4
Tetrahedron
Ph
Ph
O
References
Ca(OTf)2/Bu4NPF6
2/10 mol%, toluene
OH
+
I
+
3p
+
I2
1.
2.
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Ph
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2a
4a
1d
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Ph
COOEt
Pd(OAc)2, DABCO
CH3CN, rt, 12 h
Pd(OAc)2, Bu4NBr,
K2CO3, DMF, 80 oC, 22 h
Ph
Ph
Ph
COOEt
Ph
Ph
Ph
Ph
O
O
6, 61%
5, 66%
Scheme 5. Synthesis of Iodo-nephthopyrans and their cross-coupling reactions
We proposed the possible reaction mechanism for the synthesis
of 3 from 1 and 2 in the Scheme 6. Initially the oxophilic calcium
promotes the formation of vinyl-propargyl ether I1 from 2 and 1
3.
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(hard-hard), which then undergoes
a thermodynamic-[3,3]
rearrangement5e to furnish the allene I2. Allene I2 can readily
form the enol I3 (path-a), which then undergo a regioselective, 6-
endo cyclization to yield the desired product 3. Alternatively
(path-b), allene I2 can rearrange to a diene and then cyclize to
give the product 3.
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In summary, we developed a regioselective annulation reaction
between propargyl alcohols and ambident enols using calcium
triflate as the sustainable lewis acid catalyst, under solvent free
conditions. Through this approach, we introduced the carboxylate
derivatives of benzochromenes as new chemical entities.
Ozonolysis and X-ray data supported the structure of the
products. We extended this method for the synthesis of iodo-
pyrans, further the synthetic utlity is demonstrated through cross
coupling reactions.
6.
7.
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1
All compounds were fully characterized by H, 13C NMR, IR,
Mass, melting point and X ray data as required.10
Acknowledgements
8.
9.
Sonogashira K, Tohda Y, Hagihara N. Tetrahedron Lett.
1975;16:4467
SY thankful to UPE, AP and RD thank School of Chemistry, University of
Hyderabad for the facilities and Central University of Rajasthan for the
fellowship.
10.
Refer to the supporting information for complete details of
experimental procedures, spectral data and copies of the spectra.
4