Regio- and Stereodivergent Allylic Reductions of Bicyclic Piperidine Enecarbamate Derivatives

Article abstract of DOI:10.1021/acs.joc.8b01601

The particular nature of tetrahydropyrido[4,3-e]-1,4,2-dioxazines of type 1 allows the regio- and stereoselective obtainment of substituted N-carbamoyl tetrahydropyridines by common reducing agents. A completely novel, biologically active, bicyclic 1,3-diaza-4-oxa-[3.3.1]-nonene scaffold can be generated by the use of lithium triethylborohydride through unprecedented cascade syn-S_N2′ reduction/carbamate reduction/cyclization reactions. The remarkable regioselectivity switches in the allylic reduction process have been rationalized with the aid of computational studies.

Full text of DOI:10.1021/acs.joc.8b01601

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Note
Regio- and Stereodivergent Allylic Reductions
of Bicyclic Piperidine Enecarbamate Derivatives
Francesco Berti, Andrea Menichetti, Lucilla Favero, Fabio Marchetti, and Mauro Pineschi
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b01601 • Publication Date (Web): 13 Sep 2018
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Regio- and Stereodivergent Allylic Reductions of
Bicyclic Piperidine Enecarbamate Derivatives
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Francesco Berti,§ Andrea Menichetti, § Lucilla Favero, § Fabio Marchetti,† and Mauro
Pineschi§*
§ Dipartimento di Farmacia, Sede di Chimica Bioorganica e Biofarmacia, Università di Pisa,
Via Bonanno 33, 56126 Pisa, Italy.
† Dipartimento di Chimica e Chimica Industriale, Università di Pisa, via G. Moruzzi 3, 56124,
Pisa, Italy.
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if
required according to the journal that you are submitting your paper to)
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ABSTRACT: The particular nature of tetrahydropyrido[4,3-e]-1,4,2-dioxazines of type 1
allows the regio- and stereoselective obtainment of substituted N-carbamoyl
tetrahydropyridines by common reducing agents. A completely novel, biologically active,
bicyclic 1,3-diaza-4-oxa-[3.3.1]-nonene scaffold can be generated by the use of lithium
triethylborohydride through an unprecedented cascade syn-SN2’ reduction/carbamate
reduction/cyclization reactions. The remarkable regioselectivity switches of allylic
reduction process have been rationalized with the aid of computational studies.
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Tetrahydropyridines are important targets in synthetic organic chemistry due to their
relevance as privilege structure in medicinal chemistry.1 Therefore, considerable efforts
have been devoted to the individuation of new methods for their regio- and stereoselective
synthesis and functionalization.2 However, a regio- and stereodivergent reduction of
piperidine enecarbamates bearing an allylic leaving group has not yet been reported. The
main reason for this shortcoming can be found in the relative instability of 1,2,3,4-
tetrahydropyridine derivatives containing common leaving groups at the C4-position. We
recently became interested in the elaboration of nitroso cycloadducts derived from 2-
substituted-1,2-dihydropyridines,3 easily accessible by addition of nucleophiles to
activated pyridinium salts (eq. a, Scheme 1).4,5 The [3,3]-hetero Cope rearrangement of the
obtained nitroso Diels-Alder (NDA) cycloadducts has been reported to give 4a,7,8,8a-
tetrahydropyrido[4,3-e]-1,4,2-dioxazines of type 1 (eq. a, Scheme 1).6 We noticed that the
study of their reactivity is limited to the N-O reductive cleavage with Raney nickel to obtain
racemic aminoarabinose and aminoaltrose derivatives.6a On the other hand, palladium-
catalyzed allylic reduction have been used extensively in synthetic organic chemistry to
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introduce hydride species into a variety of allylic substrates in a regio- and stereoselective
fashion.7-9 We herein report a new method to regio- and stereoselectively reduce
piperidine enecarbamates, with the individuation of a fused 1,2,4-dioxazine framework as
an unconventional allylic leaving group (see compound of type 1, Scheme 1).
At the outset of this study, we identified optimal reaction conditions to obtain 1,2,4-
dioxazine derivatives of type 1 reproducibly and in satisfactory yields (eq. b, Scheme 1).10
While attempting to reduce the N-O bond of racemic 1,4,2-dioxazine 1a, easily prepared in
gram scale and taken as a model substrate, we serendipitously found that a heterogeneous
hydrogenation carried out with ammonium formate at 80 °C in the presence of Pd/C (20
mol%) at 80 °C readily afforded a 60/40 mixture of saturated derivatives 2a and 3a (eq. c,
Scheme 1).11
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Scheme 1. State of the art, synthesis and preliminary reduction of tetrahydropyrido[4,3-e]-
1,4,2-dioxazines (1).
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A result of this kind can be reasonably explained by admitting the intervention of 4,5-
dehydro-intermediate 4a by initial hydride transfer at the position adjacent to the nitrogen,
followed by an allylic deoxygenation of the C3-O bond.
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Stimulated by these preliminary results, we decided to carry out a deeper investigation
of the reduction of enecarbamate 1a with hydride species in order to develop selective
allylic reduction processes.8b,12 While sodium borohydride was totally ineffective, the use of
the lithium analog (LiBH4) afforded 2,3-dehydropiperidine 5a with complete
regioselectivity in attack of the hydride at the C4 position (Scheme 2). It should be noted
that 3-oxygenated-2-aryl-piperidines such as 5a are compounds of considerable
importance in medicinal chemistry.13
Scheme 2. Reduction of 1,2,4-dioxazine 1a with metal hydride species.
Much to our surprise, the reaction of 1a with an excess of highly reactive lithium
triethylborohydride (Super-H®) afforded readily a good yield of compound 6a possessing
a novel 1,3-diaza-4-oxa-[3.3.1]-bicyclic scaffold, whose structure was unequivocally
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determined by single crystal X-ray analysis (Figure 1).14 This reaction to can be easily
scaled-up to 1.0 mmol scale with only marginal deviations from the initial reaction
conditions carried out on a 0.15 mmol scale. The use of analog sodium salt (NaEt3BH)
afforded the same compound with a lower yield and an increased reaction time. The use of
LiAlH4 afforded a complex mixture of products in which only 6a could be detected.
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Figure 1. ORTEP diagram of 6a. Thermal ellipsoids are at 30% probability. Relevant bond
distances (Å): C1−N1 1.468(2), C1−C2 1.498(2), C2−C3 1.315(2), C3−C4 1.490(2), C4−O1
1.483(1), C4−C5 1.524(1), C5−N1 1.479(1), N1−C12 1.454(1), O1−N2 1.403(1), N2−C12
1.460(1).
In order to get more insight into the reaction mechanism we carried out the reaction
using LiEt3BD as the reducing agent (Figure 2).
Figure 2. Plausible mechanism for the formation of the 1,3-diaza-4-oxa-[3.3.1]-nonene
framework.
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NMR analysis of deuterated bicyclic compound 6a-D showed clearly the regio- and
stereochemistry of deuterium incorporation.15 The only way to obtain 1,3-diaza-4-oxa-
[3.3.1]-bicyclic framework 6a comes from a regioselective SN2’-addition of the hydride to
enecarbamate 1a, which turned out to be syn to the dioxazine leaving group. The syn-
stereoselectivity can be reasonably ascribed to simultaneous coordination of the cation to
the oxygen of the leaving group and the hydride of the M-HBEt3 on conformation 1a-B (see
Supporting Information).16,17 The obtained anionic 3,4-unsaturated intermediate A does
not undergo a further reduction of the oxyamido functionality arguably due to a
intramolecular chelate stabilization. It is also likely that the concomitant reduction of the
carbamate moiety of species A gives rise to zwitterionic iminium ion B that undergoes
intramolecular trapping by N-alkylation to deliver the bicyclic framework 6a-D (Figure 2).
In accordance with the proposed mechanism, the formation of such a bicyclic framework
necessarily requires the presence of a carbamate protecting group on the endocyclic
piperidine nitrogen. As a matter of fact, the LiEt3BH reduction of enamides 1b-d afforded a
complex mixture of products not containing the corresponding oxadiazabicyclononene
scaffold (Scheme 3). Hence, only using enecarbamates 1e-1k it was possible to obtain the
corresponding bicyclic structures 6b-g, albeit with variable yields (Scheme 3).
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Scheme 3. Scope of the formation of [3.3.1]-bicyclic scaffold.
The first step (SN2’-reduction) of this sequential reaction is consistent with the known
ability of highly nucleophilic and soft reagent lithium triethylborohydride to undergo
addition to double bonds of different nature including the fast reduction of pyridine to
tetrahydropyridine.18 NBO calculations showed that π*C=C (+0.0269 au) is less energetic
than σ*C-O (+0.2330 au) and therefore a softer reagent should have a preference for the C2
position. On the other hand, by examination of an electrostatic map of the fused dioxazine
1a it is clear that the C4 is more electropositive than C2 carbon (Figure 3). Evidently, the C4
is the more reactive position when dealing with harder LiBH4, a reagent that is known to
favor 1,2-addition products in conjugated systems.19
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Figure 3. Molecular electrostatic potential of dioxazine 1a.
Having established that the C4-oxygen part of the 1,2,4-dioxazine framework can act as a
leaving group in regioselective allylic displacement reactions with metal hydrides, we were
intrigued how palladium catalysis could influence such reactivity on model compound 1a.
The electrophilic Pd(II)-center activates the allyl system for the nucleophilic attack at the
allylic termini, which is usually the rate limiting step.8 It also commonly accepted that
hydride as nucleophile is initially transferred to palladium and from there to the allylic
ligand by reductive elimination.8,9 When 1a was allowed to react with sodium borohydride
in the presence of catalytic amounts of (PPh3)2PdCl2 a sluggish reaction now occurred to
afford an equimolar mixture of compounds 5a (SN2-reduction) and 4a (SN2’-reduction)
(Table 1, entry 1). The same reaction carried out with NaBD4 afforded compound 4a-D
with the deuterium atom incorporated in a trans fashion to the 2-phenyl group (entry 2).15
This clearly indicated that the syn-SN2’-reduction pathway is external to the coordination
sphere of palladium. On the other hand, in this reaction compound 5a-D was obtained with
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inversion of configuration with respect to the leaving group (i.e. from the same side of
palladium).15 When the reaction of 1a with sodium borohydride was carried out in the
presence of catalytic amounts of (CH3CN)2PdCl2, compound 5a was obtained with a
complete regiocontrol (entry 3).
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Table 1. Reduction of 1a with metal hydrides in the presence of palladium salts.
entrya
M-H
Pd
t[h]
24
8
4a/5a/6ab Yield [%]c
1
NaBH4
NaBD4
NaBH4
LiBH4
(PPh3)2PdCl2
(PPh3)2PdCl2
(CH3CN)2PdCl2
(PPh3)2PdCl2
(CH3CN)2PdCl2
(PPh3)2PdCl2
(PPh3)2PdCl2
[(PPh3)]4Pd
Pd(dba)2
60/40/0
43/57/0
0/100/0
28/72/0
0/100/0
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25
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2
3
32
1
4
5
LiBH4
1
6
LiAlH4
LiEt3BH
LiEt3BH
LiEt3BH
1
complex mixture
7
0.5
0.5
1
0/10/90
65
75
53
60
55
8
0/3/97
0/63/37
0/100/0
0/100/0
9
10
LiEt3BH (CH3CN)2PdCl2
NaEt3BH (CH3CN)2PdCl2
0.5
1
11
a Unless stated otherwise, all reactions were carried out at 0 °C using 1a (0.15 mmol), Pd-salt (0.0075 mmol),
THF (0.75 mL), metal hydride (0.9 mmol).b Determined by 1H NMR of the crude mixture (value of 0 means not
detected by NMR). c Isolated yields of products after chromatographic purification on SiO2.
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The examination of other hydrides per se able to reduce 1a and therefore in competition
with the palladium-catalyzed reduction was rather instructive. Palladium-catalysis gave a
clear acceleration of the reduction process using lithium borohydride, but only
(PPh3)2PdCl2 modified the inherent regioselectivity of the reaction with lithium
borohydride (entries 4 and 5). The inherent reactivity of Super-hydride® with 1a was only
marginally affected by the presence of (PPh3)2PdCl2 and Pd[(PPh3)]4 (entries 7 and 8). The
use of Pd(dba)2 gave a mixture of regioisomers with a prevalence of SN2-addition product
5a (entry 9). Interestingly, a complete reversal of regioselectivity with the exclusive
formation of 1,2,3,4-tetrahydropyridine 5a was obtained using LiEt3BH or NaEt3BH in the
presence of catalytic amount of (CH3CN)2PdCl2 (entries 10 and 11). Hence, it is plausible
that when only more coordinating ligands are used, the inherent reactivity of the hydride
species can compete with the reductive elimination pathway from palladium. A
computational study performed at the DFT level revealed the unsymmetrical structure of
the corresponding PdL2-allyl complex with a Pd-C4 bond strongly shorter of the Pd-C2 bond
(Figure 4), regardless of the palladium ligand, with a consequent marked carbocationic
character of C(2) atoms, as confirmed by NBO analysis (+0.188 au for L=CH3CN and +0.184
au for L=Ph3P).17
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Figure 4. DFT optimized structure and NBO Charges (Pd and C(2) atoms) of Pd-allyl
complex derived from (CH3CN)2PdCl2
At the same time Pd atom is more electropositive in the Pd-allyl complex derived from
(CH3CN)2PdCl2 (NBO Charge +0.282 au) than in the one derived from (Ph3P)2PdCl2 (NBO
Charge +0.036 au). It is plausible that a more positive palladium (L=CH3CN) undergoes
more readily the reduction and the subsequent reductive elimination at the C4-position,
whereas in the case of a less positive palladium (L=Ph3P) reduction at C(2) becomes
competitive.
In summary, we have developed new regioselective reductions of piperidine
enecarbamates bearing a stable allylic leaving group. By the use of common reducing
agents, with or without palladium catalysts, it is possible to obtain selectively ꢀ2,3- or ꢀ3,4-
piperidine derivatives with unconventional substitution patterns. By transient formation of
the latter derivative, a new stable oxadiaza-[3.3.1]-bicyclic scaffold can be obtained by a
robust cascade reaction. To our delight, compound 6a selectively stimulated GLP-1
secretion in mouse enteroendocrine-like STC-1 cells with an EC50 = 11 M. Further studies
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are ongoing to assess the biological activity of a library of compounds based on this original
scaffold.
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EXPERIMENTAL SECTION
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General Information. All reactions dealing with air or moisture sensitive compounds were
carried out under an argon atmosphere in oven dried 10 mL or 25 mL Schlenk tubes.
Analytical TLC were performed on Merck TLC Silica gel 60 silica gel sheets with detection
by exposure to ultraviolet light (254 nm) and/or by immersion in an acidic staining
solution of p-anisaldehyde in EtOH. Silica gel 60 was used for flash chromatography (230-
400 mesh). Automated column chromatography was performed using prepacked silica gel
cartridges on a Biotage Isolera 1.5.2 (27-53 μm). 1H NMR spectra were recorded on Bruker
Avance II 250 MHz spectrometer or on Bruker Avance III 300 MHz spectrometer. Chemical
shifts are reported in ppm downfield from tetramethylsilane with the solvent resonance as
the internal standard (deuterochloroform:
δ 7.26, deuteroacetonitrile: δ
1.94). 13C NMR
spectra were recorded on a Bruker Avance II 250 spectrometer (62.5 MHz) with complete
proton decoupling. Chemical shifts are reported in ppm downfield from tetramethylsilane
with the solvent resonance as the internal standard (deuterochloroform:
δ 77.16,
deuteroacetonitrile: 1.32). Melting points were determined on a Kofler apparatus and are
δ
uncorrected. Mass spectra ESIMS were measured on a Finnigan LC-Q Deca Termoquest
spectrometer, equipped with a software Xcalibur. Solvents for extraction and
chromatography were distilled before use. Compounds of type 1 were prepared in
accordance to the indicated literature procedures.10 LiBDEt3 was prepared by procedure of
Brown.20
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General procedure for the synthesis of inverse cycloadducts A1-3 (Scheme S1,
Supporting Information): a round-bottomed flask was charged with the appropriate 1,2-
dihydropyridine and N-hydroxy-2-phenylacetamide in a 3:1 solution of MeOH/H2O (0.17
M) at 0 °C. NaIO4 (1.1 eq, 99.5% purity) was slowly added and the resulting heterogeneous
mixture was allowed to stir until no 1,2-dihydropyridine was detected by TLC analysis. The
reaction crude was concentrated and diluted with CH2Cl2/H2O to give a biphasic solution.
The aqueous layer was extracted with CH2Cl2 and the combined organic layers were dried
over MgSO4, filtered and concentrated. The resulting residue was subjected to flash
chromatography.
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Methyl (1S*,4R*)-3-(2-phenylacetyl)-6-vinyl-2-oxa-3,5-diazabicyclo[2.2.2]oct-7-ene-5-
carboxylate (A1). According to the general procedure, methyl 2-vinylpyridine-1(2H)-
carboxylate (300 mg, 1.8 mmol),21 N-hydroxy-2-phenylacetamide (302 mg, 2.0 mmol),
NaIO4 (430 mg, 2.0 mmol), MeOH (7.2 mL) and H2O (3.6 mL). Subsequent flash
chromatography (petroleum ether/AcOEt 7:3 + 2% Et3N, Rf = 0.33) afforded the title
compound as a colorless oil (216 mg, 38%). 1H NMR (250 MHz, CD3CN, 60 °C) δ 7.37 – 7.17
(m, 5H), 6.80 – 6.65 (m, 2H), 6.44 – 6.34 (m, 1H), 5.61 – 5.43 (m, 1H), 5.22 (dt, J = 11.2, 1.4
Hz, 1H), 5.17 (dt, J = 4.2, 1.4 Hz, 1H), 4.93 – 4.88 (m, 1H), 4.57 – 4.50 (m, 1H), 3.69 (s, 3H),
3.66 – 3.55 (m, 2H). 13C NMR (62.5 MHz, CD3CN) δ 174.5, 156.7, 135.3, 134.8, 132.9, 130.6,
129.3, 129.2, 127.6, 118.6, 74.9, 60.0, 59.3, 53.5, 40.5. HRMS (ESI) m/z [M + Na+] Calcd for
C17H18N2O4Na 337,1164, found 337,1159.
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Methyl (1S*,4R*)-6-cyclohexyl-3-(2-phenylacetyl)-2-oxa-3,5-diazabicyclo[2.2.2]oct-7-
ene-5-carboxylate (A2). According to the general procedure, methyl 2-cyclohexylpyridine-
1(2H)-carboxylate (1.4 g, 2.5 mmol, 40 % purity),21 N-hydroxy-2-phenylacetamide (422
mg, 2.8 mmol), NaIO4 (599 mg, 2.8 mmol), MeOH (10 mL) and H2O (5 mL). Subsequent flash
chromatography (petroleum ether/AcOEt 7:3 + 2% Et3N, Rf = 0.18) afforded the title
compound as a yellow oil (444 mg, 48%). 1H NMR (250 MHz, CD3CN) δ 7.37 – 7.16 (m, 5H),
6.74 – 6.65 (m, 1H), 6.65 – 6.57 (m, 1H), 6.51 – 6.41 (m, 1H), 5.03 – 4.95 (m, 1H), 3.90 (dd, J
= 6.8, 3.6 Hz, 1H), 3.82 – 3.78 (m, 1H), 3.71 (s, 3H), 3.58 (ABq, J = 15.1 Hz, 2H), 1.73 (s, 2H),
1.68 – 1.43 (m, 4H), 1.28 – 1.01 (m, 5H). 13C NMR (62.5 MHz, CD3CN) δ 175.8, 158.5, 136.0,
133.0, 131.3, 130.9, 129.9, 128.2, 74.7, 64.0, 61.3, 54.3, 41.5, 41.3, 31.3, 31.2, 27.8, 27.7,
27.7. HRMS (ESI) m/z [M + Na+] Calcd for C21H26N2O4Na 393,1790, found 393,1786.
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Methyl (1S*,4R*)-6-allyl-3-(2-phenylacetyl)-2-oxa-3,5-diazabicyclo[2.2.2]oct-7-ene-5-
carboxylate (A3). According to the general procedure, methyl 2-allylpyridine-1(2H)-
carboxylate (716 mg, 4.0 mmol),22 N-hydroxy-2-phenylacetamide (665 mg, 4.4 mmol),
NaIO4 (940 mg, 4.4 mmol), MeOH (16 mL) and H2O (8 mL). Subsequent flash
chromatography (petroleum ether/Et2O 6:4 + 2% Et3N, Rf = 0.18) afforded the title
compound as a yellow oil (603 mg, 46%). 1H NMR (250 MHz, CD3CN, 60 °C) δ 7.38 – 7.15
(m, 5H), 6.79 – 6.69 (m, 1H), 6.67 – 6.58 (m, 1H), 6.50 – 6.39 (m, 1H), 5.90 – 5.69 (m, 1H),
5.13 (dd, J = 4.7, 3.4 Hz, 1H), 5.07 (bs, 1H), 4.96 – 4.86 (m, 1H), 4.07 – 3.95 (m, 1H), 3.60
(ABq, J = 15.3 Hz, 2H), 3.72 (s, 3H), 2.61 – 2.47 (m, 1H), 2.03 – 1.84 (m, 1H). 13C NMR (62.5
MHz, CD3CN), major rotamer δ 174.6, 156.5, 136.0, 135.3, 134.5, 132.9, 130.6, 129.2, 127.6,
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118.4, 74.3, 59.6, 58.0, 53.6, 40.6, 36.8. HRMS (ESI) m/z [M + Na+] Calcd for C18H20N2O4Na
351,1321, found 351.1317.
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General procedure for the synthesis of the dioxazine derivatives 1i-k:10 an oven-dried 10
mL pyrex vial was charged with the specified inverse cycloadduct of type A, copper(I)
chloride (0.20 eq) and 1,2-dichloroethane (DCE) (0.15 M). The resulting mixture was
allowed to react until no compound A1-3 was detected by TLC analysis. The reaction was
quenched with H2O and the aqueous phase was extracted with CH2Cl2 and the combined
organic layers were dried over MgSO4. Removal of the solvent gave a residue that was
purified by flash chromatography.
(4aR*,8S*,8aS*)-Methyl
3-benzyl-8-vinyl-8,8a-dihydropyrido[4,3-e][1,4,2]dioxazine-
7(4aH)-carboxylate (1i). According to the general procedure, A1 (193 mg, 0.61 mmol),
CuCl (12.2 mg, 0.123 mmol) in DCE (4.10 mL) reacted at 75 °C for 17 h. Subsequent flash
chromatography (petroleum ether /AcOEt 7:3, Rf = 0.23) afforded the title compound as a
colorless oil (140 mg, 72.5 %). 1H NMR (250 MHz, CD3CN, 65 °C) δ 7.39 – 7.18 (m, 5H), 6.91
(d, J = 8.5 Hz, 1H), 5.74 (ddd, J = 15.7, 10.6, 4.8 Hz, 1H), 5.26 (d, J = 10.5 Hz, 1H), 5.18 (d, J =
17.4 Hz, 1H), 4.94 (bs, 1H), 4.79 (bs, 1H), 4.66 (d, J = 8.3 Hz, 1H), 4.06 – 3.98 (m, 1H), 3.75
(s, 3H) 3.48 (s, 2H). 13C NMR (62.5 MHz, CD3CN) δ 156.5, 154.6, 136.9, 131.5, 129.6, 129.5,
127.9, 127.1, 117.9, 102.1, 67.1, 66.7, 57.3, 54.1, 38.6. HRMS (ESI) m/z [M + Na+] Calcd for
C17H18N2O4Na 337,1164, found 337.1159.
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(4aR*,8S*,8aS*)-Methyl
3-benzyl-8-cyclohexyl-8,8a-dihydropyrido[4,3-
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e][1,4,2]dioxazine-7(4aH)-carboxylate (1j). According to the general procedure, A2 (69
mg, 0.18 mmol), CuCl (3.7 mg, 0.037 mmol) in DCE (1.3 mL) reacted at 75 °C for 2 h.
Subsequent flash chromatography (petroleum ether /AcOEt 8:2, Rf = 0.25) afforded the title
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compound as a white amorphous solid (40 mg, 58 %). H NMR (250 MHz, CD3CN, 65 °C) δ
7.39 – 7.20 (m, 5H), 6.83 (d, J = 7.7 Hz, 1H), 4.92 (bs, 1H), 4.70 (d, J = 7.5 Hz, 1H), 4.19 (d, J =
8.9 Hz, 1H), 4.07 (s, 1H), 3.73 (s, 1H), 3.47 (s, 2H), 1.82 – 1.40 (m, 6H), 1.32 – 0.98 (m,
5H).13C NMR (62.5 MHz, CD3CN) δ 156.6, 155.3 and 155.2*, 136.9, 129.6, 129.5, 127.8,
127.6 and 127.3*, 103.2* and102.7, 67.5, 65.5, 60.0, 54.0 and 53.7*, 38.6, 37.9, 30.3, 26.8,
26.6, 26.5. [* minor rotamer]. HRMS (ESI) m/z [M + Na+] Calcd for C21H26N2O4Na 393,1790,
found 3931784.
(4aR*,8S*,8aS*)-Methyl
3-benzyl-8-allyl-8,8a-dihydropyrido[4,3-e][1,4,2]dioxazine-
7(4aH)-carboxylate (1k). According to the general procedure, A3 (70 mg, 0.21 mmol),
CuCl (4.2 mg, 0.043 mmol) in DCE (1.4 mL) reacted at 75 °C for 18h. Subsequent flash
chromatography (hexanes /AcOEt 7:3, Rf = 0.25) afforded the title compound as a colorless
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oil (40 mg, 58 %). H NMR (250 MHz, CD3CN, 65 °C) δ 7.42 – 7.20 (m, 5H), 6.85 (d, J = 8.4
Hz, 1H), 5.96 – 5.69 (m, 1H), 5.21 – 5.02 (m, 2H), 4.97 – 4.92 (m, 1H), 4.74 – 4.65 (m, 1H),
4.46 (td, J = 7.4, 3.0 Hz, 1H), 3.96 (dd, J = 5.9, 3.0 Hz, 1H), 3.75 (s, 3H), 3.49 (s, 2H), 2.43 –
2.18 (m, 2H). 13C NMR (62.5 MHz, CD3CN) δ 156.4, 154.6* and 154.3, 136.8, 134.2, 129.6,
129.4, 127.8, 126.7, 118.8, 102.1* and 101.7, 67.0, 66.4, 54.7, 53.9, 38.6, 34.7* and 34.3. [*
minor rotamer]. HRMS (ESI) m/z [M + Na+] Calcd for C18H20N2O4Na 351,1321, found
351.1317.
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Methyl 2-phenylpiperidine-1-carboxylate (2a). A round-bottomed flask was charged with
dioxazine derivative 1a (55 mg, 0.15 mmol), Pd/C 10 % (11 mg), ammonium formate (56
mg, 0.90 mmol) and methanol (1.2 mL). The resulting mixture was allowed to stir 1 h at 80
°C. The mixture was taken up with CH2Cl2 and filtered over a short pad of Celite. Removal of
solvent afforded a reaction crude that was purified by flash chromatography (hexanes
/AcOEt 6:4 + 1% Et3N, Rf = 0.66) to give the title compound (15 mg, 45%). The spectral
data are consistent with prior reports.23
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(2S*,3R*)-Methyl 2-phenyl-3-((2-phenylacetamido)oxy)piperidine-1-carboxylate (3a).
The second eluting fraction of the above flash chromatography (Rf = 0.15) afforded the title
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compound as a white amorphous solid (22 mg, 39 %). H NMR (250 MHz, CDCl3) δ 9.05 (s,
1H), 7.42 – 7.11 (m, 10H), 5.64 (s, 1H), 4.74 (s, 1H), 4.05 – 3.88 (m, 1H), 3.85 – 3.66 (m, 3H),
3.66 – 3.42 (m, 2H), 2.94 – 2.79 (m, 1H), 2.14 –2.00 (m, 1H), 1.96 – 1.57 (m, 2H), 1.34 – 1.17
(m, 1H). 13C NMR (62.5 MHz, CDCl3): δ 168.8, 158.6, 136.8, 134.2, 129.2, 129.0, 127.4, 127.3,
126.5, 126.4, 79.0, 54.5, 53.3, 41.1, 40.4, 23.5, 19.6. HRMS (ESI) m/z [M + Na+] Calcd for
C21H24N2O4Na 391.1634; Found 391.1631.
General procedure for the addition of metal hydrides to 1a (Scheme 1)
An over-dried 5 mL Schlenk tube was charged with the dioxazine derivative and freshly
distilled THF under argon protection. The resulting mixture was cooled at 0 °C and the
hydride source was slowly added. Upon disappearance of the dioxazine derivative, the
reaction was quenched with water and the aqueous phase was extracted with Et2O. The
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combined organic layers were dried over MgSO4, concentrated and purified by flash
chromatography.
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(2S*,3R*)-Methyl
2-phenyl-3-((2-phenylacetamido)oxy)-3,4-dihydropyridine-1(2H)-
carboxylate (5a). According to the general procedure, dioxazine 1a (55 mg, 0.15 mmol),
LiBH4 (20 mg, 0.9 mmol) and THF (0.26 mL), 0 °C for 18 h. Subsequent flash
chromatography (hexanes /AcOEt 6:4, Rf = 0.15) afforded the title compound as an
amorphous solid (26 mg, 48 %). 1H NMR (250 MHz, CDCl3) 9.04 (bs, 1H) and 8.60* (bs, 1H)
(exchangeable with D2O), 7.39 – 7.21 (m, 8H), 7.18 – 7.08 (m, 2H), 7.02 (d, J = 7.4 Hz, 1H),
5.59 (s, 1H) and 5.44* (s, 1H), 4.77 (bs, 1H), 4.44 (bs, 1H), 3.74 (s, 3H) and 3.61* (s, 3H),
3.60 - 3.48 (m, 2H), 2.38 – 2.11 (m, 1H), 2.08-1.79 (m, 1H). 13C NMR (62.5 MHz, CDCl3) δ
169.5, 154.7 and 154.4*, 138.4 and 138.2*, 134.2, 129.3, 129.0, 128.9, 127.8, 127.4, 125.7,
125.3, 124.7, 102.6 and 102.0*, 79.9 and 79.4*, 56.4 and 55.8*, 53.5, 40.9, 22.0 and 21.3*.
[*Minor rotamer] HRMS (ESI) m/z [M + Na+] Calcd for C21H22N2O4Na 389.1477; Found
389.1472.
General procedure for the synthesis of [3.3.1]-bicyclic derivatives 6a-g (Scheme 2)
An over-dried 5 mL Schlenk tube was charged with the dioxazine derivative and freshly
distilled THF under argon protection. The resulting mixture was cooled at 0 °C and a
solution of LiBHEt3 (1.0 M in THF, 6.0 eq.) was dropwise added (CAUTION PYROFORIC
REAGENT). Upon disappearance of the dioxazine derivative, the reaction was quenched
with water and the aqueous phase was extracted with Et2O. The combined organic layers
were dried over MgSO4, concentrated and purified by flash chromatography.
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2-Phenyl-1-((1R*,5S*,9R*)-9-phenyl-4-oxa-1,3-diazabicyclo[3.3.1]non-6-en-3-
yl)ethanone (6a). According to the general procedure, dioxazine 1a (365 mg, 1.0 mmol),
LiBHEt3 (6.0 ml, 6.0 mmol) and THF (1.6 mL), 0°C for 1h. Subsequent flash chromatography
(hexanes /AcOEt 8:2, Rf = 0.17) afforded the title compound as a white solid (321 mg, 76
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%). M.p. = 156-159 °C. H NMR (250 MHz, CDCl3) δ 7.39 – 7.20 (m, 10H), 6.07 – 5.95 (m,
2H), 5.61 (d, 1H, J = 13.2 Hz), 4.99 – 4.89 (m, 1H), 4.57 (d, 1H, J = 2.2 Hz), 4.48 (d, 1H, J =
13.2 Hz), 3.82 (d, 1H, J = 14.7 Hz), 3.55 (d, 1H, J = 14.7 Hz), 3.45 – 3.18 (m, 2H). 13C NMR
(62.5 MHz, CDCl3) δ 172.3, 137.0, 129.7, 128.7, 128.5, 128.4, 128.0, 127.7, 126.9, 126.8,
119.4, 72.3, 65.7, 59.6, 47.6, 39.8. HRMS (ESI) m/z [M + Na+] Calcd for C20H20N2O2Na
343.1422; Found 343.1421.
2-Phenyl-1-((1R*,5S*,8R*,9R*)-9-phenyl-4-oxa-1,3-diazabicyclo[3.3.1]non-6-en-3-yl-
2,2,8-D3)ethan-1-one (6a-D). According to the general procedure, dioxazine 1a (59 mg,
0.16 mmol), LiBDEt3 (0.97 ml, 0.97 mmol) and THF (0.28 mL), 0°C for 1 h. Subsequent
crystallization (hexanes/AcOEt) afforded the title compound as a white solid (11 mg,
21.3%). M.p.=159 °C. 1H NMR (250 MHz, CDCl3) δ 7.41 – 7.20 (m, 10H), 6.07 – 5.94 (m, 2H),
4.96 – 4.88 (m, 1H), 4.59 – 4.52 (m, 1H), 3.82 (d, J = 14.7 Hz, 1H), 3.54 (d, J = 14.7 Hz, 1H),
3.31 (s, 1H). 13C NMR (62.5 MHz, CDCl3) δ 172.3, 137.1, 137.0, 135.1, 129.7, 128.5, 128.3,
127.6, 126.9, 126.7, 119.4, 72.3, 59.5, 47.25 (J = 21.1 Hz), 39.8. HRMS (ESI) m/z [M + Na+]
Calcd for C20H17D3N2O2Na 346.1611; Found 346.1608.
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1-((1R*,5S*,9R*)-9-Ethyl-4-oxa-1,3-diazabicyclo[3.3.1]non-6-en-3-yl)-2-
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phenylethanone (6b). According to the general procedure, dioxazine 1d (50 mg, 0.15
mmol), LiBHEt3 (0.90 ml, 0.90 mmol) and THF (0.26 mL), 0°C for 30 minutes. Subsequent
flash chromatography (hexanes /AcOEt 5:5, Rf = 0.30) afforded the title compound as a
white solid (27 mg, 65 %). M.p. = 68-70 °C. 1H NMR (250 MHz, CDCl3) δ 7.45 – 7.10 (m, 5H),
6.20 – 6.13 (m, 1H), 5.91–5.77 (m, 1H), 5.48 (d, 1H, J = 13.1 Hz), 4.33 – 4.19 (m, 2H), 3.77 (d,
1H, J = 14.7 Hz), 3.61 - 3.46 (m, 2H), 3.45 – 3.31 (m, 1H), 3.25 (app-t, 1H, J = 7.4 Hz), 1.51-
1.33 (m, 2H), 0.99 (t, 3H J = 7.4 Hz). 13C NMR (62.5 MHz, CDCl3) δ 172.2, 136.5, 135.1, 129.6,
128.5, 126.8, 119.0, 73.6, 65.9, 59.5, 47.5, 39.6, 22.7, 10.6. HRMS (ESI) m/z [M + Na+] Calcd
for C16H20N2O2Na 295.1422; Found 295.1421.
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1-((1R*,5S*)-4-Oxa-1,3-diazabicyclo[3.3.1]non-6-en-3-yl)-2-phenylethanone
(6c).
According to the general procedure, dioxazine 1e (53 mg, 0.15 mmol), LiBHEt3 (0.90 mL,
0.90 mmol) and THF (0.26 mL), 0°C for 30 minutes. Subsequent flash chromatography
(hexanes /AcOEt 5:5 + 5% Et3N, Rf = 0.20) afforded the title compound as a white solid (13
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mg, 35 %). M.p. = 95-98 °C. H NMR (250 MHz, CDCl3) δ 7.39 – 7.18 (m, 5H), 6.22 (dt, 1H J
=10.0, 5.4 Hz), 6.03 - 5.91 (m , 1H), 5.39 (d, J = 13.2 Hz, 1H), 4.36 (d, J = 6.1 Hz, 1H), 4.30 (d,
J = 13.2 Hz, 1H), 3.83 – 3.42 (m, 5H), 3.0 (d, 1H, J= 13.5 Hz). 13C NMR (62.5 MHz, CDCl3) δ
172.1, 136.8, 135.0, 129.6, 128.5, 126.8, 121.1, 70.1, 64.3, 52.0, 50.0, 39.6. HRMS (ESI) m/z
[M + Na+] Calcd for C14H16N2O2Na 267.1109; Found 267.1110
Phenyl((1R*,5S*,9R*)-9-phenyl-4-oxa-1,3-diazabicyclo[3.3.1]non-6-en-3-yl)methanone
(6d). According to the general procedure, dioxazine 1h (30 mg, 0,08 mmol), LiBHEt3 (0.51
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3H), 7.36 – 7.21 (m, 5H), 5.96 (dt, J = 10.1, 2.7 Hz, 1H), 5.75 – 5.54 (m, 3H), 4.82 (d, J = 6.4
Hz, 1H), 4.69 (d, J = 13.0 Hz, 1H), 4.63 (s, 1H), 3.39 (d, J = 20.5 Hz, 1H), 3.23 (d, J = 19.6 Hz,
1H). 13C NMR (62.5 MHz, CD3CN) δ 172.3, 138.3, 137.1, 136.0, 131.2, 129.9, 129.0, 128.7,
128.2, 127.6, 121.3, 73.1, 66.9, 60.0, 48.4. HRMS (ESI) m/z [M + Na+] Calcd for
C19H18N2O2Na 329.1266; Found 329,1261.
2-Phenyl-1-((1R*,5S*,9R*)-9-vinyl-4-oxa-1,3-diazabicyclo[3.3.1]non-6-en-3-yl)ethanone
(6e). According to the general procedure, dioxazine 1i (110 mg, 0.35 mmol), LiBHEt3 (2.1
ml, 2.1 mmol) and THF (0.6 mL), 0 °C for 7 h. Subsequent flash chromatography (petroleum
ether /AcOEt 5:5, Rf = 0.20) afforded the title compound as a colorless oil (38 mg, 40%). 1H
NMR (250 MHz, CDCl3) δ 7.36 – 7.17 (m, 5H), 6.18 – 6.08 (m, 1H), 5.94 – 5.83 (m, 1H), 5.69
(ddd, J = 17.4, 10.6, 5.4 Hz, 1H), 5.47 (d, J = 13.3 Hz, 1H), 5.36 – 5.21 (m, 2H), 4.37 – 4.32 (m,
1H), 4.29 (d, J = 13.1 Hz, 1H), 3.96 (dd, J = 3.4, 1.4 Hz, 1H), 3.75 (d, J = 14.8 Hz, 1H), 3.65 –
3.53 (m, 1H), 3.48 (d, J = 14.7 Hz, 1H), 3.44 – 3.31 (m, 1H). 13C NMR (62.5 MHz, CDCl3) δ
172.2, 136.7, 135.0, 133.2, 129.6, 128.5, 126.8, 119.0, 118.7, 72.9, 65.2, 59.5, 47.9, 39.6.
HRMS (ESI) m/z [M + Na+] Calcd for C16H18N2O2Na 293.1266; Found 293.1261.
2-Phenyl-1-((1R*,5S*,9R*)-9-cyclohexyl-4-oxa-1,3-diazabicyclo[3.3.1]non-6-en-3-
yl)ethanone (6f). According to the general procedure, dioxazine 1j (155 mg, 0.42 mmol),
LiBHEt3 (2.5 ml, 2.5 mmol) and THF (0.72 mL), 0 °C for 6.5 h. Subsequent flash
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chromatography (petroleum ether /AcOEt 7:3, Rf = 0.17) afforded the title compound as a
withe solid (32 mg, 23%; m.p. = 139-142°C) and starting material 1j (Conv: ca 50%, Rf =
0.23). 1H NMR (250 MHz, CDCl3) δ 7.36 – 7.19 (m, 5H), 6.20 – 6.10 (m, 1H), 5.90 – 5.78 (m,
1H), 5.45 (d, J = 13.0 Hz, 1H), 4.41 (d, J = 5.9 Hz, 1H), 4.19 (d, J = 13.1 Hz, 1H), 3.75 (d, J =
14.7 Hz, 1H), 3.55 – 3.26 (m, 3H), 2.95 (d, J = 10.3 Hz, 1H), 1.96 (d, J = 12.5 Hz, 1H), 1.80 –
1.56 (m, 4H), 1.38 – 1.09 (m, 4H), 1.06 – 0.78 (m, 2H). 13C NMR (62.5 MHz, CDCl3) δ 172.1,
137.0, 135.2, 129.6, 128.4, 126.7, 118.9, 72.5, 66.2, 63.0, 47.6, 39.6, 36.3, 30.2, 29.1, 26.5,
25.9, 25.9. HRMS (ESI) m/z [M + Na+] Calcd for C20H26N2O2Na 349.1892; Found 349.1887.
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2-Phenyl-1-((1R*,5S*,9R*)-9-allyl-4-oxa-1,3-diazabicyclo[3.3.1]non-6-en-3-yl)ethanone
(6g). According to the general procedure, dioxazine 1k (115 mg, 0.35 mmol), LiBHEt3 (2.1
ml, 2.1 mmol) and THF (0.6 mL), 0 °C for 1.5 h. Subsequent flash chromatography
(hexane/AcOEt 6:4, Rf = 0.13) afforded the title compound as a colorless oil (72 mg, 72 %).
1H NMR (250 MHz, CDCl3) δ 7.36 – 7.13 (m, 5H), 6.17 (dt, J = 10.0, 2.6 Hz, 1H), 5.91 – 5.72
(m, 2H), 5.45 (d, J = 13.1 Hz, 1H), 5.18 – 5.04 (m, 2H), 4.30 – 4.15 (m, 2H), 3.74 (d, J = 14.7
Hz, 1H), 3.59 – 3.32 (m, 4H), 2.27 – 2.02 (m, 2H). 13C NMR (62.5 MHz, CDCl3) δ 172.1, 136.5,
135.0, 134.3, 129.6, 128.4, 126.7, 118.7, 117.6, 73.1, 65.8, 57.5, 47.5, 39.6, 34.4. HRMS (ESI)
m/z [M + Na+] Calcd for C17H20N2O2Na 307.1422; Found 307.1419.
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General procedure for the addition of metal hydrides to 1a in the presence of
palladium catalysts (Table 1)
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An over-dried 5 mL Schlenk tube was charged with the dioxazine derivative, the palladium
catalyst (0.05 eq) and freshly distilled THF under argon protection. The resulting mixture
was cooled at 0 °C and the hydride source was slowly added. Upon disappearance of the
dioxazine derivative, the reaction was quenched with water and the aqueous phase was
extracted with Et2O. The combined organic layers were dried over MgSO4, concentrated
and purified by flash chromatography.
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(5R*,6S*)-Methyl
6-phenyl-5-((2-phenylacetamido)oxy)-5,6-dihydropyridine-1(2H)-
carboxylate (4a). According to the general procedure, dioxazine 1a (54.7 mg, 0.15 mmol),
(PPh3)2PdCl2 (5.4 mg, 0.078 mmol), NaBH4 (35 mg, 0.9 mmol) and THF (0.75 mL) for 24 h
at rt. Subsequent flash chromatography (hexanes /AcOEt 6:4, Rf = 0.22) afforded the title
compound as sticky oil (16 mg, 30%). The slower eluting fractions afforded compound 5a
1
(vide supra). H NMR (250 MHz, CD3CN, 65 °C) δ 9.21 (bs, 1H), 7.39 – 7.24 (m, 8H), 7.21 –
7.13 (m, 2H), 6.14* (dd, J = 3.5, 2.7 Hz, 1H) and 6.10 (dd, J = 3.5, 2.7 Hz, 1H), 6.03 – 5.93 (m,
1H), 5.67 (s, 1H), 4.63 (d, J = 5.7 Hz, 1H), 4.42 – 4.37* (m, 1H) and 4.34 – 4.30 (m, 1H), 3.71
(s, 3H), 3.66 (dd, J = 4.0, 1.9 Hz, 1H) and 3.59* (dd, J = 4.0, 2.0 Hz, 1H), 3.48 (s, 2H). 13C NMR
(62.5 MHz, CDCl3) δ 169.0, 157.0, 136.7, 134.1, 132.2, 129.2, 128.9, 128.7, 127.9, 127.4,
127.2, 121.1, 53.5, 53.2, 41.25, 31.0, 29.8. HRMS (ESI) m/z [M + Na+] Calcd for
C21H22N2O4Na 389.1477; Found 389.1478.
Compound 4a-D. According to the general procedure, dioxazine 1a (57 mg, 0.15 mmol),
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(PPh3)2PdCl2 (5.4 mg, 0.078 mmol), NaBD4 (60 mg, 1.4 mmol) and THF (0.75 mL), rt for 8 h.
Subsequent flash chromatography (hexanes /AcOEt 6:4, Rf = 0.19) afforded the title
compound as white amorphous solid (8 mg, 15%). 1H NMR (250 MHz, CD3CN, 65 °C) δ 9.22
(s, 1H), 7.38 – 7.24 (m, 8H), 7.21-7.13 (m, 2H), 6.11 (dd, J = 10.2, 2.2 Hz, 1H), 6.03 – 5.93 (m,
1H), 5.68 (s, 1H), 4.64 (dt, J = 5.5, 1.7 Hz, 1H), 3.71 (s, 3H), 3.64-3.56 (m, 1H), 3.48 (s, 2H).
13C NMR (75 MHz, CD3CN) δ 169.6, 157.5, 139.6, 136.4, 132.7, 130.1, 129.6, 129.5, 128.5,
127.8, 127.5, 121.2, 79.5, 57.5, 55.8, 53.4, 42.1 (J = 21.9 Hz), 40.8. HRMS (ESI) m/z [M +
Na+] Calcd for C21H21DN2O4Na 390.1540; Found 390.1538.
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Compound 5a-D. From the slower eluting fraction of the above flash chromatography
(hexanes /AcOEt 6:4, Rf = 0.10) was recovered the title compound as white amorphous
1
solid (12 mg, 22%). H NMR (250 MHz, CDCl3) δ 8.92 (bs, 1H) and 8.50* (bs, 1H)
(exchangeable with D2O), 7.40 – 7.26 (m, 8H), 7.18 –7.02 (d, J = 8.4 Hz, 1H), 5.60 (s, 1H) and
5.45* (s, 1H), 4.93 – 4.71 (m, 1H), 4.45 (bs, 1H), 3.74 (s, 3H) and 3.61* (s, 3H), 3.52 (d, J =
18.3 Hz, 2H), 2.21 (d, J = 15.2 Hz, 1H). 13C NMR (75 MHz, CDCl3) δ 169.6, 154.7* and 154.5,
138.4* and 138.1, 134.1, 129.3, 129.1, 128.9, 127.8, 127.6, 125.7, 125.4, 102.5* and 101.9,
79.8, 56.5* and 55.8, 53.5, 41.1, 22.1-21.1 (m). [* minor rotamer] HRMS (ESI) m/z [M +
Na+] Calcd for C21H21DN2O4Na 390.1540; Found 390.1538.
Supporting Information Available: The Supporting Information is available free of charge
on the ACS Publications website at DOI: Computational and X-ray data, copies of 1H and 13
C
NMR spectra of all new compounds, and 1H NMR spectrum of known compound 2a.
■ AUTHOR INFORMATION
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Corresponding Author
*Eꢀmail: pineschi@farm.unipi.it
ORCID
9
Mauro Pineschi: orcid.org/0000-0001-6063-2198
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Notes
The authors declare no competing financial interest.
Acknowledgement. We gratefully acknowledge the financial support from P.R.A. 2016_27
by the University of Pisa. We also gratefully acknowledge Eli Lilly Company for providing
us biological data on GLP-1 secretion within the Open Innovation Drug Discovery Program
used with Lilly's permission. (https://openinnovation.lilly.com/dd/).
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3723-3731. (b) Hanessian, S.; Maianti, J. P.; Matia, R. D.; Feeney, L. A.; Armstrong, E. S.
Hybrid Aminoglycoside Antibiotics via Tsuji Palladium-Catalyzed Allylic Deoxygenation.
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Using Catalytic Pd/C and H2. Org. Lett. 2005, 7, 2513-2516.
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Piperidinyl Enamides and Enecarbamates by Hetero-Cope Rearrangement of Nitroso
Cycloadducts. Chem. Heterocyc. Compd. 2018, 54, 458-468 and references therein.
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11) For a seminal regioselective palladium-catalyzed hydrogenolysis of allylic formate, see:
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Stereocontrolled Synthesis of Lavandulol. Org. Lett. 2011, 13, 2682-2685. (b) Wipf, P.;
Spencer,
S.
R.
Asymmetric
Total
Syntheses
of
Tuberostemonine,
Didehydrotuberostemonine, and 13-Epituberostemonine. J. Am. Chem. Soc. 2005, 127, 225-
235.
13) See for examples: (a) Huang, W.-X.; Wu, B.; Gao, X.; Chen, M.-W.; Wang, B.; Zhou, Y.-G.
Iridium-Catalyzed Selective Hydrogenation of 3-Hydroxypyridinium Salts: A Facile
Synthesis of Piperidin-3-ones. Org. Lett. 2015, 17, 1640-1643. (b) Hardy, S.; Martin, S. F.
Multicomponent, Mannich-type assembly process for generating novel, biologically-active
2-arylpiperidines and derivatives. Tetrahedron 2014, 70, 7142-7157. (c) Huy, P. H.;
Westphal, J. C.; Koskinen, A. M. P. Concise, Stereodivergent and Highly Stereoselective
Synthesis of cis- and trans-2-Substituted 3-Hydroxypiperidines – Development of a
Phosphite-Driven Cyclodehydration. Beilstein J. Org. Chem. 2014, 10, 369-383.
14) CCDC 1587512 contains the supplementary crystallographic data for this paper. Crystal
data for 6a: C20H20N2O2, Mr = 320.38, monoclinic, space group P21/c (no. 14), a =
10.9704(3) Å, b = 6.5342(2) Å, c = 23.6337(7) Å,
β
= 101.584(2), V = 1659.62(8) Å3, Z = 4,
−
−
3
1
F(000) = 680, Dc = 1.282 g⋅cm ,
ꢁ
(Mo-K
α
) = 0.084 mm , T = 296 K, 19785 reflections
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collected, 5574 unique (Rint = 0.0197), 232 variables refined with 3963 reflections with Fo >
4σ(Fo) to R1 = 0.0451. CCDC 1587512 for more details.
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15) Relative stereochemistry determined by 2D NOESY/ROESY experiments.
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16) The hydrogen of B-H bond has an increased hydride character in Super-H® than in
-
BH4 , and it is also more tightly coordinated to the cation. For example, see : Golub, I. E.;
Filippov, O. A.; Gulyaeva, E.; Gutsul, E. I.; Belkova, N. V. The Interplay of Proton Accepting
and Hydride Donor Abilities in the Mechanism of Step-wise Boron Hydrides Alcoholysis.
Inorganica Chimica Acta 2017, 456, 113-119 and references therein.
17) See Supporting Information for details.
18) Brown, H. C.; Kim, S. C.; Krishnamurthy, S. J. Selective reductions. 26. Lithium
Triethylborohydride as an Exceptionally Powerful and Selective Reducing Agent in Organic
Synthesis. Exploration of the Reactions with Selected Organic Compounds Containing
Representative Functional Groups. J. Org. Chem. 1980, 45, 1-12.
19) Arase, A.; Hoshi, M.; Yamaki, T.; Nakanishi, H. J. Lithium Borohydride-Catalysed
Selective Reduction of Carbonyl Group of Conjugated and Unconjugated Alkenones with
Borane in Tetrahydrofuran. Chem. Soc., Chem. Commun. 1994, 855-856.
20) Brown, H. C.; Krishnamurthy, S.; Hubbard, J. L. Addition Compounds of Alkali Metal
Hydrides. 15. Steric Effects in the Reaction of Representative Trialkylboranes with Lithium
and Sodium Hydrides to Form the Corresponding Trialkylborohydrides. J. Am. Chem. Soc.
1978, 100, 3343-3349.
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