Synthesis of 3-alkenyl-2-arylchromones and 2,3-dialkenylchromones via acid-catalysed retro-Michael ring opening of 3-acylchroman-4-ones

Article abstract of DOI:10.1016/j.tetlet.2005.06.058

3-Acylchromones and 3-acylflavones, readily available by acylation of 2′-hydroxydibenzoylmethane with acid anhydrides in the presence of base, undergo efficient conjugate reduction with NaBH₄ in pyridine to give the corresponding chroman-4-ones/flavanones in high yields. The reduction is both regio- and chemoselective. Treatment of the chroman-4-ones with MeSO ₃H generates the 3-alkenyl-2-arylchromones by a dehydrative rearrangement initiated by retro-Michael cleavage of the pyranone ring. This reduction-rearrangement sequence can be extended to 2-alkyl-3-alkenoylchromones to generate 3-alkenyl-2-styrylchromones, the first examples of 2,3-dialkenylchromones.

Full text of DOI:10.1016/j.tetlet.2005.06.058

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 5515–5519
Synthesis of 3-alkenyl-2-arylchromones and
2,3-dialkenylchromones via acid-catalysed retro-Michael
ring opening of 3-acylchroman-4-ones
David S. Clarke,a Christopher D. Gabbutt,a,* John D. Hepworthb and B. Mark Herona
aDepartment of Colour and Polymer Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK
bJames Robinson Ltd, PO Box B3, Hillhouse Lane, Huddersfield HD1 6BU, UK
Received 27April 2005; revised 1 June 2005; accepted 10 June 2005
Available online 1 July 2005
Abstract—3-Acylchromones and 3-acylflavones, readily available by acylation of 20-hydroxydibenzoylmethane with acid anhydrides
in the presence of base, undergo efficient conjugate reduction with NaBH4 in pyridine to give the corresponding chroman-4-ones/
flavanones in high yields. The reduction is both regio- and chemoselective. Treatment of the chroman-4-ones with MeSO3H gener-
ates the 3-alkenyl-2-arylchromones by a dehydrative rearrangement initiated by retro-Michael cleavage of the pyranone ring. This
reduction–rearrangement sequence can be extended to 2-alkyl-3-alkenoylchromones to generate 3-alkenyl-2-styrylchromones, the
first examples of 2,3-dialkenylchromones.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Although 2-styrylchromones are well known and easily
accessible from 2-methylchromones1 other alkenylchro-
mones have been much less investigated. 3-Alkenyl-
chromones have been obtained from the reaction of
spiro-annulated chroman-4-ones with formamide ace-
tals,2a and the acid-catalysed rearrangement of 2-alkyl-
and 2,2-dialkyl-3-(hydroxymethylene)chroman-4-ones,2b
a reaction, which we have extended to the synthesis of
3,4-dihydro-9H-xanthen-9-ones3a and also to 3-isopro-
penylthiochromone.3b
ylchromone and H2C@CHOEt.5a Knoevenagel conden-
sation of cyanoacetic and malonic acids with 3-
formylchromone provides, respectively, chromone
3-acrylonitrile5b and 3-acrylic acid derivatives,5c the
latter compounds are also accessible from Heck reac-
tions of 3-bromochromones.5d Recently, (E)-3-styrylchr-
omones have been obtained by the microwave initiated
Knoevenagel reaction of phenylacetic acids with 3-form-
ylchromone.5e Missing from all these routes is an
approach which permits access to simple 3-alkenyl-2-
arylchromones. Recently, the acid-catalysed condensation
of 20-hydroxydibenzoylmethanes with phenylacetalde-
hydes has been reported to provide 3-styrylflavones in
a one-pot operation.6 However, this procedure is limited
to arylacetaldehydes and is not amenable to the synthe-
sis of simpler 3-alkenylflavones.
Reactions of phosphorus ylides with 3-formylchromone
provide 3-alkenylchromones. Thus, acylmethylenetri-
phenylphosphoranes afford the corresponding (E) alk-
enes,4a whilst benzylidenetriphenylphosphoranes give,
predominantly, the (Z)-3-styrylchromones.4b However,
the reaction with Ph2C@PPh3 is very inefficient,4c whilst
3-formylchromones react anomalously in a complex
ꢀMichael–Michael–Wittigꢁ sequence with (2,4-dioxobut-
ylidene)triphenylphosphoranes to give benzophen-
ones.4d (E)-3-(4-Oxo-1-benzopyran-3-yl)acroleins are
available via hydrolysis of the cycloadduct of 3-form-
We now report an extension of our earlier work,2b which
provides an entry to 3-alkenylchromones and flavones
from the acid-catalysed retro-Michael ring cleavage of
novel, readily available 3-aroyl-2-alkylchroman-4-ones.
Application of this dehydrative rearrangement to the
hitherto unknown 2-alkyl-3-cinnamoylchroman-4-ones
is also described. The requisite starting materials, 20-
hydroxydibenzoylmethanes 1, are easily available by
O-acylation of 20-hydroxyacetophenone followed by
Baker–Venkataraman (BV) rearrangement under standard
Keywords: Conjugate reduction; Chroman-4-ones; Chromones;
Flavones; Rearrangement.
*
Corresponding author. Tel.: +44 (0) 113 343 2936; fax: +44 (0) 113
343 2947; e-mail: c.d.gabbutt@leeds.ac.uk
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.06.058

5516
D. S. Clarke et al. / Tetrahedron Letters 46 (2005) 5515–5519
O
O
O
O
O
O
O
R
(i)
(iii)
Ar
Ar
R
(ii)
OH
OH
O
O
Ar
1
3
2
OMe
(iv)
(v), (iv)
Ar = 4-MeOC6H4
H
O
O
O
O
O
Ar
R
Ar
R
O
O
O
O
5A
4A
4B
a R = H, Ar = Ph
(vi)
b R = H, Ar = 4-MeOC6H4
c R = Me, Ar = 4-MeOC6H4
d R = H, Ar = 4-NO2C6H4
e R = Me, Ar = 4-MeC6H4
f R = H, Ar = 2-NO2C6H4
g R = Me, Ar = 2-NO2C6H4
h R = H, Ar = 2-thienyl
i R = Me, Ar = 2-thienyl
j R = H, Ar = 2-furyl
H
O
O
O
OMe
5B
k R = Me, Ar = 2-furyl
Scheme 1. Reagents and conditions: (i) ArCOCl, pyridine, rt; (ii) KOH, pyridine, rt, then AcOH; (iii) R = H: Ac2O, NaOAc, D; R = Me: (EtCO)2O,
Et3N, D; (iv) NaBH4, pyridine, rt, 52–85%; (v) (PriCO)2O, Et3N, D; (vi) MeSO3H, rt.
conditions.7 Acylation of the 1,3-diketones 1 with either
acetic or propionic anhydride gives the 2-alkyl-3-aro-
ylchromones 2 in high yields.8a–c None of the isomeric
chromones 3 were formed. Smooth and efficient conju-
gate reduction 2!4 was accomplished with NaBH4 in
pyridine (Scheme 1).9 In most cases the 3-acylchroman-
4-ones 4 were found to exist exclusively (NMR) as the
trans-diketo tautomers 4Aa–Ae and 4Ah–k. However,
reduction of 2f and 2g provided the enol tautomers
4Bf and 4Bg as the sole products; evidently conjugation
of the ortho nitro group with the C-4 carbonyl facilitates
intramolecular hydrogen bonding, so favouring the enol
tautomer.
Treatment of chroman-4-ones 4A with TsOH in hot tolu-
ene effected rearrangement to 3-alkenylchromones 6
albeit in low yields. It was found that the conversion
4A!6 proceeded more efficiently and in high yield simply
by stirring the reactant in neat MeSO3H at room temper-
ature.10a Formation of the alkenylchromone proceeded
stereoselectively since chroman-4-ones 4Ac and 4Ae
provided only 6c and 6e.10b Interestingly, the furan deriv-
atives 4j,k rearranged to the corresponding alkenylchro-
mones 6j,k without complication. The rearrangement of
4A to the alkenylchromones 6 occurs via retro-Michael
cleavage of the pyranone ring, followed by cyclisation
and dehydration. Deprotonation of the intermediate car-
bocation affords the product (Scheme 2).11 Surprisingly,
attempts to effect rearrangement of the fully enolised di-
ketones 4Bf and 4Bg, led only to their complete recovery,
even after prolonged reaction times at 75 ꢁC. The steri-
The acylation of 1 was also successful with more hin-
dered anhydrides thus 5A (78%) was obtained from
isobutyric anhydride.
O
R
MeSO3H
4A
O
Ar
-H+
6
H
O
O
O
O
O
O
R
R
+H+
Ar
R
-H2O
4A
H
O
H
Ar
OH2
Ar
Ar and R as defined in Scheme 1
Scheme 2.

D. S. Clarke et al. / Tetrahedron Letters 46 (2005) 5515–5519
5517
cally hindered chromanone 5A could not be induced to
rearrange; exposure to MeSO3H gave only the enol tauto-
mer 5B as a stable crystalline product [80%, d (CDCl3)
16.07(s, OH), 4.98 (d, J = 3.4 Hz, H-2)].
ButOK in THF, following the literature procedure.12b
Acylation of 8a,b with Ac2O or (EtCO)2O effected regio-
specific cyclisation to the 3-alkenoylchromones 9e–h.
Subsequent reduction with NaBH4–pyridine proceeded
in an entirely chemo- and regiospecific manner, with
conjugate addition of hydride to the chromone ring, to
give 10e–h. When exposed to MeSO3H, the 2-methyl-
chroman-4-ones 10e,f rearranged smoothly to the dial-
kenylchromones 11i,j (Scheme 3), which were isolated
in reasonable yields (48–60%) by simple aqueous
work-up and recrystallisation. Rearrangement of the
2-ethylchroman-4-ones 10g,h proceeded to give 12i,j as
the only identifiable products (cf. 4Ac!6c).
A simple extension of this methodology permits access
to 2,3-dialkenylchromones, of which no examples have
been reported previously. Thus, the esters 7 were ob-
tained from 20-hydroxyacetophenone by acylation with
alkenoyl chlorides in MeCN under DBU catalysis. We
found these conditions to be much more satisfactory
than the existing literature procedure.12a Aliphatic-BV
rearrangement of 7a–d to 8a–d was accomplished by
OH R2
O
O
O
O
(iv)
(ii), (iii)
(i)
R1
For 8a and 8b
OH
R2
OH
8
O
R1
7
a R1 = Ph, R2 = H
b R1 = 2-NO2C6H4, R2 = H
c R1 = Me, R2 = H
for 7 and 8
O
d R1 = R2 = Me
R3 = Me
(vi)
O
O
O
O
O
OH
11
R4
(v)
R3 R4
R3 R4
O
O
R3 = Et
Me
9
10
e R3 = Me, R4 = H
f R3 = Me, R4 = NO2
g R3 = Et, R4 = H
h R3 = Et, R4 = NO2
O
for 9 and 10
12
R4
i R4 = H
for 11 and 12
j R4 = NO2
Scheme 3. Reagents and conditions: (i) R1R2C@CHCOCl, DBU, MeCN, 0 ꢁC; (ii) KOBut, ButOH, THF, 0 ꢁC; (iii) AcOH; (iv) R3 = Me: Ac2O,
NaOAc, D, 1 h; R3 = Et: (EtCO)2O, NEt3, D, 1 h; (v) NaBH4, pyridine, rt, 60–72%; (vi) MeSO3H, rt, 1–2 h.
O
OH
Me
R
O
O
R
O
O
Ac2O, ∆
NaOAc
Me
Me
Me
Me
8c,d
+
O
O
Me
O
OH
13
R
14
15
NaBH4,
pyridine, RT
R = H
OH
O
O
O
O
OH
Me
Me
OAc O
Me
O
O
5
Ac2O
R
R
Me
Me
OH Me
2
16
17
a R = H
b R = Me
Scheme 4.

5518
D. S. Clarke et al. / Tetrahedron Letters 46 (2005) 5515–5519
O
O
O
Me
8
6
1
10
(i)
(ii)
4
O
O
O
OMe
4
11
1
6
OMe
OMe
18
6b
19
Scheme 5. Reagents and conditions: (i) hm, PhH, cat. I2, 36 h; (ii) BF3ÆOEt2 (6 equiv), Cl(CH2)2Cl, D, 24 h.
2. (a) Kabbe, H.-J.; Widdig, A. Angew. Chem., Int. Ed. Engl.
1982, 21, 247–256; (b) Gabbutt, C. D.; Hepworth, J. D.
Tetrahedron Lett. 1985, 26, 1879–1882.
3. (a) Gabbutt, C. D.; Hepworth, J. D.; Urquhart, M. W. J.;
Vazquez de Miguel, L. M. J. Chem. Soc., Perkin Trans. 1
1998, 1547–1553; (b) Gabbutt, C. D.; Hepworth, J. D.;
Heron, B. M. J. Chem. Soc., Perkin Trans. 1 2000, 2930–
2938.
Interestingly, acylation of 8c with Ac2O provided three
new compounds, which could be readily separated and
identified as chromones 14a and 15a and the pyranone
16a,13 in the ratio 1:3:5, respectively. The constitution
of 15a (and hence 14a) was confirmed by conjugate
reduction to 17. Attempts to effect rearrangement of
the latter under a variety of conditions failed to provide
a tractable product.
4. (a) Abdou, W. M.; Khidre, M. D.; Mahran, M. R.
Phosphorus, Sulfur 1991, 61, 83–90; (b) Sandulache, A.;
Silva, A. M. S.; Pinto, D. C. G. A.; Almeida, L. M. P. M.;
Cavaleiro, J. A. S. New J. Chem. 2003, 27, 1592–1598; (c)
3-(2-Phenylstyryl)chromone is more efficiently obtained
from hydrolysis of the cycloadduct of 3-formylchromone
and diphenylketene, Eiden, F.; Breugst, I. Chem. Ber.
1979, 112, 1791–1807; (d) Langer, P.; Holtz, E. Synlett
2003, 402–404.
5. (a) Ghosh, C. K.; Tewari, N.; Bhattacharyya, A. Synthesis
1984, 614–615; (b) Nohara, A.; Kuriki, H.; Saijo, T.;
Ukawa, K.; Murata, T.; Kanno, M.; Sanno, Y. J. Med.
Chem. 1975, 18, 34–37; condensations of 3-formylchro-
mone with acetylacetone and ethyl acetoacetate have also
been described, Jones, W. D.; Albrecht, W. L. J. Org.
Chem. 1976, 41, 706–707; (c) Nohara, A.; Kuriki, H.;
Saijo, T.; Sugihara, H.; Kanno, M.; Sanno, Y. J. Med.
Chem. 1977, 20, 141–145; (d) Davies, S. G.; Mobbs, B. E.;
Goodwin, C. J. J. Chem. Soc., Perkin Trans. 1 1987, 2597–
2604; (e) Silva, V. L. M.; Silva, A. M. S.; Pinto, D. C. G.
A.; Cavaleiro, J. A. S.; Patonay, T. Synlett 2004, 2717–
2720.
The chromones 14 and 15 are derived from the non-
regiospecific cyclisation of the triketone 13 (Scheme 4),
whilst 16a (from 8c) results from conjugate addition of
the enol(ate) to the alkenoyl side chain. Acylation of
8d similarly provided a mixture of 14b and 16b in a
1:1 ratio. We did not observe any of the analogous pyr-
anone 16 (R = Ph, H replaces 6-Me) from reaction of 8a
with Ac2O; presumably 1,4-addition is retarded because
of increased conjugation in this system. No reactions of
8, which provide 3-acyl-5,6-dihydropyran-4-ones have
been reported previously.14
The alkenylchromones offer considerable scope for the
synthesis of fused systems either by cycloaddition,3,15
electrocyclisation or electrophilic cyclisation. Examples
of the latter two annulation protocols with 6b are pre-
sented in Scheme 5. Thus, photodehydrocyclisation pro-
vided the benzo[c]xanthone 1816a (70%). Electrophilic
cyclisation to the indenopyran 1916b (40%) was initiated
with BF3-etherate, providing an expedient approach to
these systems. Surprisingly, compound 6h failed to react
under these conditions. Further studies of the scope and
limitations of this reaction are underway.
´
6. Lokshin, V.; Heynderickx, A.; Samat, A.; Pepe, G.;
Guglielmetti, R. Tetrahedron Lett. 1999, 40, 6761–6764.
7. Wheeler, T. S. Org. Syn., Coll. Vol. 1963, 4, 479.
8. (a) Huffman, K. R.; Burger, M.; Henderson, Jr., J. A.;
Loy, M.; Ullman, E. F. J. Org. Chem. 1969, 34, 2407–
2414; (b) Huffman, K. R.; Ullman, E. F. US Patent 1969, 3
444 212; Chem. Abstr. 1969, 71, 38807; (c) Baker, W.;
Harborne, J. B.; Ollis, W. D. J. Chem. Soc. 1952, 1294–
1302.
9. We have previously noted the efficacy with which conju-
gate reductions of activated chromones proceed under
these conditions: Gabbutt, C. D.; Hepworth, J. D.;
Urquhart, M. W. J.; Vazquez de Miguel, L. M. J. Chem.
Soc., Perkin Trans. 1 1997, 1819–1824; Reports of 3-
aroylchroman-4-ones are scarce. Apart from the present
procedure involving conjugate reduction of 3-acylchro-
mones (i.e., 2 and 9!4 and 10, respectively), the only
other convenient route involves the 1,4-addition of
R2CuLi to 3-acylchromones; Saengchantara, S. T.;
Wallace, T. W. Tetrahedron 1994, 46, 3029–3036.
10. (a) Typical procedure: 2-(4-nitrophenyl)-3-vinylchromone
6d. NaBH4 (1.89 g, 0.05 mol) was added in a single portion
to a magnetically stirred solution of 2-methyl-3-(4-nitro-
benzoyl)chromone 2d8b,c (15.46 g, 0.05 mol) in pyridine
(125 ml). The mixture became red and was allowed to stir
until all the NaBH4 had dissolved. After 2 h the mixture
was diluted with water, acidified with concd HCl and
extracted with EtOAc (3 · 100 ml). The combined extracts
were washed with 2 M HCl, water and then dried
In summary, a new route to the hitherto difficultly
accessible 3-acyl-2-substituted chroman-4-ones is
described. Rearrangement of these compounds pro-
vides 3-vinyl- and 3-prop-2-enyl-derivatives of flavones,
2-(heteroaryl)chromones and 3-alkenyl-2-styrylchro-
mones. Reactions of the new compounds will be
described more fully in due course.
Acknowledgements
We thank Lancaster Synthesis for support (D.S.C.) and
the EPSRC for access to the Mass Spectrometry Service,
University of Wales Swansea.
References and notes
1. Ellis, G. P. In Chromenes, Chromanones and Chromones;
Ellis, G. P., Ed.; Wiley-Interscience: New York, 1977, pp
581–631.

D. S. Clarke et al. / Tetrahedron Letters 46 (2005) 5515–5519
5519
(Na2SO4) and evaporated to give trans-4Ad (12.45 g, 80%)
as pale yellow crystals from MeOH, mp 164–166 ꢁC. mmax
(Nujol)/cmꢀ1 1695, 1665, 1603; dH (CDCl3, 250 MHz) 1.50
(3H, d, J = 6.4 Hz, 2-CH3), 4.60 (1H, d, J = 11.5 Hz, H-3),
5.08 (1H, dq, J = 11.5, 6.4 Hz, H-2) 7.01–8.36 (8H, m, Ar–
H). Compound 4Ad (7.78 g, 0.025 mol) was added to
MeSO3H (40 ml) and the mixture kept at rt until the
reaction was complete (ca. 2 h). Following aqueous work-
up the product was extracted into EtOAc, to give
chromone 6d (6.95 g, 95%) as an off-white microcrystalline
powder mp 147–149 ꢁC from MeOH, mmax (Nujol)/cmꢀ1
1650, 1606. dH (CDCl3, 400 MHz) 5.56 (1H, dd, J = 9.6,
4.4 Hz, –CH@CHH) 6.34–6.39 (2H, m, CH@CHH and
CH@CH2), 7.44–7.74 (3H, m, Ar–H), 7.88–7.94 (2H, m, 4-
NO2C6H4) 8.32 (1H, dd, J = 8.0, 1.6 Hz, H-5), 8.36–8.41
(2H, m, 4-NO2C6H4) Other 3-alkenylchromones were
prepared similarly. Yields (%) 6a (70), 6c (87) 6d (95), 6e
(62) 6h (30), 6i (40), 6j (66), 6k (60); (b) Representative
spectral data for trans-2-(4-methylphenyl)-3-propenylchro-
mone 6e: dH (CDCl3, 400 MHz) 1.79 (3H, dd, J = 6.7,
1.7, Hz, @CHCH3), 2.45 (3H, s, CH3), 6.11 (1H, dq,
J = 15.7, 1.7 Hz, CH@CHCH3), 6.89 (1H, dq, J = 15.7,
6.7Hz, CH @CHCH3), 7.30–7.44 (4H, m, Ar–H), 7.58–
7.64 (3H, m, Ar–H), 8.27 (1H, dd, J = 8.0, 1.5 Hz, H-5); dC
19.76, 21.53, 117.76, 117.86, 121.39, 123.50, 124.75,
126.16, 128.93, 129.51, 130.52, 132.67, 133.19, 140.69,
155.44, 161.88, 177.73; CI-HRMS [M+H]+ found
277.1223, C19H16O2+H+ requires 277.1221.
CH@CH–CH3). The low-field shift of the 2-CH3 group
in 14a and 14b is characteristic for 3-acyl-2-methylchro-
mones, see: Ghosh, C. K.; Pal, C.; Maiti, J.; Sarkar, M. J.
Chem. Soc., Perkin Trans. 1 1988, 1489–1493, Compound
16a; dH (CDCl3 250 MHz) 1.54 (3H, d, J = 6.5 Hz, 6-
CH3), 2.18 (3H, s, OCOCH3), 2.25 (3H, s, 2-CH3), 2.40–
2.65 (2H, m, H-5), 4.57–4.72 (1H, m, H-6), 7.10–7.61 (4H,
m, Ar–H).
14. The only reported example of direct formation of a
pyranone ring from 1,3-diketones 8 concerns the cycload-
dition of pyrrolidinocycloalkenes to (E)-1-(2-hydroxyphen-
yl)-5-phenylpent-4-ene-1,3-dione 8a, which provides
cycloalkeno[a]- and cycloalkeno[d]xanthones, Letcher, R.
M.; Yue, T.-Y.; Chiu, K.-F.; Kelkar, A. S.; Cheung, K.-K.
J. Chem. Soc., Perkin Trans. 1 1998, 3267–3276.
15. The cycloaddition chemistry of chromones has been
reviewed, Ghosh, C. K.; Ghosh, C. Indian J. Chem., Sect.
B 1997, 36, 968–980; For recent examples involving 3-
alkenyl(thio)chromones see: Ref. 3b and Gabbutt, C. D.;
Hepworth, J. D.; Heron, B. M.; Pugh, S. L. J. Chem. Soc.,
Perkin Trans. 1 2002, 2799–2808; Pinto, D. C. G. A.; Silva,
´
A. M. S.; Almeida, L. M. P. M.; Carrillo, J. R.; Dıaz-
Ortiz, A.; de la Hoz, A.; Cavaleiro, J. A. S. Synlett 2003,
1415–1418.
16. (a) Photochemical electrocyclisations to these ring systems
are scarce, however the cyclisation of 20-[(E)-styryl]flavone
has been reported; Parthasarathy, M. R.; Grover, N.
Indian J. Chem. Sect. B 1991, 30, 440–441, Representative
spectral data for 3-methoxy-7H-benzo[c]xanthen-7-one 18;
dH (CDCl3, 250 MHz) 3.99 (3H, s, OMe), 7.24–7.47 (3H,
m, Ar–H), 7.62–7.77 (3H, m, Ar–H), 8.26 (1H, d,
J = 8.7Hz, H-5), 8.37 (1H, dd, J = 7.8, 1.5 Hz, H-8),
8.59 (1H, d, J = 8.7Hz, H-6); (b) Representative spectral
data for 8-methoxy-10-methyl-10H,11H-indeno[1,2-b][1]-
benzopyran-11-one 19, dH (CDCl3, 250 MHz) 1.63 (3H, d,
J = 7.4 Hz, 10-Me), 3.91 (3H, s, OMe), 4.03 (1H, d, J =
7.4 Hz, H-10), 7.01 (1H, dd, J = 8.4, 2.2 Hz, H-7), 7.12
(1H, d, J = 2.2 Hz, H-9), 7.65 (1H, td, J = 8.4, 2.2 Hz, H-
4), 7.73–7.74 (2H, m, H-2, H-3), 7.75 (1H, d, J = 8.4 Hz,
H-6), 8.34 (1H, dd, J = 8.4, 2.2 Hz, H-1).
11. Similar mechanisms have been reported previously; see:
Dean, F. M.; Murray, S. J. Chem. Soc., Perkin Trans. 1
1975, 1706–1711, and Refs. 2b and 3.
12. (a) Although 7a,b could be prepared by standard proce-
dures (with pyridine or NEt3 as catalyst), compounds 7c
(57%) and 7d (63%) could only be obtained satisfactorily
by the DBU procedure; (b) Kraus, G. A.; Fulton, B. S.;
Woo, S. H. J. Org. Chem. 1984, 49, 3212–3214.
1
13. Selected H NMR data for 14a, 15a and 16a: dH (CDCl3,
250 MHz) compound 14a 1.97(3H, dd, J = 7.0, 2.5 Hz,
–CH@CH-CH3), 2.42 (3H, s, –COCH3). Compound 15a d
2.66 (3H, s, 2-CH3), 2.00 (3H, dd, J = 6.5, 2.5 Hz,