Enantio- and Diastereoselective Synthesis of 1,5-syn-(Z)-Amino Alcohols via Imine Double Allylboration: Synthesis of trans-1,2,3,6-Tetrahydropyridines and Total Synthesis of Andrachcine

Article abstract of DOI:10.1021/acs.orglett.7b00995

A stereoselective synthesis of trans-1,2,3,6-tetrahydropyridines 8 is described. This synthesis proceeds via intramolecular Mistunobu reactions of 1,5-syn-(Z)-amino alcohols 7, which were prepared by a highly diastereo- and enantioselective double-allylboration reaction of aldehyde 5 and silylimine 6. The chiral bifunctional γ-borylallylborane 9E was generated in situ by hydroboration of allene 3 with (diisopinocampheyl)borane 4. This strategy was applied to the total synthesis of andrachcine 1, thus establishing with certainty the absolute and relative configuration of the natural product.

Full text of DOI:10.1021/acs.orglett.7b00995

Letter
pubs.acs.org/OrgLett
Enantio- and Diastereoselective Synthesis of 1,5-syn-(Z)‑Amino
Alcohols via Imine Double Allylboration: Synthesis of trans-1,2,3,6-
Tetrahydropyridines and Total Synthesis of Andrachcine
Christophe Allais† and William R. Roush*
Department of Chemistry, Scripps Florida, Jupiter, Florida 33458, United States
S
* Supporting Information
ABSTRACT: A stereoselective synthesis of trans-1,2,3,6-
tetrahydropyridines 8 is described. This synthesis proceeds
via intramolecular Mistunobu reactions of 1,5-syn-(Z)-amino
alcohols 7, which were prepared by a highly diastereo- and
enantioselective double-allylboration reaction of aldehyde 5
and silylimine 6. The chiral bifunctional γ-borylallylborane 9E
was generated in situ by hydroboration of allene 3 with (diisopinocampheyl)borane 4. This strategy was applied to the total
synthesis of andrachcine 1, thus establishing with certainty the absolute and relative configuration of the natural product.
Scheme 1. Proposed Synthesis of trans-1,2,3,6-THPs
1,2,3,6-Tetrahydropyridines (THPs) are an important class of
heterocycles that are substructures of numerous biologically
active compounds.1 This scaffold occurs in many alkaloid
natural products2 and has been used as intermediates in the
synthesis of substituted piperidines3 and iminosugars.4
Consequently, numerous strategies for the stereoselective
synthesis of 1,2,3,6-tetrahydropyridines and the corresponding
piperidine derivatives have been described.5 Of these, the trans-
2,6-disubstituted 1,2,3,6-tetrahydropyridines are thermody-
namically disfavored6 owing to the pseudoaxial orientation of
one of the two substituents α to the nitrogen atom. As such,
this diastereomer must be accessed by using kinetically
controlled reaction sequences. However, relatively few methods
are available for the synthesis of enantiomerically enriched
trans-1,2,3,6-tetrahydropyridines,7 including iminium ion medi-
ated vinylsilane8 and allylsilane9 cyclizations as well as ring-
closing metathesis sequences.10 Therefore, the development of
new, highly stereochemically controlled syntheses of this ring
system remains an important challenge.
Toward this goal, we envisioned that 2,6-disubstituted trans-
1,2,3,6-tetrahydropyridines 8 could be formed by an intra-
molecular Mitsunobu cyclization of 1,5-syn-(Z)-amino alcohols
7. The latter would be synthesized via the combination of an
aldehyde 5, an aldimine 6, and a chiral γ-borylallylboronate
generated in situ by hydroboration of allenylboronic ester 3
with (diisopinocampheyl)borane ((lIpc)2BH)11 4 (Scheme 1).
During the past decade, our group has developed diastereo-
and enantioselective double-allylboration strategies for the
synthesis of substituted syn- and anti-1,5-pentenediols.12 This
powerful and convergent method has been successfully applied
to the total synthesis of polypropionate natural products.13 We
anticipated that use of an imine as the electrophilic reaction
partner in the second allylboration step would provide a highly
selective route to 1,5-amino alcohols and would enable the
scope of this method to be expanded to include the synthesis of
alkaloid natural products. This concept is far from trivial since
stereoselective allylboration of imines14 has proven to be
challenging, although some diastereo- and enantioselective15
methods have recently emerged.
We report herein the development of a highly diastereo- and
enantioselective synthesis of 1,5-syn-(Z)-amino alcohols 7 and
their conversion to enantioenriched trans-1,2,3,6-tetrahydropyr-
idines 8 via an intramolecular Mistunobu-type reaction. This
strategy was applied to the total synthesis and structure
validation of andrachcine 1, which had also previously been
assigned the stereochemistry as presented in diastereoisomer 2
(Figure 1).17
Although few isolated syntheses of acyclic 1,5-syn-(Z)-amino
alcohols have been reported,18 a general and straightforward
method to selectively form this motif is lacking in the literature.
Figure 1. Two different structures of andrachcine reported in the
literature.
Received: April 2, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00995
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Diastereo- and Enantioselective Synthesis of 1,5-syn-(Z)-Amino Alcohols 7 from Silylimine 6a*
*
a
Unless specified, all reactions were performed using 0.25 mmol of allene 3. Isolated yields of 1,5-syn-(Z)-amino alcohols 7 obtained after column
chromatography. Enantiomeric excess (% ee) and absolute configuration determined by using the Mosher ester/amide analysis.16 Diastereomer
ratio (dr) determined by H NMR analysis after filtration through silica gel and broad fraction collection. Reaction performed using 3.0 mmol of
b
c
d
1
allene 3.
Our initial efforts focused on the selection of imine 6 for use as
the second electrophilic component of our previously
established double-allylboration sequence (Scheme 2). Initial
studies demonstrated that the second allylboration step, using
in situ generated 10 as the allylborating agent, proved to be
highly sensitive to steric hindrance, as N-tosyl imines, N-acyl
imines, or N-(trimethylsilyl) imines proved to be unreactive
under standard conditions (see the Supporting Information for
details). We reasoned, as suggested originally by Brown,15u that
the much smaller N−H imine would decrease nonbonded
interactions and permit the second allylboration reaction to
proceed via TS-III. Thus, the free N−H imine was generated in
Scheme 3. Synthesis of 7f−m by Double-Allylboration
Reactions with in Situ Formation of N-Silylimines 6*
situ by methanolysis of the N−Si bond of 6a.15h,s,u
A
representative set of 1,5-syn-(Z)-amino alcohols 7 was
assembled from a panel of aldehydes 5 and (E)-N-
(trimethylsilyl) benzylidene imine (6a). Thus, treatment of in
situ generated 1012b,d,e with 1.5 equiv of imine 6a and 1.5 equiv
of MeOH at −78 °C and then at room temperature for 30 h
provided products 7a−e in good to excellent yields (79−91%).
These reactions proceeded with excellent diastereo- (dr 14:1−
16:1) and enantioselectivity (91−96% ee), and the Z/E olefin
ratio was >20:1 in all cases. This reaction sequence proved to
be readily scalable as shown for 7a (Scheme 2).
Encouraged by these results, we turned our attention to
nonaromatic silylimines 6 in an effort to increase the scope of
this double-allylboration procedure. Since aliphatic imines have
the propensity to self-condense owing to facile imine−enamine
equilibria,19 we anticipated that in situ formation of silylimines
6 might prove to be a useful strategy (Scheme 3). This was
accomplished by treating an aldehyde with LiHDMS in THF at
−40 °C for 40 min (see the SI for additional efforts to optimize
this reaction).20 Ultimately, we determined that use of 2.75
equiv of aldehyde (as precursor of imine 6) was necessary to
achieve good yields of the double-allylboration products 7f−m
summarized in Scheme 3. However, the efficiency of this
reaction sequence was still synthetically useful when 2.0 equiv
*
Unless specified, all reactions were performed using 0.25 mmol of
a
allene 3. Isolated yields of 1,5-syn-(Z)-amino alcohols 7 obtained after
b
column chromatography. Enantiomeric excess (% ee) and absolute
configuration determined by using the Mosher ester/amide analysis.18
c
Diastereomer ratio (dr) determined by 1H NMR analysis after
d
filtration through silica gel and broad fraction collection. Reaction
performed using 0.73 mmol of allene 3. Reaction performed using 3.0
mmol of allene 3.
e
B
DOI: 10.1021/acs.orglett.7b00995
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
of the imine precursor aldehyde was used (see the SI for an
optimization study).
depicted in structure 117a (Figure 1), in which both secondary
alcohols have the R absolute configuration. We believed that
our imine double allylboration reaction followed by Mitsunobu
cyclization provided an efficient strategy to synthesize both 1
and 2 in order to confirm the stereochemistry of the natural
product.
As depicted in Scheme 3, a series of in situ generated
silylimines 6 reacted with allylboronic ester intermediate 10 to
give adducts 7f−m in 74−85% yields and with high levels of
diastereo- and enantioselectivity. Achiral aliphatic (for 7f, 7i,
and 7k), α,β-unsaturated (7g and 7j), and heteroaromatic (7j)
imines were successfully generated and used in this double-
allylboration sequence. In order to test the utility of the double
allyboration in more complex synthetic contexts, we also
examined this procedure in the double-asymmetric manifold.21
As demonstrated by the formation of syn-1,5 amino alcohols 7l
and 7m, imines generated in situ from sensitive chiral aldehydes
were also successfully utilized.
Thus, advanced precursors 8d and 8e were subjected to a
two-step reductive amination and deprotection sequence, which
completed the syntheses of 2 and 1, respectively. In both cases,
the N-methylation was achieved by treating the N−H
tetrahydropyridine intermediates 8d and 8e with formaldehyde
and NaCNBH3. The two TBS ethers were subsequently
deprotected by treatment with tetrabutylammonium fluoride
1
(Scheme 5). The optical rotation as well as H and 13C NMR
We turned next to the intramolecular Mitsunobu reaction for
the preparation of trans-1,2,3,6-tetrahydropyridines, 8. Use of
standard Mitsunobu conditions was not successful here.22
Ultimately, we identified the combination of tetramethylazodi-
carboxamide (TMAD)23 and trimethylphosphine as the best
reagent combination. Mixing these two reagents in THF at 0
°C followed by addition of compounds 7 delivered the trans-
1,2,3,6-tetrahydropyridines 8 in 71−84% yields (Scheme 4).
Scheme 5. Completion of the Total Synthesis of
Andrachcine 1 and Its Diastereoisomer 2
Scheme 4. Stereospecific Cyclization of 7 to 8*
a
Isolated by column chromatography.
data obtained for 1 were in complete agreement with the data
reported for andrachcine.16a Therefore, we conclude that 1
represents the correct stereostructure of andrachcine.
In summary, we have developed a highly diastereo- and
enantioselective synthesis of 1,5-syn-(Z)-amino alcohols 7 via
double-allylboration reactions of silyl imines 6 (Schemes 2 and
3). The in situ formation of silyl imines 6 (Scheme 3) facilitated
the use of both aromatic and aliphatic aldehyde imine
precursors in this one-pot reaction sequence. Cyclization of
the 1,5-syn-(Z)-amino alcohols 7 to trans-1,2,3,6-tetrahydropyr-
idines 8 was achieved under mild conditions using TMAD and
trimethylphosphine with complete inversion of configuration.
Finally, we applied this imine double allylboration and
Mitsunobu cyclization sequence to the total synthesis of
andrachcine 1, thus confirming its stereostructure as a whole.
This work expands the utility of the double-allylboration
reaction to include the synthesis of alkaloids and potentially
scaffolds of use in drug discovery research. Further develop-
ment of this double-allylboration strategy, as well as its
application to other heterocycles and total synthesis of complex
alkaloid natural products, will be reported in due course.
*
Unless specified, all reactions were performed with 0.1 mmol of
a
amino alcohol 7. Isolated yields of trans-1,2,3,6-THPs 8 obtained
after column chromatography. Reaction performed with 0.2 mmol of
amino alcohol 7l. Reaction performed with 0.615 mmol of amino
alcohol 7m.
b
c
Chiral substrates 7l and 7m also underwent the reaction
without complication, although these reactions proceeded more
slowly and did not reach completion. The trans-tetrahydropyr-
idine stereochemistry was confirmed by hydrogenation of 8a
followed by NOESY analysis of the resulting piperidine (see the
SI).
The stimulation to develop this synthesis of trans-1,2,3,6-
tetrahydropyridines was for its potential application to the total
synthesis of alkaloid natural products. As a proof of concept, we
selected andrachcine, a 2,6-disubstituted trans-1,2,3,6-tetrahy-
dropyridine alkaloid first isolated in 1987 from the plant
Andrachne aspera Spreng. (Euphorbiaceae) in Karachi, Pakistan.
This plant is traditionally used in local medicine to improve
eyesight, and the extracts from which andrachcine was islolated
displayed antibacterial activity.17b Initially, andrachcine was
reported to possess the stereochemistry represented in
compound 217b (Figure 1), with both side-chain secondary
alcohols assigned to have S absolute configuration. In 2000,
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b00995.
Experimental procedures (PDF)
Mill and Hootele
́
revised the structure of andrachcine to that
Spectroscopic data for all new compounds (PDF)
C
DOI: 10.1021/acs.orglett.7b00995
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
2011, 67, 6497. (c) Kister, J.; Nuhant, P.; Lira, R.; Sorg, A.; Roush, W.
R. Org. Lett. 2011, 13, 1868. (d) Kister, J.; DeBaillie, A. C.; Lira, R.;
Roush, W. R. J. Am. Chem. Soc. 2009, 131, 14174. (e) Flamme, E. M.;
Roush, W. R. J. Am. Chem. Soc. 2002, 124, 13644.
(13) For some selected examples, see: (a) Nuhant, P.; Roush, W. R. J.
Am. Chem. Soc. 2013, 135, 5340. (b) Flamme, E. M.; Roush, W. R.
Org. Lett. 2005, 7, 1411. (c) Hicks, J. D.; Flamme, E. M.; Roush, W. R.
Org. Lett. 2005, 7, 5509. (d) Owen, R. M.; Roush, W. R. Org. Lett.
2005, 7, 3941.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: roush@scripps.edu.
ORCID
William R. Roush: 0000-0001-9785-5897
Present Address
†Chemical Research and Development, Pfizer Worldwide
Research and Development, Eastern Point Road, Groton, CT
06340.
(14) For recent reviews, see: (a) Diner, C.; Szabo, K. J. Am. Chem.
́ ́
Soc. 2017, 139, 2. (b) Yus, M.; Gonzalez-Gomez, J. C.; Foubelo, F.
Chem. Rev. 2013, 113, 5595. (c) Hepburn, H.; Chotsaeng, N.; Luo, Y.;
Lam, H. Synthesis 2013, 45, 2649. (d) Ramadhar, T. R.; Batey, R. A.
Synthesis 2011, 2011, 1321.
Notes
The authors declare no competing financial interest.
́
(15) (a) Alam, R.; Diner, C.; Jonker, S.; Eriksson, L.; Szabo, K. J.
Angew. Chem., Int. Ed. 2016, 55, 14417. (b) Martínez, J. I.; Smith, J. J.;
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(c) Silverio, D. L.; Torker, S.; Pilyugina, T.; Vieira, E. M.; Snapper, M.
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ACKNOWLEDGMENTS
Financial support provided by the National Institutes of Health
(GM038436) is gratefully acknowledged.
■
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