Intramolecular ketone-olefin radical cyclization with low-valent titanium reagent: Synthesis of benzopyrans

Article abstract of DOI:10.1080/00397910903340710

A novel protocol for intramolecular ketyl-olefin radical cyclization with low-valent titanium reagent is outlined. It allows the formation of the benzopyran nucleus from ortho-allyloxy propiophenones as the sole product in moderate yields via intramolecular radical cyclization. Copyright Taylor & Francis Group, LLC.

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Intramolecular Ketone-Olefin Radical
Cyclization with Low-Valent Titanium
Reagent: Synthesis of Benzopyrans
Sandip K. Nayak a , Suchitra Bhatt a & Kshama Roy a
a Bio-organic Division , Bhabha Atomic Research Centre , Mumbai,
India
Published online: 13 Sep 2010.
To cite this article: Sandip K. Nayak , Suchitra Bhatt & Kshama Roy (2010) Intramolecular Ketone-
Olefin Radical Cyclization with Low-Valent Titanium Reagent: Synthesis of Benzopyrans, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
40:20, 2969-2975, DOI: 10.1080/00397910903340710
To link to this article: http://dx.doi.org/10.1080/00397910903340710
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Synthetic Communications1, 40: 2969–2975, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903340710
INTRAMOLECULAR KETONE-OLEFIN RADICAL
CYCLIZATION WITH LOW-VALENT TITANIUM
REAGENT: SYNTHESIS OF BENZOPYRANS
Sandip K. Nayak, Suchitra Bhatt, and Kshama Roy
Bio-organic Division, Bhabha Atomic Research Centre, Mumbai, India
A novel protocol for intramolecular ketyl-olefin radical cyclization with low-valent titanium
reagent is outlined. It allows the formation of the benzopyran nucleus from ortho-allyloxy
propiophenones as the sole product in moderate yields via intramolecular radical cyclization.
Keywords: Benzopyran; intramolecular radical cyclization; low-valent titanium reagent; ortho-allyloxy
aromatic aldehydes=ketones
INTRODUCTION
Carbon–carbon bond-formation reactions are of great importance and continue
to draw attention to synthetic organic chemists. Intramolecular cyclization of the
carbon radical to multiple bonds has been extensively used as a useful tool in organic
synthesis.[1] Among those, intramolecular ketyl-olefin cyclization has many advantages,
especially in terms of high diastereoselectivities at newly formed CꢀC bond in the
cyclized product. Samarium(II) iodide[2] and organotin reagents[3] have been commonly
exploited for ketyl-olefin radical cyclization reactions relative to other low-valent
metals. Although Ti(0)[4] has been routinely used for the synthesis of 1,2-diol=olefins
from aldehydes=ketones, there is no report on its application in carbonyl-olefin=alkyne
radical cyclization reactions. Herein, we report an unprecedented Ti(0) reagent–
mediated intramolecular ketyl-olefin radical cyclization in 2-allyloxy aromatic ketones
leading to the formation of the benzopyran skeleton, which is an important motif of
many biologically active oxygen heterocycles (Scheme 1).
RESULTS AND DISCUSSION
The reductive deoxygenation of carbonyl compounds to olefins with low-valent
titanium (LVT) reagents has found wide range of applications, from the synthesis of
strained olefins to the synthesis of various complex natural products including
taxol.[4,5] The reaction proceeds by single-electron transfer (SET) from LVT to the
substrate to form a radical anion (ketyl), which undergoes fast dimerization to the
Received July 24, 2009.
Address correspondence to Sandip K. Nayak, Bio-organic Division, Bhabha Atomic Research
Centre, Mumbai 400085, India. E-mail: sknayak@barc.gov.in
2969

2970
S. K. KAYAK, S. BHATT, AND K. ROY
Scheme 1. Low-valent titanium (LVT)–mediated intramolecular carbonyl-olefin radical cyclization in
ortho-allyloxy aromatic aldehydes=ketones.
titanium pinacolate. The pinacolates are usually stable at 0 ꢁC, whereas at higher
temperatures they undergo deoxygenation to liberate olefin. The titanium oxides
formed in the reaction serve as the thermodynamic sink that drives the conversion.
So far, the McMurry reaction had been essentially confined to inter- and intramol-
ecular coupling (i) between two aldehydes=ketones and (ii) ketoesters=ketoamides,
respectively. Although intramolecular carbonyl-olefin radical cyclization with SmI2
has been amply demonstrated by Molander et al.,[2] to the best of our knowledge
there is no report on the use of Ti(0) reagent in intramolecular carbonyl-olefin rad-
ical cyclization reactions. The only report so far by Roy et al. for this cyclization is
based on the titanium(III) (Cp2TiCl) reagent.[6]
In continuation of our work on the synthetic applications of LVT reagents,[7]
we envisaged that the initially formed ketyl radical of ortho-alkenyloxy aromatic
aldehyde=ketone (2) may undergo intramolecular CꢀC bond formation to the steri-
cally close olefin moiety in the ortho position to afford a carbocyclic skeleton 5
before its dimerization to the 1,2-diol 4 or abstraction of H from solvent to form
3 (Scheme 3). To this end, a preliminary study on carbonyl-olefin coupling reaction
was carried out with ortho-allyloxybenzaldehyde (1a) as a substrate. Thus, reaction
of 1a with LVT reagent [Ti(0) prepared from TiCl4=Zn=tetrahydrofuran (THF)] at
0 ꢁC yielded a mixture of two products, which were characterized as 2-allyloxy benzyl
alcohol (3a) (possibly by the abstraction of hydrogen from THF by the initially
formed ketyl radical; path A) and 4-hydroxy-3-methyl-benzopyran (5a) (by intra-
molecular radical cyclization; path C) (Table 1, entry 1) by infrared (IR), 1H
NMR, and 13C NMR data. The structure of compound 5a was further confirmed
by its oxidation to 3-methyl-benzopyran-4-one[8] (6) by pyridinium chlorochromate
(PCC). However, under identical reaction conditions, ortho-allyloxyacetophenone
(1b), instead of intramolecular radical cyclization to chromanol 5b, underwent
reductive dimerization to afford the corresponding 1,2-diol (4b) (path B) as the sole
product in moderate yield (Table 1, entry 2). This prompted us to study the role of
different carbonyl functions ortho- to allyloxy group in carbonyl-olefin radical cycliza-
tion. Gratifyingly, the reaction of ortho-allyloxypropiophenone (1c) with the LVT
reagent under identical reaction conditions led to intramolecular radical cyclization
only (path C), affording 4-ethyl-4-hydroxy-3-methyl-benzopyran (5c) as the sole
Scheme 2. Synthesis of 50-substituted-20-allyloxypropiophenones from 4-substituted phenols.

SYNTHESIS OF BENZOPYRANS
2971
Table 1. Reactions of ortho-allyloxy aromatic aldehydes=ketones with low-valent titanium reagent
(TiCl4=Zn=THF) at 0 ꢁC
Entry
Substrate
Time (h)
Product
Yield (%)a
3.0
56.0
24.0
1
2
3
3.0
60
3.0
62.0
4
12.0
56.0
5
12.0
45.0
aIsolated yield after column chromatography.
product in moderate yield without any side product (Table 1, entry 3). More strikingly,
1H NMR and 13C NMR data revealed the formation of only one of the two possible
diastereomers of 5c. However, we could not unequivocally establish the relative stereo-
chemistry at C-3 and C-4 of 5c from nuclear Overhauser effect (NOE) spectral data.
To see the generality and scope of the reaction, two more 50-substituted
20-allyloxy propiophenones (1d and 1e) were prepared from 4-substituted phenols
in three steps: (i) esterification of phenols to corresponding arylalkanoates, (ii)

2972
S. K. KAYAK, S. BHATT, AND K. ROY
Scheme 3. Fate of LVT-generated ketyl radical in ortho-allyloxy aromatic aldehydes=ketones. (a) R2COCl,
NEt3, dichloromethane, rt; (b) AlCl3, 130 ꢁC; and (c) K2CO3, allylbromide, acetone, reflux.
AlCl3-catalyzed Fries migration to 2-acyl-4-substituted phenols, and (iii) allylation
of phenols with K2CO3=allylbromide (Scheme 2), and then subjected to the LVT
reagent at low temperature. In both the cases, corresponding 4-chromanols 5d and
5e were obtained in moderate yields (Table 1, entries 4 and 5). The results are sum-
marized in Table 1. As in the case of 5c, only one of the two possible diastereomers
1
of both 5d and 5e were formed. Although IR, H NMR, and 13C NMR data of the
compounds 5c–e corroborated their structures, attempts to record their mass spectra
remained unsuccessful for unknown reasons.
CONCLUSION
In conclusion, hitherto unknown LVT reagent [Ti(0)]–mediated intramolecular
carbonyl-olefin radical cyclization of ortho-allyloxy propiophenones has been
developed. This has led to the synthesis of benzopyran derivatives, which constitute
an important class of biologically active oxygen heterocycles. More important, only
one of the two possible benzopyran diastereomers was formed during the radical
cyclization. The mechanistic investigation is under way.
EXPERIMENTAL
IR spectra were scanned with a Jasco Fourier transform (FT)–IR 4100 spectro-
1
photometer. The H and 13C NMR spectra were recorded with a Bruker AC 200

SYNTHESIS OF BENZOPYRANS
2973
spectrometer with tetramethylsilane (TMS) as internal standard. Microanalysis was
performed with a Carlo Erba elemental analyzer (model 1110). TiCl4 was purchased
from Aldrich Chemical Co. USA and used without further purification. THF was
freshly distilled over sodium-benzophenone. All reactions were carried out under
an argon atmosphere.
Typical Procedure for Intramolecular Carbonyl-Olefin Radical
Cyclization of o-Allyloxy Propiophenone (1c) to 4-Ethyl-3-methyl-
2H-benzopyran-4-ol (5c) with Low-Valent Titanium Reagent
TiCl4 (3.0 ml, 27.0 mmol) was added slowly to a well-stirred ice-cooled suspen-
sion of zinc dust (3.5 g, 54.0 mmol) in dry THF (80 ml), and then the mixture was
allowed to attain room temperature before refluxing for 3 h. The black mixture
was then brought to room temperature and cooled in an ice-water bath. A solution
of 1c (1.7 g, 9.0 mmol) in THF (5 ml) was added dropwise to the black suspension of
LVT reagent and stirred further at 0 ꢁC. The progress of the reaction was monitored
by thin-layer chromatography (TLC). After completion of the reaction, the mixture
was diluted with ethyl acetate, quenched with 10% aqueous K2CO3 solution (10 ml),
and stirred for 2 h. The mixture was passed through a bed of celite, and the filtrate
was washed with brine and dried (Na2SO4). Removal of solvent followed by column
chromatography (SiO2) yielded 5c (1.07 g, 62% yield).
Spectral Data of Some Selected Compounds
2-Allyloxybenzylalcohol (3a). Oil; IR (neat): n ¼ 3449, 2926, 2361, 1600,
1
1491, 1454, 1238, 1023, 912 cmꢀ1; H NMR (200 MHz, CDCl3): d 3.44 (br s, 1H,
OH), 4.51 (d, 2H, J ¼ 5.0 Hz), 4.71 (s, 2H), 5.35 (m, 2H), 6.04 (m, 1H), 6.83 (d,
1H, J ¼ 8.2 Hz), 6.96 (t, 1H, J ¼ 7.2 Hz), 7.24 (m, 1H), 7.35 (d, 1H, J ¼ 7.2 Hz);
13C NMR (50 MHz, CDCl3): d 60.5, 68.2, 110.9, 116.8, 120.4, 127.9, 128.0, 129.2,
132.8, 155.6.
3-Methyl-benzopyran-4-ol (5a). Oil; IR (neat): n ¼ 3399, 2968, 2878, 2357,
1
1610, 1584, 1488, 1042, 1019 cmꢀ1; H NMR (200 MHz, CDCl3): d 1.01 (dd, 3H,
J ¼ 7.0, 14.0 Hz), 2.02 (m, 1H), 2.89 (br s, 1H, OH), 3.89 (m, 1H), 4.21 (m, 1H),
4.56 (m, 1H), 6.87 (m, 2H), 7.24 (m, 2H); 13C NMR (50 MHz, CDCl3): d 11.5,
13.6, 32.5, 34.5, 66.3, 66.4, 67.5, 69.3, 116.3, 116.5, 120.2, 120.5, 123.7, 124.2,
129.1, 129.4, 130.1, 153.9. Anal. calcd. for C10H12O2: C, 73.15; H, 7.37. Found: C,
72.97; H, 7.18.
2,3-Dihyro-3-methylchromen-4-one (6)[8]. Oil; IR (neat): n ¼ 3060, 3031,
2981, 2940, 2921, 2879, 1647, 1618, 1182, 1090, 1014, 969 cmꢀ1 1H NMR
;
(200 MHz, CDCl3): d 1.17 (d, 3H, J ¼ 8.0 Hz), 2.81 (m, 1H), 4.09 (t, 1H, J ¼ 11.0 Hz),
4.45 (dd, 1H, J ¼ 5.0, 11.2 Hz), 6.94 (m, 2H), 7.41 (m, 1H), 7.85 (dd, 1H, J ¼ 2.0,
8.0 Hz); 13C NMR (50 MHz, CDCl3): d 10.5, 40.5, 72.0, 117.6, 120.4, 121.1, 127.1,
135.5, 135.7, 161.6, 194.6.
2,3-Bis-(2-(allyloxy)phenyl)butan-2,3-diol (4b). Oil; IR (neat): n ¼ 3497,
1
3079, 3009, 2985, 2941, 2878, 1599, 1579, 1489, 1228, 1120, 1056 cmꢀ1; H NMR

2974
S. K. KAYAK, S. BHATT, AND K. ROY
(200 MHz, CDCl3): d 1.64 (s, 6H), 4.20 (ddd, 4H, J ¼ 5.3, 12.5, 17.8 Hz), 5.28 (m,
4H), 5.55 (br s, 2H, OH), 5.87 (m, 2H), 6.76 (m, 4H), 6.99 (d, 2H, J ¼ 7.4 Hz),
7.12 (m, 2H); 13C NMR (50 MHz, CDCl3): d 24.6, 69.4, 82.3, 112.6, 118.0, 120.3,
128.0, 129.9, 132.1, 132.7, 156.9. Anal. calcd. for C22H26O4: C, 74.55; H, 7.39.
Found: C, 74.35; H, 7.21.
4-Ethyl-3-methyl-benzopyran-4-ol (5c). Oil; IR (neat): n ¼ 3456, 2969,
1
2938, 2880, 1608, 1581, 1044 cmꢀ1; H NMR (200 MHz, CDCl3): d 0.74 (t, 3H,
J ¼ 7.5 Hz), 1.00 (d, 3H, J ¼ 7.0 Hz), 1.75 (brs, 1H, OH), 2.09 (m, 3H), 3.94
(m, 1H), 4.08 (dd, 1H, J ¼ 3.7, 10.9 Hz), 6.81 (d, 1H, J ¼ 8.1 Hz), 6.92 (m, 1H),
7.17 (m, 1H), 7.41 (dd, 1H, J ¼ 1.5, 7.7 Hz); 13C NMR (50 MHz, CDCl3): d 8.6,
9.9, 31.1, 33.2, 67.8, 71.2, 116.9, 120.6, 126.1, 126.7, 128.9, 154.6. Anal. calcd. for
C12H16O2: C, 74.97; H, 8.39. Found: C, 74.80; H, 8.23.
4-Ethyl-3,6-dimethyl-benzopyran-4-ol (5d). Oil; IR (neat): n ¼ 3454, 2969,
1
2938, 2880, 1498, 1136, 1036 cmꢀ1; H NMR (200 MHz, CDCl3): d 0.75 (t, 3H,
J ¼ 7.5 Hz), 1.01 (d, 3H, 6.9 Hz), 1.57 (br s, 1H, OH), 1.99 (m, 3H), 2.28 (s, 3H),
3.99 (m, 2H), 6.71 (d, 1H, J ¼ 8.2 Hz), 6.98 (d, 1H, J ¼ 8.2 Hz), 7.21 (s, 1H); 13C
NMR (50 MHz, CDCl3): d 8.6, 9.9, 20.6, 31.0, 33.3, 67.7, 71.1, 116.6, 125.8, 126.8,
129.5, 152.4. Anal. calcd. for C13H18O2: C, 75.69; H, 8.80. Found: C, 75.51; H, 8.61.
6-Bromo-4-ethyl-3-methyl-benzopyran-4-ol (5e). Oil; IR (neat): n ¼ 3457,
2971, 2938, 2881, 1485, 1461, 1136, 1089 cmꢀ1; 1H NMR (200 MHz, CDCl3): d 0.71
(t, 3H, J ¼ 7.5 Hz), 0.93 (d, 3H, J ¼ 6.9 Hz), 1.90 (m, 2H), 2.11 (m, 1H), 2.3 (br s, 1H,
OH), 3.86 (m, 1H), 4.24 (dd, 1H, J ¼ 3.5, 10.9 Hz), 6.65 (d, 1H, J ¼ 8.7 Hz), 7.20 (dd,
1H, J ¼ 2.3, 8.7 Hz), 7.47 (d, 1H, J ¼ 2.16 Hz); 13C NMR (50 MHz, CDCl3): d 8.4,
9.9, 31.2, 33.0, 67.9, 71.1, 112.5, 118.8, 128.3, 129.5, 131.6, 153.6. Anal. calcd. for
C12H15BrO2: C, 53.15; H, 5.58. Found: C, 52.92; H, 5.41.
ACKNOWLEDGMENT
One of the authors (S. B.) is thankful to the Department of Atomic Energy,
government of India, for financial support in the form of a senior research
fellowship.
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