Antineoplastic agents. 561. Total synthesis of respirantin

Article abstract of DOI:10.1021/np0680735

Total synthesis of the 18-membered ring cyclodepsipeptide believed to be respirantin (1b) has been achieved. The key step in the synthesis is an intramolecular transesterification of the β-ketoester alcohol 6 to afford the protected macrocycle 5. The synt

Full text of DOI:10.1021/np0680735

J. Nat. Prod. 2007, 70, 1073-1083
1073
Antineoplastic Agents. 561. Total Synthesis of Respirantin1a
George R. Pettit,* Thomas H. Smith, Song Feng, John C. Knight, Rui Tan, Robin K. Pettit, and Peter A. Hinrichs
Cancer Research Institute and Department of Chemistry and Biochemistry, Arizona State UniVersity, P.O. Box 872404,
Tempe, Arizona 85287-2404
ReceiVed December 15, 2006
Total synthesis of the 18-membered ring cyclodepsipeptide believed to be respirantin (1b) has been achieved. The key
step in the synthesis is an intramolecular transesterification of the â-ketoester alcohol 6 to afford the protected macrocycle
5. The synthetic product was shown to be identical to a natural product presumed to be respirantin (1b), and the absolute
stereochemistry of six of the seven asymmetric centers of cyclodepsipeptide 1b was unequivocally established. Respirantin
(1b) was found to be a remarkable inhibitor of cancer cell growth and related to the antimycin family of antibiotics.
In the preceding report, we summarized the isolation and
structures of three exceptional cancer cell growth inhibitory
cyclodepsipeptides from the bacterium Kitasatospora sp. found on
the Beaufort Sea coast of the Alaska North Slope.1a One of these
corresponded to a unique structure and was designated kitastatin 1
(1a) (Figure 1), while the other two cyclodepsipeptides on the basis
of reported NMR assignments were presumed to be respirantin (1b)
and a valeryl modification (1c).1b
Respirantin (1b) was first reported in 19931b as an insecticidal
antibiotic isolated from a Streptomyces species found in a soil
sample from Japan and shown to have cyclodepsipeptide structure
1b on the basis of analysis of its spectroscopic properties. The
stereochemistry was not determined. Kitastatin 1 (1a) and respi-
rantin (1b) contain a blastmycic acid unit also found in the
antimycins2 such as 2 and neoantimycin.3 An unusual structural
feature is the â-ketoester linkage (carbons 6-8) in the 18-membered
depsipeptide macrocycle. In order to obtain sufficient material for
more extensive biological evaluation as well as to determine the
stereochemistry and absolute configuration of kitastatin 1 (1a) and
respirantin (1b), we undertook research to develop a total synthesis
of 1b with flexibility to enable future SAR development. Herein
we report the successful results.
tible to decarboxylation, carboxylic acid 12 was acquired via
carefully controlled saponification. However all attempts to esterify
12 with alcohol 135 were deflected by either decarboxylation to
ketone 15 or intramolecular cyclization to the pyrrolidine-2,4-dione
16. The method of choice for the preparation of complex â-ke-
toesters is via transesterification. However, as this reaction proceeds
through a ketene intermediate,6,7 ester 11 is not a suitable substrate.
Nevertheless this approach was pursued in the belief that the
introduction of the gem-dimethyl groups could be postponed until
the needed â-ketoester linkage was formed. After extensive
experimentation we were able to obtain ester 14 via reaction of
ester 7 with excess alcohol 13 in refluxing cyclohexane8 in the
presence of a catalytic amount of activated zinc.9 However the
modest yield and the need for excess alcohol 13 limited this
approach. A timely report10 describing a high-yield intramolecular
â-ketoester transesterification used to form a 15-membered mac-
rocycle seemed to not only address the troublesome formation of
the required â-ketoester linkage but also simplify projected
functional group manipulations needed for macrocycle formation.
The initial approach designed to utilize the intramolecular
â-ketoester transesterification method of macrocyclization is out-
lined in Scheme 3. The diester 20 was obtained via reaction of the
acid chloride derived from silyl ester 18 under neutral conditions11,12
with alcohol 19.5 Selective hydrolytic cleavage of methyl ester 20
could not be achieved, as extensive cleavage of the internal ester
linkage occurred. The desired carboxylic acid 21 was obtained via
nucleophilic alkyl cleavage with LiI in pyridine.13 Formation of
the amide linkage leading to amide 22 proved to be problematic.
Reaction of carboxylic acid 21 with the amine derived from TFA
deprotection of Boc-protected 7 under a variety of peptide coupling
procedures (BOP,14 PyBroP,15 DEPC16) afforded at best low yields
of amide 22 along with the pyrazine 24. The formation of 24 can
be explained by dimerization of the amine free base via Schiff base
formation followed by oxidative aromatization. Subsequent experi-
mentation revealed that while the trifluoroacetate salt of the parent
amine from 7 could be isolated, we were not able to isolate the
corresponding free base. In attempts to prepare amide 22, dimer-
ization of the free base occurred in preference to reaction with the
activated carboxylic acid 21. To avoid this problem a solution of
the TFA salt in DCM was added to a solution of 21, PyBroP, and
3 equiv of DIPEA in DCM. By this method, the free base was
generated only in the presence of excess activated carboxylic acid,
and a reasonable yield of amide 22 was reproducibly obtained.
Results and Discussion
Inspection of the kitastatin 1 (1a) and respirantin (1b) macrocycle
revealed that they are composed of common amino acids, or
R-hydroxycarboxylic acids derived from them, along with the
â-ketoester unit. Since the absolute stereochemistry of 1a and 1b
was undetermined at the onset of this study, our initial target 1b
was selected by assuming the most common S-configuration for
the constituent amino acids and their presumed R-hydroxy deriva-
tives. Fortunately, that proved to be the correct choice among the
256 possible optical isomers. A retrosynthetic analysis of the
ultimately successful route to respirantin (1b) is presented in
Scheme 1. Prior antimycin syntheses2 offered good precedent for
appending the protected benzoic acid 3 to amino-substituted
macrocycles. However other issues that needed to be addressed in
developing our approach to 1b included introduction of the
â-ketoester unit, selection of appropriate esterification and peptide
bond forming methods, protecting-group strategy, and the method
and site of its macrocyclic lactonization.
Our initial approach is outlined in Scheme 2, where â-ketoester
74 represented a good starting material for incorporating carbons
6-9. Introduction of the gem-dimethyl groups was not trivial, but
after some experimentation R,R-dimethyl ester 11 was obtained in
reasonable yield. While â-ketoacids are well known to be suscep-
Deprotection of amide 22 was also nontrivial. Standard TBAF
treatment afforded a fairly complex mixture, of which the desired
alcohol 23 was the major component. Better results were obtained
using BF3‚Et2O,17 which provided alcohol 23 cleanly and in high
yield. Condensation of 23 with carboxylic acid 2518 mediated with
2-methyl-6-nitrobenzoic anhydride (MNBA)19 afforded ester 26 in
* To whom correspondence should be addressed. Tel: (480) 965-3461.
Fax: (480) 965-2747. E-mail: bpettit@asu.edu.
10.1021/np0680735 CCC: $37.00
© 2007 American Chemical Society and American Society of Pharmacognosy
Published on Web 07/04/2007

1074 Journal of Natural Products, 2007, Vol. 70, No. 7
Pettit et al.
Figure 1. Kitastatin, respirantin, and valeryl modification and (+)-antimycin A3b.
Scheme 1. Retrosynthetic Analysis of Respirantin
a reasonable yield. Desilylation of 26 using the BF3‚Et2O procedure
cleanly provided a 1:1 mixture of isomeric alcohols 27 and 28.
The spectroscopic and analytical properties of both were consistent
with the expected product and characterized as 27-28, a mixture
of diastereomers arising from racemization of the carbon bearing
the terminal hydroxyl.
convergency of the synthesis. However, efforts to deprotect either
the carboxyl (mild base or LiI/pyridine) or the hydroxyl (TBAF)
groups led to â-elimination of the leucic acid moiety, leading to
olefin 36 as the major product (Scheme 5). These results introduced
additional constraints upon the reagents available for this synthetic
approach. The desired desilylated alcohol 35 was eventually
obtained by BF3‚Et2O deprotection.
With these results in mind we embarked on the ultimately
successful route to respirantin. Scheme 6 outlines our approach to
the respirantin macrocyclic lactone 5. Condensation of the acid
chloride derived from 18 with alcohol 3722 provided ester 38. The
tert-butyl ester was chosen for carboxyl protection due to the lability
of the leucic acid portion to the conditions required for methyl ester
cleavage. Desilylation with TBAF followed by MNBA-mediated
condensation of the resulting alcohol 10 with carboxylic acid 25
provided ester 39 and ultimately alcohol 9 following TBAF
An explanation for the epimerization evident during the depro-
tection of silyl ether 26 remains obscure. Model studies (Scheme
4) did not indicate evidence of any obvious problem. The
epimerization problem and the somewhat variable results in the
presence of the â-ketoester suggested that the presence of this
potentially base labile moiety could be a problem and that delaying
its introduction should be beneficial. Concurrently additional model
studies indicated another area of concern. The C1-5 fragment 34
was prepared by condensation of the acid chloride derived from
silyl ester 3220 and alcohol 3321 with a view toward increasing the

Total Synthesis of Respirantin
Journal of Natural Products, 2007, Vol. 70, No. 7 1075
Scheme 2a
a Reagents and conditions: (a) K2CO3, MeI, DMSO, 23 °C, 48 h, 65%; (b) KOH, aq CH3OH, 23 °C, 0.25 h, 81%; (c) 13, Zn, cyclohexane, 80 °C, 33%.
Scheme 3a
a Reagents and conditions: (a) (i) Oxalyl chloride, catalytic DMF, DCM, 0-23 °C, 2 h; (ii) 19, pyridine, 23 °C, 16 h, 75%; (b) LiI, pyridine, 110 °C, 40 h, 89%;
(c) (i) 7, 1:1 TFA-DCM, 0.5 h; (ii) PyBroP, DIPEA, DCM, product from (i), 4 h, 65%; (d) BF3‚Et2O, DCM, 0.5 h, 87% for 23, 83% for 27-28; (e) 25, MNBA,
DMAP, TEA, DCM, 23 °C, 16 h, 77%.
Scheme 4a
a Reagents and conditions: (a) TBAF, THF, 0 °C, 1 h, 85%; (b) 25, MNBA, DMAP, TEA, DCM, 23 °C, 16 h, 75%; (c) BF3‚Et2O, DCM, 23 °C, 1.5 h, 92%.
deprotection. Anticipating the need for acidic conditions to achieve
deprotection of the tert-butyl ester, it was considered prudent to
utilize the more stable TBDPS protecting group rather than the usual
TBDMS group for the terminal hydroxyl protection. Condensation
of carboxylic acid 40 (available from ester 13)5 with alcohol 9 using
the MNBA procedure provided tetraester 42. Cleavage of the tert-

1076 Journal of Natural Products, 2007, Vol. 70, No. 7
Pettit et al.
Scheme 5a
a Reagents and conditions: (a) (i) oxalyl chloride, catalytic DMF, DCM, 0-23 °C, 2 h; (ii) 33, pyridine, 23 °C, 16 h, 55%; (b) BF3‚Et2O, DCM, 23 °C, 1 h, 51%;
(c) K2CO3, aqueous CH3OH or TBAF.
Scheme 6a
a Reagents and conditions: (a) (i) oxalyl chloride, catalytic DMF, DCM, 0-23 °C, 2 h; (ii) 37, pyridine, 23 °C, 16 h, 81%; (b) TBAF, THF, 23 °C, 1 h, 100%; (c)
25, MNBA, DMAP, TEA, DCM, 23 °C, 16 h, 87%; (d) TBAF, THF, 23 °C, 1 h, 100%; (e) (i) TBDPSCl or TBDMSCl, imidazole, DMF, 23 °C, 16 h; (ii) LiOH, aq
THF/CH3OH, 0-23 °C, 24 h, 80% for 40, 85% for 41; (f) 40 or 41, MNBA, DMAP, TEA, DCM, 23 °C, 16 h, 85% for 42, 83% for 43; (g) ZnBr2, DCM, 23 °C, 24
h, 80% for 44; SiO2, toluene, 110 °C, 4 h, 59% for 8; (h) (i) 7, 1:1 TFA-DCM, 0.5 h; (ii) PyBroP, DIPEA, DCM, product from (i), 4 h, 52% for 45, 65% for 46; (i)
CH3OH, AcCl, 0.5 h, 63%; (j) toluene, anhydrous CuSO4, 125 °C, 4 h, 80%.
butyl group was achieved with ZnBr2 in DCM,23 providing
carboxylic acid 44, which was coupled with the amine derived from
â-ketoester 7 employing the PyBroP coupling procedure previously
described to afford amide 45. At this point we were challenged by
attempts to remove the TBDPS group, which resulted in eliminating
the leucic acid unit. For example, amide 45 was inert to BF3‚Et2O
at ambient temperature, as well as several other acidic reagents,
and TBAF caused the expected elimination of the leucic acid
residue. Consequently, we chose to proceed with a MNBA-
promoted coupling of alcohol 9 with carboxylic acid 41 to provide
ester 43. To achieve good results with this esterfication, it was
necessary to use freshly prepared acid 41. Apparently the acidity

Total Synthesis of Respirantin
Journal of Natural Products, 2007, Vol. 70, No. 7 1077
Scheme 7a
a Reagents and conditions: (a) formamide, 150 °C, 0.5 h, 100%;27 (b) MeI, NaHCO3, DMF, 23 °C, 18 h, 83%;28 (c) BzlBr, K2CO3, DMF, 60 °C, 18 h, 95%;29 (d)
LiOH, aq Thf/CH3OH, 23 °C, 18 h, 79%.
Scheme 8a
a Reagents and conditions: (a) MeI, K2CO3, DMSO, 23 °C, 3 h, 28%; (b) H2, Pd/C, EtOAc, 23 °C, 2 h, 73%; (c) 3, EDCI, HOBt, NMM, DMF, 23 °C, 11 h, 61%;
(d) H2, Pd/C, EtOAc, 23 °C, 2 h, 82%.
of 41 is sufficient to cause decomposition to the corresponding
R-hydroxy acid. As anticipated, cleavage of the tert-butyl ester in
the presence of the TBDMS group proved to be problematic. The
ZnBr2 procedure successful with silyl ether 42 resulted in simul-
taneous cleavage of the TBDMS group and the tert-butyl ester.
Selective carboxyl deprotection was achieved by treatment of tert-
butyl ester 43 with flash silica gel24 in refluxing toluene to afford
8. PyBroP-promoted condensation of 8 with the amine derived from
â-ketoester 7 provided the key intermediate ester 46. Desilylation
once again proved to be a nontrivial operation. Similar to the results
observed with silyl ether 26, reaction of silyl ether 46 with BF3‚
Et2O afforded a 1:1 mixture of compounds with spectral and
analytical properties consistent with epimeric alcohols 6 and 47.
Better results were obtained by effecting desilylation using acetyl
chloride in CH3OH,25 which provided predominantly a single
product. While we were unable to unequivocally distinguish
between epimers 6 and 47 for the desilylation product, we
tentatively assigned isomer 6 as the structure for the predominant
product. The stage was now set for the key macrocyclization step.
Gratifyingly, treatment of alcohol 6 in refluxing toluene10 in the
26
presence of catalytic anhydrous CuSO4 smoothly afforded mac-
rocyclic lactone 5.
The synthesis of the aromatic synthon 3 was achieved in four
steps from the commercially available intermediate 48 as outlined
in Scheme 7. The completion of the synthesis is outlined in Scheme
8. Introduction of the gem-dimethyl groups at C7 (5 f 52) was
problematic. Insertion of one methyl group occurred readily, while
addition of the second methyl group to afford lactone 52 was more
difficult and occurred in only a modest yield. Hydrogenolysis of
the Cbz protecting group afforded amine 4, which was condensed
with benzoic acid 3 employing EDCI to provide amide 53. Removal
(hydrogenolysis) of the benzyl ether protecting group provided the
cyclodepsipeptide presumed on the basis of spectroscopic data to
be respirantin (1b). The synthetic specimen of cyclodepsipeptide
(1b) was found to be identical with the natural product 1b. The

1078 Journal of Natural Products, 2007, Vol. 70, No. 7
Pettit et al.
Table 1. Comparison of the Cancer Cell Growth Inhibition (GI50, µg/mL) of Kitastatin 1 (1a), Respirantin (1b), and the Valeryl
Analogue 1c against a Panel of Murine (P388, Lymphocytic Leukemia) and Human Cancer Cell Lines
leukemia
P388
pancreas
BXPC-3
breast
MCF-7
CNS
SF268
lung-NSC
NCI-H460
colon
KM20L2
prostate
DU-145
compound
1a
1b
1c
0.045
0.0037
0.033
0.0066
0.47
1.2
0.004
0.0006
0.00062
0.0035
0.0016
0.016
<0.001
0.0006
0.00063
0.0024
0.0006
0.00058
0.0026
0.00018
<0.0001
spectral properties (1H, 13C NMR, IR, HRMS) of 1b matched
perfectly with the published values for respirantin.1b
4-(tert-Butoxycarbonyl)amino-2,2,6-trimethyl-3-oxoheptanoic Acid
(12). To ester 11 (65.7 mg, 0.21 mmol) in CH3OH (0.35 mL) under
N2 was added 3.5 N KOH (0.25 mL, 0.88 mmol) and the solution stirred
at ambient temperature for 15 min. The reaction mixture was diluted
with H2O (15 mL) and washed with Et2O (2 × 15 mL). The aqueous
layer was acidified (pH 2) with 1 N H2SO4 and extracted with Et2O (3
× 15 mL). The combined extract was washed with H2O (5 mL) and 5
M NaCl (5 mL), dried, and evaporated to afford 50.8 mg (81%) of 12
as a viscous oil, which solidified on standing: mp 121 °C; TLC Rf
The absolute stereochemistry of depsipeptide 1b at carbons 2,
3, 9, 11, and 13 follows from the chirality of the starting materials.
Presumably, the 2(S),3(R)-stereochemistry of natural threonine and
the 2(S),3(S)-stereochemistry of natural isoleucine have been
retained in the biosynthesis of kitastatin 1 (1a) and respirantin
(1b). In accord with that assumption, C-5 was tentatively
assigned the R-configuration (cf. 1b), as the synthetic and natural
specimens were identical. The modular nature of this approach
should offer ready access to the scale-up synthesis of respirantin,
kitastatin, and a variety of structural modifications to develop
structure-activity relationships in this interesting class of powerful
cancer cell growth inhibitors.
1
0.52 (95:5:1 DCM-CH3OH-HOAc); IR 3268, 1714, 1655 cm-1; H
NMR (d6-DMSO) δ 7.05 (1H, d, J ) 9 Hz), 4.51 (1H, td, J ) 9, 7
Hz), 1.87 (1H, m), 1.38 (9H, s), 1.34 (3H, s), 1.26 (4H, s and m), 0.99
(1H, dd, J ) 9, 7 Hz), 0.87 (6H, d); 13C NMR (d6-DMSO) δ 214.1,
155.5, 78.0, 56.8, 38.3, 35.3, 28.1, 23.0, 21.1, 18.7, 18.2; anal. C
60.12%, H 9.27%, N 4.68%, calcd for C15H27NO5, C 59.78%, H 9.03%,
N 4.65%.
Kitastatin 1 (1a), respirantin (1b), and the valeryl analogue 1c
were evaluated as inhibitors of cancer cell growth versus the murine
P388 leukemia cell line30 and a panel of human cancer cell lines.31
The data are reported in Table 1. All three compounds displayed
an impressive spectrum of activity. An interesting observation was
the substantially better activity of kitastatin 1 (1a) against the
pancreas BXPC-3 human cancer cell line relative to the other panel
members. Whether this indicates a special selectivity against this
cancer is a question that must be explored. Pancreatic cancer is
one of the most deadly types and is notoriously refractory to current
modes of treatment. In addition to the human cancer cell line activity
cyclodepsipeptide 1b had activity against the pathogenic fungus
Cryptococcus neoformans (minimum inhibitory activity, MIC )
2).1a
1-Methoxycarbonyl-3-methylbutyl 4-tert-Butoxycarbonylamino-
6-methyl-3-oxoheptanoate (14). Ketone 7 (0.274 g, 0.95 mmol),
alcohol 13 (0.17 g, 1.13 mmol), and activated Zn (30 mg, 0.46 mmol)
were placed in cyclohexane (4 mL) under N2 and heated at reflux for
16 h with a Dean-Stark separator. Additional 13 (0.17 g, 1.13 mmol)
in cyclohexane (1 mL) was added, and heating at reflux continued for
24 h. The solution was diluted with EtOAc (40 mL), filtered through
Celite, washed with 6% NaHCO3 (3 × 10 mL), H2O (10 mL), and 5
M NaCl (10 mL), dried, and evaporated to give 0.38 g of a pale yellow
oil. This was flash chromatographed (10 g, SiO2, 93:7 hexane-EtOAc)
to afford 0.127 g (33%) of ester 14 as a colorless oil: TLC Rf 0.50
(4:1 hexane-EtOAc); IR 3368, 1751, 1712 cm-1; 1H NMR δ 5.48 (1H,
br d), 5.08 (1H, m), 4.28 (1H, br t), 3.77 and 3.46-3.80 (5H, s and
m), 1.56-1.82 (6H, m), 1.45 (9H, s), 0.95 (12H, m); 13C NMR δ 203.3,
171.1, 166.2, 155.8, 79.8, 71.6, 58.5, 52.4, 46.1, 39.7, 39.6, 28.2, 24.7,
24.4, 23.2, 22.8, 21.4, 21.3; anal. C 59.88%, H 9.02%, N 3.49%, calcd
for C20H35NO7, C 59.83%, H 8.79%, N 3.49%.
Experimental Section
General Experimental Procedures. Solvents were redistilled prior
to use. Reagents were used as received. MNBA was obtained from
TCI America. Thin-layer chromatography (TLC) was carried out with
Analtech 250 µm thick silica gel GHLF plates and visualized with H2-
SO4, phosphomolybdic acid, iodine, or UV. Organic extracts were dried
over anhydrous Na2SO4 and evaporated under reduced pressure using
a rotary evaporator. The crude products were separated by flash column
chromatography on flash (230-400 mesh ASTM) silica from E. Merck.
Melting points are uncorrected and were determined employing an
Electrothermal Mel-Temp apparatus. Optical rotations were measured
using a Perkin-Elmer 241 polarimeter. The [R]D values are given in
10-1 deg cm2 g-1. IR spectra were obtained with a Thermo Nicolet
Avatar 360 FT-IR instrument equipped with a single reflection
horizontal ATR sampling device from PIKE Technologies. HRMS data
Methyl 2-[2-(tert-Butyldimethylsilyl)oxypropionyloxy]-3-meth-
ylpentanoate (20). Silyl ether 18 (3.23 g, 10.16 mmol) was dissolved
in CH2Cl2 (10 mL) containing DMF (280 µL, 0.26 g, 3.62 mmol) under
N2 and cooled to 0 °C. Oxalyl chloride (5.6 mL of a 2 M solution in
CH2Cl2, 11.2 mmol) was added dropwise over 5 min. The solution
was stirred at 0 °C for 1.5 h and at ambient temperature for 0.5 h. The
solvent was evaporated. To the residue was added dropwise a solution
of alcohol 19 (1.27 g, 8.71 mmol) in pyridine (5 mL). The solution
was stirred under N2 for 16 h, diluted with THF (100 mL), and filtered
through Celite. The filtrate was evaporated, and the residue was
partitioned between EtOAc (200 mL) and H2O (20 mL). The organic
phase was separated, washed with H2O (20 mL), 6% NaHCO3 (2 ×
30 mL), H2O (20 mL), and 5 M NaCl (10 mL), dried, and evaporated.
The residue was flash chromatographed (90 g, SiO2, 96:4 hexane-
EtOAc) to yield 2.16 g (75%) of ester 20: TLC Rf 0.37 (95:5 hexane-
1
were recorded with a JEOL LCmate mass spectrometer. The H and
1
EtOAc); IR 1757 cm-1; H NMR δ 4.93 (1H, d, J ) 4.8 Hz), 4.41
13C spectra were recorded employing Varian Gemini 300, Varian Unity
400, or Varian Unity 500 instruments in CDCl3 unless otherwise noted
and were referenced to either TMS or the solvent. Elemental analyses
were determined by Galbraith Laboratories, Inc., Knoxville, TN.
Methyl 4-(tert-Butoxycarbonyl)amino-2,2,6-trimethyl-3-oxohep-
tanoate (11). Ketone 7 (0.47 g, 1.63 mmol), K2CO3 (2.26 g, 16.3
mmol), and MeI (0.31 mL, 0.71 g, 4.98 mmol) were placed in DMSO
(7 mL) under N2 and stirred at ambient temperature for 48 h. The
reaction mixture was diluted with H2O (30 mL) and extracted with
Et2O (3 × 20 mL). The extracts were combined, washed with H2O (10
mL) and 5 M NaCl (5 mL), dried, and evaporated. The residue was
flash chromatographed (15 g, SiO2, 95:5 hexane-EtOAc) to afford 0.34
g (65%) of ketone 11 as a colorless oil: TLC Rf 0.63 (4:1 hexane-
(1H, q, J ) 6.6 Hz), 3.72 (3H, s), 2.01 (1H, m), 1.45 (3H, d), 1.33
(2H, m), 0.97 (3H, d), 0.92 (3H, t), 0.91 (9H, s), 0.11 and 0.09 (6H,
2s); 13C NMR δ 179.1, 175.2, 81.7, 73.4, 57.3, 41.9, 31.0, 29.9, 26.7,
23.6, 20.6, 16.8, 0.4, 0.0; anal. C 58.07%, H 9.87%, calcd for C16H32O5-
Si, C 57.79%, H 9.70%.
2-[2-(tert-Butyldimethylsilanyloxy)propionyloxy]-3-methylpen-
tanoic Acid (21). Ester 20 (1.01 g, 3.04 mmol) and LiI (1.24 g, 9.25
mmol) were placed in pyridine (8.0 mL) under N2 and stirred at 105
°C for 40 h. The reaction mixture was allowed to cool, diluted with
toluene (30 mL), evaporated, and coevaporated with toluene (20 mL).
The residue was diluted with H2O (50 mL), acidified (pH 4) with
KHSO4, and extracted with EtOAc (3 × 30 mL). The extracts were
combined, washed with 10% Na2S2O3 (10 mL), H2O (10 mL), and 5
M NaCl (10 mL), dried, and evaporated. The residue was flash
chromatographed (30 g, SiO2, 99:1:0.5 DCM-CH3OH-HOAc) to
provide 0.86 g (89%) of carboxylic acid 21 as a pale yellow oil: TLC
1
EtOAc); IR 3377, 1705 cm-1; H NMR δ (4.78 1H, br d), 4.65 (1H,
m), 3.73 (3H, s), 1.70 (1H, m), 1.43 (17H, m), 0.95 and 0.92 (6H, 2 d,
J ) 6.6 Hz); 13C NMR δ 208.7, 173.5, 155.0, 79.7, 54.7, 53.8, 52.5,
41.8, 28.3, 24.6, 23.5, 22.2, 22.0, 21.33.

Total Synthesis of Respirantin
Journal of Natural Products, 2007, Vol. 70, No. 7 1079
1
Rf 0.58 (95:5:1 DCM-CH3OH-HOAc); IR 1726 cm-1; H NMR δ
4.98 (1H, d, J ) 4.4 Hz), 4.42 (1H, q, J ) 7.6 Hz), 2.04 (1H, m), 1.56
(1H, m), 1.45 (3H, d, J ) 6.6 Hz), 1.37 (1H, m), 1.01 (3H, d, J ) 7.2
evaporated. The residue was flash chromatographed (15 g, SiO2, 85:
15 hexane-EtOAc) to yield 0.36 g (77%) of ester 26 as a colorless
oil, which crystallized on standing: mp 84-86 °C; TLC Rf 0.22 (4:1
Hz), 0.94 (3H, t, J ) 7.1 Hz), 0.91 (9H, s), 0.11 and 0.08 (6H, 2s); 13
NMR δ 175.3, 173.8, 75.9, 68.1, 36.5, 25.7, 24.4, 21.3, 18.2, 15.3,
C
1
hexane-EtOAc); H NMR δ 7.37 (5H, m), 6.94 (1H, d, J ) 8.3 Hz),
5.47 (1H, d, J ) 9.4 Hz), 5.16 (4H, m), 4.67 (1H, m), 4.48 (1H, q),
4.27 (1H, m), 3.72 (3H, s), 3.52-3.62 (2H, m), 2.00 (1H, m), 1.63
(2H, m), 1.53 (4H, d and m, J ) 7.1 Hz), 1.25 (5H, m), 0.93 (12H,
m), 0.84 (9H, s), 0.06 and -0.01 (6H, 2 s); 13C NMR δ 202.0, 171.7,
168.9, 168.8, 167.5, 156.7, 136.2, 128.6, 128.3, 128.0, 78.5, 70.0, 68.7,
67.1, 60.2, 56.4, 52.3, 45.9, 38.8, 37.1, 25.6, 24.7, 24.2, 23.3, 21.2,
21.1, 17.8, 16.9, 14.8, 11.4, -4.4, -5.4; MSFAB+ 723.3890 (M +
H), calcd 723.3889; anal. C 59.95%, H 8.44%, N 3.91%, calcd for
C36H58N2O11Si, C 59.81%, H 8.09%, N 3.87%.
11.5, -5.0, -5.4.
Methyl 4-{2-[2-(tert-Butyldimethylsilyloxy)propionyloxy]-3-me-
thylpentanoylamino}-6-methyl-3-oxoheptanoate (22). Ketone 7 (0.88
g, 3.05 mmol) was placed in 1:1 TFA-DCM (12.0 mL) under N2 and
stirred at ambient temperature for 1 h. The solvent was removed and
the residue coevaporated with toluene (2 × 10 mL). Carboxylic acid
21 (0.88 g, 2.76 mmol) and PyBroP (1.29 g, 2.76 mmol) in DCM (6.0
mL) under N2 was cooled to 0 °C. Diisopropylethylamine (1.07 g, 1.4
mL, 8.28 mmol) was added over 5 min. The residue from the TFA
cleavage reaction was dissolved in DCM (10 mL) and added over 15
min. The solution was stirred at 0 °C for 4.5 h. The reaction mixture
was diluted with EtOAc (100 mL), washed with 5% citric acid (2 ×
10 mL), H2O (10 mL), 6% NaHCO3 (2 × 10 mL), H2O (10 mL), and
5 M NaCl (10 mL), dried, and evaporated. The residue was flash
chromatographed (60 g, SiO2, 90:10 f 80:20 hexane-EtOAc) to afford
0.88 g (65%) of amide 22 as a yellow oil: TLC Rf 0.39 (4:1 hexane-
Methyl 4-{2-[2-(2-Benzyloxycarbonylamino-3-hydroxybutyry-
loxy)propionyloxy]-3-methylpentanoylamino}-6-methyl-3-oxohep-
tanoate (27, 28). To silyl ether 26 (0.73 g, 1.02 mmol) in DCM (30
mL) under N2 was added BF3‚Et2O (1.44 g, 1.3 mL, 10.2 mmol) and
the solution stirred at ambient temperature for 1.5 h. The solution was
poured into 6% NaHCO3-ice (100 mL). The organic phase was
separated and the aqueous phase extracted with DCM (50 mL). The
combined extract was washed with 6% NaHCO3 (30 mL), H2O (20
mL), and 5 M NaCl (20 mL), dried, and evaporated. The residue was
flash chromatographed (20 g, SiO2, 70:30 hexane-EtOAc) to afford
0.155 g (25%) of alcohol 27 as a single isomer: TLC Rf 0.62 (50:50
hexane-EtOAc); 1H NMR δ 12.11 (0.1H, enolic H, s), 7.35 (5H, m),
6.72 (1H, d, J ) 8.2 Hz), 5.60 (1H, d, 9.9 Hz), 5.18 and 5.14 (4H, m
and s), 4.67 (1H, m), 4.53 (1H, m), 4.39 (1H, d, J ) 9.3 Hz), 3.72
(3H, s), 3.64 (1H, d, J ) 16.2 Hz), 3.50 (1H, d, J ) 16.5 Hz), 3.13
(1H, d, J ) 5.8 Hz), 2.05 (1H, m), 1.59 and 1.46-1.64 (7H, d and m),
1.32 (5H, m), 0.92 (12H, m); 13C NMR δ 202.5, 171.5, 169.9, 168.9,
167.9, 156.9, 136.2, 128.5, 128.2, 128.0, 78.5, 69.9, 67.6, 67.2, 58.8,
56.4, 52.6, 46.2, 38.8, 37.1, 24.7, 24.0, 23.2, 21.3, 20.1, 17.2, 15.0,
11.4; anal. C 59.09%, H 7.45%, N 4.49%, calcd for C30H44N2O11, C
59.20%, H 7.29%, N 4.60%. Continued elution led to 0.16 g (31%) of
alcohols 27 and 28 as a mixture of isomers. Further elution provided
0.20 g (39%) of alcohol 28 as a single isomer: TLC Rf 0.58 (50:50
hexane-EtOAc); 1H NMR δ 12.04 (0.1H, enolic H, s), 7.35 (5H, m),
6.50 (1H, d, J ) 8.3 Hz), 5.65 (1H, d, J ) 9.3 Hz), 5.18 and 5.14 (3H,
m and s), 5.03 (1H, m), 4.72 (1H, m), 4.53 (1H, m), 4.42 (1H, d, J )
9.4 Hz), 3.72 (3H, s), 3.55 (2H, m), 3.03 (1H, d, J ) 5.5 Hz), 1.99
(1H, m), 1.61 (6H, d and m, J ) 7.2 Hz), 1.29 (5H, m and d, J ) 6.6
Hz), 0.93 (12 H, m); 13C NMR δ 201.7, 171.1, 170.2, 168.5, 167.3,
156.8, 136.2, 128.5, 128.2, 128.0, 78.8, 69.7, 67.8, 67.1, 59.2, 56.5,
52.5, 46.2, 39.6, 36.9, 24.9, 24.4, 23.2, 21.6, 21.3, 19.7, 16.8, 14.8,
11.2; anal. C 59.28%, H 7.58%, N 4.24%, calcd for C30H44N2O11, C
59.20%, H 7.29%, N 4.60%.
EtOAc); IR 3341, 1750, 1665 cm-1; H NMR δ 11.98 (0.1H, s), 6.46
1
(1H, d, J ) 7.5 Hz), 5.14 (1H, d, J ) 4.5 Hz), 4.72 (1H, m), 4.43 (1H,
q), 3.86 (0.5H, s), 3.72 (3H, s), 3.57 (1H, d, J ) 16.5 Hz), 3.49 (1H,
d, J ) 15.3 Hz), 2.04 (1H, m), 1.65 (2H, m), 1.46 (6H, m and d), 1.26
(1H, m), 0.91 (21H, m), 0.12 and 0.10 (6H, 2 d); 13C NMR δ 201.5,
172.8, 169.1, 167.2, 89.5, 77.6, 68.3, 56.3, 52.4, 46.1, 39.7, 37.0, 25.7,
24.8, 24.2, 23.2, 22.4, 21.4, 21.3, 18.1, 14.9, 11.4, -4.9, -5.2; MS
APCI+ 488.30444 [M + H]+, calcd 488.3044; anal. C 58.94%, H
9.54%, N 2.94%, calcd for C24H45NO7Si, C 59.11%, H 9.30%, N 2.87%.
Methyl 4-[2-(2-Hydroxypropionyloxy)-3-methylpentanoylamino]-
6-methyl-3-oxoheptanoate (23). To amide 22 (0.51 g, 1.04 mmol) in
DCM (30 mL) under N2 was added BF3‚Et2O (1.42 g, 1.27 mL, 10
mmol) and the solution stirred at ambient temperature for 2 h. The
solution was poured into 6% NaHCO3-ice (100 mL). The organic phase
was separated and the aqueous phase extracted with DCM (40 mL).
The combined organic extract was washed with 6% NaHCO3 (30 mL),
H2O (20 mL), and 5 M NaCl (20 mL), dried, and evaporated. The
residue was flash chromatographed (15 g, SiO2, 60:40 hexane-EtOAc)
to give 0.34 g (87%) of carboxylic acid 23 as a colorless oil: TLC Rf
1
0.34 (50:50 hexane-EtOAc); H NMR δ 12.02 (0.1H, enolic H, s),
6.53 (1H, d, J ) 8.2 Hz), 5.14 (1H, d, J ) 4.9 Hz), 4.72 (1H, m), 4.40
(1H, m), 3.74 (3H, s), 3.60 (1H, d, J ) 16 Hz), 3.50 (1H, d, J ) 16
Hz), 2.95 (1H, d, J ) 5.5 Hz), 2.05 (1H, m), 1.61 (2H, m), 1.50 (4H,
d and m, J ) 7.1 Hz), 1.28 (1H, m), 0.95 (12H, m); 13C NMR δ 201.6,
174.4, 168.7, 167.4, 78.5, 67.1, 56.5, 52.6, 46.2, 40.0, 37.0, 25.0, 24.3,
23.7, 21.5, 20.2, 15.0, 11.4; FABMS 374.2189 [M + H]+, calcd
374.2179; anal. C 57.16%, H 8.56%, N 3.70%, calcd for C18H31NO7‚
0.2H2O, C 57.33%, H 8.41%, N 3.71%.
Methyl 2-(2-Hydroxypropionyloxy)-3-methylpentanoate (29). To
a solution cooled to 0 °C of silyl ether 20 (0.52 g, 1.56 mmol) in THF
(10 mL) under N2 was added a 1 M THF solution (3.2 mL) of TBAF
dropwise and the resulting solution stirred at 0 °C for 20 min. The
solution was poured into H2O and extracted with EtOAc (3 × 25 mL).
The combined extract was washed with H2O (10 mL) and 5 M NaCl
(10 mL), dried, and evaporated. The residue was flash chromatographed
(15 g, SiO2, 85:15 hexane-EtOAc) to afford 0.29 g (85%) of alcohol
29 as a colorless oil: TLC Rf 0.29 (80:20 hexane-EtOAc); IR 3488,
1744 cm-1; 1H NMR δ 5.00 (1H, d, J ) 4.4 Hz), 4.37 (1H, q, J ) 6.6
Hz), 3.75 (3H, s), 2.82 (1H, d, J ) 6.0 Hz), 2.04 (1H, m), 1.50 (4H,
d and m, J ) 6.0 Hz), 1.33 (1H, m), 0.98 (3H, d, J ) 7.1 Hz), 0.93
(3H, t, J ) 7.7 Hz); 13C NMR δ 175.44, 169.58, 66.66, 52.13, 36.51,
24.41, 20.50, 15.31, 11.49.
Methyl (3,6-Diisobutyl-5-methoxycarbonylmethylpyrazin-2-yl)-
acetate (24). Ketone 7 (0.28 g, 0.97 mmol) was placed in 1:1 TFA-
DCM (4.0 mL) under N2 and stirred at ambient temperature for 45
min. The solvent was evaporated and the residue coevaporated with
toluene (2 × 10 mL). The residue was dissolved in DCM (3.0 mL)
and cooled to 0 °C. TEA (0.41 mL, 293.3 mg, 2.91 mmol) was added
dropwise and the solution stirred at 0 °C for 4 h. The reaction mixture
was diluted with EtOAc (50 mL), washed with 5% citric acid (2 × 10
mL), H2O (10 mL), 6% NaHCO3 (2 × 10 mL), H2O (10 mL), and 5
M NaCl (10 mL), dried, and evaporated. The residue was flash
chromatographed (10 g, SiO2-, 95:5 f 90:10 hexane-EtOAc) to afford
78.2 mg (49%) of 24 as a pale yellow solid, which was recrystallyzed
from hexane (1 mL): TLC Rf 0.34 (4:1 hexane-EtOAc); mp 72-74
°C; IR 1734 cm-1; 1H NMR δ 3.87 (4H, s), 3.70 (6H, s), 2.61 (4H, d,
J ) 7.1 Hz), 2.14 (2H, m), 0.92 (12H, d); 13C NMR δ 170.6, 151.7,
146.2, 52.1, 42.6, 40.6, 28.2, 22.4; anal. C 63.82%, H 8.52%, N 8.20%,
calcd for C18H28N2O4, C 64.26%, H 8.39%, N 8.33%.
Methyl 4-(2-{2-[2-Benyloxycarbonylamino-3-(tert-butyldimeth-
ylsilyloxy)butyryloxy]propionyloxy}-3-methylpentanoylamino)-6-
methyl-3-oxoheptanoate (26). Carboxylic acid 25 (0.26 g, 0.72 mmol),
alcohol 23 (241.2 mg, 0.65 mmol), MNBA (0.25 g, 0.73 mmol), DMAP
(20.0 mg, 0.16 mmol), and TEA (0.30 mL, 0.22 g, 2.13 mmol) were
placed in DCM (3.5 mL) under N2 and stirred at ambient for 16 h. The
reaction mixture was diluted with EtOAc (50 mL), washed with H2O
(10 mL), 6% NaHCO3 (2 × 10 mL), H2O (10 mL), 5% citric acid (2
× 10 mL), H2O (10 mL), and 5 M NaCl (10 mL), dried, and
Methyl 2-{2-[2-Benyloxycarbonylamino-3-(tert-butyldimethylsi-
lyloxy)butyryloxy]propionyloxy}-3-methylpentanoate (30). Alcohol
29 (0.115 g, 0.53 mmol) and carboxylic acid 25 (0.213 g, 0.58 mmol)
were allowed to react using the MNBA esterification procedure
described for 26 to afford 0.23 g (75%) of ester 30 as a colorless oil:
1
TLC Rf 0.43 (80:20 hexane-EtOAc); H NMR δ 7.37 (5H, m), 5.47
(1H, d, J ) 9.3 Hz), 5.24 (1H, q, J ) 7.1 Hz), 5.14 (2H, s), 4.98 (1H,
d, J ) 4.4 Hz), 4.47 (1H, m), 4.30 (1H, dd, J ) 9.3, 1.6 Hz), 3.73 (3H,
s), 2.01 (1H, m), 1.57 (3H, d, J ) 6.6 Hz), 1.50 (1H, m), 1.30 (1H,
m), 1.26 (3H, d, J ) 6.0 Hz), 0.97 (3H, d, J ) 6.5 Hz), 0.91 (3H, t, J
) 7.7 Hz), 0.83 (9H, s), 0.05 and 0.00 (6H, 2 s); 13C NMR δ 170.35,
169.85, 169.63, 159.61, 136.32, 128.56, 128.19, 68.91, 68.63, 67.13,
60.39, 59.71, 52.10, 36.52, 25.70, 24.43, 21.25, 17.88, 17.10, 15.31,
11.50, -4.31, -5.34.

1080 Journal of Natural Products, 2007, Vol. 70, No. 7
Pettit et al.
Methyl 2-{2-[2-Benyloxycarbonylamino-3-hydroxybutyryloxy]-
propionyloxy}-3-methylpentanoate (31). Ester 30 (0.60 g, 1.05 mmol)
was converted employing the BF3‚Et2O desilylation procedure described
for 23 to provide 0.44 g (92%) of alcohol 31 as a colorless oil: TLC
(5H, m), 5.54 (1H, d, J ) 9.9 Hz), 5.16-5.23 (2H, m), 5.12 (2H, s),
4.86 (1H, d, J ) 4.5 Hz), 4.58 (1H, m), 4.41 (1H, dd, J ) 9.3, 3.0),
3.29 (1H, d, J ) 4.2 Hz), 1.98 (1H, m), 1.21-1.60 (18H, m), 0.84-
0.99 (9H, m); MS APCI + 496.2541 [M + H]+, calcd for C25H38NO9,
496.2547; anal. C 60.50%, H 7.67%, N 2.66%, calcd for C25H37NO9,
C 60.59%, H 7.53%, N 2.83%.
1
Rf 0.13 (80:20 hexane-EtOAc); H NMR δ 7.35 (5H, m), 5.56 (1H,
d, J ) 9.9 Hz) 5.27 (1H, q, J ) 7.1 Hz), 5.13 (2H, s), 5.02 (1H, d, J
) 4.4 Hz), 4.58 (1H, m), 4.43 (1H, dd, J ) 9.0, 1.6 Hz), 3.74 (3H, s),
3.17 (1H, d, J ) 4.4 Hz), 2.02 (1H, m), 1.63 (3H, d), 1.47 (1H, m),
1.32 (1H, m), 1.26 (3H, d, J ) 6.0 Hz), 0.98 (3H, d, J ) 6.5 Hz), 0.92
(3H, t, J ) 7.7 Hz); 13C NMR δ 171.14, 170.88, 169.59, 156.78, 136.25,
128.49, 128.08, 127.91, 77.07, 69.14, 67.73, 67.05, 59.38, 52.31, 36.59,
24.41, 18.97, 16.58, 15.20, 11.46.
2-(tert-Butyldiphenylsilyloxy)-4-methylpentanoic Acid (40). Al-
cohol 13 (3.00 g, 20.5 mmol), imidazole (2.79 g, 41.0 mmol), and
TBDPSCl (7.93 g, 28.8 mmol) were dissolved in DMF (30 mL under
N2), and the solution was stirred at ambient temperature for 18 h. The
reaction was terminated with 5 M NaCl (100 mL) and extracted with
EtOAc (2 × 100 mL). The extracts were combined, washed with cold
5% citric acid (50 mL), H2O (20 mL), and 5 M NaCl (20 mL), dried,
and evaporated, and the residue coevaporated with toluene (2 × 75
mL). The residue was flash chromatographed (270 g, SiO2, 95:5
hexane-EtOAc) to afford 6.92 g (88%) of the methyl ester: TLC Rf
0.37 (95:5 hexane-EtOAc); IR 1750, 1649 cm-1; 1H NMR δ 7.67 (4H,
m), 7.40 (6H, m), 4.23 (1H, dd, J ) 4.2, 7.2 Hz), 3.44 (3H, s), 1.43-
1.76 (3H, m), 1.09 (9H, s), 0.81 (6H, dd, J ) 6.0, 16.5 Hz); 13C NMR
δ 174.0, 136.0, 135.9, 133.9, 133.3, 129.73, 129.66, 127.6, 127.4, 71.5,
51.3, 44.3, 26.9, 24.1, 22.9, 22.2, 19.4.
A portion of this material (1.76 g, 4.58 mmol) was placed in 1:1
THF-CH3OH (40 mL) at 0 °C under N2, and LiOH (14 mL of 0.5 M
cold solution, 7.0 mmol) was added over 20 min. The mixture was
stirred at ambient temperature for 28 h, cooled to 0 °C, acidified (pH
3) with 1 M KHSO4, and extracted with EtOAc (2 × 50 mL). The
combined extract was washed with H2O (20 mL) and 5 M NaCl (20
mL) and dried, and the solvent was evaporated. The residue was flash
chromatographed (60 g, SiO2, 8:1 hexane-EtOAc) to provide 1.73 g
(98%) of carboxylic acid 40 as a colorless oil: TLC Rf 0.67 (95:5:1
DCM-CH3OH-HOAc); IR 1721 cm-1; 1H NMR δ 7.64 (4H, m), 7.41
(6H, m), 4.26 (1H, t, J ) 6.0 Hz), 1.48-1.74 (3H, m), 1.08 (9H, s),
0.69 (6H, dd, J ) 6.6, 9.3 Hz).
2-Benzyloxycarbonylamino-2-methoxycarbonyl-1-methylethyl
2-(tert-Butyldimethylsilyloxy)-4-methylpentanoate (34). Silyl ester
32 (4.5 g, 12.5 mmol) and DMF (300 µL, 3.7 mmol) were placed in
DCM (20 mL) under N2 and cooled to 0 °C. Oxalyl chloride (12.5 mL
of a 2 M solution in DCM, 25 mmol) was added dropwise. The mixture
was warmed to ambient temperature and stirred for 4.5 h, and the
solvent was evaporated. To the residue under N2 was added a solution
of alcohol 33 (2.02 g, 8 mmol), DMAP (2.9 g, 24 mmol), and TEA
(2.3 mL, 20 mmol) in DCM (15 mL) at 0 °C. The mixture was stirred
at ambient temperature for 2 h, and the reaction was terminated with
6% NaHCO3 (50 mL) and extracted with Et2O (3 × 50 mL). The
extracts were combined, dried, and evaporated. The residue was flash
chromatographed (100 g, SiO2, 6:1 hexane-EtOAc) to afford 2.17 g
(56%) of ester 34 as a colorless oil: TLC Rf 0.33 (80:20 hexane-
1
EtOAc); H NMR δ 7.37 (5H, m), 5.44 (2H, m), 5.15 (2H, s), 4.56
(1H, d, J ) 8.1 Hz), 4.15 (1H, dd, J ) 4.2, 8.1 Hz), 3.72 (3H, s), 1.78
(1H, m), 1.59 (1H, m), 1.40 (1H, m), 1.32 (3H, d, J ) 6.6 Hz), 0.87
(18H, m), 0.22 (6H, m); FABMS [M + H]+ 496.2735, calcd for C25H42-
NO7Si, 496.2731.
2-Benzyloxycarbonyl-2-methoxycarbonyl-1-methylethyl 2-Hydroxy-
4-methylpentanoate (35). The BF3‚Et2O desilylation procedure de-
scribed for 23 was applied to silyl ester 34 (265.3 mg, 0.54 mmol) to
provide 0.11 g (51%) of alcohol 35 as a colorless oil: TLC Rf 0.18
2-(tert-Butyldimethylsilyloxy)-4-methylpentanoic Acid (41). Al-
cohol 13 (1.06 g, 6.84 mmol) was treated with TBDMSCl (1.61 g,
10.26 mmol) according to the procedure described for obtaining 40
and silyl ester, which led to 1.74 g (98%) of the TBDMS ether as a
1
(80:20 hexane-EtOAc); H NMR δ 7.37 (5H, m), 5.56 (1H, d, J )
9.3 Hz), 5.49 (1H, qd, J ) 6.6, 2.2 Hz), 5.14 (2H, s), 4.55 (1H, dd, J
) 9.3, 2.5 Hz), 4.12 (1H, q, J ) 6.6 Hz), 3.73 (3H, s), 2.71 (1H, d, J
) 6.1 Hz), 1.83 (1H, hept, J ) 6.6 Hz), 1.49 (2H, t, J ) 6.6 Hz), 1.33
(3H, d, J ) 6.6 Hz), 0.94 and 0.92 (6H, 2 d); 13C NMR δ 174.5, 170.2,
156.5, 128.6, 128.3, 128.2, 71.6, 69.0, 67.4, 57.4, 52.8, 43.2, 24.3, 23.1,
21.5, 16.8.
1
colorless oil: IR 1761 cm-1; H NMR δ 4.22 (1H, dd, J ) 3.9, 8.4
Hz), 3.70 (3H, s), 1.76 (1H, m), 1.55 (2H, m), 0.93-0.98 (15H, m),
0.04 (6H, dd, J ) 4.2, 18.6). A portion of this methyl ester (1.30 g,
5.0 mmol) was hydrolyzed as described for carboxylic acid 40, which
led to 1.07 g (87%) of carboxylic acid 41 as a somewhat unstable
colorless oil: 1H NMR δ 4.27 (1H, dd, J ) 4.2, 7.2 Hz), 1.84 (1H, m),
1.62 (2H, m), 0.82-0.95 (15H, m), 0.06-0.12 (6H, m).
tert-Butyl 2-[2-(tert-Butyldimethylsilyloxy)propionyloxy]-3-me-
thylpentanoate (38). The acid chloride derivative of silyl ester 18 (8.73
g, 27.4 mmol) and alcohol 37 (3.90 g, 20.7 mmol) were allowed to
react using the catalytic DMF, oxalyl chloride esterification procedure
described for ester 20 to give 6.24 g (81%) of ester 38 as a colorless
oil: TLC Rf 0.60 (4:1 hexane-EtOAc); IR 1738, 1651 cm-1; 1H NMR
δ 4.78 (1H, d, J ) 4.5 Hz), 4.38 (1H, dd, J ) 13.8, 6.6 Hz), 1.95 (1H,
m), 1.24-1.54 (15H, m), 0.84-0.97 (15H, m), 0.10 (6H, m); MS
APCI+ 375.2567 [M + H]+, calcd for C19H38O5Si, 375.2567; anal. C
61.42%, H 10.36%, calcd for C19H38O5Si, C 60.92%, H 10.23%.
tert-Butyl 2-(2-Hydroxypropionyloxy)-3-methylpentanoate (10).
Ester 38 (0.77 g, 2.05 mmol) was transformed employing the TBAF
desilylation procedure described for alcohol 29 to provide 0.54 g (100%)
of alcohol 10 as a colorless oil: TLC Rf 0.41 (80:20 hexane-EtOAc);
2-Benzyloxycarbonylamino-2-[1-(1-tert-butoxycarbonyl-2-meth-
ylbutoxycarbonyl)ethoxycarbonyl]-1-methylethyl 2-(tert-Butyldiphe-
nylsilyloxy)-4-methylpentanoate (42). Alcohol 9 (0.83 g, 1.68 mmol)
was esterified with carboxylic acid 40 (0.78 g, 2.1 mmol) employing
the MNBA procedure described for diester 26 to afford 1.21 g (85%)
of ester 42 as a colorless oil: IR 1745 cm-1; 1H NMR δ 7.61 (4H, m),
7.35 (11H, m), 5.22 (2H, m), 5.09 (3H, m), 4.80 (1H, d, J ) 4.5 Hz),
4.43 (1H, dd, J ) 3.3, 9.3 Hz), 4.30 (1H, t, J ) 6.1 Hz), 1.94 (1H, m),
1.24-1.69 (16H, m), 1.05 (9H, s), 0.94 (6H, m), 0.74 (6H, dd, J )
4.2, 15.3 Hz); MS APCI+ 848.4402 [M + H]+, calcd for C47H66NO11-
Si, 848.4406.
2-Benzyloxycarbonylamino-2-[1-(1-tert-butoxycarbonyl-2-meth-
ylbutoxycarbonyl)ethoxycarbonyl]-1-methylethyl 2-(tert-Butyldim-
ethylsilyloxy)-4-methylpentanoate (43). By applying the preceding
method (cf. 26 and 42) alcohol 9 (3.17 g, 6.40 mmol) was esterified
with carboxylic acid 41 (1.84 g, 7.60 mmol) using MNBA, and that
reaction led to 3.85 g (83%) of ester 43 as a colorless oil: TLC Rf 0.38
(6:1 hexane-EtOAc); IR 3446, 3336, 1747 cm-1; 1H NMR δ 7.35 (5H,
m), 5.43 (2H, m), 5.11 (3H, m), 4.82 (1H, d, J ) 4.5 Hz), 4.55 (1H,
m), 4.22 (1H, m), 1.94 (1H, m), 1.24-1.59 (21H, m), 0.85-0.98 (21H,
m), 0.11 (6H, m); 13C NMR δ 173.0, 169.2, 169.1, 128.5, 128.2, 128.1,
82.2, 76.9, 70.5, 69.5, 67.3, 57.6, 43.9, 36.6, 28.0, 25.7, 24.5, 23.4,
16.9, 15.3, 11.6, -4.8, -0.5.5; anal. C 61.57%, H 8.62%, N 1.87%,
calcd for C37H61NO11Si, C 61.38%, H 8.49%, N 1.93%.
1
IR 1745 cm-1; H NMR δ 4.85 (1H, d, J ) 4.5 Hz), 4.35 (1H, dd, J
) 13.8, 6.6 Hz), 2.72 (1H, m), 2.01 (1H, m), 1.23-1.68 (15H, m),
0.95 (6H, m); anal. C 59.93%, H 9.35%, calcd for C13H24O5, C 59.98%,
H 9.29%.
tert-Butyl 2-{2-[2-Benzyloxycarbonylamino-3-(tert-butyldimeth-
ylsilyloxy)butyryloxy] propionyloxy}-3-methylpentanoate (39). Al-
cohol 10 (0.50 g, 1.92 mmol) was esterfied with carboxylic acid 25
(768 mg, 2.09 mmol) by means of the MNBA procedure described for
ester 26 to yield 1.02 g (87%) of ester 39 as a colorless oil: TLC Rf
1
0.55 (80:20 hexane-EtOAc); IR 3453, 1745, 1625 cm-1; H NMR δ
7.36 (5H, m), 5.46 (1H, m), 5.24 (1H, m), 5.13 (2H, s), 4.81 (1H, d,
J ) 4.2 Hz), 4.46 (1H, d, J ) 6.0 Hz), 1.96 (1H, m), 1.22-1.55 (18H,
m), 0.95 (6H, m), 0.82 (9H, s), 0.02 (6H, m); MS APCI+ 610.3456 [M
+ H]+, calcd for C31H52NO9Si, 610.3411.
2-Benzyloxycarbonylamino-2-[1-(1-carboxy-2-methylbutoxycar-
bonyl)ethoxycarbonyl]-1-methylethyl 2-(tert-Butyldiphenylsilyloxy)-
4-methylpentanoate (44). To tert-butyl ester 42 (1.09 g, 1.29 mmol)
in DCM (5 mL) was added ZnBr2 (1.45 g, 6.43 mmol), the solution
was stirred for 48 h, H2O (20 mL) was added, and stirring continued
for 2 h. The organic phase was separated and the aqueous phase
extracted with DCM (2 × 20 mL). The organic solutions were
tert-Butyl 2-[2-(2-Benzyloxycarbonylamino-3-hydroxybutyrylox-
y)propionyloxy]-3-methylpentanoate (9). Ester 39 (1.02 g, 1.68 mmol)
was converted using the TBAF desilylation procedure described for
alcohol 29 to afford 0.83 g (100%) of alcohol 9 as a colorless oil:
1
TLC Rf 0.46 (6:1 hexane-EtOAc); IR 1745 cm-1; H NMR δ 7.32

Total Synthesis of Respirantin
Journal of Natural Products, 2007, Vol. 70, No. 7 1081
combined, dried, and evaporated to furnish 0.82 g (80%) of carboxylic
°C for 12 h. The mixture was allowed to cool, the mixture filtered,
and the solvent evaporated. The residue was flash chromatographed
(10 g, SiO2) to afford 92 mg (80%) of lactone 5 as a colorless solid:
acid 44 as a colorless oil: TLC Rf 0.50 (50:1 DCM-CH3OH); IR 1752
1
cm-1; H NMR δ 7.62 (4H, s), 7.30 (11H, m), 4.91-5.21 (5H, m),
TLC Rf 0.61 (75:25 hexane-EtOAc); [R]25 +13.5 (c 0.68, CHCl3);
4.44 (1H, m), 4.30 (2H, m), 1.98 (1H, m), 0.73-1.66 (33H, m); FABMS
D
792.3786 [M + H]+, calcd for C43H58NO11Si, 792.3780.
IR 3336, 1735, 1717, 1684 cm-1; 1H NMR δ 7.46 (1H, d, J ) 8.7 Hz),
7.38 (5H, m), 5.91 (1H, m), 5.70 (1H, dd, J ) 7.2, 13.8 Hz), 5.54 (1H,
d, J ) 9.6 Hz), 5.20 (3H, m), 4.82 (1H, d, J ) 9.0 Hz), 4.71 (3H, m),
3.45 (1H, d, J ) 15.9 Hz), 3.23 (1H, d, J ) 15.6 Hz), 2.03 (1H, m),
1.42-1.86 (10H, m), 1.36 (6H, d, J ) 6.9 Hz), 0.87-1.02 (15H, m);
13C NMR δ 204.5, 171.7, 170.1, 169.8, 167.7, 166.4, 156.7, 135.8,
128.6, 128.4, 128.2, 81.2, 72.4, 72.2, 71.3, 67.6, 57.8, 57.2, 47.0, 41.2,
39.2, 36.7, 25.3, 24.9, 24.4, 23.5, 22.8, 21.8, 20.8, 18.5, 16.4, 14.4,
10.6; FABMS 691.3450 [M + H]+, calcd for C35H51N2O12, 691.3442;
anal. C 61.23%, H 7.30%, N 4.06%, calcd for C35H50N2O12, C 60.86%,
H 7.30%, N 4.06%.
2-Benzyloxycarbonylamino-2-[1-(1-carboxy-2-methylbutoxycar-
bonyl)ethoxycarbonyl]-1-methylethyl 2-(tert-Butyldimethylsilyloxy)-
4-methylpentanoate (8). To tert-butyl ester 43 (2.52 g, 3.10 mmol) in
toluene (70 mL) was added 230-400 mesh silica gel (5 g). The mixture
was heated at reflux under N2 for 6 h, allowed to cool, and diluted
with 4:1 DCM-CH3OH (200 mL). The solution was filtered and the
solid phase washed with 4:1 DCM-CH3OH (50 mL). The combined
DCM filtrate and washings were evaporated to dryness. The residue
was flash chromatographed (60 g, SiO2, 50:1 DCM-CH3OH) to afford
1.54 g (66%) of carboxylic acid 8 as a colorless oil: TLC Rf 0.51 (50:1
DCM-CH3OH); [R]26 -33.9 (c 1.1, CHCl3); IR 3319, 1755 cm-1
;
D
2-Hydroxy-3-formylaminobenzoic acid (49). Aniline 48 (0.51 g,
3.31 mmol) was suspended in formamide (3.0 mL under N2) and the
mixture stirred at 150 °C for 0.5 h. The resulting solution was allowed
to cool, dissolved in 6% NaHCO3 (50 mL), acidified with 1 M KHSO4,
and extracted with EtOAc (3 × 50 mL). The combined extract was
washed with 5 M NaCl (10 mL), dried, and evaporated, and the residue
was coevaporated with toluene (10 mL) to furnish 90% of phenol 49
as a greenish-gray solid: mp 168-169 °C; TLC Rf 0.20 (95:5:1 DCM-
1H NMR δ 7.34 (5H, m), 5.31 (2H, m), 4.98-5.13 (5H, m), 4.57 (1H,
dd, J ) 3.3, 9.9 Hz), 4.20 (2H, dd, J ) 3.6, 8.7 Hz), 2.01 (1H, m),
1.33-1.76 (9H, m), 0.81-1.24 (22H, m), 0.01-0.05 (6H, m); 13C NMR
δ 173.1, 169.3, 156.5, 136.0, 128.6, 128.3, 128.1, 76.2, 70.7, 70.5, 69.5,
67.4, 57.6, 43.9, 36.5, 25.7, 24.4, 24.0, 23.4, 21.5, 18.1, 16.9, 16.7,
15.3, 11.5, -4.8, -5.5; anal. C 59.49%, H 8.32%, N 1.97%, calcd for
C33H53NO11Si, C 59.35%, H 8.00%, N 2.10%.
Methyl 4-[2-(2-{2-Benzyloxycarbonylamino-3-[2-(tert-butyldiphe-
nylsilyloxy)-4-methylpentanoyloxy]butyryloxy}propionyloxy)-3-me-
thylpentanoylamino]-6-methyl-3-oxoheptanoate (45). Boc-protected
ketone 7 (0.287 g, 1.0 mmol) was deprotected (TFA-DCM) and allowed
to react with carboxylic acid 44 (0.640 g, 0.81 mmol) employing the
PyBroP-mediated amide formation procedure described for 22 to afford
0.375 g (52%) of amide 45 as a colorless oil: TLC Rf 0.45 (80:20
1
MeOH-HOAc); H NMR (d6-DMSO) δ 9.82 (1H, s), 8.38 (1H, d, J
) 9.3 Hz), 7.55 (1H, d, J ) 7.7 Hz), 6.92 (1H, t, J ) 7.7 Hz); 13C
NMR (d6-DMSO) δ 172.3, 160.3, 151.2, 126.5, 125.7, 124.5, 118.6,
112.6.
Methyl 2-Hydroxy-3-formylaminobenzoate (50). Benzoic acid
derivative 49 (1.37 g, 7.57 mmol) and NaHCO3 (1.40 g, 16.65 mmol)
were placed in DMF (20 mL) under N2. MeI (5.37 g, 2.36 mL, 37.85
mmol) in DMF (20 mL) was added and the mixture stirred at ambient
temperature for 15 h. The mixture was diluted with EtOAc (250 mL),
washed with H2O (50 mL), 6% NaHCO3 (50 mL), H2O (20 mL), and
5 M NaCl (20 mL), and dried, the solvent was evaporated, and the
residue was coevaporated with toluene (50 mL). The residue was flash
chromatographed (36 g, SiO2, 70:30 hexane-EtOAc) to supply 1.17 g
(79%) of ester 50 as an off-white solid: mp 99 °C; TLC Rf 0.66 (95:5
DCM-MeOH); IR 3248, 1693, 1651 cm-1; 1H NMR δ 11.29 and 11.18
(1H, 2 s), 8.76 and 8.56 (1H, 2 dd, J ) 7.9, 1.7 Hz), 8.51 (1H, d, J )
1.7 Hz), 7.97 (1H, br s), 7.65 and 7.57 (1H, 2 dd, J ) 8.2, 1.6 Hz),
6.90 and 6.88 (1, 2 t, J ) 8.2 Hz), 3.97 (3H, s); 13C NMR δ 170.8,
170.4, 161.2, 158.9, 151.2, 150.3, 126.5, 125.8, 125.4, 124.3, 121.7,
119.2, 113.1, 111.9, 52.7, 52.6.
1
hexane-EtOAc); IR 3367, 1755, 1682 cm-1; H NMR δ 7.59 (4H,
m), 7.33 (11H, m), 6.70 (1H, d, J ) 7.8 Hz), 5.00-5.13 (6H, m), 4.70
(1H, m), 4.41 (1H, m), 4.27 (1H, m), 3.69 (3H, d, J ) 3.0 Hz), 3.52
(2H, d, J ) 2.7 Hz); 13C NMR δ 201.7, 172.4, 169.7, 168.7, 167.3,
156.3, 136.0, 135.7, 133.1, 129.9, 128.6, 128.3, 128.1, 127.7, 78.6,
71.7, 70.7, 70.1, 67.3, 57.9, 56.4, 52.4, 46.0, 44.5, 39.2, 37.0, 29.7,
26.8, 24.8, 24.2, 24.1, 23.3, 22.9, 22.3, 21.4, 19.4, 16.9, 14.9, 11.4;
FABMS 961.4854 [M + H]+, calcd for C52H73N2O13Si, 961.4882; anal.
C 64.76%, H 7.77%, N 2.72%, calcd for C52H72N2O13Si, C 64.98%, H
7.55%, N 2.91%.
Methyl 4-[2-(2-{2-Benzyloxycarbonylamino-3-[2-(tert-butylmeth-
ylsilyloxy)-4-methylpentanoyloxy]butyryloxy}propionyloxy)-3-me-
thylpentanoylamino]-6-methyl-3-oxoheptanoate (46). Ketone 7 (1.16
g, 4.00 mmol), following cleavage of the Boc group, and carboxylic
acid 8 (2.25 g, 3.37 mmol) were coupled by PyBroP-promoted amide
formation as described for amide 22 to supply 1.83 g (65%) of amide
46 as a colorless oil: TLC Rf 0.46 (80:20 hexane-EtOAc); [R]25D -38.5
(c 0.98, CHCl3); IR 3359, 1753, 1682 cm-1; 1H NMR δ 7.35 (5H, m),
6.70 (1H, d, J ) 7.8 Hz), 5.43 (2H, d, J ) 9.3 Hz), 5.13 (4H, m), 4.68
(1H, m), 4.54 (1H, m), 4.18 (1H, dd, J ) 3.6, 9.3 Hz), 3.70 (3H, t, J
) 3.0 Hz), 3.52 (2H, s), 2.00 (1H, s), 1.27-1.74 (18H, m), 0.88-0.94
(24H, m), 0.02 (6H, m); anal. C 60.40%, H 8.52%, N 3.31%, calcd
for C42H68N2O13Si, C 60.26%, H 8.19%, N 3.35%.
Methyl 4-(2-{2-[2-Benzyloxycarbonylamino-3-(2-hydroxy-4-me-
thylpentanoyloxy)butyryloxy]propionyloxy}-3-methylpentanoylami-
no)-6-methyl-3-oxoheptanoate (6). To amide 46 (0.367 g, 0.44 mmol)
in CH3OH (5 mL) under N2 at 0 °C was added acetyl chloride (50 µL,
69.5 mg, 0.88 mmol). The solution was stirred at ambient temperature
for 30 min, diluted with DCM (40 mL), washed with 6% NaHCO3 (20
mL) and H2O (10 mL), dried, and evaporated. The residue was flash
chromatographed (10 g, SiO2, 2:1 hexane-EtOAc) to afford 0.198 g
(63%) of epimer 6 as a colorless oil: TLC Rf 0.40 (50:50 hexane-
EtOAc); [R]25D -30.4 (c 0.92, CHCl3); 1H NMR δ 7.37 (5H, m), 6.63
(1H, d, J ) 8.4 Hz), 5.45 (2H, m), 5.53 (2H, s), 5.03 (1H, d, J ) 4.8
Hz), 4.65 (2H, m), 4.10 (1H, m), 3.72 (3H, d, J ) 2.7 Hz), 3.50 (2H,
m), 1.86-2.02 (2H, m), 0.68-1.83 (33H, m); 13C NMR δ 201.7, 174.8,
169.3, 169.2, 168.6, 135.8, 128.6, 128.4, 128.2, 79.0, 78.8, 71.2, 69.9,
69.0, 68.9, 67.5, 57.5, 56.5, 56.4, 46.1, 42.8, 39.7, 36.8, 29.7, 24.9,
24.3, 24.3, 23.2, 21.5, 16.7, 14.8, 11.2; FABMS 723.3735 [M + H]+,
calcd for C36H55N2O13, 723.3704; anal. C 59.64%, H 7.81%, N 3.79%,
calcd for C36H54N2O13, C 59.82%, H 7.53%, N 3.88%.
Methyl 2-Benzyloxy-3-formylaminobenzoate (51). To methyl ester
50 (1.01 g, 5.20 mmol) and benzyl bromide (1.44 g, 1 mL, 8.42 mmol)
in DMF (20 mL under N2) was added K2CO3 (1.44 g, 10.40 mmol)
and the mixture stirred at 60 °C for 15 h. The mixture was diluted
with EtOAc (100 mL), washed with H2O (2 × 20 mL) and 5 M NaCl
(10 mL), and dried, the solvent was evaporated, and the residue was
coevaporated with toluene (20 mL). The residue was separated by flash
chromatography (50 g, SiO2, 80:20 hexane-EtOAc) to afford 1.41 g
(95%) of benzyl ester 51 as a pinkish oil, which solidified on standing.
A portion was recrystallized from toluene-hexane: mp 52-53 °C;
1
TLC Rf 0.59 (50:50 hexane-EtOAc); IR 3284, 1720, 1674 cm-1; H
NMR δ 8.53 (1H, dd, J ) 8.3, 1.6 Hz), 8.19 (1H, d, J ) 1.7 Hz), 7.65
(2H, dd, J ) 8.2, 1.6 Hz), 7.41 (5H, m), 7.18 (1H, t, J ) 8.3 Hz), 5.03
(2H, s), 3.93 (3H, s); 13C NMR δ 165.7, 158.7, 147.7, 136.4, 132.0,
128.9, 128.9, 128.5, 126.7, 124.9, 124.4, 77.8, 52.4; anal. C 67.51%,
H 5.42%, N 4.88%, calcd for C16H15NO4, C 67.36%, H 5.30%, N
4.91%.
2-Benzyloxy-3-formylaminobenzoic Acid (3). To methyl ester 51
(1.28 g, 4.48 mmol) in 3:1 THF-MeOH (20 mL) under N2 was added
LiOH (11.6 mL of 0.5 M aqueous solution, 5.8 mmol) and the mixture
stirred at ambient temperature for 18 h. The reaction mixture was
acidified (pH 3) with 1 M KHSO4, diluted with H2O (200 mL), and
extracted with EtOAc (3 × 65 mL). The combined extract was washed
with H2O (10 mL), and 5 M NaCl (20 mL), dried, and evaporated.
The residue was crystallized from EtOAc-hexane to afford 0.96 g
(79%) of benzoic acid 3 as an off-white solid: mp 133 °C; TLC Rf
1
0.51 (95:5:1 DCM-CH3OH-HOAc); IR 3343, 1697, 1636 cm-1; H
Benzyl (5-s-Butyl-8,13-diisobutyl-2,16-dimethyl-3,6,9,11,14,18-
hexaoxo-1,4,12,15-tetraoxa-7-azacyclooctadec-17-yl)carbamate (5).
A mixture of alcohol 6 (0.12 g, 0.17 mmol) and anhydrous CuSO4
(0.60 g, 3.75 mmol) in toluene (150 mL, under N2) was stirred at 120
NMR (d6-DMSO) δ 13.09 (1H, s), 9.76 (1H, s), 8.34 (1H, s), 8.30
(1H, d, J ) 8.2 Hz), 7.25-7.60 (6H, m), 7.18 (1H, t, J ) 7.7 Hz),
4.95 (2H, s); 13C NMR (d6-DMSO) δ 167.0, 160.5, 147.3, 136.8, 132.3,

1082 Journal of Natural Products, 2007, Vol. 70, No. 7
Pettit et al.
128.4, 128.0, 127.9, 126.4, 125.6, 124.7, 124.0, 75.7; anal. C 65.84%,
H 4.91%, N 5.06%, calcd for C15H13NO4‚0.1H2O, C 65.97%, H 4.88%,
N 5.12%.
(1H, t, J ) 8.0 Hz), 6.03 (1H, dd, J ) 2.7, 6.6 Hz), 5.86 (1H, q, J )
6.8 Hz), 5.21 (1H, dd, J ) 2.7, 8.7 Hz), 4.94 (1H, ddd, J ) 3.8, 9.9,
11.0 Hz), 4.86 (1H, d, J ) 9.6 Hz), 4.68 (1H, dd, J ) 4.5, 9.9 Hz),
2.12 (1H, m), 1.50-1.90 (10H, m), 1.24-1.44 (12H, m), 0.90-1.02
(16H, m); 13C NMR δ 208.1, 173.4, 171.8, 170.4, 169.9, 169.5, 167.5,
159.0, 150.6, 127.5, 125.0, 120.3, 119.1, 112.8, 80.9, 72.3, 72.0, 71.5,
56.4, 55.6, 53.0, 43.1, 39.4, 36.5, 25.3, 24.7, 24.5, 24.1, 23.6, 22.8,
21.4, 21.0, 19.8, 18.2, 16.6, 14.4, 10.4; MS APCI+ 748.3631 [M +
H]+, calcd for C37H54N3O13, 748.3657.
Benzyl (5-s-Butyl-8,13-diisobutyl-2,10,10,16-tetramethyl-3,6,9,-
11,14,18-hexaoxo-1,4,12,15-tetraoxa-7-azacyclooctadec-17-yl)car-
bamate (52). To a stirred solution of lactone 5 (76 mg, 0.11 mmol) in
DMSO (3 mL) under N2 were added K2CO3 (153 mg, 1.01 mmol) and
MeI (20 µL, 0.33 mmol). The mixture was stirred at ambient
temperature for 3 h, diluted with H2O (12 mL), and extracted with
EtOAc (3 × 20 mL). The extracts were combined, washed with H2O
(10 mL) and 5 M NaCl (5 mL), dried, and evaporated. The residue
was flash chromatographed (10 g, SiO2, 3:1 hexane-EtOAc) to afford
22 mg (28%) of Cbz-protected lactone 52 as an oil: TLC Rf 0.55 (75:
25 hexane-EtOAc); [R]25D -17.7 (c 0.90, CHCl3); IR 3330, 1738, 1718
Acknowledgment. We are pleased to acknowledge financial support
provided by Outstanding Investigator Grant CA44344-10-12 and RO1
CA90441-01-05 awarded by the Division of Cancer Treatment and
Diagnosis, National Cancer Institute, DHHS; the Arizona Disease
Control Research Commission; the Robert B. Dalton Endowment Fund;
Dr. Alec D. Keith; Dr. William Crisp; Mrs. Anita Crisp; Gary L. and
Diane R. Tooker; and Dr. John C. Budzinski. Other helpful assistance
was provided by Drs. D. L. Doubek, F. Hogan, V. J. R. V. Mukku,
and J. Chapuis, and L. Williams. We thank Dr. Zbigniew A. Cichacz
for helpful discussions. We also thank Daniel Jensen for technical
support in the preparation of starting materials and intermediates.
1
cm-1; H NMR δ 7.50 (1H, d, J ) 9.3 Hz), 7.37 (5H, m), 5.91 (1H,
m), 5.80 (1H, dd, J ) 6.6, 13.8 Hz), 5.56 (1H, d, J ) 9.0 Hz), 5.18
(2H, dd, J ) 12.0, 17.1 Hz), 4.86 (2H, m), 4.72 (1H, d, J ) 9.0 Hz),
4.59 (1H, dd, J ) 4.2, 10.2 Hz), 2.06 (1H, m), 1.13-1.83 (20H, m),
0.77-0.98 (19H, m); 13C NMR δ 208.3, 173.1, 171.8, 170.0, 169.7,
167.8, 156.6, 135.8, 128.7, 128.5, 128.2, 80.8, 72.3, 71.9, 71.1, 67.6,
57.7, 56.4, 53.0, 43.0, 39.4, 36.6, 31.9; MS APCI+ 719.3767 [M +
H]+, calcd for C37H55N2O12, 719.3755.
17-Amino-5-s-butyl-8,13-diisobutyl-2,10,10,16-tetramethyl-1,4,-
12,15-tetraoxa-7-azacyclooctadecane-3,6,9,11,14,18-hexaone (4). Ben-
zyl carbamate 52 (16 mg, 0.022 mmol) and 10% Pd/C (15 mg) in
EtOAc (3 mL) were stirred under a H2 atmosphere at ambient
temperature for 2 h. The solution phase was filtered through Celite
and the solid phase washed with CH3OH (20 mL). The combined
solvent filtrate and washings were evaporated, and the residue was flash
chromatographed (10 g, SiO2, 3:1 hexane-EtOAc) to furnish 9.5 mg
(73%) of amine 4 as a colorless oil: TLC Rf 0.40 (75:25 EtOAc-
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2004, 69, 1822-1830.
(20) Vidya, R.; Eggen, M.; Nair, S. K.; Georg, G. I.; Himes, R. H. J.
Org. Chem. 2003, 68, 9687-9693.
hexane); [R]27 -55.1 (c 0.67, CHCl3); IR 3222, 1749, 1712, 1686
D
cm-1; 1H NMR δ 7.60 (1H, d, J ) 9.3 Hz), 5.90 (1H, d, J ) 6.6 Hz),
5.78 (1H, dd, J ) 6.3, 13.5 Hz), 4.86 (2H, m), 4.63 (1H, dd, J ) 4.2,
9.9 Hz), 3.62 (1H, br s), 2.08 (1H, m), 1.26-1.82 (22H, m), 0.86-
1.18 (18H, m); 13C NMR δ 208.5, 173.1, 172.1, 171.7, 170.2, 170.1,
80.7, 72.8, 72.1, 70.6, 58.3, 56.5, 53.1, 43.0, 39.5, 36.6, 29.7, 25.3,
24.7, 24.5, 24.0, 23.6, 22.9, 21.4, 21.1, 19.8, 18.2, 16.5, 14.5, 10.5;
FABMS 585.3360 [M + H]+, calcd for C29H49N2O10, 585.3387.
2-Benzyloxy-N-(5-s-butyl-8,13-diisobutyl-2,10,10,16-tetramethyl-
3,6,9,11,14,18-hexaoxo-1,4,12,15-tetraoxa-7-azacyclooctadec-17-yl)-
3-formylaminobenzamide (53). Benzoic acid 3 (14.0 mg, 0.051 mmol),
1-hydroxybenzotriazole (7.0 mg, 0.051 mmol), EDCI (7.4 mg, 0.038
mmol), and N-methylmorpholine (20 µL, 0.18 mmol) were added
successively to a solution of amine 4 (15.0 mg, 0.026 mmol) in DMF
(1.5 mL) under N2. The reaction mixture was stirred at ambient
temperature for 11 h, and the reaction was terminated by addition of
saturated NaHSO4 (20 mL) and extracted with EtOAc (30 mL). The
extract was dried and evaporated, and the residue was flash chromato-
graphed (10 g, SiO2, 2.2:1 hexane-EtOAc) to provide 13 mg (61%)
of amide 53 as a colorless oil: TLC Rf 0.42 (2:1 hexane-EtOAc); [R]25
D
-45.7 (c 0.65, CHCl3); IR 3321, 1745, 1678 cm-1; H NMR δ 8.45
1
(1H, d, J ) 8.1 Hz), 8.20 (1H, d, J ) 9.3 Hz), 8.10 (1H, s), 7.79 (1H,
d, J ) 6.0 Hz), 7.52 (1H, d, J ) 9.3 Hz), 7.26-7.38 (7H, m), 6.05
(1H, d, J ) 6.6 Hz), 5.86 (1H, dd, J ) 8.2, 13.8 Hz), 5.45 (1H, d, J )
11.7), 5.36 (1H, m), 4.88 (3H, m), 4.56 (1H, d, J ) 9.9 Hz), 2.10 (1H,
m), 1.47-1.85 (8H, m), 1.13-1.42 (12H, m), 0.75-1.02 (18H, m);
13C NMR δ 208.2, 173.3, 171.8, 170.0, 169.8, 167.9, 165.8, 146.0,
135.3, 131.5, 129.5, 129.2, 129.1, 126.5, 126.2, 125.5, 124.9, 80.9,
79.1, 72.6, 72.0, 71.2, 56.4, 55.9, 53.1, 43.1, 39.5, 36.6, 25.3, 24.6,
24.4, 24.1, 21.0, 19.8, 18.3, 16.6, 14.5, 10.4; MS APCI+ 838.4131 [M
+ H]+, calcd for C44H60N3O13, 838.4126.
N-(5-s-Butyl-8,13-diisobutyl-2,10,10,16-tetramethyl-3,6,9,11,14,-
18-hexaoxo-1,4,12,15-tetraoxa-7-azacyclooctadec-17-yl)-3-formy-
lamino-2-hydroxybenzamide (Respirantin 1b). Amide 53 (15 mg,
0.018 mmol) and 10% Pd/C (17 mg) in EtOAc (3 mL) were stirred
under a H2 atmosphere at ambient temperature for 2 h. The solution
phase was filtered through Celite and the solid phase washed with 1:1
EtOAc-CH3OH (20 mL). The combined solvent filtrate and washings
was evaporated and the residue flash chromatographed (10 g, SiO2,
1:1 hexane-EtOAc) to afford 11 mg (82%) of 1b as a glassy solid:
(21) Golebiowski, A.; Jurczak, J. Tetrahedron 1991, 47, 1037-1044.
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G. S. Tetrahedron Lett. 2000, 41, 2847-2849.
(24) Jackson, R. W. Tetrahedron Lett. 2001, 42, 5163-5165.
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(26) Bandgar, B. P.; Sadavarte, V. S.; Uppala, L. S. Synth. Commun. 2001,
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(27) Skibo, E. K.; Gilchrist, J. H. J. Org. Chem. 1988, 53, 4209-
4218.
(28) Bocchi, V.; Casnati, G.; Dossena, A.; Marchelli, R. Synthesis 1979,
961-962.
TLC Rf 0.38 (50:50 hexane-EtOAc); [R]25 -6.0 (c 0.53, CH3OH);
D
IR 3325, 1749, 1708, 1687 cm-1; 1H NMR δ 12.51 (1H, br), 8.58 (1H,
d, J ) 8.0 Hz), 8.52 (1H, d, J ) 1.5 Hz), 7.94 (1H, s), 7.47 (1H, d, J
) 9.5 Hz), 7.36 (1H, d, J ) 9.5 Hz), 7.15 (1H, d, J ) 9.0 Hz), 6.97

Total Synthesis of Respirantin
Journal of Natural Products, 2007, Vol. 70, No. 7 1083
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NP0680735