rearrangement. It is remarkable to note that all the substrates
3a-d studied in this instance regioselectively afforded
exclusively products 4a-d.
Scheme 1
As products 4a-d possess the aryl allyl ether moiety, these
were subjected to heating in refluxing 1,2-dichlorobenzene
in the presence of N,N-diethylaniline for 12-14 h to give
phenolic products 8a-d. These were characterized from their
elemental analyses and spectral data.14 Here again the
isolation of phenolic product is quite unusual. In all other
previous instances either the formation of cyclic products
or the rearranged endo cyclic phenolic products were
reported.9,15 The formation of products 8a-d from 4a-d is
easily explained by a [3,3] sigmatropic rearrangement
followed by enolization (Scheme 3).
Scheme 3
potential sites for [3,3] sigmatropic rearrangement: an aryl
propargyl ether moiety and a vinyl propargyl sulfide moiety.
All the substrates underwent [3,3] sigmatropic rearrangement
at the vinyl propargyl sulfide moiety to give thiopyrano-
[3,2-c][1]benzopyran-5(2H)-ones. The formation of products
4a-d from substrates 3a-d may be easily explained by the
[3,3] sigmatropic rearrangement of 3a-d and rapid enoliza-
tion to form the intermediate allenylene-thiols 6a-d followed
by [1,5] hydrogen shift and 6π-electrocyclic ring closure to
give the products 4a-d (Scheme 2).
Scheme 2
Our target was to synthesize polyheterocyclic compounds.
We had earlier used pyridine hydrotribromide16 and hexa-
methylenetetramine hydrotribromide17 for regioseletive cy-
clization of o-cyclohex-2-enyl phenols. We therefore treated
products 8a-d with 1 equiv of pyridine hydrotribromide in
chloroform at 0-5 °C for 0.5 h to afford [6,6]pyranothio-
pyrans18 10a-d in almost quantitative yield. The formation
of products 10a-d from 8a-d is easily explained by the
formation of a cyclic bromonium ion 11a-d followed by a
“6-endo” cyclization (Scheme 4).
(13) (a) Majumdar, K. C.; Ghosh, M.; Jana, M. Synthesis 2002, 669. (b)
Mortensen, J. Z.; Hedegaard, B.; Lawesson, S. O. Tetrahedron 1971, 27,
3831.
Substrates 3a-d on thermal rearrangement by heating in
chlorobenzene (132 °C) could have yielded other types of
products, e.g. 3-aryloxymethyl-2-methylthieno[3,2-c]cou-
marin12 or 4′-aryloxybut-2′-ynyl-4-mercaptocoumarin13 (by
1,3-radical shift) as a consequence of the usual course of
(14) Compound 8a: mp 198 °C.; yield 75%; UV (EtOH) λmax 220, 274,
334 nm; IR (KBr) νmax 3390, 2910, 1670, 1600, 1210 cm-1 1H NMR
;
(CDCl3, 300 MHz) δ 3.33 (dd, J ) 3.3, 12.6 Hz, 1H, SCH2), 3.62 (dd, J )
7.1, 12.6 Hz, 1H, SCH2), 4.40 (dd, J ) 3.3, 7.1 Hz, 1H), 5.38 (s, 1H, d
CH2), 5.79 (s, 1H, dCH2), 6.75-6.86 (m , 2H, ArH) 7.28-7.33 (m, 2H,
ArH), 7.50-7.55 (m, 1H, ArH), 7.72-7.75 (m, 1H, ArH); MS m/z 394,
392, 390 (M+). Anal. Calcd for C19H12Cl2O3S: C, 58.46; H, 3.07. Found:
C, 58.32; H, 3.19.
(11) Compound 4a: mp 186 °C; yield 75%; UV (EtOH) λmax 220, 360
nm; IR (KBr) νmax 1690, 1580, 1240 cm-1; 1H NMR (300MHz) δ 3.47 (d,
2H, J ) 6Hz), 5.18 (d, 2H, J ) 1.0 Hz), 6.26 (tt, 1H, J ) 1, 6 Hz), 6.91-
7.85 (m, 7H); m/z 394, 392, 390 (M+). Anal. Calcd for C19H12Cl2O3S: C,
58.46; H, 3.07. Found: C, 58.31; H, 3.17.
(15) Majumdar, K. C.; Balasubramanian, K. K. ; Thyagarajan, B. S. J.
Heterocycl. Chem. 1973, 10, 159.
(16) (a) Majumdar, K. C.; Kundu A. K. Indian J. Chem. 1993, 32B,
605. (b) Majumdar, K. C.; Kundu A. K. Can. J. Chem. 1995, 73, 1727.
(17) Majumdar, K. C.; Kundu A. K.; Chatterjee, P. Synth. Commun. 1996,
26, 893.
(12) Kwart, H.; George, T. J. J. Chem. Soc., Chem. Commun. 1970, 433.
2630
Org. Lett., Vol. 4, No. 16, 2002