from an allenyl to an allyl system, the direction of the
reactions was completely reversed and only the 5-membered
acetal could be detected in the latter case (Table 2). As with
the allenyl alcohols, modifications of the aromatic ring
system did not alter the sense of the Diels-Alder reaction
(Table 2).
Table 2. Allyl Alcohol Diels-Alder Reaction
With the benefit of retrospection, an accounting of the
dramatic differences in the results in Tables 1 and 2 can be
offered. It seems likely that, from a perspective of steric strain
in the product (and of the transition state leading to it), the
formation of the 5-membered acetal would be preferred over
the 6-membered product, with its additional torquing of the
acetal carbon. However, in the allene cases there is a strong
preference for initial partial bond formation between the
unique sp carbon of the dienophile with C4 of the dienone.
In this way, radicaloid character in the transition state is
supported by the tosylate at C3 and the bromine at C1, as
well as its connected ketone. Given the orienting affinity
between C4 and the sp carbon, the formation of Table 1
products is dictated.
In conclusion, hypervalent iodine-mediated oxidative
dearomatization was used to form high energy [4 + 2]
precursor systems that readily undergo intramolecular Diels-
Alder reaction (Figure 2). Highly substituted, unactivated
responded to a possible pre-tashironin intermediate) (Scheme
3). The 6-membered acetal could not, however, be exploited
toward reaching tashironin.
Thus, we probed the scope of this undesired pathway by
investigating the consequences which perturbations of the
aromatic ring substituents might have on the regiochemical
outcome of the IMDA reaction with allenyl dienophiles.
Substrates were prepared in which the tosyl ester was
replaced with a methyl ester group (Table 1, entries 2 and
5) and the methyl ether was replaced with a benzyl ether
(Table 1, entry 4). In practice, reaction of each of these
modified substrates with allenol 12 provided the 6-membered
acetal IMDA adduct. When allenyl alcohol 13 was used as
the dienophilic coupling partner with phenol 11 (Table 1,
entry 1), a crystalline product was obtained. X-ray analysis
of the compound revealed unambiguously the 6-membered
acetal product.7
Figure 2. IMDA reactions of allenic and olefinic intermediates.
dienes are used in the reaction to produce complex Diels-
Alder adducts. Additionally, the regioselectivity of cycliza-
tion can be controlled by judicious choice of dienophile.
Applications of these findings are being pursued.
Acknowledgment. Support for this research was provided
by the National Institutes of Health (HL25848).
We sought to explore the outcome of reactions with
simpler dienophiles. Interestingly, by changing the dienophile
Supporting Information Available: Experimental pro-
cedures and physical data for compounds 6-14 and Diels-
Alder cycloadducts. This material is available free of charge
(5) Liao, C. C.; Peddinti, R. K. Acc. Chem. Res. 2002, 35, 856 and
references cited therein.
(6) Isaac, M. B.; Chan, T-H. J. Chem. Soc., Chem. Commun. 1995, 1003.
(7) Crystallographic data (excluding structural data) for the cycloaddition
product in Table 1, entry 1, have been deposited with the Cambridge
Crystallographic Data Centre (CCDC as Deposition No. CCDC 61734).
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