A Concise β-Lactam Route to Short Peptide Segments Containing β,β-Disubstituted β-Amino Acids

Article abstract of DOI:10.1021/jo9712862

An efficient epimerization-free route toward β,β-disubstituted β-amino acid-containing peptides is described. The methodology involves the use of 4,4-disubstituted-N-Boc β-lactams as acylating agents, which upon coupling with amino acid esters, promoted by potassium cyanide, give rise to dipeptides with no appreciable racemization. By this procedure short peptide segments containing a four-, five-, or six-membered ring at the β-position of the β-amino acid residue have been prepared. The method has also proven to be valuable for the preparation of tripeptides. In addition the sterically hindered amino terminus of the β-amino acid can undergo peptide couplings under standard conditions.

Full text of DOI:10.1021/jo9712862

J . Org. Chem. 1998, 63, 2469-2474
2469
A Con cise â-La cta m Rou te to Sh or t P ep tid e Segm en ts Con ta in in g
â,â-Disu bstitu ted â-Am in o Acid s
Claudio Palomo,* Mikel Oiarbide, and Simona Bindi
Departamento de Quı´mica Orga´nica, Facultad de Qu´ımica, Universidad del Paı´s Vasco, Apdo 1072,
20080 San Sebastia´n, Spain
Received J uly 14, 1997
An efficient epimerization-free route toward â,â-disubstituted â-amino acid-containing peptides is
described. The methodology involves the use of 4,4-disubstituted-N-Boc â-lactams as acylating
agents, which upon coupling with amino acid esters, promoted by potassium cyanide, give rise to
dipeptides with no appreciable racemization. By this procedure short peptide segments containing
a four-, five-, or six-membered ring at the â-position of the â-amino acid residue have been prepared.
The method has also proven to be valuable for the preparation of tripeptides. In addition the
sterically hindered amino terminus of the â-amino acid can undergo peptide couplings under
standard conditions.
The concept of structural modification in peptide
fragments to confer them specific properties is of current
interest in the study and design of new bioactive targets.1
A well-known example is the incorporation of R-ami-
noisobutyric acid (Aib) into peptides and/or proteins
which leads to important conformational restrictions,
often providing an increase in the helix content2 or the
formation of specific â-turns.3 The reduced flexibility of
the peptide backbone usually facilitates the search for
the active conformation recognized by a physiological
receptor.4 While the majority of the investigations in this
area have dealt with the synthesis and use of R-branched
R-amino acids, relatively little work has been conducted
with â,â-disubstituted â-amino acids.5 In general, the
limited metabolism of â-amino acids suggests that â-ami-
no acid analogues of R-amino acids will be metabolically
stable in vivo. The incorporation of â-amino acids into
peptides of pharmacological interest has often been found
advantageous in terms of biological activity and/or meta-
bolic stability.6 In addition, recent studies on â-peptide
oligomers have revealed new opportunities for the de-
velopment of specific helical conformations or â-sheet-
type structures.7 Within this context we have been
interested in the study of amino acids incorporating a
cycloalkane structure as a model for conformationally
restricted â,â-branched â-amino acids.
We investigated the incorporation of 1-aminocyclobu-
tylacetic acid and the higher cycloalkyl homologues into
small peptide segments.8 Although there exist many
methods for the synthesis of â-amino acids, only few of
them appear to be appropriate for the generation of a
quaternary center at the â-position.9 Herein we describe
the preparation of the desired â,â-disubstituted â-amino
acid concomitant with its incorporation into a peptide.10
Independent studies from this laboratory11 and one
other12 suggest that this goal could be achieved by using
â-lactams as acylating agents. However, earlier work in
this area13 indicated that C4-disubstituted N-sulfonyl
â-lactams, readily obtained by the sulfonyl isocyanate-
alkene cycloaddition approach,14 proved to be of very little
value in â-amino acid peptide synthesis. Recently, Bh-
upathy and co-workers15 have proposed a solution to this
(6) For some examples documenting these aspects, see: (a) Griffith,
O. W. Annu. Rev. Biochem. 1986, 55, 855. (b) Iizuka, K.; Kamijo, T.;
Harada, H.; Akahane, K.; Kubota, T.; Umeyawa, H.; Kiso, Y. J . Chem.
Soc.; Chem. Commun. 1989, 1678. (c) Rodr´ıguez, M.; Fulcrand, P.;
Laur, J .; Aumelas, A.; Bali, J . P.; Mart´ınez, J . J . Med. Chem. 1989,
32, 522. (d) Stachowiak, K.; Khosla, M. C.; Plucinska, K.; Khairallah,
P. A.; Bumpus, F. M. J . Med. Chem. 1979, 22, 1128. (e) Mierke, D. F.;
No¨ssner, G.; Schiller, P. W.; Goodman, M. Int. J . Pept. Protein Res.
1991, 35, 35. (f) Yamazaki, T.; Probstl, A.; Schiller, P. W.; Goodman,
M. Int. J . Pept. Protein Res. 1991, 37, 364. (g) Huang, Z.; Probstl, A.;
Spencer, J . R.; Yamazaki, T.; Goodman, M. Int. J . Pept. Protein Res.
1993, 42, 352.
(7) For detailed information on this topic, see: (a) Ferna´ndez-Santin,
J . M.; Mun˜oz-Guerra, S.; Rodr´ıguez-Gala´n, A.; Aymami, J .; Lloveras,
J .; Subirana, J . A.; Giralt, E.; Ptak, M. Macromolecules 1987, 20, 62.
(b) Seebach, D.; Overhand, M.; Ku¨hnle, F. N. M.; Martinoni, B.; Oberer,
L.; Hommel, U.; Widmer, H. Helv. Chim. Acta 1996, 79, 913. (c)
Appella, D. H.; Christianson, L. A.; Karle, I. L.; Powell, D. R.; Gellman,
S. H. J . Am. Chem. Soc. 1996, 118, 13071. (d) Hintermann, T.; Seebach,
D. Synlett 1997, 437. (e) Daura, X.; van Gunsteren, W. F.; Rigo, D.;
J ann, B.; Seebach, D. Chem. Eur. J . 1997, 3, 1410.
(1) For comprehensive reviews, see: (a) Degrado, W. F. Adv. Protein
Chem. 1988, 39, 51. (b) Toniolo, C. Int. Pept. Protein Res. 1990, 35,
287. (c) Giannis, A.; Kolter, T. Angew. Chem., Int. Ed. Engl. 1993, 32,
1244. (d) Liskamp, R. M. J . Recuil. Trav. Chim. Pays. Bass. 1994, 113,
1. (e) Gante, J . Angew. Chem., Int. Ed. Engl. 1994, 33, 1699.
(2) For some representative examples, see: (a) Toniolo, C. CRC Crit.
Rev. Biochem. 1980, 9, 1. (b) Mutter, M. Angew. Chem. 1985, 97, 639.
(c) Obrecht, D.; Spiegler, C.; Schoo¨nholzer, P.; Mu¨ller, K.; Heimgartner,
H.; Stierli, F. Helv. Chim. Acta 1992, 75, 1666.
(3) For some representative examples, see: (a) Wipf, P.; Heimgart-
ner, H. Helv. Chim. Acta 1988, 71, 258. (b) Wipf, P.; Kunz, R. W.;
Prewo, R.; Heimgartner, H. Helv. Chim. Acta 1988, 71, 268. (c) Obrecht,
D.; Bohdal, U.; Broger, C.; Bur, D.; Lehmann, C.; Ruffieux, R.;
Scho¨nholzer, P.; Spiegler, C.; Mu¨ller, K. Helv. Chim. Acta 1995, 78,
563. (d) Obrecht, D.; Karajrannis, H.; Lehmann, C.; Scho¨nholzer, P.;
Spiegler, C.; Mu¨ller, K. Helv. Chim. Acta 1995, 78, 703. (e) Obrecht,
D.; Lehmann, C.; Ruffieux, R.; Scho¨nholzer, P.; Muller, K. Helv. Chim.
Acta 1995, 78, 1567. (f) Obrecht, D.; Bohdal, U.; Lehmann, C.;
Scho¨nholzer, P.; Mu¨ller, K. Tetrahedron 1995, 51, 10883. (g) Obrecht,
D.; Abrecht, C.; Altorfer, M.; Bohdal, U.; Grieder, A.; Kleber, M.; Pfyffer,
P.; Mu¨ller, K. Helv. Chim. Acta 1996, 79, 1315.
(8) For a review on amino acids of the cyclobutane series, see:
Avotins, F. Russ. Chem. Rev. 1993, 62, 897.
(9) For recent reviews on this topic, see: (a) Cole, D. C. Tetrahedron
1994, 50, 9517. (b) J uaristi, E.; Quintana, D.; Escalante, J . Aldrichim.
Acta 1994, 27, 3. (c) Cardillo, G.; Tomassini, C. Chem. Soc. Rev. 1996,
117. (d) Enantioselective Synthesis of â-Amino Acids; J uaristi, E., Ed.,
Wiley VCH: New York, 1997.
(4) Hruby, V. J . In Perspectives in Peptide Chemistry; Eberle, A.,
Geiger, R., Wielland, T., Eds.; Karger: Basel, 1981, p 207.
(5) For more information on this subject, see: (a) ref 1. (b) Hruby,
V. J .; Al-Obeidi, F.; Kazmierski, W. Biochem. J . 1990, 268, 249. (c)
Heimgartner, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 238.
(10) For another approach to â-amino acids enabling direct coupling
with R-amino acids, see: Podlech, J .; Seebach, D. Angew. Chem., Int.
Ed. Engl. 1995, 34, 471. Also, see: ref 7b.
S0022-3263(97)01286-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/20/1998

2470 J . Org. Chem., Vol. 63, No. 8, 1998
Palomo et al.
alkenes 1 according to established methods.17 Subse-
quent treatment of each â-lactam product 2, 3,18 and 4
with di-tert-butyl dicarbonate and DMAP19 in acetonitrile
overnight provided the N-Boc â-lactams 5, 6, and 7 in
yields of 80%, 78%, and 84%, respectively. With these
â-lactams in hand, our first aim was to establish the
optimum reaction conditions to achieve their coupling
with some representative R-amino acid esters. With
regard to this, we had previously observed that the ring
opening of both 3-oxy and 3-amino â-lactams is strongly
influenced by the substitution pattern at the C4 position.11
Thus, while 3-oxy and 3-amino 4-monoalkylated â-lac-
tams were efficiently coupled in DMF with R-amino acid
esters without any additives or using sodium azide as
the promoter, the corresponding 4-dialkylated counter-
parts were unreactive under these conditions, and potas-
sium cyanide was required to catalyze the coupling
reaction.11 Taking these observations into account, it was
not surprising to observe that C4-disubstituted â-lactams
5, 6, and 7, upon treatment with (L)-LeuOBn either in
the presence or the absence of sodium azide, were
completely resistant to â-lactam ring opening. However,
contrary to our expectations, the coupling also did not
proceed in the presence of potassium cyanide, and only
unreacted â-lactam was recovered after 16 h of stirring
at room temperature. We were gratified to observe that,
by simply performing the reaction at 40 °C in the
presence of a stoichiometric amount of potassium cya-
nide, the corresponding dipeptide products were obtained
in excellent yields and without loss of optical purity,20
vide infra. For example, under these conditions the
coupling of 5 with both (L)-LeuOBn and (L)-ValOBn gave
8a and 8b after 16 h of reaction in 90% and 93% yields,
respectively. Even the bulky AibOBn could efficiently
be coupled with 5 to afford the dipeptide 8c in 70% yield.
Similarly, the assistance of potassium cyanide proved to
be crucial for the coupling of both O-benzyl (L)-SerOBn
and (L)-AspOBn with the â-lactam 5 to give rise to
products 8d (76%) and 8e (75%), respectively. Likewise,
treatment of both 6 and 7 with (L)-LeuOBn afforded, after
column chromatography, the respective 9a and 10a in
98% and 90% isolated yields although in the latter case
a longer reaction time was needed. The lower reactivity
of â-lactam 7 could be overcome by performing the
reaction at higher temperatures. Thus coupling of 7 with
(L)-LeuOBn was complete at 70 °C in 16 h affording the
corresponding peptide with little reduction in chemical
Sch em e 1
Sch em e 2a
problem based on the reaction of a N-chlorosulfonyl
â-lactam with an amino compound to give a highly
reactive intermediate (Scheme 1) which reacts rapidly
with methanol to give the corresponding â-methoxysul-
fonyl protected amino derivative. We wish to present
here a complementary route toward short peptide seg-
ments containing â,â-disubstituted â-amino acids that
involves the coupling of N-Boc â-lactams with R-amino
acid esters. The key to this approach is the use of an
N-activated â-lactam and potassium cyanide as the
promoter of the acylation reaction.16 In our approach
(Scheme 2) the chlorosulfonyl moiety of the corresponding
â-lactam is replaced by the more readily removable tert-
butoxycarbonyl (Boc) group often employed as a protect-
ing group in amino acid chemistry.
The starting â-lactams were prepared by the addition
of chlorosulfonyl isocyanate (CSI) to the corresponding
(17) (a) Ave, D. H.; Iwahashi, H.; Shellhamer, D. F. Tetrahedron
Lett. 1973, 3719. (b) Durst, T.; O’Sullivan, M. J . J . Org. Chem. 1970,
35, 2043. (c) Barrett, A. G. M.; Betts, M. J .; Fenwick, A. J . Org. Chem.
1985, 50, 169.
(18) During preparation of compound 3 an impurity (10-20%) was
detected in the reaction. This impurity could be separated from 3 only
after forming the Boc-derivative. From the spectroscopic data we
assigned structure 3′, which would likely have arised from an acid-
catalyzed isomerization of the terminal olefin to the endocyclic olefin,
to this impurity.
(11) (a) Palomo, C.; Aizpurua, J . M.; Cuevas, C. J . Chem. Soc.; Chem.
Commun. 1994, 1957. (b) Palomo, C.; Aizpurua, J . M.; Galarza, R.;
Mielgo, A. J . Chem. Soc.; Chem. Commun. 1996, 633. Ibid. Idem. 1996,
1400.
(12) Ojima, I.; Sun, C. M.; Park, Y. H. J . Org. Chem. 1994, 59, 1249.
(13) Drey, C. N. C.; Lowbridge, J .; Ridge, R. J . J . Chem. Soc.; Perkin
Trans. 1 1973, 2001.
(14) For reviews on sulfonyl isocyanates, see: (a) Rasmussen, J . K.;
Hassner, A. Chem. Rev. 1976, 76, 389. (b) Dhar, D. N.; Murthy, K. S.
K. Synthesis 1986, 437. (c) Kamai, A.; Sattur, P. B. Heterocycles 1987,
26, 1051. For a general review on â-lactams, see: (d) Backes, J . In
Houben-Weyl, Methoden der Organischem Chemie; Muller, E., Baeyer,
O., Eds; Thieme: Stuttgart, 1991; BandE16B, p 31.
(15) Bhupathy, M.; Bergan, J . J .; McNamara, J . M.; Volante, R. P.;
Reider, P. J . Tetrahedron Lett. 1995, 36, 9445.
(19) Baldwin, J . E.; Adlington, R. M.; Gollins, D. W.; Schofield, C.
J . Tetrahedron 1990, 46, 4733.
(20) Potassium cyanide has also been employed as a catalyst in
transesterification processes to remove peptides from the Merrifield
resin without racemization of the C-terminal amino acid. For more
detailed information, see: Moore, G.; McMaster, D. Int. J . Pept. Protein
Res. 1978, 11, 140.
(16) For the use of NaCN to promote ring opening of â-lactams,
see: (a) Cavagna, F.; Linkies, H.; Pietsch, H.; Reuschling, D. Angew.
Chem., Int. Ed. Engl. 1980, 19, 129. (b) Bose, A. K.; Krishman, L.;
Wagle, D. R.; Manhas, M. S. Tetrahedron Lett. 1986, 27, 595. (c) Wagle,
D. R.; Monteleone, M. G.; Krishman, L.; Manhas, M. S.; Bose, A. K. J .
Chem. Soc.; Chem. Commun. 1989, 8915.

Peptide Segments Containing â,â-Disubstituted â-Amino Acids
J . Org. Chem., Vol. 63, No. 8, 1998 2471
Sch em e 3
Sch em e 4
yield and, again, without appreciable racemization. Al-
ternatively, coupling reactions could also be performed
at room temperature in DMF-H2O (10:1) albeit chemical
yields of isolated peptides fell to 50-60%. Decreasing
the amount of potassium cyanide from 1 to 0.1 equiv
resulted in the incomplete reaction (25% conversion after
16 h).
Sch em e 5
As Scheme 3 illustrates, exposure of the dipeptide
product 8b to H2 and 10% Pd/C provided the N-Boc
dipeptide 11 in 95% yield and ready for a subsequent
coupling step with R-amino acid esters. For example, the
DCC-assisted coupling of 11 with (L)-ValOMe furnished
the tripeptide 12 in 70% isolated yield and without
formation of epimerization products. In this context, it
should be mentioned that coupling of (L)-ValOMe with
11 via its acid fluoride, prepared according to Carpino’s
procedure,21 gave the corresponding coupling product as
an equimolar mixture of diastereomers. The next ques-
tion we addressed was whether â-lactam 5 could be
coupled with dipeptides. If so, tripeptide units incorpo-
rating â,â-disubstituted â-amino acids at the N-terminus
would be produced efficiently and, therefore, in better
chemical yields. Under the same conditions employed
for the preparation of dipeptides, â-lactam 5 efficiently
coupled with (L)-Val-ValOMe to produce, in 83% isolated
yield, the tripeptide 12 without detectable epimerization.
As mentioned above, the potential of â-lactams to act
as acylating agents of amino acid esters seems to be
influenced by the presence of electron-withdrawing sub-
stituents at the C3 position of the azetidinone nucleus.
To confirm this, the 3-chloro â-lactam 13 was prepared
by treatment of the lithium enolate of 5 with an excess
of p-toluensulfonyl chloride according to the method of
Ku¨hlein,22 and the product 13 was then submitted to
treatment with (S)-LeuOBn in DMF as solvent (Scheme
4). Indeed, unlike the result with 3-unsubstituted â-lac-
tams, attempted amino acid coupling at room tempera-
ture for 16 h afforded detectable dipeptide formation (15%
conversion judged by 1H NMR) in the absence of any
promoter. Further, when the same coupling was carried
out under identical conditions but with the assistance of
1 equiv of sodium azide, the yield rose to 85%. Finally,
substituting potassium cyanide for sodium azide afforded
complete reaction at room temperature and gave rise to
dipeptide 14 in 83% isolated yield. Subsequent exposure
of the crude product 14 to H2 and 10% Pd/C led to the
compound 15 which showed identical chiroptic and
spectroscopic data to those of the product previously
obtained from 8a . Therefore, these results along with
those previously reported11 clearly indicate, from a
qualitative standpoint, that the presence of an electron-
withdrawing group at the C3 position of the â-lactam ring
facilitates the N1-C2 carbon bond cleavage by nitrogen
nucleophiles.23
The preceding results also demonstrate that both
dipeptide and tripeptide units incorporating â,â-disub-
stituted â-amino acids are directly accessible from â-lac-
tams and R-amino acid derivatives. We then examined
the formation of peptide bonds at the hindered N-
terminus of â,â-branched dipeptides.24 To this end we
again evaluated the acid fluoride methodology developed
by Carpino and found that the dipeptide 16 coupled
efficiently with Boc-ValF in the presence of N-methyl-
morpholine to give 17 in 80% yield (Scheme 5).
A remaining question related to this coupling meth-
odology concerns the optical integrity of the compounds
under the reaction conditions used. To assess this latter
aspect the crude products 8a , 8b, 9a , and 10a , selected
as representative examples, were subjected to HPLC
analysis using a chiral column. In all cases, judged by
the unique absortion signal exhibited on the respective
chromatogram, the above-described procedure appeared
to be free of racemization. To ensure the validity of this
optical purity assay, we have also prepared the corre-
sponding peptides 8a , 8b, 8c, 9a , and 10a in their
racemic forms by potassium cyanide mediated coupling
of each N-Boc â-lactam 5, 6, and 7 with the corresponding
(23) For detailed information on the factors determining the chemi-
cal reactivity of â-lactams, see: The Chemistry of â-Lactams, Page,
M. I., Ed.; Chapman and Hall: London, 1992.
(24) For pertinent information on methods to couple hindered
R-amino acids, see: (a) Spencer, J . R.; Antonenko, U. V.; Delaet, N. G.
J .; Goodman, M. Int. J . Pept. Protein 1992, 40, 282. (b) Carpino, L. A.
J . Am. Chem. Soc. 1993, 115, 4397. (c) Angell, Y. M.; Garc´ıa-
Echevarr´ıa, C.; Rich, D. H. Tetrahedron Lett. 1994, 35, 5981. (d)
Wenschuch, H.; Beyermann, M.; Winter, R.; Bienert, M.; Ionescu, D.;
Carpino, L. A. Tetrahedron Lett. 1996, 37, 5483.
(21) (a) Carpino, L. A.; Sadat-Aalaee, D.; Chao, H. G.; DeSelms, R.
H. J . Am. Chem. Soc. 1990, 112, 9651. Also, see: (b) Bertho, J .-N.;
Loffet, A.; Pinel, C.; Reuther, F.; Sennyey, G. Tetrahedron Lett. 1991,
32, 1303.
(22) Ku¨hlein, K.; J ensen, H. Liebigs Ann. Chem. 1974, 369.

2472 J . Org. Chem., Vol. 63, No. 8, 1998
Palomo et al.
Kiesegel 60 (230-400 Mesh) using mixtures of ethyl acetate
and hexane as eluants. Ether was distilled over sodium and
benzophenone. Methylene chloride was shaken with concen-
trated sulfuric acid, dried over potasium carbonate, and
distilled. DMF was distilled over BaO. CH3CN was dried by
refluxing over CaH2 and distilled. MeOH was dried over
magnessium metal and iodine. Commercially available com-
pounds were used without further purification. Caution!
During the workup of the N-Boc â-lactam opening reaction
some quantities of HCN might be produced. A good ventilated
hood was used, and the remaining aqueous phases were
combined and treated as indicated in The Sigma-Aldrich
Library of Chemical Safety Data, 2nd ed.; Vol. 2, p 2898B,
before wasting.
Gen er a l P r oced u r e for th e P r ep a r a tion of â-La cta m s
2 a n d 4. Chlorosulfonyl isocyanate (1.40 g, 9.9 mmol) was
added dropwise at 0 °C to a solution of the corresponding
alkene (10 mmol) in dry Et2O (4 mL). The reaction mixture
became semisolid with fine needles. The oily residue resulting
from the evaporation of the volatiles was dissolved in Et2O
(23 mL) and added dropwise to a mixture of two parts 25%
aqueous Na2SO3 (13 mL) and one part of Et2O. The aqueous
phase was kept slightly basic by addition of 10% KOH solution
as the reduction proceeded. After 1 h of stirring, the Et2O
phase was separated and dried and the solvent evaporated to
give the corresponding â-lactam as a colorless oil.
1-Aza sp ir o[3.3]h ep ta n -2-on e (2): yield, 0.80 g (72%). Oil.
IR (film) 1767 cm-1, 1734 cm-1 (CO). 1H NMR (CDCl3, δ) 6.9-
6.5 (bs, 1H), 2.92 (d, 2H, J ) 1.46 Hz), 2.5-2.1 (m, 4H), 1.8-
1.5 (m, 2H). 13C NMR (CDCl3, δ) 167.6, 55.5, 49.8, 33.9, 12.9.
EIMS m/z 112 (M + 1)+.
F igu r e 1. Determination of enantiomeric purity of coupling
products by chiral HPLC analysis. Examples of chromato-
grams corresponding to (A) nonracemic 10a and (B) racemic
10a .
racemic R-amino acid esters. The resulting crude prod-
ucts were then submitted to HPLC analysis under the
same conditions as above. Comparison of each pair of
chromatograms corresponding to the nonracemic and
racemic samples, as in the example shown in Figure 1,
confirmed that no appreciable epimerization had occurred
during coupling reactions.25 It, thus, appears that potas-
sium cyanide facilitates the ring opening of â-lactams by
generating an acyl cyanide intermediate which then
reacts with the corresponding R-amino acid ester under
racemization free conditions. Although this intermediate
has not been isolated yet, the results presented herein
indicate that the procedure should be readily extended
to further applications in â-lactam-derived â-amino acid
peptide synthesis.26 An additional attractive feature of
the method is that the overall process involves a very
concise way to build-up peptide fragments from alkenes.
1-Aza sp ir o[3.4]octa n -2-on e (3): yield, 0.81 g (65%). Oil.
IR (film) 1743 cm-1 (CO). 1H NMR (CDCl3, δ) 6.7-6.5 (bs,
1H), 2.82 (d, 2H, J ) 1.7 Hz), 2.0-1.5 (m, 8H). 13C NMR
(CDCl3, δ) 167.8, 59.5, 48.1, 36.0, 22.9. EIMS m/z 126 (M +
1)+.
1-Aza sp ir o[3.5]n on a n -2-on e (4): Yield, 1.15 g (83%). Oil.
IR (film) 1748 cm-1 (CO). 1H NMR (CDCl3, δ) 6.9-6.7 (bs,
1H), 2.57 (d, 2H, J ) 1.66 Hz), 1.8-1.3 (m, 10H). 13C NMR
(CDCl3, δ) 168.1, 54.3, 47.5, 36.2, 24.3, 23.5. EIMS m/z 140
(M + 1)+.
Gen er a l P r oced u r e for th e P r ep a r a tion of N-Boc
â-La cta m s 5, 6, a n d 7. Boc2O (4.32 g, 20 mmol) and DMAP
(0.12 g, 1 mmol) were added to a solution of the corresponding
azetidin-2-one (10 mmol) in acetonitrile (24 mL) at 0 °C, and
the mixture was stirred at room temperature overnight. Then,
methylene chloride (25 mL) was added, and the mixture was
washed with 1 M NaHSO3 (2 × 25 mL), a saturated solution
of NaHCO3 (25 mL), and a saturated solution of NaCl (25 mL).
The organic layer was dried over MgSO4, and the solvent was
removed in vacuo. Products were purified by column chro-
matography (silica gel, hexane/EtOAc 5:1) and crystallized
from hexane.
Exp er im en ta l Section
Melting points were determined with a Bu¨chi SMP-20
capillary apparatus and are uncorrected. Proton nuclear
magnetic resonance spectra (200 and 300 MHz) and 13C spectra
(75 MHz) were recorded at room temperature in CDCl3, unless
otherwise stated. All chemical shifts are reported as δ values
(ppm) relative to residual CHCl3 δH (7.26 ppm) and CDCl3 δC
(77.0 ppm) as internal standards. Mass spectra (EIMS) were
obtained on a Finnigan GCQ mass spectrometer (70 eV) using
GC-MS coupling (column: fused silica gel, 15 m, 0.25 mm, 0.25
mm phase SPB-5). Optical rotations were measured at 25 (
0.2 °C in methylene chloride unless otherwise stated. HPLC
analyses were performed on analytical columns (25 cm, phase
Lichrosorb-Si60) and (25 cm, phase Chiralcel OD) with flow
rates of 1 mL/min and 0.5 mL/min, respectively, using a DAD
detector. Flash chromatography was executed with Merck
N-Boc-1-Aza sp ir o[3.3]h ep ta n -2-on e (5). The general
procedure was followed starting from 2 (0.80 g, 7.2 mmol).
Yield, 1.22 g (80%). Mp: 61-62 °C. IR (KBr) 1787 (CO), 1710
cm-1 (CO). 1H NMR (CDCl3, δ) 2.99 (s, 2H), 2.87 (ddd, 2H, J
) 19.8, 9.9, 2.7 Hz), 2.2-2.0 and 1.9-1.6 (m, 2H), 1.52 (s, 9H).
13C NMR (CDCl3, δ) 164.4, 147.7, 82.9, 59.1, 50.0, 31.2, 28.0,
12.9. Anal. Calcd for C11H17NO3 (211.26): C, 62.54; H, 8.11;
N, 6.63. Found: C, 62.30; H, 8.09; N, 6.68.
N-Boc-1-Aza sp ir o[3.4]octa n -2-on e (6). The general pro-
cedure was followed starting from 3 (0.81 g, 6.5 mmol). Yield,
1.02 g (70%). Mp: 64-65 °C. IR (KBr) 1804 (CO), 1713 cm-1
(CO). 1H NMR (CDCl3, δ) 2.87 (s, 2H), 2.4-2.2 (m, 2H), 1.9-
1.6 (m, 6H), 1.52 (s, 9H). 13C NMR (CDCl3, δ) 164.7, 147.1,
82.4, 65.4, 51.4, 34.4, 27.7, 24.0. Anal. Calcd for C12H19NO3
(225.29): C, 63.98; H, 8.50; N, 6.22. Found: C, 63.70; H, 8.68;
N, 6.35.
(25) Chiral HPLC analyses were performed using a 25 cm phase
Chiralcel OD analytical column, with a 0.5 mL/min flow of a mixture
of ethyl acetate and hexane. In all cases the R enantiomer showed a
shorter retention time.
(26) For a recent review on â-amino acids and their derivatives from
â-lactams, see: Palomo, C.; Aizpurua, J . M.; Ganboa, I., in ref 9d, p
279.
N-Boc-1-Aza sp ir o[3.5]n on a n -2-on e (7). The general pro-
cedure was followed starting from 4 (1.15 g, 8.3 mmol). Yield,
1.67 g (84%). Mp: 87-88 °C. IR (KBr) 1814 (CO), 1708 cm-1
(CO). 1H NMR (CDCl3, δ) 2.69 (s, 2H), 2.3-2.0 and 1.9-1.8
(m, 2H), 1.7-1.6 (m, 3H), 1.52 (s, 9H), 1.3-1.1 (m, 3H). 13C
NMR (CDCl3, δ) 165.1, 147.7, 82.7, 60.9, 47.9, 33.9, 28.0, 24.7,

Peptide Segments Containing â,â-Disubstituted â-Amino Acids
J . Org. Chem., Vol. 63, No. 8, 1998 2473
23.9. Anal. Calcd for C13H21NO3 (239.31): C, 65.25; H, 8.84;
N, 5.85. Found: C, 65.62; H, 8.90; N, 6.07.
2H), 2.01-1.70 (m, 2H), 1.7-1.0 (m, 11H), 1.39 (s, 9H), 0.86
(d, 6H, J ) 4.0 Hz). 13C NMR (CDCl3, δ) 172.5, 170.3, 154.9,
135.4, 128.4, 128.2, 128.1, 79.1, 66.7, 53.7, 50.6, 44.6, 41.1, 35.2,
35.1, 28.3, 25.3, 24.6, 22.7, 21.7, 21.4. EIMS m/z M+ 460.
P r ep a r a tion of 12. Meth od A. The â-Lactam 5 (0.5 mmol)
was treated with (L)-Val-(L)-ValOMe (0.75 mmol) in DMF
under the same conditions described above for coupling reac-
Gen er a l P r oced u r e for th e Cou p lin g of N-Boc-â-La c-
ta m s w ith r-Am in o Acid Ester s. To a solution of the N-Boc-
â-lactam (1 mmol) in dry DMF were added the corresponding
R-amino acid benzyl ester (1.2 mmol) and KCN (1 mmol), and
the resulting mixture was stirred under a N2 atmosphere at
40 °C. When no more starting material was observable by
TLC, Et2O (20 mL) was added and the mixture washed with
brine (2 × 20 mL), 1 N HCl (20 mL), and NaHCO3 (saturated
solution, 20 mL). The organic layer was dried over MgSO4
and filtered, and the solvent was evaporated at reduce pres-
sure to give the corresponding peptide, which was purified by
column chromatography.
tions. Yield, 83%. Mp: 138-139 °C. [R]25 ) -15.6 (c ) 1.0,
D
CH2Cl2). IR (KBr) 1744 cm-1 (CO), 1709 cm-1 (CO), 1686 cm-1
(CO), 1645 cm-1 (CO). 1H NMR (CDCl3, δ) 6.87 (d, 1H, J )
8.6 Hz), 6.40 (d, 1H, J ) 8.2 Hz), 5.56 (s, 1H), 4.47 (dd, 1H, J
) 8.6 Hz, 5.2 Hz), 4.29 (dd, 1H, J ) 8.2 Hz, 6.8 Hz), 3.69 (s,
3H), 2.74 (m, 2H), 2.4-2.0 (m, 6H), 2.0-1.7 (m, 2H), 1.38 (s,
9H), 1.0-0.8 (m, 12H). 13C NMR (CDCl3, δ) 172.3, 171.2,
171.1, 154.7, 79.3, 58.7, 57.13, 55.0, 52.1, 43.2, 33.5, 32.9, 31.1,
30.5, 28.3, 19.2, 18.9, 18.10; 17.9; 14.4. Anal. Calcd for
8a . Yield, 90%. Mp: 79-80 °C. [R]25D -9.23 (c ) 1.0, CH2-
Cl2). IR (KBr) 1748 (CO), 1679 cm-1 (CO). 1H NMR (CDCl3,
δ) 7.35 (s, 5H), 5.94 (d, 1H, J ) 5.4 Hz), 5.15 (m, 2H), 5.07 (s,
1H), 4.63 (ddd, 1H, J ) 12.3, 5.8, 3.2 Hz), 2.75 (s, 2H), 2.3-
2.1 (m, 4H), 2.0-1.7 (m, 2H), 1.7-1.5 (m, 3H), 1.42 (s, 9H),
0.91 (d, 6H, J ) 4.2 Hz). 13C NMR (CDCl3, δ) 172.7, 170.7,
154.7, 135.4, 128.6, 128.4, 128.2, 79.2, 67.0, 54.9, 50.6, 43.1,
41.4, 33.4, 33.2, 28.4, 24.8, 22.8, 21.8, 14.6. Anal. Calcd for
C24H36N2O5 (432.56): C, 66.64; H, 8.39; N, 6.48. Found: C,
66.84; H, 8.63; N, 6.43.
C
22H39N3O6 (441.57): C, 59.84; H, 8.90; N, 9.52. Found C,
60.15; H, 9.11; N, 9.59.
Meth od B. To a solution of peptide 8b (0.417 g, 1 mmol)
in MeOH (10 mL) was added 10% palladium on charcoal (42
mg), and the mixture was kept under hydrogen (1 atm) at room
temperature for 18 h. The suspension was then filtered
through a pad of Celite, and evaporation of the solvent gave
product 11. Yield, 95%. 1H NMR (CD3OD, δ) 7.0-6.8 (bs, 1H),
6.2-6.0 (bs, 1H), 6.0-5.7 (bs, 1H), 4.3-4.1 (m, 1H), 2.8-2.6
(m, 2H), 2.4-2.0 (m, 5H), 1.9-1.6 (m, 2H), 1.39 (s, 9H), 1.0-
1.8 (m, 6H). This crude product was coupled with (L)-ValOMe
(0.125 g, 0.95 mmol) by using DCC (0.206 g, 1 mmol) and HOBt
(0.128 g, 0.95 mmol) in THF (3 mL) at 0 °C for 1 h and
additional 1 h at rt. The reaction mixture was filtered, the
solvent evaporated, and the residue redissolved in EtOAc (40
mL) and washed with a saturated solution of NaHCO3 (20 mL),
1 N citric acid (20 mL), saturated NaHCO3 (20 mL), and water
(20 mL). The solution was dried over MgSO4 and filtered, and
the solvent was evaporated at reduced pressure to give 12,
which was purified by column chromatography. Yield, 0.296
g (70%). All physical and spectroscopic data were identical to
those of the material obtained following method A.
P r ep a r a tion of 13. To a solution of iPr2NH (0.08 mL, 0.6
mmol) in dry THF (1 mL) at -78 °C under a nitrogen
atmosphere was added 1.6 M BuLi in hexane (0.37 mL, 0.6
mmol), and the solution was stirred for 30 min. â-Lactam 5
(0.106 g, 0.5 mmol) in THF (0.5 mL) was dropped in, and the
mixture was stirred at -78 °C for 30 min. A solution of TsCl
(0.154 g, 0.8 mmol) in THF (0.5 mL) was added dropwise, and
the cold bath was removed to allow the mixture to rise to rt
over 1 h. The resulting mixture was partitioned between a
saturated solution of NH4Cl (10 mL) and Et2O (10 mL), the
organic phase was dried over MgSO4, and the solvent was
evaporated under reduced pressure. The residue obtained was
purified by flash column chromatography (eluant CH2Cl2/
EtOAc 200:1. Yield, 0.754 g (61%). Mp: 67-68 °C. IR (KBr)
1809 (CO), 1720 cm-1 (CO). 1H NMR (CDCl3, δ) 4.67 (s, 1H),
3.1-2.8 (m, 1H), 2.8-2.5 (m, 2H), 2.3-2.1 (m, 1H), 2.0-1.6
(m, 2H), 1.55 (s, 9H). 13C NMR (CDCl3, δ) 160.9, 147.4, 84.1,
65.2, 64.7, 31.0, 28.0, 26.8, 13.2. Anal. Calcd for C11H16ClNO3
(245.70): C, 53.77; H, 6.56; N, 5.70. Found C, 53.53; H, 6.59;
N, 5.75.
8b. Yield, 83%. Mp: 93-94 °C. [R]25D) -2.4 (c ) 1.0, CH2-
Cl2). IR (KBr) 1723 (CO), 1691 (CO), 1666 cm-1 (CO). 1H
NMR (CDCl3, δ) 7.35 (s, 5H), 6.06 (d, 1H, J ) 8.8 Hz), 5.16
(m, 2H), 5.11 (s, 1H), 4.58 (dd, 1H, J ) 8.8, 4.8 Hz), 2.78 (s,
2H), 2.3-2.1 (m, 5H), 2.0-1.7 (m, 2H), 1.42 (s, 9H), 0.91 (d,
3H, J ) 6.8 Hz), 0.85 (d, 3H, J ) 6.8 Hz). 13C NMR (CDCl3,
δ) 171.6, 170.7, 154.5, 135.1, 128.4, 128.3, 128.2, 79.1, 66.8,
56.7, 54.8, 43.0, 33.1, 33.0, 30.9, 28.2, 18.9, 17.5, 14.5. Anal.
Calcd for C23H34N2O5 (418.53): C, 66.00; H, 8.19; N, 6.69.
Found C, 65.68; H, 8.30; N, 6.43.
8c. Yield, 66%. Mp: 117-119 °C. IR (KBr) 1743 (CO),
1680 (CO), 1659 cm-1 (CO). 1H NMR (CDCl3, δ) 7.33 (s, 5H),
6.17 (s, 1H), 5.14 (s, 2H), 4.96 (s, 1H), 2.66 (s, 2H), 2.3-2.0
(m, 4H), 1.9-1.7 (m, 2H), 1.51 (s, 6H), 1.41 (s, 9H). 13C NMR
(CDCl3, δ) 174.0, 170.2, 154.7, 135.7, 128.4, 128.1, 128.1, 79.1,
67.0, 56.1, 55.0, 43.1, 33.3, 28.4, 24.8, 14.6. Anal. Calcd for
C
22H32N2O5 (404.50): C, 65.32; H, 7.97; N, 6.92. Found C,
65.09; H, 8.10; N, 7.20.
8d . Yield, 76%. Mp: 76-78 °C. [R]25 ) +2.89 (c ) 1.0,
D
CH2Cl2). IR (KBr) 1738 (CO), 1703 (CO), 1674 cm-1 (CO). H
1
NMR (CDCl3, δ) 7.4-7.1 (m, 10H), 6.40 (d, 1H, J ) 8.2 Hz),
5.3-5.0 (m, 3H), 4.78 (dt, 1H, J ) 8.2, 3.0 Hz), 4.45 (m, 2H),
3.91 (dd, 1H, J ) 9.4, 3.0 Hz), 3.65 (dd, 1H, J ) 9.4, 3.0 Hz),
2.75 (m, 2H), 2.3-2.1 (m, 4H), 2.0-1.7 (m, 2H), 1.40 (s, 9H).
13C NMR (CDCl3, δ) 170.7, 169.9, 154.6, 137.2, 135.2, 128.5,
128.4, 128.2, 127.8, 127.6, 79.1, 73.2, 69.6, 67.2, 54.8, 52.4, 43.1,
33.2, 32.8, 28.3, 14.6. Anal. Calcd for C28H36N2O6 (496.60):
C, 67.72; H, 7.31; N, 5.64. Found C, 68.18; H, 7.31; N, 5.75.
8e. Yield, 75%. Mp: 86-88 °C. [R]25 ) +3.72 (c ) 1.0,
D
CH2Cl2). IR (KBr) 1753 (CO), 1737 (CO), 1731 (CO), 1674 cm-1
(CO). 1H NMR (CDCl3, δ) 7.4-7.2 (m, 10H), 6.65 (d, 1H, J )
8.2 Hz), 5.16 (bs, 1H), 5.11 (s, 2H), 5.05 (s, 2H), 4.91 (dt, 1H,
J ) 8.2, 4.7 Hz), 3.05 (dd, 1H, J ) 17.0, 4.7 Hz), 2.85 (dd, 1H,
J ) 17.0, 4.7 Hz), 2.71 (s, 2H), 2.3-2.0 (m, 4H), 2.0-1.7 (m,
2H), 1.43 (s, 9H). 13C NMR (CDCl3, δ) 170.5, 170.5, 170.3,
154.5, 135.1, 134.9, 128.5, 128.3, 128.3, 128.2, 128.2, 79.1, 67.4,
66.7, 54.7, 48.3, 43.0, 36.2, 33.0, 32.8, 28.3, 14.5. Anal. Calcd
for C29H36N2O7 (524.61): C, 66.39; H, 6.92; N, 5.34. Found C,
66.86; H, 6.57; N, 5.61.
P r ep a r a tion of 15. Meth od A: A solution of 8a (0.432 g,
1 mmol) in MeOH (10 mL) was kept under H2 in the presence
of 0.043 g of Pd on charcoal (10% w/w) at rt until disappearance
of the starting material (16 h). The catalyst was then filtered
through a pad of Celite and the solvent removed under reduced
pressure to give 15 which was crystallized from Et2O to afford
a white foamy solid. Yield, 0.325 g (93%). Mp: 164-165 °C.
9a . Yield, 80%. Syrup. [R]25 ) -15.22 (c ) 1.0, CH2Cl2).
D
IR (CH2Cl2) 1736 (CO), 1706 (CO), 1671 cm-1 (CO). 1H NMR
(CDCl3, δ) 7.35 (s, 5H), 6.04 (d, 1H, J ) 5.4 Hz), 5.15 (m, 2H),
4.69 (s, 1H), 4.61 (ddd, 1H, J ) 11.4, 6.0, 3.4 Hz), 2.71 (m,
2H), 2.0-1.5 (m, 11H), 1.43 (s, 9H), 0.91 (d, 6H, J ) 5.2 Hz).
13C NMR (CDCl3, δ) 172.6, 170.9, 155.2, 135.4, 128.5, 128.3,
128.2, 79.2, 66.9, 62.1, 50.7, 43.6, 41.3, 38.4, 38.1, 28.4, 24.7,
23.1, 23.1, 22.7, 21.8. EIMS m/z M+ 446.
[R]25 ) -14.5 (c ) 1.0, MeOH). IR (KBr) 1729 (CO), 1682
D
(CO), 1635 cm-1 (CO). 1H NMR (CD3OD, δ) 4.5-4.4 (m, 1H),
2.72 (m, 2H), 2.3-2.1 (m, 4H), 1.9-1.6 (m, 5H), 1.44 (s, 9H),
0.96 (d, 3H, J ) 6.3 Hz), 0.93 (d, 3H, J ) 6.3 HZ). 13C NMR
(CD3OD, δ) 176.2, 173.2, 156.8, 80.2, 56.3, 52.2, 44.0, 42.0, 34.0,
33.7, 28.9, 26.0, 23.3, 22.0, 15.3. Anal. Calcd for C17H30N2O5
(342.43): C, 59.63; H, 8.83; N, 8.18. Found C, 60.01; H, 8.93;
N, 8.10.
10a . Yield, 86%. Syrup. [R]25 ) -11.13 (c ) 1.0, CH2-
D
Cl2). IR (NaCl, film) 1736 cm-1 (CO), 1706 cm-1 (CO), 1669
cm-1 (CO). 1H NMR (CDCl3, δ) 7.30 (s, 5H), 6.13 (d, 1H, J )
7.8 Hz), 5.10 (m, 2H), 4.6-4.4 (m, 1H), 4.5 (s, 1H), 2.62 (m,
Meth od B. The coupling of 13 (0.246 g, 1 mmol) with (S)-
LeuOBn was carried out first by the same procedure than that

2474 J . Org. Chem., Vol. 63, No. 8, 1998
Palomo et al.
used for coupling of 3-unsubstituted â-lactams, except the
reaction was conducted at room temperature. Yield of the 1:1
epimeric mixture of 14 after column chromatography was
0.388 g (83%). To a solution of chloride 14 (0.388 g, 0.83 mmol)
in MeOH (10 mL) were added Et3N (0.210 g, 2.1 mmol) and
10% Pd/C, and the resulting mixture was kept under H2 at rt
until the starting material was consumed (24 h). Filtration
of the catalyst through a pad of Celite and evaporation of the
solvent gave an oil which was partitioned between CH2Cl2 and
0.1 N HCl. The organic layer was separated and dried over
MgSO4, and the solvent was evaporated to afford 15 which
exhibited identical physical and spectroscopic data to those of
the product obtained by the method A. Yield, 0.145 g (51%).
P r ep a r a tion of 17. To a solution of peptide 8a (0.143 g,
0.33 mmol) in CH2Cl2 (2 mL) was added CF3CO2H (1.0 mL,
13 mmol), and the resulting solution was stirred at rt for 1.5
h. After evaporation of all the volatiles, the resulting residue
was dissolved in CH2Cl2 (5 mL), the solution was washed with
NaHCO3 (saturated, 4 mL), the organic layer was dried over
MgSO4, and the solvent was evaporated under reduce pressure
to afford 16. Yield, 0.075 g (70%). To crude 16 in dry CH2Cl2
(1 mL) were added a solution of N-Boc-Val-F (0.053 g, 0.24
mmol) in CH2Cl2 (0.5 mL) and N-methylmorpholine (0.04 mL,
0.33 mmol). After 2 h of stirring at rt, additional CH2Cl2 (1
mL) was added and the resulting solution washed with H2O
(1 mL), 1 N HCl (1 mL), and NaHCO3 (saturated, 1 mL). The
organic layer was dried over MgSO4 and the solvent evapo-
rated under reduced pressure. Yield, 0.079 g (70%). Syrup.
[R]25 ) +27.6 (c ) 1.0, CH2Cl2). IR (KBr) 1742 (CO), 1721
D
(CO), 1651 (CO). 1H NMR (CDCl3, δ) 7.34 (s, 5H), 7.00 (d,
1H, J ) 8.6 Hz), 6.85 (s, 1H), 5.15 (m, 2H), 5.06 (d, 1H, J )
8.0 Hz), 4.8-4.6 (m, 1H), 3.6-3.5 (m, 1H), 2.84 (d, 1H, J )
12.8 Hz), 2.50 (d, 1H, J ) 12.8 Hz), 2.5-2.3 (m, 2H), 2.3-2.1
(m, 1H), 2.1-1.8 (m, 4H), 1.8-1.5 (m, 3H), 1.41 (s, 9H), 0.97
(m, 6H), 0.89 (d, 6H, J ) 5.7 Hz). 13C NMR (CDCl3, δ) 174.4,
171.3, 171.1, 156.6, 135.4, 128.5, 128.3, 128.0, 79.9, 67.1, 61.0,
56.2, 50.9, 43.5, 39.4, 33.9, 31.2, 30.1, 28.2, 24.8, 22.9, 21.1,
19.5, 18.7, 15.4. HRMS (FAB) m/z calcd for C29H46N3O6 (M +
H) 532.3387, found 532.3388.
Ack n ow led gm en ts. This work was financially sup-
ported by Comisio´n Interministerial de Ciencia y Tec-
nolog´ıa. (Project SAF: 95/0749), and in part by Gobierno
Vasco (Project PI 95/93) and Diputacio´n Foral de Gipuz-
koa. A grant from Gobierno Vasco to S.B. is gratefully
acknowledged.
J O9712862