C O M M U N I C A T I O N S
test this hypothesis stable crystalline complexes of BF3 with methyl
cinnamate, benzylidene acetone, and dibenzylidene acetone were
prepared and subjected to X-ray single-crystal structure determina-
tion. The structures found for these complexes 5, 6, and 7, respec-
tively, involve coordination of BF3 to a lone pair on the carbonyl
group syn to an R,â-double bond, as proposed for complex 4. In
result was obtained with the cyclic R,â-enal 8 which follows the
expected pathway via a complex of type 3. Diethyl fumarate (9)
and 2-butenolide (10) both afforded the adducts shown in high yield
and ee, the absolute stereochemical course indicating reaction via
complexes of type 4, as expected. Remarkably good results were
obtained with the cyclic R,â-enones 11, 2-cyclopentenone, 2-cy-
clohexenone, and 2-cycloheptenone, to give the products predicted
from the complex 4 model. To the best of our knowledge, these
are the first examples of chiral Lewis-acid-catalyzed Diels-Alder
reactions with these fundamental cyclic R,â-enone structures.10
Finally, the quinone monoketals 12 and 13 provide Diels-Alder
adducts with good yields and enantioselectivities, thus demonstrat-
ing the effectiveness of catalysts 1 and 2 in promoting the hitherto
difficult, but important, quinone Diels-Alder subtype.11 The
absolute stereochemical course of these reactions of quinone
monoketals 12 and 13 can also be explained in terms of a favored
reaction channel via a complex of type 4.
In conclusion, the studies reported herein provide additional
evidence of the utility of the recently introduced chiral Lewis acids
1 and 2 in enantioselective synthesis. We believe that the
mechanistic models implied by complexes 3 (for 2-substituted R,â-
enals) and 4 (for R,â-enones and esters) have useful predictive
power. The application of 1 and 2 to other types of reactions will
be reported in due course.
addition the distances between the R-olefinic hydrogen and the
nearest fluorine range between 2.44 and 2.52 Å, as shown. These
distances are appreciably shorter than the sum of the van der Waals
radii for H and F, 2.67 Å (H ) 1.20 Å, F ) 1.47 Å), which points
to the likelihood of an attractive H-F interaction (coordinatively
induced hydrogen bond). A similar H-O interaction may help orga-
nize the pretransition-state assemblies for the reactions shown in
Table 1.4
Acknowledgment. We are grateful to Boehringer Ingelheim for
a fellowship to T.W.L. and Pfizer Inc. for a generous grant. Thanks
are also due to Dr. Richard Staples for the X-ray analyses.
Further confirmation of the unusually wide scope and promise
of the enantioselective Diels-Alder reactions catalyzed by 1 and
2 is provided by the data displayed in Table 2. An outstanding
Supporting Information Available: General procedure for the
enantioselective reactions and product characterization for the cases
reported in Tables 1 and 2 (PDF). X-ray crystallographic data for
complexes 5, 6, and 7 (CIF). This material is available free of charge
Table 2. Diels-Alder Reactions of Cyclopentadiene with Various
Dienophiles Catalyzed by Chiral Lewis Acid 1 or 2 in CH2Cl2
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a Isolated yield. b Endo-exo ratios determined by 1H NMR or GC
analysis or both. c Enantioselectivities determined by 1H NMR MTPA
analysis or GC analysis. d Side products corresponding to further Diels-
Alder reactions of desired product with cyclopentadiene obtained (ca. 43%).
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