Asymmetrie synthesis of allylsilanes by the borylation of lithiated carbamates: Formal total synthesis of (-)-decarestrictine D

Article abstract of DOI:10.1002/anie.201001223

Chemical equation represented The flip side: (-)-Sparteine-complexed lithiated carbamates react with β-silylvinylboranes with inversion of configuration, whereas the diamine-free lithiated carbamates react with retention of configuration, thereby providing enantiocomplementary routes to β-hydroxy allylsilanes (see scheme). The methodology has been applied in a concise formal total synthesis of (-)-decarestrictine D.

Full text of DOI:10.1002/anie.201001223

Communications
DOI: 10.1002/anie.201001223
Asymmetric Synthesis
Asymmetric Synthesis of Allylsilanes by the Borylation of Lithiated
Carbamates: Formal Total Synthesis of (ꢀ)-Decarestrictine D**
Michael Binanzer, Guang Yu Fang, and Varinder K. Aggarwal*
Chiral allylsilanes are highly valuable nucleophiles in stereo-
selective organic synthesis because of the multitude of
asymmetric transformations that they can undergo.[1] For
example, b-hydroxy allylsilanes have been used extensively by
the groups of both Panek and Roush in natural product
synthesis. Amongst the most useful applications in recent
years are the [3+2] annulation[2] and the [4+2] annulation[3] as
they allow facile access to furans and pyrans.[4] However, the
synthesis of substituted b-hydroxy allylsilanes is not always
straightforward and usually requires multiple steps.[5] The
broad synthetic utility of b-hydroxy allylsilanes motivated us
to develop more efficient synthetic routes to such intermedi-
ates.
We recently reported a new method for the homologation
Scheme 1. Proposed synthesis of b-hydroxy allylsilanes 7 and anti-diols
9; Cb=N,N-diisopropylcarbamoyl, sp=(ꢀ)-sparteine, Cy=cyclohexyl.
of boronic esters and boranes[6] by employing the lithiated
carbamates reported by Hoppe et al.[7] Our method involved
the use of carbamates derived from primary and secondary
alcohols, which led to the formation of secondary and
tertiary[8] alcohols in high enantiomeric ratios after oxidation.
The reaction could be extended to a one-pot, multiple
homologation process and its application in the synthesis of
(+ )-faranal was demonstrated.[9] In extending this method-
ology further, we considered its application in the stereocon-
trolled, one-pot synthesis of b-hydroxy allylsilanes. We
envisioned that the reaction of a lithiated carbamate with b-
silyl vinyl borane 3[10] would form the intermediate allylbor-
ane 5[11] which could react with an aldehyde to give an Z-
configured anti-allylsilane 7 (Scheme 1).[12] Subsequent epox-
idation and elimination/ring-opening could then provide a
stereocontrolled route to 2-ene-anti-1,4-diols 9, a common
motif in natural products (Scheme 1). Herein we detail our
Scheme 2. Initial studies of the one-pot reaction.
success in developing this methodology and its application in
synthesis.
Our initial studies, however, revealed some unexpected
results (Scheme 2). For example, the reactions of lithiated
carbamates 2a,b with B-Ph-9-BBN and subsequent oxidation
gave the corresponding alcohols 11a,b in 97:3 e.r. and with
complete retention of configuration.[6] Surprisingly, reaction
of the lithiated carbamate 2b with borane 3 and subsequent
trapping with benzaldehyde gave allylsilane 7b in only 71:29
e.r. More alarmingly, lithiated carbamate 2a gave allylsilane
ent-7a in 93:7 e.r. but now with inversion of the expected
stereochemistry.
Through a careful set of control experiments we estab-
lished that the recalcitrant step causing the unexpected
selectivity was the reaction of the lithiated carbamate with
the borane, and that the reaction was critically dependent
upon the nature of the amine that was complexed to the
lithiated carbamate.
[*] Dipl.-Chem. M. Binanzer, Dr. G. Y. Fang, Prof. Dr. V. K. Aggarwal
School of Chemistry, University of Bristol
Cantock’s Close, Bristol, BS8 1TS (UK)
Fax: (+44)117-929-8611
E-mail: v.aggarwal@bristol.ac.uk
Homepage: http://www.chm.bris.ac.uk/org/aggarwal/
aggarhp.html
In our detailed studies we used the related stannane 12
since the stereochemistry associated with the synthesis of 12,
its subsequent lithiation, and electrophilic trapping had been
reported and rigorously proven by Hoppe et al.[13] We first
explored diamine-free reactions by initial formation of
[**] We thank the EPSRC for support of this work. V.K.A. thanks the Royal
Society for a Wolfson Research Merit Award and the EPSRC for a
Senior Research Fellowship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001223.
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Chemie
stannane 12 and subsequent tin–lithium exchange.[14] After
the addition of borane 3 and benzaldehyde, allylsilane 7a was
formed in 94:6 e.r., but now with essentially complete
retention of configuration (Table 1, entry 1). Addition of
ated carbamate leads to predominantly inversion of stereo-
chemistry, is especially useful since it obviates the need to use
the (+)-sparteine surrogate ligand.[17] Furthermore, the pro-
cess was general for a range of aldehydes, thereby providing a
simple method for accessing both sets of enantiomeric b-
hydroxy allylsilanes (Table 2, entries 1–6).
Table 1: Detailed studies of the lithiation/borylation reaction.
Table 2: Stereocontrolled one-pot synthesis of b-hydroxy allylsilanes.[a]
Entry R (substrate)
R’
Yield 7/ent-7[e] Z/E[h]
anti/syn[h]
[%][d]
1
2
3
4
5
6
7
8
9
CH3[b] (1a)
CH3[b] (1a)
CH3[b] (1a)
nBu 67
7:93[f]
7:93[f]
7:93[f]
95:5[g]
95:5[g]
94:6[g]
98:2[g]
98:2[g]
98:2[g]
> 25:1 > 25:1
> 25:1 > 25:1
> 25:1 > 25:1
> 25:1 > 25:1
> 25:1 > 25:1
> 25:1 > 25:1
> 25:1 > 25:1
> 25:1 > 25:1
> 25:1 > 25:1
Cy
Ph
65
64
[c]
CH3 (12)
nBu 95
[c]
CH3 (12)
Cy
Ph
96
95
[c]
Entry
Additive
Stereochemical
outcome
7a/ent-7a
Yield
[%]
CH3 (12)
PhCH2CH2 (13) nBu 81
PhCH2CH2 (13) Cy
PhCH2CH2 (13) Ph
[c]
[c]
74
81
1
2
none
retention
retention
94:6
91:9
95
78
[c]
TMEDA[a]
[a] Reaction conditions: lithiated carbamate (1.0 equiv), borane
3
(1.3 equiv), aldehyde (2.0 equiv), and workup with H2O2/NaOH.
[b] Method A was used. [c] Method B was used. [d] Yield of isolated
product (see the Supporting Information). [e] The e.r. value was
determined by GC or HPLC methods using a chiral stationary phase.
[f] Major enantiomer originates from inversion of the configuration
during the reaction of lithiated carbamate with borane 3. [g] Major
enantiomer originates from retention of the configuration in the reaction
of lithiated carbamate with borane 3. [h] Ratio was determined by
1H NMR spectroscopy.
3
4
retention
inversion
92:8
8:92
87
83
5
6
inversion
inversion
35:65
35:65
50
94
[a] TMEDA=N,N,N’,N’-tetramethylethylenediamine.
The new, diamine-free protocol also solved the problem of
low enantioselectivity observed with lithiated carbamate 2b.
Thus, employing stannane 13[15] (Method B) led to b-hydroxy
allylsilanes with high e.r. values (Table 2, entries 7–9). These
reactions proceed via transition state 6 in which the R group
occupies an axial position to avoid steric repulsion from the
bulky 9-BBN moiety.
The stereochemical outcome of the reactions of the
lithiated carbamates with boranes can be rationalized by
considering the steric environment around the lithiated
carbamate (Figure 1). In general, there is an inherent
preference for nonmesomeric stabilized lithiated carbamates
(with or without (ꢀ)-sparteine complexed) to react with
boranes, and indeed all other electrophilic reagents (e.g.,
SiMe3Cl, CO2, Me3SnCl, MeI,[13] or boronic esters[8,9]) with
stereoretention.[7a] Evidently, reactions with alkenyl boranes
are more finely balanced. In the absence of bulky diamines
the lithiated carbamates react with retention of configuration
as expected (Figure 1, A and C). However, the unhindered
sterically undemanding diamines to the lithiated carbamate
led to the same major enantiomer (Table 1, entries 2 and 3),
whereas addition of the hindered diamine (ꢀ)-sparteine led to
almost complete inversion of the configuration (Table 1,
entry 4). The stereochemistry of the diamine did not influence
the stereochemical outcome of the reaction since the (+)-
sparteine surrogate, reported by OꢀBrien and co-workers,[15]
also led to inversion of configuration, albeit with lower
selectivity (Table 1, entry 5). Similar inversion of configura-
tion was obtained with the achiral, hindered diamine bispi-
dine[16] (Table 1, entry 6). From these results it was clear that
the main factors influencing the stereochemical outcome of
the reaction were the bulk [(ꢀ)-sparteine is more hindered
than (+)-sparteine surrogate] and rigidity of the diamine and
not its stereochemistry.
The discovery that in the case where R is methyl (which
constitutes the most important example), the diamine-free
lithiated carbamate leads predominantly to retention of
configuration and the (ꢀ)-sparteine complexed to the lithi-
ꢀ
C Me lithiated carbamate possessing a bulky diamine reacts
with the borane with inversion since the face bearing the
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Communications
Scheme 3. Synthesis of syn-diol 18 through (E)-b-hydroxy allylsilanes
17: a) sBuLi, (ꢀ)-sparteine, Et2O, ꢀ788C, 5 h; b) boronic ester 14,
ꢀ788C!238C, 1 h; c) MgBr2, 238C, 1 h; d) PhCHO, ꢀ258C, 18 h;
e) H2O2/NaOH; 62% yield (a–e, one pot; e.r. =98:2, E/Z=20:1);
f) mCPBA, NaHCO3, CH2Cl2; g) AcOH, MeOH, 85%, E-configured syn:
Z-configured syn: E-configured anti ꢁ 15:4:1. mCPBA=meta-chloro-
perbenzoic acid.
Figure 1. Rationale for the stereochemical outcome considering the
steric environment.
ligand on boron allows the methyl group to occupy the less
hindered equatorial position, thereby resulting in the forma-
tion of the E double bond. Subsequent epoxidation/olefina-
tion gave syn-diol 18. The moderate diastereoselectivity was
expected since the conformation of E allylsilanes is less well-
controlled than their Z counterparts, leading to lower diaste-
reoselectivity in the epoxidation step.[19]
Finally the potential of this methodology is illustrated in
the synthesis of (ꢀ)-decarestrictine D (28), a 10-membered
lactone that has been isolated from Penicillium corylophilum
and Polyporus tuberaster.[22] It displays selective and strong
inhibition of liver cell cholesterol biosynthesis (HEP cells,
IC50 = 100 nm). From a structural point of view the four
stereogenic centers, the unsaturated segment, and the 10-
membered macrolactone pose significant synthetic challenges
(Scheme 4).[23] A retrosynthetic analysis of 28 led to seco acid
lithium (retentive pathway) is blocked by the amine (Figure 1,
B). As the substituent on the carbamate becomes larger, both
faces become rather hindered and as such, reactions occur
with both retention and inversion leading to low enantiose-
lectivity (Figure 1, D).[18a] It should be noted that the electro-
philic substitution of benzyl[8] and alkenyl carbamates[18b] is
less predictable and depends upon the substituents of the
carbanion, the electrophile, and the ligands complexing the
lithium cation.[18c]
The silyl group is a powerful stereocontrolling element in
synthesis. This was demonstrated by epoxidation of allylsi-
lanes 7 using mCPBA[19] and subsequent acid-catalyzed
elimination[20] which gave the 2-ene-1,4-anti-diols 9 in high
yields and with excellent diastereoselectivity in all cases
(Table 3).
1,4-syn-Diols can also be prepared using this methodology
simply by using an alternative boron reagent (Scheme 3).
Thus, reaction of lithiated carbamate 2a with boronic ester 14
in the presence of MgBr2 and then addition of benzaldehyde
gave the E-configured anti-b-hydroxy allylsilanes 17 in 20:1
d.r. and 98:2 e.r. In contrast to reactions with B-alkenyl-9-
BBN derivatives, the reaction of the lithiated carbamate with
boronic ester 14 occurred with retention of configuration,
even in the presence of the bulky diamine.[21] The reaction
proceeds via transition structure 16 in which the small ester
Table 3: Synthesis of 2-ene-1,4-diols 9 by the epoxidation/olefination of
b-hydroxy allylsilanes.
Scheme 4. Synthesis of seco acid 27; a) SEMCl, DIPEA, CH2Cl2, 93%;
b) LiAlH4, THF, 95%; c) ClC(O)NiPr2, Et3N, CH2Cl2, 57% (95% brsm);
d) (+)-sparteine surrogate, sBuLi, Et2O then Bu3SnCl, 72%, d.r. >97:3;
e) nBuLi, Et2O, then borane 3, then aldehyde 22, 85%, (Z/E >25:1,
anti/syn >25:1); f) 1. mCPBA, NaHCO3, CH2Cl2 then aqueous
Na2S2O3; 2. AcOH, MeOH, 78% (two steps), E/Z >25:1, anti/syn
>25:1); g) Dudley reagent (25), MgO, PhCF3, 57%; h) LiOH, MeOH/
THF/H2O (2:2:1), 90%; i) TFA, CH2Cl2, 77%. Bn=benzyl,
Entry
R
R’
Yield [%][a]
E/Z[b]
anti/syn[b]
1
2
3
4
5
6
CH3
CH3
CH3
PhCH2CH2
PhCH2CH2
PhCH2CH2
nBu
Cy
Ph
nBu
Cy
Ph
96
89
93
90
82
88
> 25:1
> 25:1
> 25:1
> 25:1
> 25:1
> 25:1
> 25:1
> 25:1
> 25:1
> 25:1
> 25:1
> 25:1
brms=based on recovered starting material, DIPEA=diisopropylethyl-
amine, Dudley reagent=2-benzyloxy-1-methylpyridinium triflate,
SEM=2-(trimethylsilyl)ethoxymethyl), TFA=trifluoroacetic acid.
[a] Yield of isolated product (2 steps). [b] Ratio was determined by
1H NMR spectroscopy.
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Lett. 2000, 41, 9413 – 9417; c) I. E. Wrona, J. T. Lowe, T. J.
Turbyville, T. R. Johnson, J. Beignet, J. A. Beutler, J. S. Panek, J.
Org. Chem. 2009, 74, 1897 – 1916.
27 as a suitable target for a formal synthesis since it had been
converted into the natural product in two steps by a
Yamaguchi macrolactonization and subsequent debenzyl-
ation.[24] The synthesis commenced with commercially avail-
able b-hydroxy ester 19 (Scheme 4). After protection using
SEMCl, reduction, and carbamoylation, compound 20 was
deprotonated with sBuLi/(+)-sparteine-surrogate and trap-
ped with Bu3SnCl to yield stannane 21.[25] The key step was
the three-component coupling involving lithiation of 21 and
then sequential addition of vinyl borane 3 and aldehyde 22,[26]
which gave the desired allylsilane 23 in 85% yield and
essentially perfect stereoselectivity. Epoxidation and olefina-
tion led to diol 24, which was doubly benzylated using the
Dudley reagent (25).[27] This diol was found to be especially
sensitive since alternative reagents led to complete decom-
position. Saponification and removal of the SEM group gave
seco acid 27 which completed the formal total synthesis.[24]
In conclusion we have developed a novel, high-yielding,
one-pot procedure for the stereocontrolled synthesis of
allylsilanes with almost complete selectivity over the three
elements of stereogenicity. In particular, it has been discov-
ered that sparteine-complexed lithiated carbamates react
with b-silylvinylboranes with inversion of configuration
whereas the diamine-free lithiated carbamates react with
retention of configuration. Epoxidation of the allylsilanes and
subsequent acid-catalyzed elimination/ring-opening gave 2-
ene-1,4-anti-diols with excellent yields and diastereoselectiv-
ity. Related syn diols could also be obtained albeit with lower
levels of stereoselectivity. We have demonstrated the appli-
cation of this methodology in a concise formal total synthesis
of (ꢀ)-decarestrictine D. Additional applications of this
efficient and highly stereoselective methodology in natural
product synthesis are underway.
[6] J. L. Stymiest, G. Dutheuil, A. Mahmood, V. K. Aggarwal,
Angew. Chem. 2007, 119, 7635 – 7638; Angew. Chem. Int. Ed.
2007, 46, 7491 – 7494.
[7] a) D. Hoppe, T. Hense, Angew. Chem. 1997, 109, 2376 – 2410;
Angew. Chem. Int. Ed. Engl. 1997, 36, 2282 – 2316; b) E.
Beckmann, V. Desai, D. Hoppe, Synlett 2004, 2275 – 2280; see
also: c) G. Besong, K. Jarowicki, P. J. Kocienski, E. Sliwinski,
F. T. Boyle, Org. Biomol. Chem. 2006, 4, 2193 – 2207.
[8] J. L. Stymiest, V. Bagutski, R. M. French, V. K. Aggarwal, Nature
2008, 456, 778 – 782.
[9] G. Dutheuil, M. P. Webster, P. A. Worthington, V. K. Aggarwal,
Angew. Chem. 2009, 121, 6435 – 6437; Angew. Chem. Int. Ed.
2009, 48, 6317 – 6319.
[10] a) D. A. Singleton, J. P. Martinez, Tetrahedron Lett. 1991, 32,
7365 – 7368; b) J. C. Colberg, A. Rane, J. Vaquer, J. A. Soder-
quist, J. Am. Chem. Soc. 1993, 115, 6065 – 6071.
[11] For related double allylboration reagents based on boron and
silicon: a) F. Peng, D. G. Hall, J. Am. Chem. Soc. 2007, 129,
3070 – 3071; b) E. M. Flamme, W. R. Roush, J. Am. Chem. Soc.
2002, 124, 13644 – 13645; c) R. Lira, W. R. Roush, Org. Lett.
2007, 9, 4315 – 4318.
[12] Reactions of sulfur ylides with alkenyl 9-BBN derivatives and
subsequent trapping with aldehydes: a) G. Y. Fang, V. K. Aggar-
wal, Angew. Chem. 2007, 119, 363 – 366; Angew. Chem. Int. Ed.
2007, 46, 359 – 362; Reactions of lithiated carbamates with
unfunctionalized alkenyl 9-BBN derivatives and alkenyl boronic
esters: b) M. Althaus, A. Mahmood, J. R. Suꢁrez, S. P. Thomas,
V. K. Aggarwal, J. Am. Chem. Soc. 2010, 132, 4025 – 4028.
[13] D. Hoppe, F. Hintze, P. Tebben, Angew. Chem. 1990, 102, 1457 –
1459; Angew. Chem. Int. Ed. Engl. 1990, 29, 1424 – 1425. The e.r.
of stannane 12 is 95:5 (Chiral HPLC). Details of the optimiza-
tion of the e.r. of the stannane (nature of Sn and N substituents)
are given in the Supporting Information.
[14] W. C. Still, C. Sreekumar, J. Am. Chem. Soc. 1980, 102, 1201 –
1202.
[15] M. J. Dearden, C. R. Firkin, J.-P. R. Hermet, P. OꢀBrien, J. Am.
Chem. Soc. 2002, 124, 11870 – 11871; (+)-sparteine-surrogate
behaves in an enantiocomplementary fashion to (ꢀ)-sparteine.
[16] M. J. McGrath, P. OꢀBrien, J. Am. Chem. Soc. 2005, 127, 16378 –
16379.
[17] There are a few examples where (ꢀ)-sparteine can be used to
generate either enantiomer of a product, for example: Y. S. Park,
M. L. Boys, P. Beak, J. Am. Chem. Soc. 1996, 118, 3757 – 3758.
Received: March 1, 2010
Revised: March 25, 2010
Published online: May 5, 2010
Keywords: boranes · borates · homologation · lithium ·
.
natural products
=
[18] a) Reaction of less hindered vinyl boranes (e.g. trans-H3CCH
CH-B-9BBN) show intermediate reactivity with the same
carbamates: 2b reacts with complete retention of configuration
but 2a reacts to give a mixture of products derived from
retention and inversion of configuration. Carbamate 12 (dia-
mine-free) reacts with complete retention of configuration; see:
Ref. [12b]; b) D. Hoppe, F. Marr, M. Brꢂggemann in Organo-
lithiums in Enantioselective Synthesis Topics Organomet. Chem.,
Vol. 5, (Ed.: D. M. Hodgson), Springer, Heidelberg, 2003, 5,
pp. 61 – 138; The triisopropoxytitanation of 1-oxy-2-alkenyl-
lithium compounds proceeds with retention of configuration
with the TMEDA complexes whereas the sparteine complexes
react with inversion: c) O. Zschage, J.-R. Schwark, T. Krꢃmer, D.
Hoppe, Tetrahedron 1992, 48, 8377 – 8388.
[1] a) C. E. Masse, J. S. Panek, Chem. Rev. 1995, 95, 1293 – 1316;
b) I. Fleming, A. Barbero, D. Walter, Chem. Rev. 1997, 97, 2063 –
2192; c) L. Chabaud, P. James, Y. Landais, Eur. J. Org. Chem.
2004, 3173 – 3199.
[2] a) Methodology: J. S. Panek, M. Yang, J. Am. Chem. Soc. 1991,
113, 9868 – 9870; b) A recent application: P. Va, W. R. Roush, J.
Am. Chem. Soc. 2006, 128, 15960 – 15961.
[3] a) Methodology: H. Huang, J. S. Panek, J. Am. Chem. Soc. 2000,
122, 9836 – 9837; b) A recent application: Q. Su, J. S. Panek, J.
Am. Chem. Soc. 2004, 126, 2425 – 2430.
[4] Some other selected applications are epoxidation/elimination:
a) W. R. Roush, P. T. Grover, Tetrahedron Lett. 1990, 31, 7567 –
7570; Fleming-Tamao-oxidation: b) J. A. Hunt, W. R. Roush, J.
Org. Chem. 1997, 62, 1112 – 1124; fluorination: c) G. T. Giuf-
fredi, S. Purser, M. Sawicki, A. L. Thompson, V. Gouverneur,
Tetrahedron: Asymmetry 2009, 20, 910 – 920; addition to acti-
vated acetals: d) J. S. Panek, M. Yang, J. Am. Chem. Soc. 1991,
113, 6594 – 6600.
[19] a) I. Fleming, A. K. Sarkar, A. P. Thomas, J. Chem. Soc. Chem.
Commun. 1987, 157 – 159; b) Y. Landais, L. Parra-Rapado,
Tetrahedron Lett. 1996, 37, 1205 – 1208.
[20] See Ref. [4a] and W. R. Roush, P. T. Grover, Tetrahedron 1992,
48, 1981 – 1998.
[5] a) M. A. Sparks, J. S. Panek, J. Org. Chem. 1991, 56, 3431 – 3438;
b) W. R. Roush, A. N. Pinchuk, G. C. Micalizio, Tetrahedron
[21] In comparison to reactions with boranes, reactions of lithiated
carbamates with boronic esters have a greater tendency to occur
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Communications
with retention of configuration. This is probably because the
[24] P. R. Krishna, P. V. N. Reddy, Tetrahedron Lett. 2006, 47, 7473 –
7476; We thank Dr. P. R. Krishna for a 1H NMR spectrum of 27
for comparison.
lithium can complex with the oxygen atom of the boronic ester
so that the boronic ester is delivered from the same face as the
metal; see Ref. [8].
[25] Use of sBuLi/TMEDA and then Bu3SnCl gave a 1:1 ratio of
ꢀ
[22] a) S. Grabley, E. Granzer, K. Hꢂtter, D. Ludwig, M. Mayer, R.
Thiericke, G. Till, J. Wink, S. Philipps, A. Zeeck, J. Antibiot.
1992, 45, 66 – 73; b) W. A. Ayer, M. Sun, L. M. Browne, L. S.
Brinen, J. Clardy, J. Nat. Prod. 1992, 55, 649 – 653; c) S. Grabley,
E. Granzer, K. Hꢂtter, D. Ludwig, M. Mayer, R. Thiericke, G.
Till, J. Wink, S. Philipps, A. Zeeck, J. Antibiot. 1992, 45, 56 – 65.
[23] V. B. Riatto, R. A. Pilli, M. M. Victor, Tetrahedron 2008, 64,
2279 – 2300; and references cited therein.
diastereomeric stannanes showing that the O SEM group does
not influence the lithiation step.
[26] a) G. E. Keck, M. B. Andrus, D. R. Romer, J. Org. Chem. 1991,
56, 417 – 420; the quality of MgBr2·OEt2 in the regioselective
reduction is crucial, see: b) H. Angert, R. Czerwonka, H.-U.
Reißig, Liebigs Ann./Recl. 1997, 2215 – 2220.
[27] K. W. C. Poon, G. B. Dudley, J. Org. Chem. 2006, 71, 3923 – 3927.
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