3 with PPTS in MeOH gave the δ-hydroxy compounds 4
(78-92%) and 5 (80-93%), respectively, which were then
reacted under ring closure conditions. For initial evaluation
of the endo mode cyclization of allenyl derivatives 4 and 5,
the sulfoxide 4a, producing the formation of the furan
derivative, was chosen. Upon exposure to tBuOK in tBuOH9
at room temperature, the sulfoxide 4a unexpectedly under-
went a rapid ring closure (within 5 min) to provide furan
derivative 6a10 in a quantitative yield (Table 1, entry1).
The next phase of this program was to determine whether
the construction of medium-sized oxacycles would be
realized. Thus, sulfoxide 4d was exposed to the standard
basic conditions at room temperature, anticipating the
formation of the medium-sized oxacycle 6d. However,
nothing happened and the starting 4d was recovered com-
pletely intact. When the reaction temperature was raised to
60 °C, consecutive retro [2,3]-sigmatropic rearrangement and
desulfenylation occurred, leading to the exclusive production
of 2-octyne-1,8-diol (desilylated 1d) in 74% yield (entry 4).
These results are summarized in Table 1.
This ring closure might tentatively be rationalized in terms
of the endo mode Michael-type reaction initiated by the
attack of the terminal hydroxy anion at the electron-deficient
sp-hybridized carbon of the allenyl functionality, followed
by double bond migration. Therefore, conversion of the
sulfinyl group of 4 into the sulfonyl one would increase the
reactivity of the allenyl moiety as a formal Michael acceptor.
On the basis of the above hypothesis, we investigated the
ring closure of the sulfonyl derivative 5. Exposure of 5a-c
to the basic conditions described for 4 effected efficient ring
closure to give 7a-c10 in 78-90% yield (see Table 1, entries
5-7). The eight-membered ring formation that could not be
attained using 4 was next investigated. Upon treatment with
Table 1. Ring Closure of Allenes 4 and 5
t
tBuOK in BuOH at room temperature, 5d underwent ring
closure in a manner similar to that of the sulfonyl derivatives
5a-c, resulting in the exclusive formation of oxocane 7d10
in 79% yield (entry 8). To evaluate this procedure for the
oxocane skeleton formation, two additional allenyl sulfones,
8 and 9,11 were examined. Sulfone 8 was reacted under
standard basic conditions at room temperature, giving rise
to the easy endo mode cyclization as expected and produced
oxocane 10 with the (E)-ethylidene moiety12 in 74% yield
as the sole product (Scheme 2).13 A similar result was
Scheme 2
observed when 9 was treated with base, affording 1113,14 in
78% yield.
(5) Denmark reported intramolecular reaction of allenyl sulfones with
alkoxides: Denmark, S, E.; Harmata, M. A.; White, K. S. J. Org. Chem.
1987, 52, 4031.
a 2-Octyne-1,8-diol was obtained.
(6) Parsons noted that exo mode ring closure of allenyl sulfoxide
derivatives possessing a carbon side chain at the C-3 position4 under basic
conditions resulted in the formation of the five- and six-membered oxacycles
(benzofuran and pyran derivatives, respectively): (a) Pairaudeau, G.;
Parsons, P. J.; Underwood, J. M. J. Chem. Soc., Chem. Commun. 1987,
1718. (b) Gray, M.; Parsons, P. J.; Neary, A. P. Synlett 1992, 597.
(7) Dai described an exo mode cyclization of the allenyl sulfone
derivatives leading to compounds having a furan framework: Dai, W.-M.;
Lee, Y. H. Tetrahedron 1998, 54, 12497.
Similar treatment of the carbon chain elongated analogue
4b produced the pyran derivative 6b10 in 79% yield (entry
2). Oxepane formation was also realized when 4c was treated
with the base to furnish 6c9 in 81% yield (entry 3).
(8) Horner, L.; Binder, V. Ann. Chem. 1972, 37, 757.
3386
Org. Lett., Vol. 3, No. 21, 2001
Mukai
