2
112
K. C. Majumdar et al. / Tetrahedron Letters 43 (2002) 2111–2113
Hitherto unreported 1-alkyl-4-mercaptoquinolin-2(1H)-
ones (1) in turn were synthesised by the reaction of
Thio-Claisen rearrangement of aryl propargyl sulphide
is known to give a mixture of products, viz. 2-
11
1
-alkyl-4-chloroquinolin-2(1H)-ones
with
sodium
methylthionaphthene derivatives and 2H-thiochrom-3-
12
hydrosulfide in ethanol at 0–10°C for 6 h.
ene derivatives. [1,3] Radical shifts also occur in some
cases when thio-Claisen rearrangements are attempted.
It is interesting to note that in the present instance only
Substrate 3a was refluxed in chlorobenzene (bp 132°C)
for 5 h to give a white crystalline solid, 4a, mp 120°C in
2
H-thiopyrano derivatives are obtained so making this
9
0% yield. The products 3a and 4a were characterised
methodology a general regioselective synthesis.
by their elemental analyses and spectroscopic data. The
1
H NMR spectrum of 3a showed signals at l 3.8 (d,
2
H, J=2.5 Hz), l 3.7 (s, 3H) and l 2.3 (t, 1H, J=2.5
Acknowledgements
1
Hz). The H NMR spectrum of 4a showed a two
proton double doublet at l 3.34 (J=6 Hz, 1.5 Hz); a
one proton double triplet at l 6.20 (J=10 Hz, J=6
Hz); and a one proton multiplet at l 6.85 indicating the
formation of the six-membered thiopyran ring. Encour-
aged by this result, other substrates 3b–f were similarly
subjected to thermal rearrangement to give the prod-
ucts 4b–f in 82–90% yields (Scheme 2).
We thank the CSIR (New Delhi) for financial
assistance.
References
1
2
. Jurd, L.; Benson, M. J. Chem. Soc., Chem. Commun.
1983, 92.
. (a) Brown, R. F. C.; Hobbs, J. J.; Hughes, G. K.; Ritchie,
E. Aust. J. Chem. 1954, 7, 348; (b) Brown, R. F. C.;
Hughes, G. K.; Ritchie, E. Chem. Ind. (London) 1955,
The formation of the products 4a–f may be
1
0
rationalised by an initial [3,3] sigmatropic shift of the
propynyl group to form the intermediate allene 5, fol-
lowed by enolisation to ene-thiol 6, a 1,5-hydrogen shift
to 7 and electrocyclic ring closure to give the cyclic
products 4a–f (Scheme 3).
1385; (c) Danieli, L. N.; Weitman, R.; Glotter, E. Tetra-
hedron 1968, 24, 3011; (d) Dreyer, D. L.; Lee, A. Phyto-
chemistry 1972, 11, 763; (e) Taylor, D. R.; Warner, J. M.
Phytochemistry 1973, 12, 1359; (f) Reisch, J.; Korosi, J.;
Szendrei, K.; Novak, I.; Minker, E. Phytochemistry 1975,
14, 1678.
3
. (a) Grundon, M. F.; Green, R. J.; Caston, J. C. J. Chem.
Res. (M) 1985, 5, 1877; (b) Kappe, Th.; Fritz, P. F.;
Ziegler, E. Chem. Ber. 1973, 106, 1927; (c) Rao, V. S.;
Darbarwar, M. Synth. Commun. 1989, 19, 2713; (d)
Reish, J.; Bethe, A. Arch. Pharm. (Weinheim) 1987, 320,
737; Chem. Abstr. 1988, 108, 55862; (e) Reisch, J. Arch.
Scheme 2.
Pharm. (Weinheim) 1967, 300, 533.
Scheme 3.