Enantioselective synthesis of oasomycin A, part II: Synthesis of the C29-C46 subunit

Article abstract of DOI:10.1002/anie.200603654

(Chemical Equation Presented) Putting the pieces together: The total synthesis of the natural macrolide oasomycin A has been realized. Key fragment couplings include an anti-Felkin selective aldol addition (green), Kociensky-Julia olefinations (red), and

Full text of DOI:10.1002/anie.200603654

Angewandte
Chemie
DOI: 10.1002/anie.200603654
Natural Products Synthesis
Enantioselective Synthesis of Oasomycin A, Part II: Synthesis of the
C29–C46 Subunit**
David A. Evans,* Pavel Nagorny, Dominic J. Reynolds, and Kenneth J. McRae
Dedicated to Professor Y. Kishi on the occasion of his 70th birthday.
Syntheses of the C1–C12 and C13–C28 oasomycin A subunits
were described in the preceding Communication.[1] Herein we
describe the synthesis and assemblage of the C29–C46 portion
of this polyketide natural product. According to the synthesis
plan,[2] the C29–C46 fragment targeted as aldehyde I is
considered as one of the complex subgoals.
of the elegant studies of Wasserman et al., the decision was
made to mask the C46 carboxy terminus in sulfone II as its
derived 4,5-diphenyloxazole,[3] thus preserving its oxidation
state. The singlet-oxygen-mediated liberation of this carboxy
moiety could, in principle, be executed at numerous stages in
the synthesis because of the compatibility of this transforma-
tion with the multitude of other oxygen-protecting groups in
the assembled or partially assembled subunits. The C29–C38
fragment III (Scheme 1) is composed of both polyacetate and
polypropionate subunits. The latter motif could be introduced
by a SnII-mediated syn-selective aldol addition of dipropionyl
synthon IV to aldehyde V—a reaction which was developed
by us some years ago.[4]
Julia disconnection of the D38 olefin in I affords fragments
II and III of comparable complexity (Scheme 1). On the basis
The synthesis of aldehyde V began with a chiralLewis
acid catalyzed aldol addition of the Chan diene[5] 1 to
benzyloxy acetaldehyde 2 promoted by the CuII complex 3
(5 mol%) that was previously developed by our research
group (Scheme 2).[6] The resultant ketoester 4 (95% ee) was
[7]
reduced with Me4NBH(OAc)3 to afford a 1,3-anti diol
(91:9 d.r.). Silylation of the diol (TBSCl, imidazole) followed
by a reduction using DIBALH provided aldehyde 5 (77%,
3 steps). The dipropionylsynthon IV was next introduced by a
SnII-mediated aldol addition of b-ketoimide 6 to aldehyde 5[4]
thus providing 7 as a 95:5 mixture of diastereomers. Imme-
[7]
diate treatment of 7 with Me4NBH(OAc)3 afforded the
anticipated anti diol 8a (90:10 d.r.)[8] which was readily
purified by flash chromatography. Selective protection
(TBSOTf, lutidine) of the less sterically hindered C33
hydroxy group gave the TBS ether 8b in 75% yield
(2 steps). Since we were unable to directly protect the
hindered C31 hydroxy group as the PMB ether, the well-
precedented three-step procedure consisting of reductive
removalof the chiralauxiilary with LiBH 4, protection of the
diolas the
p-methoxybenzylidene acetal, and selective
[9]
reduction of the acetalwith borane, catayl zed by Sc(OTf)
,
3
Scheme 1. Retrosynthetic analysis of oasomycin A. Bn=benzyl.
was then accomplished (80%, 3 steps). Interestingly, when
the aforementioned acetalreduction was attempted with
DIBALH, none of the desired product was obtained and the
reaction resulted in loss of the TBS group at C37. Alcohol 9
was then silylated (TESOTf, lutidine) and the resulting
product was hydrogenated (H2, dry Pd(OH)2/C, EtOAc) to
give the alcohol at C38 that was then oxidized with Dess–
Martin reagent[10] to afford the desired C29–C38 subunit 10.
The construction of sulfone II (Scheme 1) began with the
preparation of a,b-unsaturated aldehyde 12 from the known
4,5-diphenyloxazole 11 (Scheme 3).[11] The aldol addition of
oxazolidinone 13 to aldehyde 12 catalyzed by magnesium
chloride[12] afforded the corresponding anti aldol adduct that
[*] Prof. D. A. Evans, P. Nagorny, Dr. D. J. Reynolds, Dr. K. J. McRae
Department of Chemistry & Chemical Biology
Harvard University
Cambridge, MA 02138 (USA)
Fax: (+1)617-495-1460
E-mail: evans@chemistry.harvard.edu
[**] Financial support has been provided by the National Institutes of
Health (GM-33327-19), the Merck Research Laboratories, Amgen,
and Eli Lilly. A postdoctoral fellowship was provided to D.J.R. by the
Glaxo Foundation.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2007, 46, 541 –544
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
541

Communications
Scheme 2. Synthesis of the C29–C38 fragment 10. Reagents and conditions: a) 1. 3 (0.05 equiv), CH2Cl2, ꢀ958C; 2. PPTS, MeOH, (95% ee);
b) Me4NBH(OAc)3, MeCN/AcOH, ꢀ258C, (91:9 d.r.); c) TBSCl, imidazole, DMF, RT; d) DIBALH, toluene, ꢀ90!ꢀ788C, (77%, 3 steps); e) 1. 6, Sn(OTf)2,
NEt3, CH2Cl2, ꢀ20!ꢀ788C; 2. 5, CH2Cl2, (95%, 95:5 d.r.); f) Me4NBH(OAc)3,
MeCN/AcOH, ꢀ208C, (90:10 d.r.); g) TBSOTf, lutidine, CH2Cl2, 08C, (75%,
2 steps); h) LiBH4, THF, H2O, 08C; i) PMPCH(OMe)2, PPTS, CH2Cl2; j) Sc(OTf)3
(0.1 equiv), BH3·THF (5 equiv), CH2Cl2, 08C, (80%, 3 steps); k) TESOTf, lutidine,
THF, 08C; l) Pd(OH)2/C (0.1 equiv), H2, EtOAc; m) DMP, Py, CH2Cl2, (69%,
3 steps). DIBALH=diisobutylaluminum hydride, DMF=dimethylformamide,
DMP=Dess–Martin Periodinane, PMB=4-methoxybenzyl, PMP=4-methoxy-
phenyl, PPTS=pyridinium p-toluenesulfonate, Py=pyridine, TBS=tert-butyldi-
methylsilyl, TES=triethylsilyl, TMS=trimethylsilyl, Tf=trifluoromethanesulfonyl.
was then hydrogenated (Pd/C, H2, EtOAc) to give alcohol
14[13] (88%, 2 steps). Remarkably, the diastereoselectivity of
the aldol addition was counterintuitively temperature depen-
dent. Thus, when the reaction temperature was raised from
ꢀ10 to 778C, the diastereoselectivity for the anti product
increased from 1:1 to 13:1. The anti aldol adduct 14 obtained
was then silylated[14] (TESOTf, lutidine) and the chiral
auxiliary was removed by a two-step procedure to provide
the corresponding aldehyde, which was treated with ethyl
(triphenylphosphoranylidene)acetate to give the a,b-unsatu-
rated ester 16. Cleavage of the TES group (HCl, MeOH)
followed by an intramolecular heteroconjugate addition[15] of
the hemiacetal p-anisaldehyde adduct of 17 (Scheme 3)
resulted in the formation of acetal 18 (59%, 94:6 d.r.), in
accord with our previous findings.[16] We found that a non-
polar solvent system (Et2O/PhMe) was required for this
reaction to proceed with significant conversion.
Incorporation of the phenyltetrazole sulfone moiety at the
C38 terminus of 18 was then executed by a three-step
procedure: 1) reduction of the ester with LiAlH4, 2) Mitsu-
nobu reaction with 1-phenyl-1H-tetrazole-5-thiol, and 3) oxi-
Scheme 3. Synthesis of the C39–C46 fragment 20. Reagents and
dation of the derived sulfide[17] to give 19 in 53% yield over
conditions: a) 1. nBuLi, THF, ꢀ788C; 2. DMF, ꢀ78!208C;
the three steps. The p-methoxybenzylidene acetal was
removed (AlBr3, EtSH)[18] and silylation of the resultant
unstable diol (TMSCl, imidazole) afforded the fully elabo-
rated C39–C46 fragment 20 in good yield (65%, 2 steps).
With fragments 10 and 20 in hand, their coupling was then
addressed (Scheme 4). Kocienski–Julia olefination proved to
be optimalunder Barbier conditions and proceeded with
excellent stereoselectivity (> 95:5 E/Z).[19] However, this
transformation was highly dependent on the nature of the
=
b) PPh3 CHCHO, CH2Cl2, (61%, 2 steps); c) 1. 13, MgCl2, TMSCl,
NEt3, EtOAc, 778C; 2. TFA, MeOH, (13:1 d.r.); d) Pd/C (10%), H2,
EtOAc, (88%, 2 steps); e) TESOTf, lutidine, CH2Cl2, 08C, 90% f) EtSLi,
=
THF, ꢀ208C; g) DIBALH, CH2Cl2, ꢀ908C; h) PPh3 CHCO2Et, CH2Cl2,
(75%, 3 steps); i) HCl (0.05n), MeOH; 90%; j) PMPCHO, KOtBu,
Et2O/toluene, ꢀ208C, 59%; k) LiAlH4, Et2O, 08C; l) 1-phenyl-1H-
tetrazole-5-thiol, DEAD, PPh3, THF; m) (NH4)6Mo7O24, H2O2, EtOH,
(53%, 3 steps); n) AlBr3, EtSH, CH2Br2/CH2Cl2; o) TMSCl, imidazole,
CH2Cl2, (65%, 2 steps). DEAD=diethyl azodicarboxylate, TFA=tri-
fluoroacetic acid.
542
www.angewandte.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 541 –544

Angewandte
Chemie
Scheme 4. Assembly of C29–C46 subunit 25. Reagents and conditions: a) KHMDS, DME, ꢀ48!208C, (>95:5 E/Z); b) PPTS, MeOH/CH2Cl2 (1:1), 08C,
(71%, 2 steps); c) Rose Bengal, O2, hn, (CH2Cl)2, 90%; d) TMSCl, imidazole, CH2Cl2; e) PPTS, Py, MeOH/CH2Cl2 (1:2); f) DMP, Py, CH2Cl2, (55%, 3 steps).
Bz=benzoyl, DME=1,2-dimethoxyethane, HMDS=hexamethyldisilazide, PT=5-phenyltetrazole.
R. J. Gambale, M. J. Pulwer, Tetrahedron Lett. 1981, 22, 1737 –
protecting groups on the sulfone fragment 20, with TMS
1740; d) H. H. Wasserman, R. J. Gambale, J. Am. Chem. Soc.
1985, 107, 1423 – 1424; e) H. H. Wasserman, R. W. DeSimone,
groups affording the optimalyield. [20] The unpurified product
was then treated with PPTS to remove the primary TES group
and the two TMS groups to provide triol 22 in 71% yield
(2 steps). This successfulcross-coupilng reaction confirmed
our prediction that the CH kinetic acidity conferred on 20 by
the sulfone moiety would be greater than the acidity
contributed by the oxazole synthon. The subsequent singlet-
oxygen oxidation of the 4,5-diphenyloxazole moiety in 22
proceeded with concomitant lactonization via 23 to provide
lactone 24 in 90% yield. The hydroxy groups at C29 and C41
of compound 24 were then protected as TMS ethers (TMSCl,
imidazole) and the product subjected to PPTS buffered with
pyridine to selectively remove the primary TMS group at
C29.[21] The product was then oxidized to afford the targeted
C29–C46 subunit of oasomycin A (55%, 3 steps).
W. B. Ho, K. E. McCarthy, K. Spencer Prowse, A. P. Spada,
Tetrahedron Lett. 1992, 33, 7207 – 7210.
[4] D. A. Evans, J. S. Clark, R. Metternich, V. J. Novack, G. S.
Sheppard, J. Am. Chem. Soc. 1990, 112, 866 – 868.
[5] G. A. Molander, O. K. Cameron, J. Am. Chem. Soc. 1993, 115,
830 – 846.
[6] a) D. A. Evans, M. C. Kozlowski, J. A. Murray, C. S. Burgey,
K. R. Campos, B. T. Connell, R. J. Staples, J. Am. Chem. Soc.
1999, 121, 669 – 685; for a recent review on catalytic, enantio-
selective, vinylogous aldol reactions, see: b) S. E. Denmark, J. R.
Heemstra, Jr., G. L. Beutner, Angew. Chem. 2005, 117, 4760 –
4777; Angew. Chem. Int. Ed. 2005, 44, 4682 – 4698.
[7] The reduction of 4 is described in Ref. [6]. For the original
procedure for Me4NBH(OAc)3 reduction, see: D. A. Evans,
K. T. Chapman, E. M. Carreira, J. Am. Chem. Soc. 1988, 110,
3560 – 3578.
The study described above provided an efficient route to
the C29–C46 portion of oasomycin A, and led to the
culmination of the total synthesis of oasomycin A that is
addressed in the following Communication.
[8] The diolwas alctonized to
26 and its configuration was
determined by NOESY experiments.
Received: September 6, 2006
Published online: December 8, 2006
[9] C. C. Wang, S. Y. Luo, C. R. Shie, S. C. Hung, Org. Lett. 2002, 4,
847 – 849.
[10] D. B. Dess, J. C. Martin, J. Am. Chem. Soc. 1983, 105, 4155 –
4156.
[11] W. W. Pei, S. H. Li, X. P. Nie, Y. W. Li, J. Pei, B. Z. Chen, J. Wu,
X. L. Ye, Synthesis 1998, 1298 – 1304.
[12] a) D. A. Evans, J. S. Tedrow, J. T. Shaw, C. W. Downey, J. Am.
Chem. Soc. 2002, 124, 392 – 393; for a review of recent studies
with chiralimides, see: b) D. A. Evans, J. T. Shaw, Actual. Chim.
2003, 4–5, 35 – 38.
[13] The stereochemistry of 14 was proven by
X-ray crystallography of its derivative 27:
[14] Protection as the silyl ether simplified the
purification of 15 from the minor diaste-
reomer giving > 18:1 d.r. after flash chro-
matography.
Keywords: aldol reaction · Kocienski–Julia olefination ·
macrolactonization · natural products · total synthesis
.
[1] D. A. Evans, P. Nagorny, K. J. McRae, D. J. Reynolds, L.-S.
Sonntag, F. Vounatsos, R. Xu, Angew. Chem. 2007, 119, 543 –
546; Angew. Chem. Int. Ed. 2007, 46, 537 – 540.
[2] For the discussion of the synthesis plan, refer to the following
Communication: D. A. Evans, P. Nagorny, K. J. McRae, L.-S.
Sonntag, D. J. Reynolds, F. Vounatsos, Angew. Chem. 2007, 119,
551 – 554; Angew. Chem. Int. Ed. 2007, 46, 545 – 548.
[3] For examples using 4,5-diphenyloxazole as a protecting group,
see: a) H. H. Wasserman, R. J. Gambale, M. J. Pulwer, Tetrahe-
dron 1981, 37, 4059 – 4067; b) H. H. Wasserman, R. J. Gambale,
Tetrahedron Lett. 1981, 22, 4849 – 4852; c) H. H. Wasserman,
Angew. Chem. Int. Ed. 2007, 46, 541 –544
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
543

Communications
[15] D. A. Evans, J. A. Gauchet-Prunet, J. Org. Chem. 1993, 58,
[19] a) P. J. Kocienski, A. Bell, P. R. Blakemore, Synlett 2000, 365 –
366; for a recent review on this topic, see: b) P. R. Blakemore, J.
Chem. Soc. Perkin Trans. 1 2002, 2563 – 2585.
[20] A similar coupling reaction with the TES-protected versions of
20 proceeded in 35% yield whereas the corresponding cyclo-
pentylidene ketal afforded the Julia coupling product in 65%
yield.
[21] The diol 24 resulting from deprotection of both TMS ethers was
also recovered in 10–25% yield, and recycled. The yield was
calculated after one such recycling of 24.
2446 – 2453.
[16] The stereochemistry of acetal 18 was proven by 1D NOE
experiments.
[17] H. S. Schultz, H. B. Freyermuth, S. R. Buc, J. Org. Chem. 1963,
28, 1140 – 1142.
[18] For the use of this reagent combination in the deprotection of
benzylethers, see: D. A. Evans, J. L. Katz, G. S. Peterson, T.
Hintermann, J. Am. Chem. Soc. 2001, 123, 12411 – 12413.
544
www.angewandte.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 541 –544