; Suzuta, Y.; Ine, H. Heler-oc).cles, 1981, 16, 969.
25. Bunnell, A. E.; Flippin, L. A,; Liu, Y. J. Org. Chem., 1997, 62, 9305
26. (a) Yagudaev, M. P.; Bessonova, 1. A. Chem. Nut. Compd. (Engl. Trans.)., 1989, 25, 20. (b) Rozsa, Zs.; Rabik. M.; Szendrei, K.; Aynechi, M.; Pelczer, I. Phytochemistry, 1988,27,2369.
27. (a) Pita, B.; Masaguer, C. F.; Ravina, E. Tetrahedron Leu., 2002, 43, 7929. (b) Kim, S. R.; Hwang, S. Y.; Jang, Y. P.; Park, M. J.; Markelonis, G. J.; Oh, T. H.; Kim, Y. C. Planta Med., 1999, 65, 218. (c) Onodera, K.; Kojima, W. M. Nihon Shinkei Yakurigaku Zasshi., 1998, 18, 33. (d) Limbert, M.; Isert, D.; Klesel, N.; Markus, A,; Seeger, K.; Seibert, G.; Schrinner, E. Antimicrob. Agents Chemother., 1991, 35, 14. (e) Curran, A. C. W.; Shepherd, R. G. Ger. Par., 1975, John Wyeth 2459 631, CA 83: 131483w.
28. (a) Rao, H. S. P.; Jeyalakshmi, K.; Senthilkumar, S. P. Tetrahedron, 2002, 58, 2189. (b) Rao, H. S. P.; Jeyalakshmi, K.; Bharathi. B.; Pushpalatha, L.; Uma Maheswari, V. J, Indian Chem. Soc, (Golden Jubilee Issue), 2001, 78, 787.
29. Joule, J. A,; Mills, K. In Heteroo'clic Chemistq, 4" Ed. Blackwell Science Ltd., London, 2000, p 7 1.
30. Reichardt, C. In Solvents and Solvent effects in Organic Chemistq: 2"* Ed. VCH Ltd., New York, 1990, p 408.
31. Tietze, L. F.; Petersen, S. Etrr: J. Org. Chon., 2000, 1827
32. Eliel, E. L. Stereochemisty of Carbon Compounds, Tata McGraw-Hill Publishing Co. Ltd., New Delhi, 1975, p 248.
33. Strurnza, J. ; Altschuler, S. Israel J. Chem. 1963, 1, 106; CAN: 1964, 60:60367.
34. Lutz, R. E.; Chi-Kang, D. J. Org. Chetn., l956,21, 551
35. Tamura, Y.; Sumoto, K.; Matsushirna, H.; Taniguchi, H.; Ikda, M. J. Org. Chem., 1973,38,4324.
36. (a) Rapson, W. S.; Robinson, R. J. Chem. Soc., 1935, 1285. (b) Gawley, R. E. S~nfhes i s , 1976, 777.