A biomimetic approach for polyfunctional secocholanes: Tuning flexibility and functionality on peptidic and macrocyclic scaffolds derived from bile acids

Article abstract of DOI:10.1021/jo800708m

(Chemical Equation Presented) Bile acids are important scaffolds in medicinal and supramolecular chemistry. However, the use of seco bile acids, i.e., bile acids with opened rings, as cores or building blocks for the assembly of complex peptide conjugates or macrocycles has remained elusive so far. A biomimetic approach to secocholanes, based on an oxidative ring-expansion/ring- opening sequence, offers efficient access to novel structures with tunable flexibility and functionality. The process preserves selected portions of the original stereochemical and functional information of the steroid, while additional structural elements are incorporated in further (diversity- generating) steps. The potential of these building blocks for peptide and macrocycle chemistry is exemplified by the attachment of relevant α-amino acids and by the production of various complex macrocycles obtained by conventional (e.g., macrolactonization and macrolactamization) and multicomponent (e.g., Ugi four-component) macrocyclizations. This combination of secocholanic skeleton manipulation with, e.g., varied types of macrocyclization protocols, produces high levels of skeletal diversity and complexity. Therefore, this approach may have applicability either for the synthesis of biologically active ligands or as artificial receptors ( hosts ).

Full text of DOI:10.1021/jo800708m

A Biomimetic Approach for Polyfunctional Secocholanes: Tuning
Flexibility and Functionality on Peptidic and Macrocyclic Scaffolds
Derived from Bile Acids
Daniel G. Rivera,*,†,‡ Orlando Pando,† Rayser Bosch,† and Ludger A. Wessjohann*,‡
Center for Natural Products Study, Faculty of Chemistry, UniVersity of HaVana, 10400 HaVana, Cuba,
and Department of Bioorganic Chemistry, Leibniz Institute of Plant Biochemistry, Weinberg 3,
D-06120 Halle/Saale, Germany
dgr@fq.uh.cu; wessjohann@ipb-halle.de
ReceiVed March 30, 2008
Bile acids are important scaffolds in medicinal and supramolecular chemistry. However, the use of seco
bile acids, i.e., bile acids with opened rings, as cores or building blocks for the assembly of complex
peptide conjugates or macrocycles has remained elusive so far. A biomimetic approach to secocholanes,
based on an oxidative ring-expansion/ring-opening sequence, offers efficient access to novel structures
with tunable flexibility and functionality. The process preserves selected portions of the original
stereochemical and functional information of the steroid, while additional structural elements are
incorporated in further (diversity-generating) steps. The potential of these building blocks for peptide
and macrocycle chemistry is exemplified by the attachment of relevant R-amino acids and by the production
of various complex macrocycles obtained by conventional (e.g., macrolactonization and macrolactam-
ization) and multicomponent (e.g., Ugi four-component) macrocyclizations. This combination of
secocholanic skeleton manipulation with, e.g., varied types of macrocyclization protocols, produces high
levels of skeletal diversity and complexity. Therefore, this approach may have applicability either for the
synthesis of biologically active ligands or as artificial receptors (“hosts”).
Introduction
biological and/or pharmacological properties. Rigidity and
chirality arise from the distinctive fused-rings system known
as the steroidal nucleus (the gonan core structure). This
polycyclic skeleton (especially when it is highly substituted)
not only possesses a high number of natural stereogenic
centers but also can be easily functionalized in two different
basic forms (i.e., R and ꢀ orientations) at each nonbridging
carbon atom. Accordingly, the stereochemical information
embedded into the steroid can be conserved, adjusted, or
transferred into acyclic systems by a variety of ring-opening
procedures.
The steroid skeleton is a readily available source of rigidity,
chirality, and lipophilicity combined in a single aliphatic
system. These chemical features are the basis of many of
the biological functions exhibited by steroidal compounds.
Accordingly, (bio)synthetic manipulation of available sub-
strates to vary these characteristics is a common approach
employed by both nature and chemists to pursuit new
† University of Havana.
‡ Leibniz Institute of Plant Biochemistry.
10.1021/jo800708m CCC: $40.75
Published on Web 07/22/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 6229–6238 6229

Rivera et al.
The partial loss of rigidity of ring-opened systems compared
to the steroidal nucleus usually leads to the appearance of new
biological functions, which frequently are quite different from
those of the original steroid. Biologically active secosteroids
(i.e., steroids with opened rings) commonly occur in nature,
especially in animals and marine organisms.1,2 In addition, it
seems that some privileged substructural elements are common
in biologically active natural products (analogues) structurally
related to opened steroids.3 Synthetic work on secosteroids has
also traditionally rendered novel compounds displaying a wide
variety of biological activities, ranging from hormone antago-
nists4 to tyrosine phosphatase inhibitors.5 Other types of
interesting scaffolds derived from steroids are the macrocycles
obtained by cleavage of the ABC ring system. Two distinct
approaches rely on this methodology: the Winterfeldt approach
toward ansa-steroids6 and the fragmentation of endoperoxides
that produces the “stereoklastanes”.7 These approaches have the
advantage of conserving most of the stereochemical information
of the original steroidal nucleus once the rings have been opened
or the ring fusion has been cleaved. Nevertheless, further
procedures that increase the number of functionalities during
ring-opening process may be preferred as they lead to a higher
number of diversity elements per synthetic operation.
receptors featuring macrocyclic scaffolds,8,9 and to achieve rigid
arrays of peptide strands with interesting biological, recognition
and catalytic properties.8,10 However, with the exception of a
single example,11 the use of seco bile acids as scaffolds to build
up either peptide chains or relatively flexible macrocyclic
structures has remained elusive so far.
Herein we report on a biomimetic approach to produce a
variety of secocholanic scaffolds very amenable either for
positioning multiple amino acids, and prospectively peptides,
as well as for assembling diverse types of macrocyclic frame-
works. The focus is posed mainly on producing skeletal diversity
by enabling a tunable variation of the flexibility, stereochemistry,
and functionality of the (originally) cholanic skeleton. This gives
access to a wide variety of scaffolds that provide alternative
topologies and functionalization patterns compared to the
classical, fully conserved skeleton of the bile acids.
Results and Discussion
The partial cleavage of the steroidal nucleus to produce less
rigid skeletons is a common approach in medicinal chemistry.
This is mostly due to the fact that adjusting the conformational
flexibility required for an appropriate binding to a biological
target is a crucial issue in drug design. However, approaches
that produce secosteroids having a large number of function-
alities suitable for further derivatization and thus acting as
amenable building blocks are very rare. Secosteroids have been
usually considered as synthetic targets and not as substrates or
building blocks or novel scaffolds that allow the introduction
of binding or reactive motifs. Accordingly, we envisaged to
implement this possibility through a simple and versatile
approach that allows to tune the flexibility and functionality of
the steroid-derived scaffolds while keeping most of the stere-
ochemical information of the original nucleus.
The approach relies on a sequential ring-expansion/ring-
opening procedure that ensures the production of multiple
functional groups for subsequent derivatization. The Baeyer-
Villiger reaction of ketocholanes was chosen for the ring
expansion, as the process incorporates an additional oxygen atom
and the resulting lactone ring can be submitted to standard ring-
opening procedures. This method can be referred to as biomi-
metic, as both the Baeyer-Villiger and lactone ring-opening
reactions are ubiquitous in biosynthetic pathways of many
A quite attractive type of steroidal nucleus is the cholanic
skeleton of bile acids. These readily available compounds have
been widely exploited for the construction of supramolecular
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6230 J. Org. Chem. Vol. 73, No. 16, 2008

Biomimetic Approach for Polyfunctional Secocholanes
SCHEME 1. Biomimetic Approach towards Polyfunctional Secocholanes Based on the Baeyer-Villiger Oxidation/Lactone
Ring-Opening Sequence of Different Keto Bile Acid Derivatives
natural products, including biologically active steroids.12 Other
advantages of the Baeyer-Villiger reaction are the complete
retention of the configuration and its known regioselectivity for
ketosteroids. Both issues have been studied in detail before,13
thus offering possibilities for a reliable synthetic design.
As depicted in Scheme 1, the procedure can be implemented
either for producing protected tricarboxylic acids or a triazido
secocholane, which can be subsequently converted into a
triamino compound. The first two entries rely on a double
lactonization reaction followed by acid-catalyzed lactone ring
opening to afford two different polycarboxylic scaffolds. The
regioisomeric mixtures of lactones 2 and 6 are a consequence
of the unselective nature of the Baeyer-Villiger reaction on
3-ketosteroids. The resulting regioisomeric ratios are in agree-
ment with previous results that explain the more favorable
migration of C-2 compared to C-4, thus affording the 3-oxa-
4-keto-4a-homocholanes 2a and 6a as the main products of the
lactonization of diketones 1 and 5, respectively. The regioiso-
meric lactones were difficult to separate by column chroma-
tography; thus the mixtures were submitted to ring opening by
refluxing in an acidic methanolic solution, and the resulting
secocholanic esters were easily separated by flash chromatog-
raphy. The harsh conditions used comprise the concomitant
dehydration of the forming secondary and tertiary hydroxyl
groups, which helps shifting the process toward the opened
compounds. Interestingly, esters 3 and 4 were formed by a
stereospecific nucleophilic attack of the primary hydroxyl group
to the incipient carbocation at the R-face at C-8. The stereo-
chemistry of the newly formed stereocenter at C-8 was revealed
in NOESY experiments by confirming the cross-peaks between
H-8 and the methyl groups 18 and 19 (see Supporting Informa-
tion). For compounds 7 and 8, protection of the primary
hydroxyl group as tert-butyldimethylsilyl ether (TBDMS) was
required to avoid renewed lactone ring closing during the
subsequent deprotection of the methyl esters.
Synthesis of the secocholanic triazide 12 also started with
the Baeyer-Villiger oxidation of ketosteroid 9 to furnish the
regioisomeric mixture of lactones 10,13b which was subjected
to reductive opening of both ester functionalities to yield triol
11 as a single product. Subsequently, 3-fold mesylation and
substitution by azide according to standard procedures furnished
triazide 12, which can be subsequently converted into the
corresponding triamino secocholane-based scaffold. The great
value of the general approach based on a ring-expansion/ring-
opening sequence relies on the close control of the core’s
flexibility and stereochemistry and location of the functional
groups. Of course, the approaches employed for polycarboxylic
secocholanes can be easily adapted to produce polyamino
skeletons.
To illustrate the scope of this idea in the production of
multivalent peptidic scaffolds, we focused on the introduction
of multiple R-amino acids into the different secocholanic
skeletons. As shown in Scheme 2, esters 3, 4, 7, and 8 were
deprotected and subsequently submitted to a 3-fold coupling to
C-protected R-amino acids by standard coupling protocols. The
peptido-secosteroidal conjugates 14, 16, 18, and 20 were thus
obtained in good to excellent yields, considering the 3-fold
character of the coupling processes and the additional cleavage
of the TBDMS groups required to afford compounds 18 and
20. Alternatively, triazide 12 was reduced by 3-fold Staudinger
reaction giving rise to the corresponding triamine, which was
identified by ESI-MS and subjected to peptide coupling without
further purification and characterization. Accordingly, the tri-
histidinic secocholanic skeleton 21 was produced in 67% yield
via the standard EDC/HOBt protocol and without affecting the
(13) (a) Takatsuto, S.; Ikekawa, N. Tetrahedron Lett. 1983, 24, 917–920.
(b) Rivera, D. G.; Pando, O.; Suardiaz, R.; Coll, F. Steroids 2007, 72, 467–473,
and references therein.
J. Org. Chem. Vol. 73, No. 16, 2008 6231

Rivera et al.
SCHEME 2. Synthesis of Trivalent Peptidic Compounds with a Polyfunctional Secocholane Core
configuration of the di-Boc-L-histidine building block. It might
be noticed that R-amino acids bearing bulky and functional side
chains were utilized in most procedures in order to probe the
reactivity of these novel polyfunctional skeletons and also to
explore the possible assembly of biologically relevant, larger
peptide strands. In addition, the variable character of the
topology, directionality of functional groups and flexibility of
these secocholanic scaffolds shows promise toward aligned,
multivalent clusters of oligosaccharides.
As mentioned before, the rigid steroidal nucleus of bile acids
has been one of the most exploited options for imposing
preorganization and to align multiple recognition motifs within
macrocyclic scaffolds.8,9 To probe the more flexible seco-
cholanes in macrocyclizations, we turned to the separate
cleavage of rings B and C that produces bifunctional building
blocks suitable to assemble macrocycles of varied cavity size,
shape and flexibility. Also, to show applicability toward
accessing chemical entities of either biological or supramolecular
interest, three different and powerful macrocyclization protocols
were tested, i.e., macrolactonization, macrolactamization, and
multicomponent macrocyclization.
Scheme 3 illustrates the route toward C-secocholanes and
their subsequent use in the synthesis of a macrobislactone and
a macrobislactam by Yamaguchi and Schmidt macrocycliza-
tions, respectively. Hence, Baeyer-Villiger oxidation of keto-
cholane 22 furnished lactone 23 as the single regiosiomer, as a
result of the higher migration aptitude of the quaternary C-13
compared to the secondary C-11. To gain access to the
selectively functionalized polyhydroxylic secocholane 24, si-
multaneous protection of the secondary hydroxyl groups at C-3
and C-7 was needed prior to the reductive opening of the lactone
ring. Because of the hindered character of the axial 7R-OH,
the use of tert-butyldimethylsilyl triflate (TBDMSOTf) was
required to accomplish the silylation process. Next, the simul-
taneous reduction of both the lactone moiety on ring C and the
ester function at C-24 afforded the diprotected pentahydroxy-
6232 J. Org. Chem. Vol. 73, No. 16, 2008

Biomimetic Approach for Polyfunctional Secocholanes
SCHEME 3. Synthesis of a Macrobislactone and a Macrobislactam Including Secocholanic Skeletons
secocholane 24 in 82% yield. The two primary hydroxyls of
24 were converted into amino groups by consecutive mesylation,
substitution by azide, and Staudinger reduction to afford the
secocholanic diamine 26 in 67% overall yield. The bifunctional
secocholanes 24 and 26 were next subjected to macrocycliza-
tions by using different dicarboxylic acids as counterparts in
ester and amide bond formation reactions, respectively.
The efficient Yamaguchi macrolactonization protocol gave
the macrobislactone 25 in 63% yield from compound 24 and
adipic acid previously activated via the 2,4,6-trichlorobenzoyl
mixed anhydride method.14 Alternatively, the macrobislactam
27 was produced in 57% yield from diamine 26 and pyridine-
2,6-dicarboxylic acid via the Schmidt macrolactamization
protocol, which involves the carboxylate activation by forma-
tion of the pentafluorophenol ester.15 As both macrocyclization
protocols were originally designed for intramolecular cyclization
under high dilution conditions, the synthetic procedures required
adjustment for bifunctional building blocks. That is, one of the
building blocks (activated if required) is slowly added to a ca.
10 mM solution of the other one. This provides a suitable high
dilution condition for the acyclic intermediate formed in situ,
which subsequently can cyclize via the second ester or amide
bond forming reaction. Remarkably, both reactions proceeded
well under these conditions with a variety of dicarboxylic acids
of both aromatic and aliphatic origin. Compounds 25 and 27
are but examples that illustrate the possibility of obtaining
macrocycles of either medicinal or supramolecular relevance,
as both natural product- and receptor-type scaffolds can be
mimicked with easiness. For example, macrocycle 27 seems to
be especially suitable for anion binding upon protonation, as a
convergent array of functionalities with hydrogen-donor char-
acter is provided within the cavity.
As depicted in Scheme 4, an alternative approach was carried
out to synthesize the ꢀ-secocholanic dicaboxylic acid 30, which
was subsequently employed in two different multicomponent
macrocyclization protocols. As expected, the Baeyer-Villiger
oxidation of ketocholane 28 furnished exclusively the regiosi-
omer 29, as a result of the favored migration of the tertiary C-8
compared to the secondary C-6. Consecutively, a reaction
sequence consisted in base-catalyzed lactone ring opening,
followed by TBDMS protection of all OH functionalities and
finally selective deprotection of the silyl ester led to the
secocholanic di-acid 30 in 68% overall yield. The choice of a
base-catalyzed hydrolysis aims to preserve the full stereochem-
ical information embedded in the original cholic acid skeleton,
i.e., avoiding dehydration of the secondary hydroxyl at C-8 as
it occurred previously when acid-catalyzed hydrolysis was
utilized.
(14) (a) Dommerholt, F. J.; Thijs, L.; Zwanenburg, B. Tetrahedron Lett. 1991,
32, 1499–1502. (b) Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi,
M. Bull. Chem. Soc. Jpn. 1979, 52, 1989–1993.
The resulting diacid 30 is an ideal building block for multiple
multicomponent macrocyclizations, a recently developed meth-
odology with great capacity to generate skeletal diversity in one
step.11,16 If the Ugi four-component reaction (Ugi-4CR) is
employed, three different combinations could be accomplished
with the use of diacid 30.11 As shown in Scheme 4, examples
of the diacid/diisocyanide and diacid/diamine combinations were
successfully implemented by conducting the double Ugi-4CR-
based macrocyclizations under typical pseudodilution condi-
tions.17 Macrocycles 32 and 33 were thus obtained in 47% and
54% yield, respectively, after full deprotection of the hydroxyl
groups under mild conditions that did not affect the integrity
of the macrocyclic cavity. Whereas the overall yields are lower
than those of macrocycles 25 and 27, it must be noticed that
(15) Grieꢀer, H.; Kroner, M.; Schmidt, U. Synthesis 1991, 294–300.
(16) For examples of multiple multicomponent macrocyclizations, see: (a)
Wessjohann, L. A.; Ruijter, E. Mol. DiVersity 2005, 9, 159–169. (b) Leo´n, F.;
Rivera, D. G.; Wessjohann, L. A. J. Org. Chem. 2008, 73, 1762–1767. (c)
Wessjohann, L. A.; Rivera, D. G.; Coll, F. J. Org. Chem. 2006, 71, 7521–7526.
(d) Rivera, D. G.; Vercillo, O. E.; Wessjohann, L. A. Synlett 2007, 308–312. (e)
Michalik, D.; Schaks, A.; Wessjohann, L. A. Eur. J. Org. Chem. 2007, 149–
157. (f) Rivera, D. G.; Wessjohann, L. A. J. Am. Chem. Soc. 2006, 128, 7122–
7123.
(17) The diacid/dialdehyde combination was not considered because of the
poor availability and stability of dialdehydes. Also, due to the nonselective nature
of the Ugi-4CR, mixtures of diastereoisomeric macrocycles are produced, thereby
making the characterization process very difficult. For example, see: Wessjohann,
L. A.; Rivera, D. G.; Leon, F. Org. Lett. 2007, 9, 4733-4336.
(18) Wessjohann, L. A.; Andrade, C. K. Z.; Vercillo, O. E.; Rivera, D. G.
Targets Heterocycl. Syst. 2006, 10, 24–53.
(19) (a) Sui, Q.; Borchardt, D. L. J. Am. Chem. Soc. 2007, 129, 12042–
12048. (b) Fischer, G. Chem. Soc. ReV. 2000, 29, 119–127.
J. Org. Chem. Vol. 73, No. 16, 2008 6233

Rivera et al.
SCHEME 4. Synthesis of Peptoid-Secocholane Hybrid Macrocycles by Ugi MiBs
eight new covalent bonds are formed during this type of
multicomponent synthesis. In contrast, only two new bonds are
formed during the “classical” synthesis of 25 and 27 by the
Yamaguchi and Schmidt macrocyclizations, which are indeed
much less “diversity-generating”.
and macrocyclic compounds has been developed. The method
relies on the use of a ring-expansion/ring-opening sequence that
utilizes the Baeyer-Villiger oxidation of ketosteroids as the
ring-expansion step. This sequence allows a very straightforward
and variable access to compounds with tunable flexibility,
stereochemistry and functionality of the (subelements of)
cholanic skeletons, thus creating structural diversity in very few
steps. The potential of the polyfunctional secocholanes as core
units of multivalent conjugates was proved by the synthesis of
trivalent peptidic scaffolds. The polyfunctional building block
character was proven by the synthesis of macrocycles of varied
cavity shapes and sizes. The easy access to high levels of skeletal
diversity and complexity with a concentration of biologically
privileged structural elements, low synthetic cost, and the easy
availability of the starting materials demonstrate the versatility
of this diversity oriented approach. Finally, these results
strengthen the use of renewable natural products such as steroids,
not only for medicinal chemistry semisynthetic manipulation
but also for the production of scaffolds featuring novel topolo-
gies and functionalization patterns. An appropriate selection of
these functionalities ideally paired with a fixation of a suitable
conformational flexibility can provide new applications in
supramolecular chemistry or a biological tool.
Interestingly, the NMR spectra of macrocycles 32 and 33
show broad sets of signals due to the occurrence of various
conformers in solution. As previously discussed for this type
of macrocyclic peptoids,18 the N-substitution facilitates the
amide bond isomerization by decreasing the energy barrier
between the s-cis and s-trans configurations.19 This intrinsic
feature of peptoids, along with the presence of nonsymmetric
structures, often leads to poorly resolved spectra. Fortunately,
these macrocycles contain quasi-preorganized endocyclic moi-
eties (i.e., fused aliphatic rings) capable to introduce certain
conformational restrictions into the macrocyclic cavity. How-
ever, the “peptoid effect” is still significant since the NMR
spectra of macrocycles 32 and 33 are poorly resolved as
compared to, e.g., that of macrocycle 27 (see Supporting
Information).
The prospect of combining the skeletal diversity arising from
the wide variety of available secocholanes with that of the
multiple peptoid moiety variations derived from MiBs is
immense. For example, the diacid/diisocyanide combination
used to obtain compound 32 produces only endocyclic amide
bonds, while the diacid/diamine one used to obtain compound
33 produces peptoid backbones with an exocyclic amide bond.
The tactic of varying the number of endo- and exocyclic amide
bonds is strengthened by the easy incorporation of various
functional groups at the secocholanic building blocks. As these
latter skeletons are endocyclic, an appropriate adjustment of their
preorganization/flexibility ratio is of high significance for tuning
the overall conformational flexibility of the target macrocycles.
Experimental Section
Methyl 4,7-Diketo-3,8-dioxa-4a,8a-dihomo-5ꢀ-cholan-24-oate (2a)
and Methyl 3,7-Diketo-4,8-dioxa-4a,8a-dihomo-5ꢀ-cholan-24-oate
(2b). A heterogenic mixture of trifluoroacetic anhydride (2.1 mL,
15 mmol, 3 equiv) and urea-H2O2 adduct (1.41 g, 15.0 mmol, 3
equiv) in 30 mL of CH2Cl2 was stirred at 0 °C for 10 min. A
solution of diketone 1 (2.01 g, 5.0 mmol) in CH2Cl2 (30 mL) was
then added dropwise, and the reaction mixture was stirred vigor-
ously for 4 h at 0 °C. The mixture was diluted with CH2Cl2 (100
mL) and suction-filtered, and the filtrate was washed with aqueous
10% Na2SO3 (2 × 100 mL), aqueous 10% NaHCO3 (2 × 100 mL)
and brine (100 mL). The organic phase was dried over anhydrous
Na2SO4 and evaporated under reduced pressure to dryness. Flash
Conclusions
A biomimetic approach toward polyfunctional secocholanic
skeletons suitable as a core component of multivalent peptidic
6234 J. Org. Chem. Vol. 73, No. 16, 2008

Biomimetic Approach for Polyfunctional Secocholanes
column chromatography purification (n-hexane//EtOAc 2:1) af-
forded the mixture of the regioisomeric lactones 2 (2.04 g, 94%,
ratio 2a/2b, 2.3:1) as a white foam. Rf ) 0.25 (n-hexane/EtOAc
1:1). IR (KBr, cm-1): 3363, 2953, 1739, 1711, 1432, 1289, 1273,
1243, 1170, 1122, 1006. 1H NMR (CDCl3,*signals assigned to 2a):
δ 0.69 (s, 3H); 0.92 (d, 3H, J ) 6.4 Hz); 1.18, 1.17* (s, 3H); 2.74,
2.88* (dd, 1H, J ) 14.0/11.9 Hz); 3.08*, 3.02 (d, 1H, J ) 13.8
Hz); 3.67 (s, 3H); 4.07-4.14* (m, 1H); 4.18-4.36 (m, 2H). 13C
NMR (CDCl3): δ 174.8, 174.4*, 173.8, 172.6 (CO); 79.4, 79.0*
(CH); 66.2, 62.7 (CH2); 55.6, 54.3 (CH); 51.5 (CH3O); 45.1 (CH);
43.1 (C); 40.3, 40.25 (CH); 40.2 (CH2); 39.5 (C); 38.2, 36.6 (CH2);
34.9 (CH); 34.2, 34.0 30.9, 30.6, 27.6 (CH2); 24.9 (CH2); 24.9*,
24.5 (CH3); 22.9 (CH2); 18.1, 11.5 (CH3). HRMS (ESI-FT-ICR)
m/z: 457.2564 [M + Na]+; calcd for C25H38NaO6 457.2567.
tography (n-hexane/EtOAc 8:1) to give the pure triazide 12 (791
mg, 87%) as a white foam (CAUTION: Polyazido compounds may
be explosive upon heating). Rf ) 0.62 (n-hexane/EtOAc 5:1). IR
(KBr, cm-1): 2974, 2098, 2090, 2085, 1454, 1203, 1174. 1H NMR
(CDCl3): δ 0.68 (s, 3H); 0.94 (d, 3H, J ) 6.6 Hz); 0.95 (s, 3H);
3.07-3.25 (m, 6H). 13C NMR (CDCl3): δ 58.9, 58.4, 57.9 (CH2);
46.1, 44.3 (CH); 40.7 (C); 39.1, 38.0 (CH); 37.8 (CH2) 36.5 (CH);
36.1, 35.8, 35.2 (CH2); 34.6 (C); 33.5 (CH); 31.8, 29.4, 28.3 (CH2);
28.0 (CH); 24.2 (CH2); 23.0 (CH3); 21.0 (CH2); 18.7, 12.1 (CH3).
HRMS (ESI-FT-ICR) m/z: 456.3569 [M + H]+; calcd for C24H42N9:
456.3564.
Peptidosecosteroid 14. LiOH (158 mg, 3.75 mmol, 7.5 equiv)
was added to a solution of ester 3 (240 mg, 0.5 mmol) in 50 mL
of THF/H2O (2:1, v/v) at 0 °C. The reaction mixture was stirred at
0 °C for 6 h and then acidified with aqueous 10% NaHSO4 to pH
3. The layers were separated and the aqueous phase was extracted
with EtOAc (2 × 80 mL). The combined organic phase was dried
over anhydrous Na2SO4 and concentrated under reduced pressure
to dryness. The resulting tricarboxylic acid 13 was added to a stirred
suspension of Nꢀ-Boc-L-lysine methyl ester hydrochloride (890 mg,
3.0 mmol, 6 equiv) and NaHCO3 (252 mg, 3.0 mmol, 6 equiv) in
dry DMF (30 mL) under nitrogen atmosphere. EDC (228 mg, 3.0
mmol, 6 equiv) was then added and the stirring was continued at
room temperature for 24 h. The reaction mixture was diluted with
EtOAc (200 mL), washed with aqueous 10% HCl (2 × 50 mL)
and brine (50 mL), and then dried over anhydrous Na2SO4 and
evaporated under reduced pressure. The crude product was purified
by flash column chromatography (CH2Cl2/EtOAc 3:1) to afford the
peptidosecosteroid 14 (437 mg, 75%) as a white solid. Rf ) 0.50
(EtOAc). Mp (from MeOH): 224-226 °C. IR (KBr, cm-1): 3433,
2972, 2937, 1734, 1732, 1702, 1680, 1521, 1453, 1367, 1247, 1170,
1027. 1H NMR (CDCl3): δ 0.77 (s, 3H); 0.89 (d, 3H, J ) 6.3 Hz);
1.00 (s, 3H); 1.44 (s, 27H); 2.48-5.57 (m, 3H); 2.80-2.74 (m,
2H); 3.10 (m, 7H); 3.48-3.37 (m, 2H); 3.73 (s, 3H); 3.76 (s, 6H);
4.80 (m, 2H); 4.54-4.61 (m, 6H); 6.05 (d, 1H, J ) 7.5 Hz); 7.87
(m, 2H). 13C NMR (CDCl3): δ 176.0, 175.8, 174.1, 173.1, 172.9,
172.45, 155.9, 155.7 (CO); 79.0 (C); 75.5 (CH); 64.5 (CH2); 56.3,
55.8, 55.2 (CH); 52.9 (CH3O); 52.4, 52.1, 51.8 (CH); 44.4 (C);
40.1, 40.0, 39.9, 39.4, 39.0, 35.7 (CH2); 35.2, 34.9 (CH); 34.6 (C);
33.3, 32.1, 31.5, 31.0, 30.7, 29.5, 29.6 (CH2); 28.45 (CH3); 24.0
(CH3); 23.8, 23.3, 23.2, 22.5, 22.4 (CH2); 18.3, 12.55 (CH3). HRMS
Methyl 3,13-Dioxa-4,12-diketo-4a,13a-dihomo-5ꢀ-cholan-24-oate
(6a) and Methyl 3,12-diketo-4,13-dioxa-4a,13a-dihomo-5ꢀ-cholan-
24-oate (6b). Diketone 5 (2.01 g, 5.0 mmol) was submitted to
Baeyer-Villiger oxidation in a similar way as described in the
synthesis of the mixture of lactones 2. Flash column chromatog-
raphy purification (CH2Cl2/EtOAc 3:1) afforded the mixture of the
regioisomeric lactones 6 (2.04 g, 94%, ratio 6a/6b, 3.1:1) as a white
foam. Rf ) 0.41 (CH2Cl2/EtOAc 2:1). IR (KBr, cm-1): 2968, 2955,
1
2880, 1739, 1733, 1436, 1379, 1250, 1111, 1053, 1026. H NMR
(CDCl3,*signals corresponding to 6a): δ 0.87 (s, 3H); 1.05 (d, 3H,
J ) 6.3 Hz); 1.26, 1.30* (s, 3H); 2.65*, 3.06 (m, 1H); 3.66 (s,
3H); 3.96* (d, J ) 13.2 Hz); 4.04-4.20 (m, 1H), 4.65* (dd, 1H, J
) 13.1/10.2 Hz). 13C NMR (CDCl3): δ 174.5, 174.0, 173.9, 173.5
(CO); 87.2 (C); 70.0, 64.7 (CH2); 60.6, 54.7 (CH); 54.5 (CH2);
51.5 (CH3O); 50.9 (CH2); 46.0 (CH); 42.8 (C); 44.2, 42.8 (CH);
37.8, 37.4 (CH2); 36.8 (CH); 36.2, 34.2, 32.6, 30.7, 30.2, 29.0, 25.2,
24.1 (CH2); 21.3, 18.7, 17.2, 14.5, 14.2 (CH3). HRMS (ESI-FT-
ICR) m/z: 457.2569 [M + Na]+; calcd for C25H38NaO6 457.2567.
2,3-Seco-5ꢀ-cholane-2,3,24-triol (11). Ketone 9 (972 mg, 2.5
mmol) was submitted to Baeyer-Villiger as described in ref 13b
to afford the regioisomeric mixture of lactones 10. This crude
product was dried in vacuo, dissolved in 50 mL of dry THF, and
added dropwise under nitrogen to a stirred suspension of LiAlH4
(576 mg, 15 mmol) in dry THF (100 mL) at 0 °C. Stirring was
continued for 1 h at room temperature and finally for 2 h at reflux.
The mixture was treated cautiously with aqueous 10% NaOH (80
mL), and the resulting white powder was filtered off and washed
several times with EtOAc. The combined organic phase was washed
with aqueous 10% HCl (50 mL) and brine (50 mL), dried over
anhydrous Na2SO4, and concentrated under reduced pressure to
dryness. Flash column chromatography purification (CH2Cl2/EtOAc
5:1) afforded the pure triol 11 (984 mg, 94%) as a white powder.
Rf ) 0.60 (CH2Cl2/MeOH 2:1). Mp (from n-hexane/CH2Cl2):
211-213 °C. IR (ATR, cm-1): 2931, 2862, 1457, 1448, 1376, 1062,
(ESI-FT-ICR) m/z: 1187.7397 [M
C60H104N6NaO16: 1187.7401. Anal. Found: C 62.0 H 9.3 N 7.5;
Calcd.: C 61.8 H 9.0 N 7.2.
+
Na]+; calcd for
Peptidosecosteroid 16. Ester 4 (240 mg, 0.5 mmol) was depro-
tected to tricarboxylic acid 15, which was coupled with O-t-butyl-
L-tyrosine methyl ester hydrochloride (863 mg, 3.0 mmol) in a
similar way as described in the synthesis of 14. Flash column
chromatography purification (CH2Cl2/EtOAc 5:1) yielded the pep-
tidosecosteroid 16 (440 mg, 69%) as a white foam. Rf ) 0.55
(CH2Cl2/EtOAc 1:1). Mp (from acetone): 246-247 °C. IR (KBr,
cm-1): 3415, 3085, 3029, 2964, 2932, 1738, 1732, 1674, 1667,
1
1041, 1032. H NMR (CDCl3): δ 0.65 (s, 3H); 0.92 (d, 3H, J )
6.6 Hz); 0.95 (s, 3H); 3.69-3.64 (m, 4H); 3.65-3.69 (m, 2H). 13
C
NMR (CDCl3): δ 63.6, 62.9, 62.4, 42.7 (CH2); 40.1 (C); 39.9 (CH);
38.4, 37.8, 36.6 (CH2); 36.3, 35.9, 35.6 (CH); 35.0 (CH2); 34.6
(C); 31.8, 29.4, 28.3, 24.2 (CH2); 23.0 (CH3); 21.0 (CH2); 18.7,
12.1 (CH3). HRMS (ESI-FT-ICR) m/z: 403.3184 [M + Na]+; calcd
for C24H44NaO3 403.3186.
1
1448, 1248, 1205, 1167, 1033. H NMR (CDCl3): δ 0.62 (s, 3H);
0.77 (s, 3H); 0.85 (d, 3H, J ) 6.3 Hz); 1.30 (s, 27H); 2.91-2.97
(m, 2H); 3.02-3.07 (m, 4H); 3.23 (t, 1H, J ) 9.7 Hz); 3.31 (t, 1H,
J ) 11.5 Hz); 3.63 (s, 3H); 3.64 (s, 3H); 3.67 (s, 3H); 3.72 (dd,
1H J ) 11.5/4.5 Hz); 4.69-4.78 (m, 2H); 4.79-4.84 (m, 1H); 5.90
(d, 1H, J ) 7.6 Hz); 6.50 (d, 1H, J ) 7.6 Hz); 6.86-6.90 (m, 6H);
6.95 (d, 2H, J ) 8.4 Hz); 6.99 (d, 2H, J ) 8.6 Hz); 7.04 (d, 2H,
J ) 8.6 Hz). 13C NMR (CDCl3): δ 173.9, 172.8, 172.45, 172.4,
172.1 (CO); 131.2, 130.8, 130.3 (C); 128.7 (CH); 124.8 (C); 123.3
(CH); 78.3 (C); 74.5 (CH); 66.0 (CH2); 55.15, 54.25, 52.9, 52.8
(CH); 52.3, 51.8, 51.6 (CH3O); 50.2 (C); 46.0, 44.6 (CH); 37.9
(C); 38.4 (CH2); 38.0 (CH); 36.6, 36.45, 36.4 (CH2); 34.35 (CH);
32.6, 32.55, 31.3, 30.8, 29.0 (CH2); 28.1 (CH3); 27.7, 22.9, 19.85
(CH2); 17.6, 14.8, 11.6 (CH3). HRMS (ESI-FT-ICR) m/z: 1138.6942
[M + H]+; calcd for C66H96N3O13 1138.6938. Anal. Found: C 69.8,
H 8.5, N 3.8. Calcd: C 69.6, H 8.4, N 3.7.
2,3-Seco-2,3,24-triazido-5ꢀ-cholane (12). A solution of triol 11
(807 mg, 2.0 mmol) in dry CH2Cl2 (60 mL) at 0 °C was treated
with Et3N (3.72 mL, 27.0 mmol) and mesyl chloride (1.08 mL,
9.0 mmol). The reaction mixture was stirred at 0 °C for 2 h, then
diluted with 150 mL of CH2Cl2, and washed with brine (3 × 50
mL). The organic phase was dried over anhydrous Na2SO4 and
concentrated under reduce pressure to dryness. The resulting crude
product was dissolved in DMF (50 mL), and the solution was treated
with NaN3 (504 mg, 9.0 mmol). The reaction mixture was stirred
vigorously under nitrogen atmosphere at 50 °C for 48 h and then
diluted with 200 mL of Et2O. The organic phase was washed with
aqueous 10% HCl (2 × 50 mL) and brine (2 × 50 mL), dried over
anhydrous Na2SO4, and evaporated under reduced pressure to
dryness. The crude product was purified by flash column chroma-
J. Org. Chem. Vol. 73, No. 16, 2008 6235

Rivera et al.
Peptidosecosteroid 18. Ester 7 (298 mg, 0.5 mmol) was depro-
tected to tricarboxylic acid 17, which was coupled with L-valine
methyl ester hydrochloride (503 mg, 3.0 mmol) in a similar way
as described in the synthesis of 14. The resulting crude product
was dissolved in 30 mL of THF, and tetra-n-butylammonium
fluoride trihydrate (357 mg, 1.0 mmol) was added. The reaction
mixture was stirred at room temperature for 12 h and then diluted
with 100 mL of EtOAc. The solution was washed with aqueous
10% NaHCO3 (2 × 20 mL) and brine (30 mL), dried over
anhydrous Na2SO4, and evaporated under reduced pressure to
dryness. Flash column chromatography purification (CH2Cl2/EtOAc
8:1) yielded the peptidosecosteroid 18 (315 mg, 81%) as a white
foam. Rf ) 0.26 (CH2Cl2/EtOAc 6:1). Mp (from MeOH): 217-218
°C. IR (KBr, cm-1): 3314, 2954, 2930, 1741, 1650, 1527, 1463,
10% HCl (40 mL), and brine (50 mL) and then dried over anhydrous
Na2SO4 and evaporated under reduced pressure to dryness. The
crude product was purified by flash column chromatography
(CH2Cl2/EtOAc 10:1) to give the peptidosecosteroid 21 (465 mg,
67%) as a white foam. Rf ) 0.37 (CH2Cl2/EtOAc 5:1). Mp (from
MeOH/CH2Cl2): 233-235 °C. IR (KBr, cm-1): 3411, 3065, 3033,
1
2939, 2871, 1727, 1672, 1659, 1583, 1451, 1247, 1050, 1027. H
NMR (CDCl3): δ 0.67 (s, 3H); 0.92 (d, 3H, J ) 6.4 Hz); 0.91 (s,
3H); 1.43 (s, 27H); 1.45 (s, 27H); 3.37-3.48 (m, 6H); 4.67-4.71
(m, 1H); 4.74-4.79 (m, 2H); 5.67 (m, 1H); 5.84 (m, 1H); 5.88 (m,
1H); 5.91-5.99 (m, 2H); 6.76-6.79 (m, 3H); 7.38-7.43 (m, 3H).
13C NMR (CDCl3): δ 172.4, 172.1, 171.8, 155.8, 154.9 (CO); 136.0,
135.6, 135.3, 134.7, 133.6 (CH); 80.6, 80.1, 79.8 (C); 54.4, 54.3,
44.2, 43.7 (CH); 42.5, 42.2, 41.9 (CH2); 40.0 (C); 39.6, 37.9 (CH);
37.3 (C); 36.2 (CH2); 35.8 (CH); 35.1, 34.7 (CH2); 33.5 (CH); 33.0,
32.7, 32.7, 32.6 (CH2); 31.5, 29.4 (CH2); 28.5, 28.4, 28.3 (CH3);
28.1, 24.2, 21.9 (CH2); 20.2 (CH3); 17.6, 12.6 (CH3). HRMS (ESI-
FT-ICR) m/z: 1411.8590 [M + Na]+; calcd for C72H116N12NaO15
1411.8584. Anal. Found: C 62.4, H 8.65, N 12.3. Calcd: C 62.2, H
8.4, N 12.1.
1
1435, 1373, 1257, 1202, 1153, 1074, 836. H NMR (CDCl3): δ
0.89 (m, 6H); 0.87, 0.88, 0.89, 0.90, 0.91, 0.92 (6 × d, 18H, J )
6.8 Hz); 3.69-3.71 (m, 2H); 3.72 (s, 3H); 3.73 (s, 6H); 4.48 (dd,
1H, J ) 8.3/4.7 Hz); 4.52 (dd, 1H, J ) 8.8/4.9 Hz); 4.56 (dd, 1H,
J ) 8.9/4.9 Hz); 5.81 (d, 1H, J ) 8.5 Hz); 5.88 (d, 1H, J ) 9.0
Hz); 5.92 (d, 1H, J ) 9.0 Hz). 13C NMR (CDCl3): δ 173.2, 173.15,
172.8, 172.7, 172.6, 172.1 (CO); 139.7, 133.8 (C); 60.3 (CH2); 56.8,
56.7, 55.6 (CH); 52.0 (CH3O); 43.5, 39.4, 38.5 (CH); 37.6 (C);
36.1, 35.2 (CH2); 33.9, 31.9, 31.3, 31.2 (CH); 30.6, 29.65, 26.2,
25.2, 22.7, 21.5 (CH2); 18.7, 17.9, 17.8, 17.7, 12.8 (CH3). HRMS
(ESI-FT-ICR) m/z: 800.5068 [M + Na]+; calcd for C42H71N3NaO10
800.5037. Anal. Found: C 64.7, H 9.5, N 5.6. Calcd: C 64.8, H
9.2, N 5.4.
3r,7r-Di(tert-butyldimethylsilyloxy)-12,13-seco-5ꢀ-cholane-
12,13r,24-triol (24). TBDMSOTf (1.72 mL, 7.5 mmol, 3 equiv)
was added to a stirred solution of lactone 23 (1.09 g, 2.5 mmol)
and Et3N (1.04 mL, 7.5 mmol) in dry CH2Cl2 (50 mL) under
nitrogen atmosphere. Stirring was continue for 12 h, and then the
solution was diluted with 200 mL of CH2Cl2 and washed sequen-
tially with with aqueous 10% NaHCO3 (2 × 50 mL), aqueous 10%
HCl (50 mL), and brine (50 mL). The organic layer was dried over
anhydrous Na2SO4 and evaporated under reduced pressure. The
resulting crude product was dried in vacuo, then dissolved in 50
mL of dry THF, and added dropwise to a stirred suspension of
LiAlH4 (576 mg, 15 mmol) in dry THF (100 mL). The reaction
mixture was stirred at reflux under nitrogen atmosphere for 5 h
and then treated cautiously with aqueous 10% NaOH (50 mL). The
resulting white powder was filtered off and washed several times
with EtOAc. The combined filtrate was washed with aqueous 10%
HCl (50 mL) and brine (50 mL), then dried over anhydrous Na2SO4,
and concentrated under reduced pressure to dryness. Flash column
chromatography purification (CH2Cl2/EtOAc 5:1) afforded the triol
24 (1.31 g, 82%) as a white powder. Rf ) 0.22 (CH2Cl2/EtOAc
5:1). Mp (from EtOAc): 206-207 °C. IR (ATR, cm-1): 3301, 1944,
Peptidosecosteroid 20. Ester 8 (298 mg, 0.5 mmol) was depro-
tected to tricarboxylic acid 19, which was coupled with L-
phenylalanine methyl ester hydrochloride (647 mg, 3.0 mmol) in a
similar way as described in the synthesis of 14. The resulting crude
product was dissolved in 45 mL of THF, and tetra-n-butylammo-
nium fluoride trihydrate (357 mg, 1.0 mmol) was added. The
reaction mixture was stirred at room temperature for 12 h and then
diluted with 150 mL of EtOAc. The solution was washed with
aqueous 10% NaHCO3 (2 × 30 mL) and brine (30 mL), dried over
anhydrous Na2SO4, and evaporated under reduced pressure to
dryness. Flash column chromatography purification (CH2Cl2/EtOAc
10:1) yielded the peptidosecosteroid 20 (314 mg, 68%) as a white
foam. Rf ) 0.42 (CH2Cl2/EtOAc 6:1). Mp (from acetone): 238-240
°C. IR (ATR, cm-1): 3307, 3006, 2950, 2928, 1738, 1651, 1522,
1436, 1250, 1202, 1176, 835, 729, 700. 1H NMR (CDCl3): δ 0.82
(d, 3H, J ) 6.8 Hz); 0.87 (s, 3H); 0.93 (s, 3H); 3.01-3.15 (m,
6H); 3.62-3.67 (m, 2H); 3.66 (s, 3H); 3.69 (s, 3H); 3.71 (s, 3H);
4.77-4.81 (m, 1H); 4.83-4.89 (m, 2H); 5.76 (d, 1H, J ) 7.2 Hz);
5.84 (d, 1H, J ) 7.6 Hz); 5.92 (d, 1H, J ) 7.8 Hz); 7.06-7.09 (m,
5H); 7.23-7.31 (m, 10H). 13C NMR (CDCl3): δ 173.4, 132.1, 172.9,
172.6, 172.2, 171.8 (CO); 139.8, 138.4, 136.1, 135.7 (C); 133.8
(C); 129.3, 129.2, 128.5, 128.45, 128.4, 127.1, 127.0, 126.9 (CH);
60.1 (CH2); 57.0, 55.3, 55.1, 53.9 (CH); 52.25, 52.0, 52.0 (CH3O);
44.3, 40.4, 39.4 (CH); 38.7 (C); 38.5, 38.3, 36.4, 35.4 (CH2); 35.2
(CH); 34.6 (CH2); 30.5 (CH); 26.2 (CH2); 20.2, 18.4, 12.8 (CH3).
HRMS (ESI-FT-ICR) m/z: 944.5035 [M + Na]+; calcd for
C54H71N3NaO10 944.5032. Anal. Found: C 70.5 H 7.5 N 4.2. Calcd:
C 70.3 H 7.8 N 4.55.
Peptidosecosteroid 21. Trimethylphosphine (1 M in THF, 1.8
mL, 1.8 mmol) was added to a solution of azide 12 (228 mg, 0.5
mmol) in dry THF (10 mL), and the reaction mixture was stirred
at room temperature under nitrogen for 5 h. Then water (0.2 mL)
was added, and stirring was continued for additional 8 h. The
volatiles were evaporated under reduced pressure, and the crude
product was dried by repeated addition of toluene and evaporation.
The resulting triamine (identified by ESI-MS) in 15 mL of dry DMF
was added to a stirred suspension of N,N-Di-Boc-L-histidine (1.07
g, 3.0 mmol) and HOBt (168 mg, 3.0 mmol) in dry DMF (20 mL)
under nitrogen. The reaction mixture was then treated with EDC
(233 mg, 3.0 mmol) and stirred at room temperature under nitrogen
atmosphere for 24 h. The mixture was diluted with EtOAc (150
mL) and washed with aqueous 10% NaHCO3 (2 × 40 mL), aqueous
1
1205, 1046, 1024, 829, 770. H NMR (CDCl3): δ 0.03 (s, 3H);
0.04 (s, 3H); 0.06 (s, 6H); 0.76 (s, 3H); 0.86 (s, 9H); 0.91 (s, 9H);
0.93 (d, 3H, J ) 6.5 Hz); 0.94 (s, 3H); 3.38 (br. m, 1H); 3.72-3.66
(m, 4H); 3.90 (m, 1H). 13C NMR (CDCl3): δ -4.7, -4.5, -3.3,
-3.2, 12.8, 13.3 (CH3); 17.9, 18.0 (C); 23.4, 26.0, 26.2, 26.3, 26.4
(CH3); 28.5, 30.0, 32.8 (CH2); 33.3 (CH); 33.7, 34.9, 36.5 (CH2);
37.9 (C); 39.4 (CH2); 42.0, 42.9, 55.1 (CH); 62.8, 63.3 (CH2); 72.8,
72.3 (CH); 78.4 (C). HRMS (ESI-FT-ICR) m/z: 663.4809 [M +
Na]+; calcd for C36H72NaO5Si2 663.4810.
Macrocycle 25. To an ice-cold solution of adipic acid (73 mg,
0.5 mmol, 1.0 equiv) and Et3N (0.165 mL, 1.2 mmol, 2.4 equiv) in
5 mL of dry THF was added 2,4,6-trichlorobenzoyl chloride (0.23
mL, 1.1 mmol, 2.2 equiv). The reaction mixture was allowed to
reach room temperature, stirred for 2 h, and then diluted with 80
mL of dry toluene. This suspension was added dropwise over 8 h
to a stirred solution of DMAP (430 mg, 3.5 mmol, 7.0 equiv) and
diol 24 (320 mg, 0.5 mmol, 1.0 equiv) in dry toluene (50 mL) at
reflux. After addition was completed, the reaction mixture was
cooled to room temperature and concentrated under reduced
pressure. The crude product was dissolved in 200 mL of Et2O, and
the solution was washed with aqueous 10% HCl (50 mL), aqueous
10% NaHCO3 (2 × 50 mL), and brine (50 mL). The organic layer
was dried over anhydrous Na2SO4 and evaporated under reduced
pressure to dryness. The resulting crude product was dissolved in
50 mL of THF, and tetra-n-butylammonium fluoride trihydrate (356
mg, 1.0 mmol) was added. The reaction mixture was stirred at room
temperature for 12 h and then diluted with 150 mL of EtOAc. The
solution was washed with aqueous 10% NaHCO3 (2 × 30 mL)
6236 J. Org. Chem. Vol. 73, No. 16, 2008

Biomimetic Approach for Polyfunctional Secocholanes
and brine (50 mL), dried over anhydrous Na2SO4, and evaporated
under reduced pressure to dryness. Flash column chromatography
purification (CH2Cl2/EtOAc 10:1) yielded the macrocycle 25 (165
mg, 63%) as a colorless oil. IR (ATR, cm-1): 2948, 2889, 1734,
reduced pressure. Flash column chromatography purification (CH2Cl2/
EtOAc 1:1) afforded the dicarboxylic acid 30 (1.064 g, 68%) as pale
brown oil. Rf ) 0.22 (CH2Cl2/EtOAc 1:1). IR (ATR, cm-1): 3446,
1
3332, 2934, 2876, 1732, 1454, 1436, 1245, 1171, 1067. H NMR
1
1732, 1205, 1141, 1106, 1046. H NMR (CDCl3): δ 0.74 (s, 3H);
(CDCl3/CD3OD 95:5): δ 0.05 (s, 6H); 0.06 (s, 6H); 0.09 (s, 6H); 0.85
(s, 9H); 0.88 (s, 9H); 0.93 (s, 9H); 0.92 (d, 3H, J ) 6.5 Hz); 0.67 (s,
3H); 1.02 (s, 3H); 3.61 (m, 1H); 3.63 (m, 1H); 3.92 (m, 1H). 13C NMR
(CDCl3/CD3OD 95:5): δ -5.05, -5.0, -4.7, -3.3, 12.7, 17.7 (CH3);
17.8, 18.3 (C); 23.5, 25.7, 26.2 (CH3); 28.1, 29.6, 30.1, 30.7, 31.3,
31.9 (CH2); 35.9 (CH); 37.6 (C); 38.3, 46.5 (CH); 48.6 (C); 49.3 (CH);
66.9, 73.5, 73.9 (CH); 174.8, 175.4 (CO). HRMS (ESI-FT-ICR) m/z:
781.5295 [M - H]-; calcd for C42H81O7Si3 781.5291.
0.92 (d, 3H, J ) 6.4 Hz); 1.04 (s, 3H); 3.33 (br. m, 1H); 3.78-3.72
(m, 4H); 3.92 (m, 1H). 13C NMR (CDCl3): δ 12.6, 13.8, 17.4 (CH3);
23.3, 24.0, 24.3 (CH2); 24.7 (CH); 28.1, 29.6, 30.5, 33.3, 34.0
(CH2); 35.3, 35.8 (CH); 37.9 (C); 39.7, 40.8 (CH); 54.7 (CH2);
45.6 (CH); 48.5 (C); 49.0, 49.4 (CH); 58.9 (CH); 64.8, 65.2 (CH2);
72.3, 73.8 (CH); 78.2 (C); 174.9, 175.8 (CO). HRMS (ESI-FT-
ICR) m/z: 545.3559 [M + Na]+; calcd for C30H50NaO7 545.3554.
Macrocycle 27. Dicyclohexylcarbodiimide (260 mg, 1.25 mmol,
2.5 equiv) was added protionwise to a stirred solution of pyridine-
2,6-dicarboxylic acid (84 mg, 0.5 mmol, 1.0 equiv) and pentafluo-
rophenol (276 mg, 1.5 mmol, 3.0 equiv) in dry CH2Cl2 (20 mL) at
0 °C. Stirring was continued for 20 h at room temperature, and
then the resulting precipitate was filtered off. The filtrate was diluted
with CH2Cl2 (100 mL) and saturated aqueous NaHCO3 (50 mL).
A solution of diamine 26 (320 mg, 0.5 mmol, 1.0 equiv) in 20 mL
of CH2Cl2 was slowly added to the reaction mixture using a syringe
pump (flow rate 0.5 mL h-1). After addition was completed, the
reaction mixture was stirred for additional 24 h and diluted with
100 mL of CH2Cl2. The organic phase was washed with aqueous
10% NaHCO3 (50 mL) and brine (50 mL), dried over anhydrous
Na2SO4, and concentrated under reduced pressure. The resulting
crude product was dissolved in 50 mL of THF, and tetra-n-
butylammonium fluoride trihydrate (356 mg, 1.0 mmol) was added.
The reaction mixture was stirred at room temperature for 12 h and
then diluted with 150 mL of EtOAc. The solution was washed with
aqueous 10% NaHCO3 (2 × 50 mL) and brine (50 mL), dried over
anhydrous Na2SO4, and evaporated under reduced pressure. The
crude product was purified by flash column chromatography
(CH2Cl2/MeOH 25:1) and then recrystallized from EtOAc to yield
the macrocycle 27 (195 mg, 47%) as pale yellow solid. Mp (from
EtOAc): 169-170 °C. IR (ATR, cm-1): 3307, 3102, 3056, 2944,
Macrocycle 32. A solution of L-alanine methyl ester hydrochloride
(139.6 mg, 1.0 mmol), Et3N (0.14 mL, 1.0 mmol) and paraformalde-
hyde (30 mg, 1.0 mmol) in MeOH (100 mL) was stirred at room
temperature for 1 h. Two solutions, one of diacid 30 (392 mg, 0.5
mmol) and another of diisocyanide 3116f (78 mg, 0.5 mmol) in 20
mL of MeOH each, were simultaneously, slowly added to the reaction
mixture using syringe pumps (flow rate 0.5 mL h-1). After addition
was completed, the reaction mixture was stirred for 8 h and
concentrated under reduced pressure to dryness. The resulting crude
product was dissolved in 50 mL of THF, and tetra-n-butylammonium
fluoride trihydrate (535 mg, 1.5 mmol) was added. The reaction mixture
was stirred at room temperature for 12 h and then diluted with 150
mL of EtOAc. The solution was washed with aqueous 10% NaHCO3
(2 × 30 mL) and brine (50 mL), dried over anhydrous Na2SO4, and
evaporated under reduced pressure to dryness. Flash column chroma-
tography purification (CH2Cl2/MeOH 25:1) yielded the macrocycle
32 (195 mg, 47%) as pale yellow solid. Rf ) 0.62 (CH2Cl2/MeOH
4:1). Mp (from CH2Cl2): 216-218 °C. IR (ATR, cm-1): 3323, 2935,
2871, 1733, 1652, 1646, 1436, 1205, 1022. 1H NMR (CDCl3): δ 0.80
(d, 3H, J ) 6.8 Hz); 0.82 (s, 3H); 0.90 (s, 3H); 1.50 (d, 3H, J ) 7.0
Hz); 1.52 (d, 3H, J ) 7.2 Hz); 2.80-3.40 (m, 6H); 3.52 (br. m, 1H);
3.67 (s, 3H); 3.70 (s, 3H); 3.92-3.98 (m, 2H); 4.12-4.24 (m, 4H);
4.34-4.47 (m, 2H); 4.61-4.82 (m, 4H); 7.04 (m, 1H); 7.22-7.31
(m, 5H). 13C NMR (CDCl3): δ 174.7, 174.3, 172.8, 171.6, 169.7, 168.4
(CO); 139.8, 139.2 (C); 128.4, 128.2, 128.1, 128.0 (CH); 72.0, 71.8,
70.9 (CH); 58.1, 56.7 (CH2); 55.4, 55.1, 53.2 (CH); 52.9, 52.7 (CH3O);
51.4, 49.3 (CH); 43.2, 43.0 (CH2); 42.2 (C); 41.7, 39.9 (CH2); 39.7
(C); 36.3, 36.2 (CH2); 36.0, 35.7 (CH); 34.9, 34.6 (CH2); 34.4, 34. 1,
33.8 (CH); 32.7 (CH2); 30.8, 30.5 (CH3); 30.0, 29.6 (CH); 28.8, 23.7
(CH2); 18.0, 16.4, 12.6 (CH3). HRMS (ESI-FT-ICR) m/z: 849.4630
[M + Na]+; calcd for C44H66N4NaO11 849.4627. Anal. Found: C 64.1,
H 8.3, N 7.0. Calcd: C 63.9, H 8.0, N 6.8.
1
1203, 1159, 1086, 1025. H NMR (CDCl3): δ 0.81 (s, 3H); 0.97
(d, 3H, J ) 6.5 Hz); 1.05 (s, 3H); 2.54-2.69 (m, 4H); 2.92-3.04
(m, 2H); 3.11-3.20 (m, 2H); 3.35 (br. m, 1H); 3.94 (m, 1H); 7.61
(d, 1H, J ) 7.7 Hz); 6.92 (m, 1H); 7.20 (m, 1H); 7.94 (t, 1H, J )
7.7 Hz); 8.08 (d, 1H, J ) 7.7 Hz). 13C NMR (CDCl3): δ 12.8,
17.7, 18.2 (CH3); 23.8, 24.0 (CH2); 25.2 (CH); 28.2, 29.6, 33.7
(CH2); 35.7, 36.3 (CH); 39.8 (C); 40.2, 40.8 (CH); 46.6 (CH); 48.9,
49.4 (CH); 52.4, 54.9 (CH2); 72.1, 73.2 (CH); 78.5 (C); 155.4, 152.4
(C); 136.8, 123.7, 119.1 (CH); 171.1, 170.5 (CO). HRMS (ESI-
FT-ICR) m/z: 542.3591 [M + H]+; calcd for C31H48N3O5 542.3597.
Anal. Found: C 68.4, H 8.6, N 7.6. Calcd: C 68.7, H 8.7, N 7.8.
Macrocycle 33. A solution of the 2,6-dioxa-1,8-diaminooctane
(74 mg, 0.5 mmol) and paraformaldehyde (1.0 mmol) in MeOH
(200 mL) was stirred at room temperature for 2 h. Two solutions,
one of diacid 30 (392 mg, 0.5 mmol) and another of cyclohexyl-
isocyanide (90 µL, 1.0 mmol) in 20 mL of MeOH each, were
simultaneously, slowly added to the reaction mixture using syringe
pumps (flow rate 0.5 mL h-1). After addition was completed, the
reaction mixture was stirred for 8 h and concentrated under reduced
pressure to dryness. The resulting crude product was dissolved in
50 mL of THF and tetra-n-butylammonium fluoride trihydrate (535
mg, 1.5 mmol) was added. The reaction mixture was stirred at room
temperature for 12 h and then diluted with 150 mL of EtOAc. The
solution was washed with aqueous 10% NaHCO3 (2 × 30 mL)
and brine (50 mL), dried over anhydrous Na2SO4 and evaporated
under reduced pressure to dryness. Flash column chromatography
purification (CH2Cl2/MeOH 20:1) yielded the macrocycle 33 (224
mg, 54%) as a pale yellow oil. Rf ) 0.58 (CH2Cl2/MeOH 4:1). IR
(ATR, cm-1): 3304, 2930, 2856, 1653, 1638, 1634, 1449, 1272,
3r,8r,12r-Tri(tert-butyldimethylsilyloxy)-7,8-seco-5ꢀ-cholane-
7,24-dioic Acid (30). KOH (0.55 g, 10 mmol) was added to a solution
of lactone 29 (872 mg, 2.0 mmol) in 40 mL of the mixture MeOH/
H2O (4:1, v/v). The reaction mixture was stirred at reflux for 6 h, then
cooled to room temperature, and poured into 100 mL of saturated
aqueous NH4Cl. The organic phase was extracted with EtOAc (4 ×
50 mL), and the combined organic extracts were dried over anhydrous
Na2SO4 and concentrated under reduced pressure to dryness. The
resulting crude product was dried in vacuo for 2 h and suspended in
80 mL of dry CH2Cl2 under nitrogen. The suspension was treated with
Et3N (1.3 mL, 12.0 mmol) and TBDMSOTf (2.75 mL, 12.0 mmol, 6
equiv) and stirred under nitrogen atmosphere for 48 h. The reaction
mixture was diluted with 150 mL of CH2Cl2 and washed sequentially
with saturated aqueous NH4Cl (2 × 50 mL) and brine (50 mL). The
organic phase was dried over anhydrous Na2SO4 and evaporated under
reduced pressure to dryness. The crude product was then dissolved in
50 mL of the mixture THF/H2O (2:1, v/v) and treated with LiOH (120
mg, 5 mmol) at 0 °C. The suspension was allowed to reach room
temperature, stirred for 4 h, and then acidified with 10% aqueous
NaHSO4 to pH 3. EtOAc (200 mL) was added, and the organic phase
was washed sequentially with aqueous 5% HCl (50 mL) and brine
(50 mL), then dried over anhydrous Na2SO4, and evaporated under
1
1098, 1026. H NMR (CDCl3): δ 0.81 (s, 3H); 0.87 (s, 3H); 0.92
(d, 3H, J ) 6.6 Hz); 3.44-3.78 (m, 21H); 3.84-4.17 (m, 4H);
6.21 (d, J ) 8.0 Hz); 6.66 (d, J ) 8.0 Hz). 13C NMR (CDCl3): δ
174.8, 172.7, 167.9, 167.7 (CO); 72.2, 72.0, 71.9 (CH); 70.8, 70.4,
69.2, 67.5, 59.6, 53.4 (CH2); 52.0, 50.3, 48.8 (CH); 48.5, 48.1
(CH2); 47.6, 41.7, (CH); 40.6 (C); 39.5, 36.9 (CH2); 36.5 (C); 33.5,
33.2, 33.0, 32.8, 32.4 (CH2); 31.5, 29.7 (CH); 25.5, 24.9, 24.8, 24.7,
J. Org. Chem. Vol. 73, No. 16, 2008 6237

Rivera et al.
22.8 (CH2); 22.3, 18.9, 14.2 (CH3). HRMS (ESI-FT-ICR) m/z:
853.5662 [M + Na]+; calcd for C46H78N4NaO9 853.5669.
Supporting Information Available: Experimental proce-
dures for the preparation of secocholanic building blocks and
NMR spectra of new compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
Acknowledgment. We are grateful to the Deutsche Fors-
chungsgemeinschaft for financial support (Programs GRK-894,
CERC-3). We also thank Dr. Andrea Porzel for recording the
NOESY spectra.
JO800708M
6238 J. Org. Chem. Vol. 73, No. 16, 2008