Total synthesis of (+)-calyculin A and (-)-calyculin B: Cyanotetraene construction, asymmetric synthesis of the C(26-37) oxazole, fragment assembly, and final elaboration

Article abstract of DOI:10.1021/ja992135e

A convergent total synthesis leading to (+)-calyculin A and (-)-calyculin B (1 and 2), antipodes of the potent, highly selective and remarkably cell-permeable phosphatase inhibitors calyculins A and B, has been achieved. In the preceding paper we outlined the asymmetric synthesis of the C(9-25) spiroketal dipropionate subunit (+)-BC; herein we describe construction of the C(1-8) cyanotetraene, an asymmetric synthesis of the C(26-37) oxazole, fragment assembly and final elaboration to (+)-1 and (-)-2. Highlights of the synthesis include: application of a one-pot three-component Suzuki reaction for the construction of phosphonate A, a bifunctional triene precursor of the light sensitive C(1-8) cyanotetraene subunit, an asymmetric synthesis of the C(26-32) oxazole (-)-D, exploiting the Silks-Odom ⁷⁷Se NMR protocol to assess enantiomeric purity, construction of the C(33-37) subtarget (-)-E in a highly stereocontrolled fashion via an acyliminium ion, and a concise, highly efficient sequence for fragment assembly and elaboration to (+)-calyculin A and (-)-calyculin B. The synthesis of (-)-2 also confirms the structure of calyculin B, previously based only on spectral comparison with calyculin A.

Full text of DOI:10.1021/ja992135e

10478
J. Am. Chem. Soc. 1999, 121, 10478-10486
Total Synthesis of (+)-Calyculin A and (-)-Calyculin B:
Cyanotetraene Construction, Asymmetric Synthesis of the C(26-37)
Oxazole, Fragment Assembly, and Final Elaboration
Amos B. Smith, III,* Gregory K. Friestad, Joseph Barbosa, Emmanuel Bertounesque,
James J.-W. Duan, Kenneth G. Hull, Makoto Iwashima, Yuping Qiu, P. Grant Spoors, and
Brian A. Salvatore
Contribution from the Department of Chemistry, the Laboratory for Research on the Structure of Matter,
and the Monell Chemical Senses Center, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104
ReceiVed June 22, 1999
Abstract: A convergent total synthesis leading to (+)-calyculin A and (-)-calyculin B (1 and 2), antipodes of
the potent, highly selective and remarkably cell-permeable phosphatase inhibitors calyculins A and B, has
been achieved. In the preceding paper we outlined the asymmetric synthesis of the C(9-25) spiroketal
dipropionate subunit (+)-BC; herein we describe construction of the C(1-8) cyanotetraene, an asymmetric
synthesis of the C(26-37) oxazole, fragment assembly and final elaboration to (+)-1 and (-)-2. Highlights of
the synthesis include: application of a one-pot three-component Suzuki reaction for the construction of
phosphonate A, a bifunctional triene precursor of the light sensitive C(1-8) cyanotetraene subunit, an asymmetric
synthesis of the C(26-32) oxazole (-)-D, exploiting the Silks-Odom 77Se NMR protocol to assess enantiomeric
purity, construction of the C(33-37) subtarget (-)-E in a highly stereocontrolled fashion via an acyliminium
ion, and a concise, highly efficient sequence for fragment assembly and elaboration to (+)-calyculin A and
(-)-calyculin B. The synthesis of (-)-2 also confirms the structure of calyculin B, previously based only on
spectral comparison with calyculin A.
The calyculins comprise a family of potent, highly selective
Scheme 1
serine-threonine phosphatase inhibitors possessing a striking
array of stereochemical and functional elements. In this, the
second of a two-part full account,1 we describe the completion
of a highly convergent total synthesis of (+)-1 and the first
total synthesis of (-)-calyculin B (2).2-4 From the outset, we
envisioned an approach (Scheme 1) which would provide both
1 and 2 from a common advanced intermediate, avoid extensive
manipulations of the light-sensitive5 C(1-9) cyanotetraene, and
permit flexibility in fragment coupling. Accordingly, discon-
nections at the C(2) and C(8) olefins led to phosphonate A
(Scheme 1), possessing a latent C(3) carbonyl for penultimate
Peterson olefination to access both 1 and 2. Disconnection of
BCDE at the C(25-26) olefin revealed subtargets BC and DE.
Having described an efficient route to (+)-BC in the preceding
(1) Part 1: Smith, A. B., III; Friestad, G. K.; Barbosa, J.; Bertounesque,
E.; Hull, K. G.; Iwashima, M.; Qiu, Y.; Salvatore, B. A.; Spoors, P. G.;
Duan, J. J.-W. J. Am. Chem. Soc. 1999, 121, XXXX.
(2) At the start of our work, the absolute stereochemistry was unknown.
(3) Matsunaga, S.; Fusetani, N. Tetrahedron Lett. 1991, 32, 5605. (b)
Hamada, Y.; Tanada, Y.; Yokokawa, F.; Shioiri, T. Tetrahedron Lett. 1991,
32, 5983.
(4) Preliminary communications: (a) Smith, A. B., III; Friestad, G. K.;
Duan, J. J.-W.; Barbosa, J.; Hull, K. G.; Iwashima, M.; Qiu, Y.; Spoors, P.
G.; Bertounesque, E.; Salvatore, B. A. J. Org. Chem. 1998, 63, 7596. (b)
Smith, A. B., III; Cho, Y. S.; Friestad, G. K. Tetrahedron Lett. 1998, 39,
8765. (c) Iwashima, M.; Kinsho, T.; Smith, A. B., III. Tetrahedron Lett.
1995, 36, 2199. (d) Smith, A. B., III; Iwashima, M. Tetrahedron Lett. 1994,
35, 6051. (e) Salvatore, B. A.; Smith, A. B., III. Tetrahedron Lett. 1994,
35, 1329. (f) Smith, A. B., III; Salvatore, B. A.; Hull, K. G.; Duan, J. J.-W.
Tetrahedron Lett. 1991, 32, 4859. (g) Smith, A. B., III; Duan, J. J.-W.;
Hull, K. G.; Salvatore, B. A. Tetrahedron Lett. 1991, 32, 4855.
(5) Matsunaga, S.; Fujiki, H.; Sakata, D.; Fusetani, N. Tetrahedron 1991,
47, 2999.
paper, we detail here the synthesis and reactivity of phosphonate
A, a concise, highly stereocontrolled route to (+)-DE, the union
of subtargets A, (+)-BC, and (+)-DE, and completion of the
total synthesis of (+)-calyculin A and (-)-calyculin B.
10.1021/ja992135e CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/30/1999

Synthesis of (+)-Calyculin A and (-)-Calyculin B
J. Am. Chem. Soc., Vol. 121, No. 45, 1999 10479
Scheme 2
Scheme 4
the three steps was 55%. Unfortunately, R-methylation of 12
could not be achieved; treatment of 12 with a variety of bases
led to severe olefin isomerization. This result did not build
confidence in our ability to generate metalated phosphonate
reagents in the presence of an unsaturated nitrile. We therefore
decided to postpone Peterson olefination until after C(8-9)
olefin construction.9
At this stage we also learned that introduction of the
phosphonate group could not be readily achieved when C(8)
was substituted with a methyl group (Scheme 5). For example,
Evolution of Phosphonate A. Initially we envisioned
introduction of the C(1-9) cyanotetraene in one step employing
Horner-Emmons reactions of unsaturated phosphonates 3a and
3b (Scheme 2) with a C(9) aldehyde derived from intermediate
BCDE. Phosphonates 3a and 3b in turn would arise via Peterson
olefination of ketophosphonate 4 with trimethylsilylacetonitrile.
To fabricate 4 we planned to exploit a one-pot three-component
Suzuki coupling6 of E-bromovinyl boronate 7 with vinyl iodide
6a or 6b and organozinc 8, followed by introduction of the
phosphonate and mild acid hydrolysis of the intermediate enol
ether. Importantly, the C(8) methyl could be present from the
outset (i.e., 6b), or introduced via R-methylation of the
corresponding primary phosphonate.
Scheme 5
We initiated the synthesis of 3a and 3b with known vinyl
iodides 6a and 6b, available via Negishi carbometalation
(Scheme 3).7 Exposure of E-bromovinyl boronate 7 to vinyl
Scheme 3
attempts to convert allylic chloride 14 to phosphonate 4 using
the standard Arbuzov8 or Becker10 conditions proved unsuc-
cessful. We therefore modified our approach to include the
phosphonate prior to Suzuki coupling. In particular, we chose
to employ a delicate balance of reactive functionality at the
termini of the C(1-8) synthon (Scheme 6), which would require
Scheme 6
zinc 8 in the presence of bis(triphenylphosphine)palladium(II)
chloride (5 mol %) provided dienylboronate 9 (not isolated),
which upon addition of a lithium hydroxide-methanol solution
and vinyl iodide 6a (or 6b) established the C(3-8) carbon
skeleton in the form of enol ethers 5a,b; hydrolysis with dilute
aqueous hydrochloric acid furnished ketones 10a (or 10b), albeit
in variable but good yields.
To test the viability of the Horner-Emmons tactic in the
presence of the conjugated nitrile (i.e., after Peterson olefina-
tion), 10a was treated with trimethylsilylacetonitrile and lithium
bis(trimethylsilyl) amide; a mixture of E/Z olefins ( ∼2:1)
resulted (Scheme 4), with the major isomer possessing the
requisite C(2,3) E geometry as in calyculin B. Removal of the
TBS group followed by conversion to the primary mesylate and
Arbuzov reaction8 with neat trimethyl phosphite and excess
sodium iodide furnished phosphonate 12; the overall yield for
minimal manipulations after the Suzuki protocol. In the end,
the successful strategy comprised direct use of the delicate enol
ether derived from the Suzuki coupling (i.e., A), containing the
phosphonate moiety for subsequent C(8) methylation and C(8-
9) Horner-Emmons coupling. Peterson olefination would then
be performed after generation of the C(8,9) olefin. Inclusion of
the requisite phosphonate on the vinyl iodide (16) would also
minimize the number of transformations performed on the
sensitive trienol ether.
(6) Ogima, M.; Hyuga, S.; Hara, S.; Suzuki, A. Chem. Lett. 1989, 1959.
(b) For a review on Suzuki coupling, see: Miyaura, N.; Suzuki, A. Chem.
ReV. 1995, 95, 2457.
(7) Rand, C. L.; Van Horn, D. E.; Moore, M. W.; Negishi, E.-I. J. Org.
Chem. 1981, 46, 4093. Zirconium-catalyzed carboalumination, followed by
quenching with iodine and protection of the allylic hydroxyl as the
t-butyldimethylsilyl ether furnished 6a,b from the corresponding propargylic
alcohols.
(9) The difficult methylation was consistent with reports by Evans et al.
of the low reactivity of 13, prepared by a different method, which did not
undergo normal Horner-Emmons coupling, see: Evans, D. A.; Gage, J. R.;
Leighton, J. L. J. Am. Chem. Soc. 1992, 114, 9434.
(10) Engel, R. Synthesis of Carbon-Phosphorous Bonds; CRC Press:
Boca Raton, FL, 1988; p 7.
(8) Bhattacharya, A. K.; Thyagarajan, G. Chem. ReV. 1981, 81, 415.

10480 J. Am. Chem. Soc., Vol. 121, No. 45, 1999
Smith et al.
Scheme 7
Scheme 9
Preparation of phosphonate A is illustrated in Scheme 7.
Conversion of alcohol 187 to the allylic chloride, followed by
Arbuzov reaction with neat trimethyl phosphite and sodium
iodide provided primary phosphonate 16 in 70% yield for two
steps. This iodide proved quite unstable to the usual Suzuki
coupling promoters (e.g., lithium methoxide and thallium
hydroxide).6b,11 However silver(I) oxide utilized by Kishi was
found to be both mild and efficient in promoting the coupling
of 16 with dienylboronate intermediate 19.11 In the event,
treatment of vinyl iodide 16 at reflux with 19 (2 equiv, formed
in situ) and Ag2O (5-6 equiv) in aqueous THF provided 20 in
approximately 60% yield. Although 20 could not be fully
purified without significant decomposition, C(8) methylation
(n-BuLi, MeI, -78 °C) occurred without complications, thereby
completing assembly of phosphonate A.12
Construction of the C(26-32) Oxazole via Davidson
Cyclization. Reaction of commercially available 4-chlorobutyryl
chloride (24, Scheme 10) with lithiated oxazolidinone 2513a
Scheme 10
With phosphonate A in hand, we undertook a model study
to explore both the Horner-Emmons coupling and Peterson
olefination. Metalation of A (n-BuLi/THF), followed by intro-
duction of model aldehyde 21, provided a 5:1 (E/Z) mixture of
C(8-9) olefins; hydrolysis of the enol ether (silica gel) gave
ketone 22 in 74% yield. Peterson olefination then furnished
cyanotetraene 23 as a 2:1 E/Z mixture (Scheme 8). These results
followed by displacement of the primary chloride with sodium
azide furnished (+)-26 in 95% yield for the two steps. Evans
asymmetric methylation13b of the sodium enolate of (+)-26 then
led to (+)-27 in high yield with excellent diastereoselectivity
(94.6% de; HPLC). After separation by flash chromatography,
displacement of the chiral auxiliary from (+)-27 (â isomer) with
the alkoxide derived from known allylic alcohol 2814 and
subsequent ozonolysis of the exomethylene gave keto ester 29.
Unfortunately, all attempts at Davidson cyclization15 resulted
only in low yields ( ∼7%)16 of oxazole 30.
Scheme 8
Construction of the C(26-32) Oxazole via Oxazoline 31.
This approach would entail removal of the chiral auxiliary from
(+)-27 (Scheme 11), coupling with a serine derivative, cyclo-
dehydration, and then oxidation. Hydrolytic removal of the chiral
auxiliary from (+)-27 (â isomer), exploiting the Evans proto-
(13) (a) Evans, D. A.; Bartroli, J.; Shih, T. L. J. Am. Chem. Soc. 1981,
103, 2127. (b) Evans, D. A.; Ennis, M. D.; Mathre, D. J. J. Am. Chem.
Soc. 1982, 104, 1737.
suggested that phosphonate A, containing the sensitive bifunc-
tional triene, would serve us well.
(14) Tanner, D.; Somfai, P. Tetrahedron 1986, 42, 5985. (b) Eliel, E.
L.; Badding, V. G.; Rerick, M. N. J. Am. Chem. Soc. 1962, 84, 2371.
(15) Davidson, D.; Weiss, M.; Jelling, M. J. Org. Chem. 1937, 2, 328.
(16) The Davidson oxazole cyclization has been employed successfully
in a variety of cases in which the product oxazoles are 2,4,5-trisubstituted;
low yields are typical for 2,4-disubstituted or monosubstituted oxazoles.
This is likely due to reduced regiocontrol in enamine formation (i.e., i vs
ii) during the cyclization; the reaction is more successful when the substituent
at C(5) is aromatic. See Theilig, G. Chem. Ber. 1953, 86, 96.
Evolution of the DE Subunit. Disconnection at the amide
linkage of DE revealed two fragments, the C(26-32) oxazole
D and lactam E (Scheme 9). Reduction of the azide in oxazole
D followed by reaction with the acyl-activated lactam E would
provide the DE subunit.
(11) Uenishi, J.-I.; Beau, J.-M.; Armstrong, R. W.; Kishi, Y. J. Am. Chem.
Soc. 1987, 109, 4756.
(12) Unstable phosphonate A was best stored in a frozen benzene
solution, from which anhydrous A could be obtained upon concentration
in vacuo (with azeotropic removal of adventitious moisture). Under these
conditions, storage of A for up to one month had no detrimental effect on
subsequent Horner-Emmons reactions.

Synthesis of (+)-Calyculin A and (-)-Calyculin B
J. Am. Chem. Soc., Vol. 121, No. 45, 1999 10481
col,17 provided carboxylic acid (-)-32. Condensation with
L-serine methyl ester was then attempted with dicyclohexyl-
carbodiimide (DCC) and 4-(dimethylamino)pyridine (DMAP);
amide (-)-33 was produced in moderate yield (∼65%). Use of
diethylcyanophosphonate18 (DECP) as the coupling reagent
proved preferable, furnishing (-)-33 in 87% yield. Conversion
of the latter to primary chloride 34 with SOCl2 and treatment
with silver triflate induced cyclization to afford oxazoline 31.19
Oxidation with nickel peroxide, following the precedent of
Meyers et al.,20 furnished D in 40-50% yield (Scheme 11).
Scheme 11
Figure 1.
To our delight, 77Se NMR clearly resolved the diastereomeric
resonances (Figure 2). Spectrum (a) depicts the 77Se NMR of
37 derived from a known enantiomeric mixture (1.4:1) of
oxazole D. Importantly, the resonances from the two diastere-
omers display baseline separation (∆δ > 2 ppm!). Parts b and
Scheme 12
Unfortunately, the 13C NMR spectrum of 31 revealed a 6:1
diastereomeric mixture. Subsequent HPLC analysis of alcohol
(-)-33 (>95%) and chloride 34 indicated that epimerization
had occurred during chloride formation, not during amide
coupling or cyclization. For example, cyclization of pure (-)-
34 under the above conditions led to a single oxazoline
diastereomer. However, a report on the configurational instabil-
ity of substituents R to the 2-position in oxazolines21 suggested
that additional epimerization might be occurring during oxazo-
line oxidation.
c of Figure 2 depict the corresponding spectra of 37 derived
from D prepared via the synthetic routes outlined in Schemes
10 and 11. The diastereomeric resonances in Figure 2b appear
in the same ratio ( ∼6:1) as previously observed for chloride
34. We therefore concluded that epimerization did not occur at
the crucial C(30) stereogenic center during the Meyers nickel
peroxide oxidation (Scheme 11).
The Enantiomeric Purity of Oxazole D. To define the
enantiomeric purity of oxazole D, we explored the attachment
of a number of chiral auxiliaries to the C(26) carboxylic acid,
to the corresponding C(26) alcohol, and to the C(32) amine.
For example, reduction of (-)-D to the corresponding amine
(H2, Pd/C) and reaction with the Anderson-Shapiro chiral
chlorodioxaphospholane22 furnished phosphoramide 35. Unfor-
tunately, 31P NMR analysis of an authentic diastereomeric
mixture revealed only broad lines due to P-H(N) coupling;
resolution of the NMR resonances was not feasible. Similarly,
it was not possible to resolve the diastereomeric resonances of
Three subsequent observations led to the most efficient
calyculin oxazole synthesis reported to date (Scheme 13). First,
Scheme 13
1
the Mosher esters derived from (-)-D (i.e., 36) by H and 19F
the degree of epimerization at C(30) in 34 could be reduced
(∼< 5%) by employing the milder conditions of MsCl in
pyridine. Second, cyclodehydration at 55 °C with the Burgess
reagent,24 as introduced by Wipf et al.,25 proved superior to silver
triflate and avoided the use of the intermediate chloride and
the expensive silver triflate. Third, the Barrish-Singh CuBr2
oxidation26 proceeded in higher yield. Taken together these
conditions afforded (-)-D in 67% yield (two steps), with little
or no epimerization.27
NMR (Figure 1). We therefore turned to the Silks-Odom
selenium-77 NMR technique23 which called for the preparation
of 37 (Scheme 12).
(17) Evans, D. A.; Britton, T. C.; Ellman, J. A. Tetrahedron Lett. 1987,
28, 6141.
(18) Yamada, S.; Kasai, Y.; Shioiri, T. Tetrahedron Lett. 1973, 14, 1595.
(19) For example, see: (a) Hamada, Y.; Shibata, M.; Shioiri, T.
Tetrahedron Lett. 1985, 26, 5155. (b) Hamada, Y.; Shibata, M.; Shioiri, T.
Tetrahedron Lett. 1985, 26, 6501.
(20) Evans, D. L.; Minster, D. K.; Jordis, U.; Hecht, S. M.; Mazzu, A.
L., Jr.; Meyers, A. I. J. Org. Chem. 1979, 44, 497.
(21) Yonetani, K.; Hirotsu, Y.; Shiba, T. Bull. Chem. Soc. Jpn. 1975,
48, 3302.
(22) Anderson, R. C.; Shapiro, M. J. J. Org. Chem. 1984, 49, 1304.
(23) Silks, L. A., III; Dunlap, R. B.; Odom, J. D. J. Am. Chem. Soc.
1990, 112, 4979. (b) Silks, L. A., III; Peng, J.; Odom, J. D.; Dunlap, R. B.
J. Org. Chem. 1991, 56, 6733.
(24) Burgess E. M.; Penton, H. R., Jr.; Taylor, E. A.; Williams, W. M.
Organic Syntheses; Wiley & Sons: New York, 1988; Collect. Vol. VI, p
788. (b) Burgess, E. M.; Penton, H. R., Jr.; Taylor, E. A. J. Org. Chem.
1973, 38, 26.
(25) Wipf, P.; Miller, C. P. Tetrahedron Lett. 1992, 33, 907.
(26) Barrish, J. C.; Singh, J.; Spergel, S. H.; Han, W.-C.; Kissick, T. P.;
Kronenthal, D. R.; Mueller, R. H. J. Org. Chem. 1993, 58, 4494.

10482 J. Am. Chem. Soc., Vol. 121, No. 45, 1999
Smith et al.
Construction of Lactam E. Initial efforts to prepare the
C(33-37) acid (43) focused on a variation of the Hamada/
Shioiri synthesis28 of the γ-amino acid moiety found in the
gastroprotective substance AI-77-B. Beginning with benzylidene
aminal 44,28 a known compound available from L-glutamic acid
(Scheme 15), selective hydrogenolysis29 followed by methyla-
tion of the derived primary alcohol furnished (+)-46 in 90%
yield. However, neither hydrolytic opening of the lactam ring
nor removal of the N-benzyl group proved viable (Scheme 15).
Scheme 15
Although the N-benzyl group could not be removed from
lactam (+)-46, the benzylidene group in 44 was easily removed
via transfer hydrogenation to provide (+)-47 (Scheme 16).28
Scheme 16
Figure 2.
To facilitate exploratory work, we prepared two additional
oxazoles (Scheme 14). Reduction of the azide in (-)-D provided
Scheme 14
Installation of p-methoxybenzyl groups on both nitrogen and
oxygen, followed by selective removal of the O-PMB group
and O-methylation, furnished (+)-49.
Oxidative removal of the N-PMB group from (+)-49 could
then be achieved with cerium(IV) ammonium nitrate (CAN) in
aqueous acetonitrile30 to furnish lactam (-)-50 in 61% yield,
accompanied by the over-oxidized imide (-)-51 (24%). The
amine 40, which proved to be unstable as the free base.
Alternatively, ester reduction to alcohol (-)-41, followed by
azide reduction furnished amine (-)-42 which could be stored
without event.
(28) Hamada, Y.; Kawai, A.; Kohno, Y.; Hara, O.; Shioiri, T. J. Am.
Chem. Soc. 1989, 111, 1524. (b) Hamada, Y.; Hara, O.; Kawai, A.; Kohno,
Y.; Shioiri, T. Tetrahedron 1991, 47, 8635.
(29) Sakura, N.; Ito, K.; Sekiya, M. Chem. Pharm. Bull. 1972, 20,
1156.
(27) Barrish-Singh oxidation did not compromise the integrity of the
C(30) stereocenter, as judged by analysis of the optical rotation of (-)-D
produced in this fashion.

Synthesis of (+)-Calyculin A and (-)-Calyculin B
J. Am. Chem. Soc., Vol. 121, No. 45, 1999 10483
Table 1
latter could be readily redirected to the synthetic sequence by
hydrolysis with LiOOH; a combined yield of 80% for lactam
(-)-50 was obtained. The tert-butyl carbamate derivative (-)-E
was then generated from lactam (-)-50 in 91% yield; the t-Boc
moiety would serve to activate the lactam for ring opening.
Although successful, the synthesis of lactam (+)-49 (11 steps
from L-glutamic acid, 8.6% overall yield) proved both lengthy
and inefficient. We therefore explored an alternate route
(Scheme 17), which was envisioned to take advantage of a
stereocontrolled addition of an appropriate one-carbon nucleo-
phile to the convex face of N-acyliminium ion 52 derived from
hemiaminal 53. The latter would be prepared from erythrono-
lactone; the elegant indolizidine syntheses of Overman and
Heitz31 provided precedent for this tactic.
The reactivity of 56 was tested with n-butylcopper, vinyl-
trimethylsilane, and trimethylsilylacetylene;35,36 no reaction
occurred! Allyltrimethylsilane however reacted efficiently to
furnish exclusively the â-allyl adduct (+)-57 in 86% yield (Table
1; entry a). Encouraged by this success, we explored nucleo-
philes which could be more readily transformed to the required
methoxymethyl moiety. Reaction of 56 with trimethylsilylcya-
nide promoted by boron trifluoride etherate in various solvents
(entries b-d) led to 58 in good yield, but with only modest
â-selectivities.37
We next explored enol ethers as the nucleophile. Shono and
co-workers demonstrated that enol ethers add intermolecularly
to N-acyliminium cations to furnish ketones after hydrolysis.38
We reasoned that a ketone which could enolize only in one
direction would serve as an effective hydroxymethyl synthon,
assuming the sequence of addition, enolization, ozonolysis, and
reduction. In the event, treatment of 56 with the trimethylsilyl
(TMS) enol ether of pinacolone (61)39 in the presence of BF3‚
OEt2 furnished ketone (+)-59 as a single diastereomer in 81%
yield (entry e). The analogous transformation with acetophenone
was equally selective, but less efficient (entry f). Generation of
the trimethylsilyl enol ether of 59 (Scheme 19) followed by
ozonolysis with reductive workup (NaBH4) led as expected to
alcohol (+)-48, identical (1H and 13C NMR) to that prepared
previously from L-glutamic acid. Importantly, this sequence
proceeded in 54% overall yield for the 3 steps.
Scheme 17
Toward this end, treatment of commercially available iso-
propylidene-D-erythronolactone [(-)-54]32 (Scheme 18) with the
Scheme 18
(35) Ludwig, C.; Wistrand, L.-G. Acta Chem. Scand. 1990, 44, 707. (b)
Skrinjar, M.; Wistrand, L.-G. Tetrahedron Lett. 1990, 31, 1775.
(36) Additions of Grignard species to sulfonylpyrrolidinones i prepared
from 53 [MsCl, Et3N; PhSH; monoperoxyphthalic acid magnesium salt
hydrate (MMPP); 67% overall] were similarly unsuccessful.
aluminum amide33 derived from p-methoxybenzylamine, fol-
lowed by Parikh-Doering oxidation and acetylation led to 56
as a 7:1 anomeric mixture in 53% for the three steps.34
(30) Yoshimura, J.; Yamaura, M.; Suzuki, T.; Hashimoto, H. Chem. Lett.
1983, 1001.
(31) Heitz, M.-P.; Overman, L. E. J. Org. Chem. 1989, 54, 2591.
(32) Lactone (-)-54 can be prepared from D-isoascorbic acid, see:
Cohen, N.; Banner, B. L.; Laurenzano, A. J.; Carozza, L. Org. Synth. 1985,
63, 127.
For related examples, see: Brown, D. S.; Hansson, T.; Ley, S. V. Synlett
1990, 48. Brown, D. S.; Charreau, P.; Ley, S. V. Synlett 1990, 749.
(37) The â orientation of nitrile 58 (major isomer) was assigned on the
basis of 1H NMR coupling constant analysis and confirmed by single-crystal
X-ray analysis.
(38) Shono, T.; Matsumura, Y.; Tsubata, K. J. Am. Chem. Soc. 1981,
103, 1172. (b) Shono, T.; Tsubata, K.; Okinaga, N. J. Org. Chem. 1984,
49, 1056.
(33) Basha, A.; Lipton, M.; Weinreb, S. M. Tetrahedron Lett. 1977, 18,
4171.
(34) Single-crystal X-ray analysis of 56 (major anomer) identified it as
the â-anomer as expected on steric grounds.
(39) House, H. O.; Czuba, L. J.; Gall, M.; Olmstead, H. D. J. Org. Chem.
1969, 34, 2324.

10484 J. Am. Chem. Soc., Vol. 121, No. 45, 1999
Smith et al.
Scheme 19
functionality at C(26) also appeared prudent until after the C(34,-
35) interchange.
Synthesis of the second generation C(26-37) subtarget
(Scheme 21) began with the union of amino ester 40 with (-)-
Scheme 21
Union of Fragments C(26-32) and C(33-37). Two meth-
ods were investigated to couple the C(26-32) oxazole 30 with
the C(33-37) lactam (-)-E (Scheme 20). Reaction of the
Scheme 20
65 to furnish amide (+)-68 in 75% yield. Removal of the N-Boc
group with TMSOTf,44 followed by reductive amination (HCHO,
NaBH3CN, aqueous HOAc, CH3CN) introduced the two methyl
groups at nitrogen with partial acetonide hydrolysis. Complete
acetonide hydrolysis was readily achieved with HCl in anhy-
drous MeOH/ether. Silylation with diethylisopropylsilyl triflate
(DEIPSOTf) in the presence of 2,6-lutidine then furnished (+)-
69 in 50% yield for the four steps, with only one chromato-
graphic purification.
Reduction of the ester was next achieved with LiAlH4 at -78
°C to afford alcohol (+)-70 (Scheme 21); low-temperature
workup with ethyl acetate was essential to prevent loss of one
of the DEIPS groups. Turning next to ylide formation, use of
both the C(26) iodide or bromide (e.g., I2, PPh3 or CBr4, PPh3)
was feasible, albeit they proved unstable. On the other hand
the chloride obtained by treatment of (+)-70 with methane-
sulfonyl chloride in the presence of triethylamine and benzyl-
triethylammonium chloride was stable at 0 °C for weeks.
Displacement with tri-n-butylphosphine in the mixed solvent
system of CH3CN/THF (2:1) led to clean conversion to
phosphonium salt (+)-DE in 95% yield.
Wittig Coupling: A Model Study. Initially we explored the
triphenyl- and tributylphosphonium salts 72a and 72b, generated
in situ from iodide (+)-67 (Scheme 22). Of particular concern
was the E/Z coupling selectivity and the need for C(17) hydroxyl
protection. Reaction with hydrocinnamaldehyde revealed a
dramatic increase in the E/Z ratio with the tri-n-butylphospho-
aluminum amide derived from amino oxazole 63 (prepared by
Lindlar reduction40 of azide 30) with (-)-E was explored
initially. This reaction proved extremely slow, furnishing
consistent yields of 64 only when a large excess of the aluminum
amide (>1.5 equiv) was employed. Better results were obtained
by coupling γ-amino oxazole (-)-42 with γ-amino acid (-)-
65, available by lithium hydroxide hydrolysis of (-)-E;41
diethylcyanophosphonate (DECP) proved to be the coupling
agent of choice. With concentrations of (-)-65 and (-)-42 near
0.3-0.4 M, amide (+)-66 was routinely obtained in ∼70%
yield. With (+)-66 in hand, formation of primary iodide (+)-
67, progenitor of the phosphonium salt, was readily ac-
complished with triphenylphosphine, imidazole, and iodine.
A Second Generation C(26-37) Subtarget. In the end, we
found that the acetonide at C(34,35) was not a viable protecting
group due to its hydrolytic stability in the completely function-
alized calyculin framework.42 Although the related cyclopen-
tylidene ketal could be more readily removed, eventual success
was achieved with diethylisopropylsilyl (DEIPS) groups43a
deployed at the C(34,35) diol. We also opted to transform the
C(36) N-Boc moiety to the required N,N-dimethylamine prior
to union with the C(9-25) spiroketal, based on preliminary
experiments suggesting that 2-trimethylsilylethyl protection of
the C(17) phosphate ester would not survive the Boc deprotec-
tion-reductive amination sequence.43b
(42) The acetonide required strongly acidic conditions (CSA, MeOH,
45 °C, 24 h or 1.2 M aqueous HCl, THF, 50 °C, 12 h) to effect removal
from model compound i, casting uncertainty upon its successful removal
from fully protected calyculin. Indeed, although model compound iii could
be deprotected, removal of the acetonide was unsuccessful with fully
protected calyculin A. (b) Proton NMR comparison showed that cyclopen-
tylidene hydrolysis from ii was 4.9 times faster than acetonide hydrolysis
from i.
Two additional considerations came to the fore. Interchange
of the acetonide and DEIPS groups could not be effected with
lactam (+)-49 or (-)-E; this operation was therefore postponed
until after establishing the amide linkage. Retaining the ester
(40) Corey, E. J.; Nicolaou, K. C.; Balanson, R. D.; Machida, Y. Synthesis
1975, 590.
(41) Flynn, D. L.; Zelle, R. E.; Grieco, P. A. J. Org. Chem. 1983, 48,
(43) (a) Toshima, K.; Mukaiyama, S.; Kinoshita, M.; Tatsuta, K.
Tetrahedron Lett. 1989, 30, 6413. (b) Spoors, P. G.; Smith, A. B., III,
unpublished results.
2424.
(44) Hamada, Y.; Kato, S.; Shioiri, T. Tetrahedron Lett. 1985, 26, 3223.

Synthesis of (+)-Calyculin A and (-)-Calyculin B
J. Am. Chem. Soc., Vol. 121, No. 45, 1999 10485
Scheme 24
Scheme 22
nium salt, consistent with the observations subsequently reported
by Armstrong and co-workers.45,46 We also observed that the
yield increased to 69% without significantly altering the E/Z
selectivity when three equivalents of n-BuLi were employed.
With the more advanced model spiroketals (+)-74 and (+)-
7547 (Scheme 23), moderate yields of the expected alkene
Scheme 23
Introduction of the C(1-8) Cyanotetraene. To attach the
C(3-8) subunit, the pivaloate group was removed from the C(9)
terminus of (+)-BCDE via reduction (DIBAL or LiEt3BH)
(Scheme 25). Initially this process proved capricious;51 reliable
Scheme 25
products (+)-76 and (+)-77 were obtained with similar E
selectivities. Although the experiment with (+)-74 established
the compatibility of the Wittig coupling without protection at
C(17),48 better yields were obtained with protection.
Union of (+)-BC with (+)-DE. Aldehyde (+)-78, readily
obtained in high yield from (+)-BC1 by removal of the C(25)
O-benzyl protecting group and Ley oxidation [catalytic perru-
thenate (TPAP), NMO],49 was submitted to Wittig olefination
(Scheme 24). As with the model studies, olefination with
triphenylphosphonium salt 7950 (LiHMDS, THF) furnished (+)-
BCDE with 2:1 E/Z selectivity, whereas the tri-n-butylphos-
phonium salt (+)-DE provided a 5:1 E/Z mixture. With N,N-
dimethylformamide as solvent, the E/Z ratio improved further
to 9:1, consistent with the results of Armstrong et al.45 Evans
and co-workers reported Wittig olefination with a related
substrate (LDA, DMF, 0 °C) to be completely stereoselective.
In our hands, their conditions afforded 10:1 E/Z selectivity, albeit
the reaction was less efficient (∼32-69%). For material
advancement we employed LiHMDS (1.5 eq) in DMF at 0 °C;
isomer separation was achieved via radial chromatography.
results were eventually obtained by minimizing the amount of
DIBAL followed by workup with sodium potassium tartrate.
Ley oxidation49 then afforded aldehyde (+)-80 in high yield
(84%). It should be emphasized that this oxidation proceeded
in excellent yield despite the C(36) tertiary amine.52
Horner-Emmons coupling was then achieved via metalation
of A with n-BuLi; a deep red-orange solution resulted. Addition
of (+)-80 led to olefination within 30 min, with high selectivity
(>10:1).53 With basic workup (aqueous NaHCO3), the product
containing the C(3) carbonyl function masked as an enol ether
could be isolated ( ∼50-60%) and chromatographed. However,
best results entailed direct conversion via acid treatment (0.5
N HCl) to ketone (+)-81 (92% yield; E/Z ) 15:1).
(45) Zhao, Z.; Scarlato, G. R.; Armstrong, R. W. Tetrahedron Lett. 1991,
32, 1609.
(46) The role of the phosphorous ligands on the stereoselectivity in Wittig
reactions has been discussed in detail, see: Vedejs, E.; Marth, C. F. J. Am.
Chem. Soc. 1988, 110, 3948.
(47) Compounds 74 and 75 were prepared from i, the latter prepared
during the synthesis of the C(9-25) fragment (+)-BC (see ref 1).
O-Methylation, hydrogenation (Pearlman’s catalyst) to saturate fully the
alkyne, and removal of the C(25) O-benzyl afforded the C(25) primary
alcohol. Ley oxidation (TPAP/NMO), with and without prior removal of
the C(17) TBS group provided 74 and 75, respectively.
(51) This reductive deprotection initially provided variable yields and
low mass balance. We speculate that chelation of the Lewis acidic boron
and aluminum reagents by the C(36-37) permethylated 1,2-amino alcohol
increased the product hydrophilicity and interfered with isolation; related
C(9-25) substrates not possessing the 1,2-amino alcohol (i.e., before Wittig
coupling) behaved normally.
(52) If the C(36) tertiary amine is oxidized, the N-oxide product may be
expected to be functionally similar to NMO. We did not detect the N-oxide.
(53) Determination of the E/Z selectivity by 1H NMR spectroscopy was
unreliable at this stage due to decomposition and the presence of interfering
resonances.
(48) Independently, Armstrong demonstrated similar transformations with
the C(17) hydroxyl unprotected, see: Ogawa, A. K.; DeMattei, J. A.;
Scarlato, G. R.; Tellew, J. E.; Chong, L. S.; Armstrong, R. W. J. Org.
Chem. 1996, 61, 6153.
(49) Ley, S. V.; Norman, J.; Griffith, W. P.; Marsden, S. P. Synthesis
1994, 639.
(50) Phosphonium salt 79 was prepared from the corresponding alkyl
chloride (+)-71 (PPh3, MeCN/THF; 28% yield).

10486 J. Am. Chem. Soc., Vol. 121, No. 45, 1999
Smith et al.
Completion of the Calyculin Synthetic Venture. The stage
was now set for Peterson olefination and global deprotection.
In the event (Scheme 25), the desired cyanotetraene emerged
in 94% yield as a 1.7:1 E/Z mixture. The Z and E isomers,
corresponding to fully protected ent-calyculin A and ent-
calyculin B respectively, were easily separable by careful radial
chromatography.
All that remained was global deprotection. We recount here
two important experiments which had earlier suggested the
feasibility of this scenario. Oxazole (+)-69, when subjected to
hydrolysis with HF (1.6 M in 8:1 CH3CN/H2O),54 effectively
afforded 82 (Scheme 26). In a similar fashion, treatment of (+)-
after 2 d, the major component present contained only one
trimethylsilylethyl-protecting group.55 Prolonged exposure to HF
(2 M in 8:1 CH3CN/H2O, 7 d) cleanly provided synthetic (+)-1
in 69% yield. In a similar fashion, fully protected ent-calyculin
B afforded synthetic (-)-calyculin B (2) (84% yield). Synthetic
(+)-calyculin A and (-)-calyculin B were identical in all
respects (1H and 13C NMR, IR, UV, HRMS, TLC, HPLC),
except for chiroptic properties,56 to natural (-)-1 and (+)-2,
respectively.57
Acknowledgment. Support was provided by the National
Institutes of Health (National Cancer Institute) through Grant
CA-19033 and postdoctoral fellowships to G.K.F. and K.G.H.
We also thank DuPont Pharmaceuticals, Inc. and Zeneca
Pharmaceuticals, Inc. for support of this work. In addition, we
thank Drs. George T. Furst and Patrick J. Carroll and Mr. John
Dykins of the University of Pennsylvania Spectroscopic Service
Center for assistance in securing and interpreting high-field
NMR spectra, X-ray crystal structures, and mass spectra,
respectively.
Scheme 26
Supporting Information Available: Spectroscopic and
analytical data and selected experimental procedures for all
synthetic intermediates (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
JA992135E
(55) Interestingly, this monoprotected intermediate was more polar (TLC)
than fully deprotected calyculin A. Together with precedent, this TLC
behavior suggests that a fully deprotected phosphate is required for formation
of a folded tertiary structure which exposes a less polar surface. Evans and
coworkers observed that an intermediate with one t-butyldimethylsilyl group
remaining coeluted with calyculin A on silica gel; it was proposed that this
silyl group [tentatively assigned as C(11)-OTBS] was buried within a folded
calyculin structure, minimizing the effect on the apparent overall polarity
and accessibility to external reagents.9 In our case, the phosphate group is
last to be deprotected. Deprotection at C(11) appears normal; this implicates
the free phosphate as critical for the folding process. Similar behavior was
observed during deprotections of calyculins A and B.
BCDE with the same HF conditions led to 83 in 74% yield;
prolonged exposure to HF was required to remove the remaining
protecting group from the phosphate. The latter reaction
established both the lability of the phosphate protection and the
stability of the phosphate P-O(17) bond under the HF condi-
tions.
Bolstered by these results, fully protected ent-calyculin A
(Scheme 25) was exposed to HF (1.5 M in 10:1 CH3CN/H2O);
(56) Synthetic (+)-calyculin A and (-)-calyculin B had optical rotations
and CD spectra of equal magnitudes and opposite signs relative to the
corresponding natural products.
(54) We benefited significantly in this regard from the previous successes
of Evans (see ref 9) and Masamune (see: Tanimoto, N.; Gerritz, S. W.;
Sawabe, A.; Noda, T.; Filla, S. A.; Masamune, S. Angew. Chem., Int. Ed.
Engl. 1994, 33, 673), each of whom demonstrated that calyculin A is stable
to HF in aqueous acetonitrile.
(57) We thank Professors Nobuhiro Fusetani and Shigeki Matsunaga
(University of Tokyo) for authentic samples of natural (-)-1 and (+)-2,
and for confirming that natural 2 has an [R] of +15° (c 0.07). Natural 2
was originally reported to have [R] -60° (c 0.05). We also thank Professor
David Evans (Harvard University) for a sample of synthetic (+)-1.