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Modular Construction of Protected 1,2/1,3-Diols, -Amino Alcohols, and -Diamines via Catalytic Asymmetric Dehydrative Allylation: An Application to Synthesis of Sphingosine Shinji Tanaka, Ramachandran Gunasekar, Tatsuya Tanaka, Yoko Iyoda, Yusuke Suzuki, and Masato Kitamura J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b01181 • Publication Date (Web): 25 Jul 2017 Downloaded from http://pubs.acs.org on July 25, 2017
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The Journal of Organic Chemistry
Modular Construction of Protected 1,2/1,3-Diols, -Amino Alcohols, and -Diamines via Catalytic Asymmetric Dehydrative Allylation: An Application to Synthesis of Sphingosine Shinji Tanaka, Ramachandran Gunasekar, Tatsuya Tanaka, Yoko Iyoda, Yusuke Suzuki, and Masato Kitamura* Graduate School of Pharmaceutical Sciences, Research Center for Materials Science, and Graduate School of Science, Nagoya University Chikusa, Nagoya, 464-8601 (Japan)
Supporting Information Placeholder O
ABSTRACT: A new enantioselective catalysis has been
developed for the one-step construction of methylenebridged chiral modules of 1,2- and 1,3-OH and/or NH function(s) from δ- or λ-OH/NHBoc-substituted allylic alcohols and “H2C=O”/”H2C=NBoc.” A protonic nucleophile, either in situ generated CH2OH or CH2NHBoc, is intramolecularly allylated to furnish eight possible 1,2- or 1,3-O,O, -O,N, -N,O, and -N,N chiral modules equipped with an ethenyl group in high yields and enantioselectivities. The utility of this method has been demonstrated in the five-step synthesis of sphingosine.
OH
–
N
R
O
H+
Ru
Cl
Cl
(R)-Cl-Naph-PyCOOH
(R)-1
a Previous work OH
n
O
n
(R)- or (S)-1
n
+
A
AH
O
+ H 2O
A O
A = O, NCOR, etc. n = 1–3
quant, up to >99:1 er
b This work
H+
Chiral 1,2- and 1,3-oxygen- and/or nitrogenunits are ubiquitous in natural products ranging from small molecules, such as sugars, amino acids, and lipids, to huge molecules.1 Efficient construction of these chiral modules with an appropriate functionality for the continuing reaction is needed toward a target molecule, and many excellent methods have been developed on the basis of asymmetric oxydation,2 reduction,3 aldol reaction,4 and allylation.5 However, these methods lack generality; therefore, we have aimed to develop a method that can furnish all eight possible 1,2- and 1,3-O,O, O,N, -N,O, and -N,N modules with a single chiral catalyst. Figure 1 illustrates our basic concept. The monocationic CpRu(II) complex 1 of chiral picolinic acid, ClNaph-PyCOOH (6-(2-chloronaphthalen-1-yl)-5methylpyridine-2-carboxylic acid), catalyzes an intramolecular asymmetric dehydrative Tsuji-Trost type allylation of protonic nucleophiles, e.g., OH, NHCOR, and COOH, to furnish 5- and 6-membered cyclic ethers, amides, and esters with high enantiomeric ratios (er) (Figu re 1a).6 Figure 1b illustrates our envisioned strategy that catalyzes upon the slightly acidic conditions under
N
R
PF 6
O
A Z
Z=A
OH
n
AH cat 1
H+ Z=A
1
HA β
HA
nγ
OH α
A Z
n
A
Z O A Z
AH
n = 1 or 2
cat 1
HA
chiral modules
Ru
*
1,2-A,A n
AH
H+ Z=A
n
2
Z A
Ru
1
3
* A Z 1,3-A,A
A
A = O or NR
undesired pathways by-products
Figure 1. Dehydrative asymmetric allylation approach for construction of chiral 1,2- and 1,3-bifunctional modules.
which the CpRu/H+ operates. Addition of the protonic nucleophile AH (A = O or NCOR) in a δ- or λ-AHsubstituted allylic alcohol to an electron-deficient unsaturated “Z=A” bridging unit should be accelerated by H+ to generate the two-atom inserted A–Z–AH species; this species would undergo a smooth cyclization to the
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corresponding 5- and 6-membered 1,2- and 1,3-A,A motifs bridged by “Z.” Although many possible species exist in equilibria, catalyst 1 will only furnish the product-forming species. However, if a π-allyl intermediate is generated more quickly than A–Z–AH formation, the undesired pathways would become predominant. The appropriate selection of “Z=A” is the key in the complex network system. The bridging unit “Z=A” was screened in the reaction of (E)-but-2-ene-1,4-diol (2) under the standard conditions of “[2] = 100 mM; [(R)-1] = 1 mM; 10 mol amount of “Z=A”; DMA; 100 °C.” Among many potentially usable “Z=A,” “Z” was fixed to a C1 unit, R2C, and “A” was selected as “O.” Table 1 summarizes the representative results. With the generally utilized bridging units, O=C=O, CH3CH=O, CCl3CH=O, and (CH3)2C=O, substrate 2 was completely consumed, but none of the desired 1,2-O,O module, 4-ethenyl-1,3dioxorane, was obtained (entries 1–4). Most likely, the intermediary π-allyl complex underwent β-elimination to form a dienal and/or self-condensation with 2 to form allyl ethers. Use of highly electrophilic (CF3)2C=O·nH2O furnished the unprotected but-3-ene1,2-diol in a 77:23 er and 52% yield (entry 5), qualitatively confirming the validity of H+-catalyzed A–Z–AH generation, as illustrated in Figure 1b. Finally, use of a 37% aqueous solution of formaldehyde, HCH=O + H2O ⇆ HOCH2OH,7 furnished the desired methylenebridged 1,2-O,O module (3) with a 98:2 R/S er in 96% NMR yield (entry 6). Enantioselectivity decreased in tBuOH, THF, CH2Cl2, and toluene. Yields tended to be lower in hydrophobic solvents (entries 7–10). Anhydrous paraformaldehyde lowered the yield,8 and the addition of H2O increased the yield and er (entry 11). Trioxane showed low reactivity no matter of whether the H2O exist or not.8 Furthermore, HOCH2NHBoc (4) was found to be an excellent bridging unit that facilitated the efficient construction of methylene-bridged 1,2-O,N module (5) with a 96:4 R/S er in 92% isolated yield (entry 12). Use of 2 mol amount of 4 was sufficient. In both cases, the absolute configurations matched the enantioselectivity observed in the (R)-1-catalyzed intramolecular OH and NHBoc allylation.6 Replacement of Boc with Cbz, Bz, Piv, TFA, and Ts dramatically decreased the reactivity.8 A complicated equilibria exists between various chemical species that are generated from 2, the π-allyl intermediate and “HCH=A” (A = O or NCOR) (Figure 1b). The equilibrium concentration of the desired product-forming species should be strongly affected by the acidity, polarity, and the H2O content of the reaction media. A subtle change in the bridging unit significantly affects reactivity. The H2C=O andH2C=NBoc bridging units satisfy the requirements for the desired reaction pathway, as shown in Figure 1b.
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Table 1: Screening of ‘‘Z=A’’ in the O and N Allylation of (E)-But-2-ene-1,4-diol (2).a OH
HO 2 100 mM
1 mol% (R)-1 ’’Z=A’’ 100 °C
O * O C A R 1,2-O,A module R
O 3
*
O
* NBoc 5
entry
‘‘Z=A’’b
solvent
% yieldc
R:Sd
1
O=C=Oe
DMA
—f
—
f
—
2
CH3CH=O
DMA
—
3
CCl3CH=O
DMA
—f
—
f
—
4
(CH3)2C=O
DMA
—
5
(CF3)2C=O·nH2O
DMA
(52)g
(77:23)g
6
HCH=O (37% formalin)
DMA
96
98:2
7
HCH=O (37% formalin)
t-BuOH
94
76:24
8
HCH=O (37% formalin)
THF
94
90:10
9
HCH=O (37% formalin)
CH2Cl2
40
87:13
10
HCH=O (37% formalin)
toluene
45
88:12
11
HCH=O (praformaldehyde)h
DMA
66
12
g
HO-CH2-NHBoc(4)
i
DMA
92
96:4 j
96:4
a
Conditions: [2] = 100 mM; [(R)-1] = 1 mM; 100 °C; 0.5 mmol scale. Allyl ester of Cl-Naph-PyCOOH was used for easier manipulation. The (R)-1 catalyst is generated in situ. b10 mol amount of Z=A was used otherwize specified. cDetermined by 1H-NMR analysis otherwise specified. dDetermined by HPLC analysis. e8 atm. fThe substrate 2 was completely consumed, but no desired module was obtained. gThe values in parentheses are the data of but-3-ene-1,2-diol obtained under the reaction conditions. h100 mol amount of H2O was added. i 2 mol amount of 4. jIsolated yield.
The generality of the present method was investigated with the use of 2, λ-OH-substituted allylic alcohol 6, δ-BocNH-substituted allylic alcohol 7, and λBocNH-substituted allylic alcohol 8. The results are shown in Figure 2. All of the eight possible chiral modules, 1,2-O,O (3), 1,2-O,N (5), 1,3-O,O, 1,3-O,N, 1,2-N,O, 1,2-N,N, 1,3-N,O, and 1,3-N,N were obtained with >94:6 ers in >86% yields. Under the standard conditions, the yields of 1,3-O,N and 1,3-N,N were halved and the enantioselectivity of 1,3-O,N was low (R/S = 25:75). However, increasing the catalyst loading from 1 mol% to 10 mol% improved the yields and ers to 86% with 98:2 er and 88% with 99:1 er for 1,3-O,N and 1,3N,N, respectively. The reliability of the present method was confirmed by the synthesis of 1,2-O,O, 1,2-O,N, and 1,3-O,O on a 1–5 g scale of 2 and 6. The present method could not be applied to the synthesis of 1,4-type modules, because intramolecular cyclization of HO(CH2)3CH=CHCH2OH more quickly occurres.6a
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The Journal of Organic Chemistry OH
HO
HO
I O
O O
NBoc
1,2-O,O (3) 96% NMR yield 98:2 er 5-g
scaleb
R
O
I
II
O
II
O
1,3-O,O 92% yield 96:4 er
scaleb
scaleb
1-g
NBoc 5 96:4 er
O
1,2-O,N (5) 92% yield 96:4 er 5-g
10 mol amt (Z)-but-2-ene-1,4-diol 3 mol% HG-II
OH 6
2
BocN O
NBoc 1,2-N,N 92% yield 99:1 er
1,2-N,O 91% yield 96:4 er
BocN
O
I
II OH
BocHN 7
II
Figure 2. Synthesis of eight possible chiral modules. All of yields except for that of 3 are isolated ones. Conditions I: [allylic alcohol] = 100 mM, [HCHO aq] = 1 M, [(R)-1] = 1 mM, DMA, 100 °C, 3 h. Conditions II: [allylic alcohol] = 100 mM, [4] = 200 mM, [(R)-1] = 1 mM, DMA, 100 °C, 3 h. aResults obtained by use of 10 mM (R)-1. b[2] or [6] = 500 mM, [HCHO aq] = 5 M or [4] = 1 M, [(R)-1] = 5 mM, DMA, 100 °C, 12 h. A longer time was required for completion probably due to less efficient phase separation of H2O. Isolation of 3 was problematic.
(S)-1:
29% yield, 78:22 dr
Ru
An ethenyl group that was simultaneously installed into the chiral modules during the course of the asymmetric cyclization process is advantageous for the subsequent reaction toward a target molecule.9 For example, metathesis with an allylic alcohol can furnish the δ-O/N substituted allylic alcohol moiety, which could be used for the next O/N introduction with our strategy. Among many potentially applicable target molecules, Derythro-sphingosine (9), which is representative of sphingolipids, was selected for demonstration of our strategy because of its structural expediency and significant biological characteristics.10 Figure 3 shows the synthetic pathways. The 1,2-O,N module ((R)-5) obtained above as a 96:4 R/S mixture was subjected to olefin metathesis using 3 mol% of 2nd-generation Hoveyda-Grubbs catalyst (HG-II) in acetone containing 10 mol amount of (Z)-but-2-ene-1,4-diol11 gave 10 as a 95:5 E/Z mixture, the E product of which was isolated in 65% yield. (E)-10 was converted to 11 in a 80:20 diastereomeric ratio (dr) in the presence of (R)-1 in DMA containing 60 mol amount of H2O without the use of H2C=O. The “NCH2OH” unit was in situ generated from the methylene-bridged and δ-NBoc-substituted allylic alcohol (E)-10, realizing an atom economical methylene relay process. The second allylation was roughly one order slower than the first one (100 °C, 3 h: 100% conversion vs 100 °C, 15 h: 78% conversion). An 80:20 mixture of 11 (98:2 er) and epi-11 (98:2 er) was isolated in 74% yield together with ca 15% recoverty of
O
O
O
NHBoc 12 not obtained
O
CH2Cl2, rt, 4 h
NHBoc epi-12
11
a) 10 mol amt HONH 2·HCl HO
O BocN
OH HO
O
+
13 78% yield 1
S
BocN epi-11
74% yield, 80:20 dr 11: 98:2 er epi-11: 98:2 er
10 mol amt 1-pentadecene 3 mol% HG-II 11 + epi-11
S
HO
(R)-1:
NHBoc Int OCH22OH
8
DMA, 100 °C, 15 h
+
O
O
k6
OH
BocHN
R
60 mol amt H 2O 3 mol% cat 1
BocN 11
OH
NBoc
OH I
S
HO
HO
1,3-N,Na 88% yield 99:1 er
1,3-N,O 90% yield 94:6 er
OH
Ru
k5 BocN
BocN
β
NBoc 10 65% yield 96:4 er
acetone, rt, 4 h
IntNCH22OH BocN
α
δ
NBoc
1,3-O,N a 86% yield 98:2 er
γ
R
O
3 S R 2
74:1 CH 3OH–H2O reflux, 19 h quantitative yield b) separation by silica-gel column chromatography
NH 2 D -erythro-sphingosine
(9)
Figure 3. Synthesis of sphingosine (9) from 1,2-O,N module (5) via methylene-relayed dehydrative allylation combined with olefin metathesis. All of yields are isolated ones. amt: amount.
(E)-10 with a 93:7 er. The reactivity of the enantiomeric catalyst (S)-1 was significantly lowered (29% yield), and the dr hardly changed. Thus, the slight enhancement of the product er from 96:4 to 98:2 can be ascribed to the kinetic resolution of the 96:4 R/S mixture of (E)-10 by (R)-1. Inthis specific case, the substrate control is predominant over the catalyst control.12,13 In this second diastereoselective allylation, the OCH2Otype product 12 was not generated at all, indicating that the reactivity of the 5-membered ring formation (k5) of the NCH2OH-type intermediate IntNCH2OH is much higher than that of the 6-membered ring formation (k6) of the OCH2OH-type intermediate IntOCH2OH (k5 >> k6). The oxazolidine product 11 and 1-pentadecene (10 mol amount) were coupled in CH2Cl2 in the presence of HGII (3 mol%) to give 13 as a sole product in 78% isolated yield. The methylene bridge and Boc protecting group were quantitatively removed using Harding’s conditions (HONH2·HCl, 74:1 CH3OH–H2O, reflux, 19 h)14 to furnish a 80:20 mixture of D-erythro-sphingosine (9) and epi-9. The mixture was purified to give the target molecule. The 1H- and 13C-NMR spectra, as well as the optical rotation ([α]D22=–3.99 (c=0.465 in CHCl3)), were all consistent with authentic 9 ([α]D21=–3.29 (c=0.460 in CHCl3)).15
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In summary, we have successfully developed a general method for the construction of chiral 1,2and 1,3-O,O, -O,N, -N,O, and -N,N eight modules equipped with a terminal ethenyl group; the reaction proceeds in the presence of our CpRu complex of chiral picolinic acid, Cl-Naph-PyCOOH, in combination with “H2C=O” or “HOCH2NHBoc” with δ- or λ-OH/NHBocsubstituted allylic alcohol substrates. The success of this method can be ascribed to two factors: i) the Ru/H+catalyzed allylation functioning under slightly acidic conditions; and ii) the use of a methylene bridging unit, H2C=A (A = O or NBoc). The slightly acidic conditions facilitate the formation an acetal, aminal, and a Ru-πallyl species directly from the allylic alcohol. The methylene bridging unit enables the facile generation of the sterically less-demanding ACH2OH acetal and ACH2NHBoc aminal (A = O or NBoc). Removal of the methylene group is straightforward, and the ethenyl group can function as the basis for the generation of the next allylic alcohol motif via HG-II-catalyzed metathesis. By using these advantages, we have realized a unique and short-step asymmetric synthesis of sphingosine from commercially available (E)-but-2-ene-1,4-diol. On-going projects in our group include the application of this strategy to other O/N-multi-functionalized natural products.
EXPERIMENTAL SECTION All of Figure S1–S35 and Table S1 were attached in the supporting information.8 (1) Instruments. Nuclear magnetic resonance (NMR) spectra were recorded on JEOL JNM-ECA-600 (600 MHz for 1H, 152 MHz for 13C), and the chemical shifts are expressed in parts per million (ppm) downfield from Si(CH3)4 or in ppm relative to the solvent peaks (δ 7.26 (CHCl3), 2.49 (CD3SOCHD2), and 3.31 (CHD2OD) in 1H NMR; δ 77.0 (CDCl3) and 49.0 (CD3OD) in 13C NMR, respectively). The signal patterns of 1H NMR are indicated as follows: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; and br, broad signal. The resolutions of 1H- and 13C-NMR spectra are 0.69 Hz and 1.44 Hz, respectively. NMR spectra were measured with samples of 5–15 mM at 25 °C unless otherwise specified. High-resolution mass spectra (HRMS) were measured by ESI ionization method on a Bruker Daltonics microTOF-QII system. High performance liquid chromatography (HPLC) analyses were performed on Shimadzu LC-10AD, LC-20AD, and LC20TA systems. Gas chromatography (GC) analyses were performed on a Shimazu GC-2014 system. Optical rotations were measured on a JASCO P-1010-GT system. (2) Materials. Gases. Argon (Ar) gas was purified by being passed through a column of BASF R3-11 catalyst at 80 °C and then through a column of granular CaSO4. Carbon dioxide (CO2, >99.9% purity, H2O 99%. The reaction mixture obtained under the above conditions in a different batch was partitioned between pentane (5 mL) and H2O (10 mL). The aq
layer was extracted with pentane (5 mL x 3). The combined organic layers were washed with H2O (5 mL x 2), dried over Na2SO4 (ca. 2 g), and filtrated. The filtrate was concentrated at 600 hPa by use of a rotary evaporator to ca. 1 mL, which contains ca. 10 mg of 3. During the concentration process, a large part of 3 was evaporated. The enantiomeric ratio (er) was determined by HPLC analysis of the residue to be 98:2 (R:S) (conditions: CHIRALCEL OD-H column (0.46 cm φ x 250 mm); hexane– iPrOH 99.9:0.1 eluent; 1.0 mL/min flow rate; 254-nm light detection; tR = 9.8 min (S) and 10.9 min (R)) (Figure S2). Reaction with 500-mM 2. The reaction mixture obtained under the above conditions except for the concentration of 2 (1.14 mL (500-mM DMA solution of 2), 0.570 mmol (50.2 mg)) was used for preparation of a 100–300 mM solution of 3. The reaction mixture was partitioned between pentane (3 mL) and H2O (5 mL). The aq layer was further extracted with pentane (3 mL x 4). The combined organic layers were washed with H2O (5 mL), and dried over Na2SO4 (ca. 2 g). Filtration gave a ca. 15 mL pentane solution of 3 containing 30% of DMA. The amount of 3 in the solution was determined to be ca. 40 mg/15 mL (ca. 25 mM) by the mesitylene method (0.100 mL; 0.500 mL (100-mM CDCl3 mesitylene solution, 0.0500 mmol (6.00 mg)); 0.50 mL CDCl3; 3.00:1.60 signal ratio at δ 4.04 and δ 6.78), indicating that the extraction efficiency was 80%. A 20-mL Young-type Schlenk tube was charged with the above solution (5 mL), closed, and connected to a vacuum line via a trap. The Young-type Schlenk tube was cooled to –78 °C and the trap was inserted into a liquid N2 bath. After the inner pressure of the vacuum line reached to
