A R T I C L E S
Sculimbrene et al.
Scheme 1
library were a number of catalysts that selectively phosphory-
lated the 1-position to give 3(1-P), as well as some that were
selective for the enantiotopic 3-position, albeit in lower selectiv-
ity. The members of the initial 39-member library were chosen
randomly, largely on the basis of sequences we had prepared
that were soluble in organic solvents. In addition, they were
intended to be highly diverse, since we were searching for leads
in the absence of any type of “lead information” on stereose-
lectivity (Vide infra). Because such a high percentage of the
library exhibited some degree of enantioselectivity, we suspected
that expanding the screen to include other unrelated sequences
might afford structure-selectivity relationships that could suggest
sequences that were particularly suited to phosphorylation of
the 3-position. The results of the expanded screen are shown in
Figure 1B. As in our previous studies, unpurified peptides were
screened at room temperature.
Results and Discussion
Peptides for both the initial and expanded screens (peptides
1-136) were selected on the basis of sequences that we had
previously prepared in our laboratory. In addition to sequences
that were biased to form â-turns and â-hairpins in organic
solvents,14,15 we wished to include others that were highly
diverse, since we were not sure at the outset what sequences
would be appropriate. To achieve diverse sequences, we turned
to a randomization algorithm that would afford sequences that
were in principle unrelated.16 We chose pentapeptide 5 as the
core structure and then assigned each of the 16 amino acid
monomers a letter. The algorithm then delivered 80 random sets
Discovery of Enantiodivergent Peptide Catalysts. While
the “ideal” synthesis of D-I-3P (or D-I-3P) could rely on a one-
step synthesis of the target monophosphate from free myo-
inositol, we chose triol 1 as a test substrate, since it (a) exhibits
excellent solubility in the organic solvents where the peptide-
based catalysts we have studied function best9 and (b) triol 1
contains three, rather than six, potential sites of derivatization.
In previously communicated results, we reported that pentapep-
tide 2 is an effective catalyst for the desymmetrization of 2,4,6-
tribenzyl-myo-inositol (1) through enantioselective phosphory-
lation (Scheme 1).10 When 2 mol % catalyst, diphenylchlorophos-
phate (DPCP) as the phosphorylating reagent, and triethylamine
as an HCl scavenger are employed, a highly selective reaction
is observed at 0 °C (>98% ee for 3(1-P), 65% isolated yield).
Using this catalyst, we completed a concise total synthesis of
D-myo-inositol-1-phosphate. Because the catalysis proceeds
through a putative phosphoryl imidazolium ion (4),11 we referred
to the peptide as a kinase mimic, in analogy to the histidine-
dependent class of kinases that participates in cell-signaling
pathways.12,13
of three-letter combinations. These were inserted into structure
5, and the members were synthesized for screening.
The results of the expanded screen were striking in that the
distribution of catalysts that were selective for the enantiotopic
1- and 3-positions was nearly statistical. For example, in the
expanded screen, two new sequences (6 and 7) afforded 3(3-P)
in >55% ee; likewise, two new sequences were selective for
the enantiotopic 3(1-P) in >55% ee. It is important to note that,
in desymmetrization reactions such as these, the extent to which
the selective formation of the product occurs is related to the
overall conversion of the reaction.18,19 Mindful of this issue,
we are cautious not to overinterpret small differences in the ee
of the isolated 3(3-P). The conditions of the screen were intended
to produce the monophosphate at about 70% conversion.
Nevertheless, inspection of Figure 1A and B reveals that the
Our approach to the discovery of enantiodivergent phospho-
rylation catalysts employed screening a combination of random
and focused libraries. Our initial screen (Figure 1A) of small-
peptide catalysts for asymmetric phosphorylation of substrate
1 was based on 39 peptides (tetra- through octapeptides) that
contained the nucleophilic residue L-π(Me)-histidine. Within the
(6) For other representative metal-free peptide and peptide-like enantioselective
catalysts, see: (a) Sigman, M. S.; Jacobsen, E. N. J. Am. Chem. Soc. 1998,
120, 4901. (b) Iyer, M. S.; Gigstad, K. M.; Namdev, N. D.; Lipton, M. J.
Am. Chem. Soc. 1996, 118, 4910. (c) For a review, see: Jarvo, E. R.; Miller,
S. J. Tetrahedron 2002, 58, 2481.
(7) (a) Agranoff, B. W.; Fisher, S. K. In Inositol Phosphates and DeriVatiVes:
Synthesis, Biochemistry and Therapeutic Potential; Reitz, A. B., Ed.; ACS
Symposium Series 463; American Chemical Society: Washington, DC,
1991; p 20. (b) Billington, D. C. The Inositol Phosphates: Chemical
Synthesis and Biological Significance; VCH: New York, 1993.
(8) We have observed enantiodivergent peptide catalysis for acyl transfer.
See: Copeland, G. T.; Jarvo, E. R.; Miller, S. J. J. Org. Chem. 1998, 63,
6784.
(14) (a) Gellman, S. H. Curr. Opin. Chem. Biol. 1998, 2, 717. (b) Venkatraman,
J.; Shankaramma, S. C.; Balaram, P. Chem. ReV. 2001, 101, 3131.
(15) â-Turns and â-hairpins have been found to exhibit enantioselectivity in a
number of other catalytic processes. For example, see: (a) Reference 5.
(b) Gilbertson, S. R.; Collibee, S. E.; Agarkov, A. J. Am. Chem. Soc. 2000,
122, 6522.
(16) (a) Urbaniak, G. C.; Plous, S. Research Randomizer, version 2.1 [Internet-
based computer program]; Middletown, CT. Retrieved most recently June
prepared to screen asymmetric acylations. See: Jarvo, E. R.; Evans, C.
A.; Copeland, G. T.; Miller, S. J. J. Org. Chem. 2001, 66, 5522.
(17) A comprehensive list of the sequences and the screening results are provided
in the Supporting Information.
(18) The extent to which the bis(phosphorylated) product is formed serves as a
correction mechanism for enantioselectivity. See: Schreiber, S. L.;
Schreiber, T. S.; Smith, D. B. J. Am. Chem. Soc. 1987, 109, 1525.
(19) For reviews of enantioselective desymmetrization, see: (a) Poss, C. S.;
Schreiber, S. L. Acc. Chem. Res. 1994, 27, 9. (b) Willis, M. C. J. Chem.
Soc., Perkin Trans. 1 1999, 1765.
(9) There are often advantages to employing enzymes in organic solvents also.
See: Klibanov, A. M. Nature 2001, 409, 241-246.
(10) Sculimbrene, B. R.; Miller, S. J. J. Am. Chem. Soc. 2001, 123, 10125.
(11) For the spectroscopic observation of the NMI-DPCP adduct, see: Mal’tseva,
T. V.; Ivanova, E. M.; Korobeinicheva, I. K. IzV. Sib. Otd. Akad. Nauk.
SSSR, Ser. Khim. Nauk. 1985, 112-116.
(12) Pirrung, M. C. Chem. Biol. 1999, 6, R167.
(13) For several examples of His-dependent kinases, see: (a) Bilwes, A. M.;
Alex, L. A.; Crane, B. R.; Simon, M. I. Cell 1999, 96, 131. (b) Mizuguchi,
H.; Cook, P. F.; Tai, C. H.; Hasemann, C. A.; Uyeda, K. J. Biol. Chem.
1999, 274, 2166. (c) Fraser, M. E.; James, M. N.; Bridger, W. A.; Wolodko,
W. T. J. Mol. Biol. 1999, 285, 1633.
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