9970 J. Am. Chem. Soc., Vol. 122, No. 41, 2000
Nicolaou et al.
Scheme 2. Standard Method for Elaboration of Benzopyran
Olefins
of the primary library and, at the same time, possibly enrich its
physical and pharmacological properties.
Development of Parallel Solution-Phase Methods. While
conceptually attractive, the implementation of such a proposal
was contingent upon several key factors, including the follow-
ing: (a) definition of a suitable method for the simultaneous
epoxidation of diverse substrates in microtiter plates with no
byproducts; (b) definition of suitable methods for opening of
these epoxides with various nucleophiles (i.e. alcohols, amines,
thiols, etc) wherein any excess or unreacted reagent could be
readily scavenged; (c) definition of suitable methods for
electrophilic derivatization of the products resulting from the
epoxide opening such that any excess or unreacted reagent could
be easily scavenged; and (d) development of suitable experi-
mental techniques for transfer of starting materials and reagents,
evaporation of solvents, sequestering of excess reagents, and
characterization of final compounds. These challenges and their
practical solutions will be discussed consecutively, culminating
in the application of the developed techniques to the derivati-
zation of two representative benzopyran libraries generated by
the previously described solid-phase methods. Through the latter
two studies, we will demonstrate the continuity (solid phase f
solution phase) and generality of the described chemistry.
The first task required finding an efficient method to
epoxidize the benzopyran olefins found in the original library.
A critical aspect of this step required that the reagent be
sufficiently general so that substrates with diverse electronic
and steric environments could be epoxidized under similar
reaction conditions, since one could imagine that a given
microtiter plate would contain a heterogeneous mixture of
compounds in its wells. Moreover, since the epoxidation reagent
and its byproducts had to be easily removed from the epoxides
after the reaction, it was apparent that either a solid supported
epoxidizing reagent or a volatile solution phase oxidant would
be the reagent of choice.
A search of the literature revealed multiple examples where
organometallic epoxidation catalysts (typically chiral salen
manganese complexes) have been immobilized on solid supports
and used for the epoxidation of various olefinic substrates.19
Very recently, Song et al. described such a system containing
a polymer-bound (pyrrolidine salen)manganese(III) complex
which, in the presence of a co-oxidant, could epoxidize
benzopyran ring systems in high yield and good enantiopurities.19a
A slight limitation of this and other related methods, however,
is that the catalytic complex typically cannot be recycled and
that one must also employ a stoichiometric co-oxidant (i.e.
m-CPBA, NMO, NaOCl, or PPNO) in the reaction mixture.19
Thus, while the potential of such polymer-bound reagents in
the current study was significant, we decided (for demonstration
purposes) to pursue a nonchiral solution-phase epoxidation
method at least in the initial phases. This decision simplified
the chemistry. Furthermore, once the complete protocol was
Houghten in 1994 when he described a strategy whereby a
primary peptide library was constructed and then used, in
essence, as the starting material for further chemical transforma-
tions, resulting in a second combinatorial library with enhanced
properties.5 Houghten’s work focused on peptide-based libraries
which were constructed and subsequently peralkylated, exhaus-
tively reduced, and/or transformed into various heterocycles.5
Given the success of these efforts, we anticipated that a similar
strategy might be employed with nonoligomeric small organic
molecules such as the current benzopyrans which represent a
desirable class of potential ligands and drug candidates. It
seemed intuitive that enhancing the size (and presumably the
diversity) of a library through the introduction of additional sites
of diversity is a superior strategy as compared to simply creating
an equally large library by using a greater number of building
blocks to vary fewer overall sites of diversity.17 Hence, we
deliberated on potential methods to effect such transformations
on these small molecule libraries. It was obvious that since the
pyran olefin was generated during cleavage from the solid
support, these changes could most likely not be introduced via
our solid-phase methodology; consequently, we required a
solution-phase method whereupon one could use copies of the
split-and-pool library as starting points for new libraries.
A brief search of the literature revealed that one of the most
established routes for derivatizing the olefinic site of benzopyran
rings was through epoxidation as outlined in Scheme 2.18 As
shown, pyran 4 is converted to the corresponding epoxide 17
which is reacted with nucleophiles giving rise to alcohol 18.
This secondary alcohol can then be reacted with electrophiles
to provide derivatized benzopyran 19. It was envisaged that this
same technique might be applied in parallel to libraries as
outlined in Figure 3, whereby the compounds of the original
pyran library, cleaved from solid support directly into 96-well
microtiter plates, could be diluted and partitioned into multiple
plates, thereby creating copies of the primary library. To each
of the reaction wells would then be added a suitable epoxidizing
agent to effect, in parallel, the transformation of each benzopyran
to its corresponding epoxide. These epoxides could then be
opened with various nucleophiles (one nucleophile for each copy
of the library being used) and the resulting alcohols could also
be further derivatized with electrophilic reagents if necessary.
The overall effect of this strategy would be to multiply the size
(19) (a) Song, C. E.; Roh, E. J.; Yu, B. M.; Hi, D. Y.; Kim, S. C.; Lee,
K.-J. Chem. Commun. 2000, 615-616. (b) Kim, G.-J.; Shin, J.-H.
Tetrahedron Lett. 1999, 40, 6827-6830. (c) Angelino, M. D.; Laibinis, P.
E. Macromolecules 1998, 31, 7581-7587. Pozzi, G.; Cinato, F.; Montanari,
F.; Quici, S. Chem. Commun. 1998, 877-878. (d) Janssen, K. B. M.;
Laquiere, I.; Dehaen, W.; Parton, R. F.; Vankelecom, I. F. J.; Jacobs, P. A.
Tetrahedron: Asymmetry 1997, 8, 3481-3487. (e) Vankelecom, I. F. J.;
Tas, D.; Parton, R. F.; de Vyver, V. V.; Jacobs, P. A. Angew. Chem., Int.
Ed. Engl. 1996, 35, 1346-1348. (f) Sabater, M. J.; Corma, A.; Domenech,
A.; Forne´s, V.; Garc´ıa, H. Chem. Commun. 1997, 1285-1286. (g) Minutolo,
F.; Pini, D.; Salvadori, P. Tetrahedron Lett. 1996, 37, 3375-3378. (h)
Minutolo, F.; Pini, D.; Petri, A.; Salvadori, P. Tetrahedron: Asymmetry
1996, 7, 2293-2302. (i) De, B. B.; Lohray, B. B.; Sivaram, S.; Dhal, P. K.
Tetrahedron: Asymmetry 1995, 6, 2105-2108. (j) De, B. B.; Lohray, B.
B.; Dhal, P. K. Tetrahedron Lett. 1993, 34, 2371-2374.
(17) For an insightful review of library diversity, see: Kauvar, L. M.;
Laborde, E. Curr. Opin. Drug DiscoVery DeV. 1998, 1, 66-70 and
references therein.
(18) For a representative example, see: Rovnyak, G. C.; Ahmed, S. Z.;
Ding, C. Z.; Dzwonczyk, S.; Ferra, F. N.; Humphyreys, W. G.; Grover, G.
J.; Santafianos, D.; Aywal, K. S.; Baird, A. J.; McLaughlin, L. G.;
Normandin, D. E.; Sleph, P. G.; Traeger, S. C. J. Med. Chem. 1997, 40,
24-34 and references therein.