Domino alkene-isomerization-Claisen rearrangement strategy to substituted allylsilanes

Article abstract of DOI:10.1021/jo202560g

A one-pot isomerization-Claisen protocol has been developed for the synthesis of highly substituted allylsilanes. Monosilylated divinyl ethers can be isomerized using a cationic iridium(I) catalyst followed by a thermal Claisen rearrangement to provide the allylsilanes in excellent yields and diastereoselectivities.

Full text of DOI:10.1021/jo202560g

Note
pubs.acs.org/joc
Domino Alkene-Isomerization−Claisen Rearrangement Strategy to
Substituted Allylsilanes
Mark G. McLaughlin and Matthew J. Cook*
School of Chemistry and Chemical Engineering, Queen’s University Belfast, Belfast BT9 5AG, Northern Ireland
S
* Supporting Information
ABSTRACT: A one-pot isomerization−Claisen protocol has
been developed for the synthesis of highly substituted allylsilanes.
Monosilylated divinyl ethers can be isomerized using a cationic
iridium(I) catalyst followed by a thermal Claisen rearrangement
to provide the allylsilanes in excellent yields and diastereose-
lectivities.
llylsilanes bearing stereogenic centers at the carbon−silicon bond
have been shown to be very versatile synthetic intermediates in
A
organic synthesis.1 They have been utilized in many stereospecific and
stereoselective reactions including Sakurai allylations,2 oxidations,3
aminations,4 tetrahydrofuran and cyclopentane annulations,5 cyclo-
propanations,6 and protodesilylations7 and as traceless stereodirecting
groups for boron aldol reactions.8 They have also been impressively
demonstrated in the arena of total synthesis, with many polyketide-
derived natural product syntheses benefiting from these flexible
intermediates.9
We recently reported a new method for the hydrosilylation of
propargylic alcohols which works especially well for sterically
encumbered tertiary alcohols.10 We wanted to use these compounds
to synthesize highly substituted allylsilanes via a Claisen rearrangement
strategy. Panek reported the conversion of secondary hydrosilylated
alcohols to chiral allylsilanes using both Johnson11 and Ireland−
Claisen12 rearrangements forming two contiguous stereogenic centers
in the process. When the Ireland−Claisen approach is used the
diastereoselctivity can be directed by controlling the enol ether
geometry to provide either syn or the anti isomer selectively, generally
in good diastereoselctivities (eqs 1 and 2). Alternatively, the product of
the Johnson−Claisen approach can undergo base-mediated α-
alkylation with alkyl halides with the anti diastereomer being favored
(eq 3).13
catalyst and ethyl vinyl ether (which forms an isomerization catalyst in
Our stategy was to perform an isomerization−Claisen reaction of a
monosilylated bis-allylic ether to provide products complementary to
those previously reported (eq 4).14 This approach would afford highly
substituted chiral allylsilanes as the syn diastereoisomer at the aldehyde
oxidation level. The major advantages of this are the mild, neutral
reaction conditions and the tolerance of tertiary-substituted allyl ethers
which generate trisubstituted alkenes on the allylsilane moiety. Herein
we report a simple procedure for the synthesis of allylsilanes with high
diastereoselectivity, predominately as the syn diastereoisomer.
We began investigating whether we could isomerize the allyl ethers
to the corresponding vinyl ether using basic conditions (Table 1). All
attempts at performing this were unsuccessful with extensive
decompsition observed. Several isomerization−Claisen rearrangements
have been reported using a variety of transition-metal catalysts,
including ruthenium and iridium.15 An isomerization−Claisen reaction
has been reported previously using a similar substrate and
(Ph3P)3RuCl2 as the catalyst; however, this required very high
temperature (190 °C) and led to a near 1:1 mixture of
diastereomers.16 We began investigating the use of these catalysts
and found that the combination of Grubbs’ first-generation metathesis
situ) gave good reactivity; however, the resulting product was a 58:42
mix of diastereoisomers.17 Similarly, the combination of [Ir(COD])-
Cl]2 and Cs2CO3 gave good reactivity and a 55:45 mix of
diastereoisomers.18 Both these conditions use elevated temperatures
(ca. 100 °C) for the isomerization, which leads to significant
epimerization of the resulting aldehyde center.
Nelson found a powerful solution to this problem by using a highly
active cationic Ir(I) catalyst, which performs the isomerization at
ambient temperature.19 Following isomerization, PPh3 is added which
sequesters the Ir catalyst and negates its Lewis acidic properties.20
Gratifyingly, when this procedure was used, the isomerization−Claisen
product could be isolated in 95:5 dr. A minor modification of the
solvent from CH2Cl2 to PhCl was required as dichloromethane was
not high enough boiling to perform the Claisen rearrangement
efficiently. The use of 1,2-dichloroethane, the other solvent used by
Nelson, resulted in a very slow isomerization reaction. However, when
Received: December 19, 2011
Published: January 10, 2012
© 2012 American Chemical Society
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Note
Table 1. Optimization Studies
a
b
entry
conditions
yield (%)
dr
1
2
3
4
5
6
t-BuOK, toluene, 90 °C
Grubbs I, EtOCHCH2, toluene, 100 °C
[Ir(COD)Cl]2, CS2CO3, toluene, 90 °C
dec
87
73
45
20
91
c
d
e
e
e
58:42
53:47
95:5
[Ir(COE)2Cl]2, PCy3, NaBPh4, CH2Cl2/acetone (20:1) then PPh3, 40 °C
[Ir(COE)2Cl]2, PCy3, NaBPh4, (CH2Cl)2/acetone (20:1) then PPh3, 80 °C
[Ir(COE)2Cl]2, PCy3, NaBPh4, PhCl/acetone (20:1) then PPh3, 90 °C
95:5
95:5
a
b
c
d
e
Isolated yield. Determined by 1H NMR. 5 mol %. 2 mol % dimer. 1 mol % of dimer, 6 mol % of PCy3, 2 mol % of NaBPh4 then 6 mol % of
PPh3.
chlorobenzene was used, a fast isomerization occurred. This, coupled
with its high boiling point, allowed a thermal Claisen rearrangement to
take place at 90 °C to provide a yield of 91% in a 95:5 diastereomeric
ratio.
We began to examine the scope of the reaction (Table 2) and
discovered allyl ethers derived from secondary alcohols worked with
both aryl and alkyl substituents tolerated. The phenyl substituent gave
excellent yields and diastereoselectivities of the requisite allylsilane 2b.
Alkyl substituents gave the corresponding allylsilane 2c−e in moderate
yields and diasteroselectivities. The isomerization of these substrates
generally occurs within 1 h; however, the Claisen rearrangement is
slower to occur. The lower diastereoselectivity observed is due to the
prolonged reaction times thus allowing the aldehyde product to
epimerize. The use of a more bulky cyclohexyl substituent accelerates
the Claisen rearrangement and provides allylsilane 2e in good yield
and excellent dr.
We next investigated the tertiary alcohol derived substrates. As was
mentioned previously, the dimethyl derivative could be converted to
the allylsilane in excellent yields and diastereoselctivities. Similarly, the
diphenyl 2f and cycloalkyl 2g,h substrates performed the Claisen
rearrangement very quickly, though the isomerization was slow, to
provide the corresponding allylsilanes 2f−h. Once again, in excellent
yields and diastereoselectivities were obtained. The isomerization does
not appear to be adversely effected by basic heteroatoms with good
yields and diastereoselectivities being obtained for the carbamate
protected piperidine 2i and pyran 2j.
Table 2. Substrate Scope
Unsymmetrical tertiary allylic ethers were also examined to see
whether we could control E/Z isomers in the corresponding allylsilane.
We examined the phenyl methyl substrate 1k, and under standard
conditions we observed a 93% yield; however, the allylsilane 2k was
produced as a 55:45 mixture of E/Z isomers (eq 5). We discovered
that by performing the Claisen rearrangement we could improve this
ratio to 80:20 with excellent diastereoselectivity.21 We next turned our
attention to the methyl isobutyl derivative 1l, which gave an excellent
yield of 2l as a 52:48 mix of E/Z isomers at 90 °C. Lowering the
temperature to 40 °C did not alter this ratio significantly with a 56:44
ratio being observed (eq 6).
These findings are consistent with a chair-type transition state
which correlate to the substituent A values (eq 7). There is a large
difference in A values between phenyl and methyl (Ph = 3.1; Me =
a
Conditions: 1 mol % of [Ir(COE)Cl]2, 6 mol % of PCy3, 2 mol % of
NaBPh4, PhCl/acetone (20:1), rt, 0.75−35 h, then 6 mol % of PPh3,
90 C, 0.75−35 h. For full details, see the Experimental Section.
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Note
1.8), which would account for increased selectivity at lower
temperatures, as conformation A will be more favored than B,
whereas the difference between isobutyl and methyl is very small (i-Bu
= 2.1; Me = 1.8), and therefore, very little selectivity is observed.22
We also investigated substitution on the allylic ether. The
crotylation reaction of the allylic alcohol with crotyl bromide provides
the corresponding crotyl products as a mixture of geometric isomers
(75:25 secondary phenyl and 85:15 for the tertiary cyclohexyl
substrates) (eqs 8 and 9). When these were subjected to the standard
reaction conditions described previously, they underwent the isomer-
5.25 (1H, dd, J = 17.3, 1.7 Hz), 5.11 (1H, dd, J = 10.3, 1.7 Hz), 3.83
(2H, dt, J = 5.5, 1.7 Hz), 1.29 (6H, s), 0.35 (6H, s); 13C NMR (100
MHz, CDCl3) δ 153.1, 138.7, 136.0, 133.8, 128.9, 127.7, 126.2, 115.8,
76.5, 64.2, 25.9, −2.5; HRMS (ES+) calcd for C16H27NOSi [M +
NH3] 278.1940, found 278.1945.
Synthesis of (E)-(3-(Allyloxy)-3-phenylprop-1-en-1-yl)-
dimethyl(phenyl)silane (1b). Alcohol (200 mg, 0.746 mmol) was
used to afford 1b (161 mg, 70%) as a colorless oil: Rf (9:1 hexane−
ethyl acetate) = 0.80; IR νmax (thin film)/cm−1 2900, 1522, 1400, 999,
1
767 565, 490; H NMR (400 MHz, CDCl3) δ 7.57−7.52 (2H, m),
7.42−7.30 (8H, m), 6.26 (1H, ddd, J = 18.8, 5.8, 1.7 Hz), 6.12 (1H, d,
J = 18.8 Hz), 6.05−5.94 (1H, m), 5.33 (1H, ddt, J = 17.3, 3.0, 3.0 Hz),
5.23 (1H, ddt, J = 10.5, 2.8, 2.8 Hz), 4.89 (1H, d, J = 5.8 Hz), 4.03
(1H, m), 0.39 (3H, d, J = 1.5 Hz), 0.38 (3H, d, J = 1.5 Hz) 13C NMR
(100 MHz, CDCl3) δ 147.6, 140.8, 138.5, 134.8, 133.8, 129.2, 128.9,
128.4, 127.7, 127.6, 127.1, 116.9, 83.7, 69.3, −2.6, −2.7; HRMS (EI+)
calcd for C19H21OSi [M − CH3]+ 293.1374, found 293.1362.
(E)-(3-(Allyloxy)-5-methylhex-1-en-1-yl)dimethyl(phenyl)-
silane (1d). Alcohol (213 mg, 0.869 mmol) was used to afford 1d
(200 mg, 80%) as a colorless oil: Rf (9:1 hexane−ethyl acetate) = 0.74;
IR νmax (thin film)/cm−1 2955, 1616, 1427, 1248, 1082, 920, 842, 782,
1
698, 467; H NMR (400 MHz, CDCl3) δ 7.56−7.51 (2H, m), 7.40−
7.34 (3H, m), 5.99−5.88 (3H, m), 5.27 (1H, dq, J = 17.1, 1.5 Hz),
5.18 (1H, dq, J = 10.3, 1.5 Hz), 5.07 (1H, ddt, J = 12.8, 5.3, 1.5 Hz),
3.87−3.79 (2H, m), 1.84−1.73 (1H, m), 1.61−1.53 (1H, m), 1.36−
1.27 (2H, m), 0.93 (6H, dd, J = 6.7, 0.6 Hz), 0.38 (3H, s), 0.37 (3H,
s); 13C NMR (100 MHz, CDCl3) δ 148.9, 138.6, 135.2, 133.8, 129.7,
129.0, 127.8, 116.6, 80.8, 69.4, 44.5, 24.4, 23.0, 22.5, −2.5, −2.6;
HRMS (EI+) calcd for C18H28OSi [M]+ 288.1909, found 288.1905.
(E)-(3-(Allyloxy)-3,3-diphenylprop-1-en-1-yl)dimethyl-
(phenyl)silane (1f). Alcohol (472 mg, 1.37 mmol) was used to afford
1f (405 mg, 77%) as a waxy yellow solid: Rf (9:1 hexane−ethyl
acetate) = 0.85; IR νmax (thin film)/cm−1 2956, 2955, 1646, 1446,
1247, 1114, 842, 469; 1H NMR (400 MHz, CDCl3) δ 7.48−7.44 (2H,
m), 7.38−7.20 (13H, m), 6.64 (1H, d, J = 18.8 Hz), 6.08 (1H, d, J =
18.8 Hz), 5.99−5.88 (1H, m), 5.36 (1H, dd, J = 17.3, 1.6 Hz), 5.14
(1H, dd, J = 10.5, 1.6 Hz), 3.74 (2H, dt, J = 5.1, 1.6 Hz), 0.35 (6H, s);
13C NMR (125 MHz, CDCl3) δ 149.2, 143.9, 138.7, 135.3, 133.8,
ization and subsequent Claisen rearrangement to provide the α-ethyl
products. The phenyl substrate 3a afforded the ethyl product 4a in
51% yield as a 97:3 mixture of diastereomers and the cyclohexyl
derivative 3b performs excellently to give the ethyl product 4b in 93%
yield and 97:3 dr.
Both of these reactions take a mixture of E/Z isomers and during
the isomerzation procedure convert them to a single E isomer in the
vinyl ether intermediate. This can then undergo the rearrangement
with near complete stereocontrol. To prove this, we performed the
isomerization and isolated the vinyl ether 5 which was formed
exclusively as the E isomer. Furthermore, the pure E vinyl ether was
subjected to the rearrangement conditions and we obtained the same
diastereoselectivity (eq 10), which further supports the fact that the
diastereoselectivity is entirely derived from the Claisen rearrangement
of the geometrically pure vinyl ether.
In conclusion, we have developed a rapid and reliable method for
the synthesis of highly substituted chiral allylsilanes containing two
contiguous stereogenic centers based on the isomerization−Claisen
work developed by Nelson. We have demonstrated the formation of
trisubstituted alkenes and the use of internal allylic ethers. Our
approach gives the syn-isomer and is complementary to approaches
developed by others. The use of these compounds in synthesis is
currently being explored.
129.0, 128.5, 127.9, 127.9, 127.8, 127.0, 115.4, 85.2, 65.0, −2.4; HRMS
(ES+) calcd for C19H36NO6Si [M + NH3]+ 402.2312, found 402.2305.
(E)-Ethyl 4-(allyloxy)-4-(2-(dimethyl(phenyl)silyl)vinyl)-
piperidine-1-carboxylate (1i). Alcohol (205 mg, 0.616 mmol)
was used to afford 1i (200 mg, 87%) as a colorless oil
(chromatography eluent: 1:1 hexane−EtOAc): Rf (1:1 hexane−ethyl
acetate) = 0.39; IR νmax (thin film)/cm−1 2925, 1700, 1428, 1236,
1
1063, 898, 826; H NMR (400 MHz, CDCl3) δ 7.52−7.46 (2H, m),
7.38−7.31 (3H, m), 6.02 (1H, d, J = 19.1 Hz), 5.95 (1H, d, J = 19.1
Hz), 5.95−5.84 (1H, m), 5.26 (1H, dd, J = 17.1, 1.5 Hz), 5.12 (1H,
dd, J = 10.5, 1.5 Hz), 4.12 (2H, q, J = 7.0 Hz), 3.94−3.81 (2H, br),
3.79 (2H, d, J = 5.5 Hz), 3.30−3.14 (2H, br m), 1.86−1.75 (2H, br m)
1.65−1.55 (2H, br m) 1.25 (3H, t, J = 7.0 Hz), 0.35 (6H, s); 13C NMR
(100 MHz, CDCl3) δ 155.6, 150.8, 138.3, 135.4, 133.8, 129.1, 128.8,
127.9, 116.0, 75.3, 63.3, 61.2, 39.7, 14.7, −2.6; HRMS (EI+) calcd for
C21H31NO3Si [M]+ 373.2073, found 373.2113.
(E)-(3-(Allyloxy)-3-phenylbut-1-en-1-yl)dimethyl(phenyl)-
silane (1k). Alcohol (403 mg, 1.43 mmol) was used to afford 1k (424
mg, 92%) as a pale yellow oil: Rf (9:1 hexane−ethyl acetate) = 0.87; IR
νmax (thin film)/cm−1 2955, 2855, 1646, 1446, 1248, 1114, 995, 919,
EXPERIMENTAL SECTION
■
General Procedure A: Formation of Allyl Ethers. To a solution
of vinylsilane alcohols10 in DMF (0.08 M) cooled to 0 °C was added
NaH (2 equiv, 60% unwashed) in one portion followed by allyl
bromide (2 equiv). The mixture was stirred for 1 h, quenched with
water, extracted with EtOAc (3 × 25 mL), washed with water (25 mL)
and brine (50 mL), and dried over MgSO4 to afford the requisite allyl
ether following column chromatography (9:1 hexane−EtOAc).
(E)-(3-(Allyloxy)-3-methylbut-1-en-1-yl)dimethyl(phenyl)-
silane (1a). Alcohol (1.91 g, 8.67 mmol) was used to afford 1a (2.03
g, 90%) as a colorless oil: Rf (9:1 hexane−ethyl acetate) = 0.91; IR νmax
1
791, 469; H NMR (400 MHz, CDCl3) δ 7.53−7.49 (2H, m), 7.45−
7.40 (2H, m), 7.37−7.32 (5H, m), 7.29−7.23 (1H, m), 6.30 (1H, d, J
= 18.8 Hz), 6.08 (1H, d, J = 18.8 Hz), 6.00−5.90 (1H, m), 5.32 (1H,
dq, J = 17.2, 1.8 Hz), 5.14 (1H, dq, J = 10.3, 1.8 Hz), 3.90−3.78 (2H,
m), 1.64 (3H, s), 0.36 (6H, s); 13C NMR (100 MHz, CDCl3) δ 151.7,
144.6, 138.7, 135.7, 133.8, 129.0, 128.1, 127.8, 127.0, 126.6, 126.3,
115.7, 80.4, 64.3, 24.7, −2.5, −2.5; HRMS (EI+) calcd for C20H23OSi
[M − CH3]+ 307.1505, found 307.1518.
(E)-3-((E)-But-2-en-1-yloxy)-3-phenylprop-1-en-1-yl)-
dimethyl(phenyl)silane (3a). Alcohol (272 mg, 1.015 mmol) was
used to afford 3a (256 mg, 78%, 75:25 E/Z isomers) as a colorless oil
(using crotyl bromide): Rf (9:1 hexane−ethyl acetate) = 0.78; IR νmax
(thin film)/cm−1 3069, 1646, 1111,1084, 993, 731, 488 ; H NMR
1
(400 MHz, CDCl3) δ 7.54−7.48 (2H, m), 7.37−7.32 (3H, m), 6.11
(1H, d, J = 19.1 Hz), 5.91 (1H, d, J = 19.1 Hz), 5.96−5.84 (1H, m),
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Note
1
(thin film)/cm−1 3067, 2956, 1377, 1248, 1113, 731, 524; H NMR
(400 MHz, CDCl3) δ 7.50−7.46 (2H, m), 7.38−7.28 (8H, m), 6.21
(minor, 0.25H, dd, J = 13.1, 5.8 Hz), 6.20 (major, 0.75H, dd, J = 18.7,
5.8 Hz), 6.03 (minor, 0.25H, dd, J = 18.6, 1.2 Hz), 6.02 (major, 0.75H,
dd, J = 18.6, 1.2 Hz), 5.73−5.56 (2H, m), 4.82 (1H, dd, J = 5.8, 1.2
Hz), 3.91−3.87 (2H, m), 1.70 (major, 2.25H, dq, J = 6.0, 1.0 Hz), 1.57
(minor, 0.75H, dq, J = 6.0, 1.0 Hz) 0.33 (minor, 0.75H, s), 0.33
(major, 2.25H, s), 0.32 (minor, 0.75H, s), 0.31 (major, 2.25H, s); 13C
NMR (100 MHz, CDCl3) δ 147.9, 141.0, 138.6, 133.8, 129.5 129.1,
129.0, 128.4, 127.7, 127.6, 127.6, 127.2, 127.0, 83.6, 69.2, 17.8, −2.6,
−2.6; HRMS (ES+) calcd for C21H30NOSi [M + NH4]+ 340.2097,
found 340.2097.
raphy (1:1 CH2Cl2−hexane) to afford the title compound (63 mg,
63%, 81:19 dr) as a yellow oil: Rf (1:1 DCM−hexane) = 0.61; IR νmax
(thin film)/cm−1 2900, 1650, 1147, 900, 865, 735, 444; 1H NMR (400
MHz, CDCl3) δ 9.50 (1H, d, J = 2.5 Hz), 7.53−7.47 (2, m), 7.37−7.33
(3H, m), 5.38−5.19 (2H, m), 2.48 (1H, qd, J = 7.0, 2.5 Hz), 1.35−
1.24 (6H, m), 1.00 (3H, d, J = 7.0 Hz), 0.89 (3H, t, J = 7.0 Hz), 0.36
(3H, s), 0.34 (3H, s); 13C NMR (100 MHz, CDCl3) δ 205.4, 137.8,
133.9, 132.1, 129.1, 128.0, 127.1, 47.0, 35.6, 32.4, 22.1, 14.4, 13.9,
−2.9, −3.8; HRMS (EI+) calcd for C18H28OSi [M]+ 288.1909, found
288.1915.
(2S,3S,E)-3-(Dimethyl(phenyl)silyl)-2,7-dimethyloct-4-enal
(2d). The title compound was prepared from 1d (150 mg, 0.521
mmol). Isomerization was complete in 3 h, and heating for 28 h
achieved the Claisen rearrangement. Purification was performed using
chromatography (1:1 CH2Cl2−hexane) to afford the title compound
(97 mg, 63%, 97:3 dr) as a colorless oil: Rf (1:1 DCM−hexane) = 0.6;
((E)-2-(1-((E)-But-2-en-1-yloxy)cyclohexyl)vinyl)dimethyl-
(phenyl)silane (3b). Alcohol (306 mg, 1.13 mmol) was used to
afford 3b (272 mg, 74%, 85:15 E/Z isomers) as a colorless oil (using
crotyl bromide): Rf (9:1 hexane−ethyl acetate) = 0.81; IR νmax (thin
film)/cm−1 2956, 1600, 1499, 1144, 953, 800, 479; H NMR (400
1
1
IR νmax (thin film)/cm−1 2955, 1699, 1111, 850, 753, 700; H NMR
MHz, CDCl3) δ 7.59−7.53 (2H, m), 7.41−7.37 (3H, m), 6.08 (minor,
0.15H, d, J = 19.3 Hz), 6.07 (major, 0.85H, d, J = 19.3 Hz), 5.98
(minor, 0.15H, d, J = 19.3 Hz), 5.97 (major, 0.85H, d, J = 19.3 Hz),
5.76−5.57 (2H, m),3.88−3.86 (minor, 0.15H, m), 3.76−3.73 (major,
0.85H, m), 1.85−1.76 (2H, m), 1.74 (3H, dd, J = 6.0, 1.0 Hz), 1.70−
1.62 (4H, m), 1.57−1.46 (4H, m), 0.39 (6H, s); 13C NMR (100 MHz,
CDCl3) δ 152.8, 138.8, 133.7, 128.9, 128.7, 128.1, 127.7, 76.8, 62.6,
57.2, 34.3, 26.7, 21.9, 17.8, −2.5; HRMS (ES+) calcd for C20H34NOSi
[M + NH4]+ 332.2410, found 332.2410.
General Procedure B: Isomerization−Claisen Reaction.
Chlorobis(cyclooctene)iridium(I) dimer (1 mol %), tricyclohexyl-
phosphine (6 mol %), and sodium tetraphenylborate (2 mol %) were
measured into a flask and purged with argon. A 20:1 mixture of
chlorobenzene−acetone (0.7 M) was added and the resulting yellow
solution stirred at room temperature for 5 min followed by addition of
the allyl ether 1. The mixture was stirred until completion as judged by
TLC, after which triphenylphosphine (6 mol %) was added and stirred
at room temperature for 10 min. The flask was then fitted with a reflux
condenser and heated to 90 °C. The reaction was concentrated and
chromatographed to afford the requisite aldehyde.
(400 MHz, CDCl3) δ 9.50 (1H, d, J = 2.5 Hz), 7.53−7.47 (2H, m),
7.38−7.31 (3H, m), 5.36−5.20 (2H, m), 2.48 (1H, qd, J = 7.0, 2.5
Hz), 1.98 (1H, dd, J = 9.0, 7.0 Hz), 1.87 (2H, td, J = 6.3, 1.2 Hz),
1.61−1.50 (1H, m), 0.98 (3H, d, J = 7.0 Hz), 0.85 (3H, d, J = 3.0 Hz),
0.84 (3H, d, J = 3.0 Hz), 0.35 (3H, s), 0.35 (3H, s); 13C NMR (100
MHz, CDCl3) δ 205.4, 137.8, 133.9, 130.8, 129.1, 128.2, 127.8, 47.0,
42.2, 35.7, 28.6, 22.3, 22.2, 14.4, −2.8, −3.8; HRMS (EI+) calcd for
C18H28OSi [M]+ 288.1909, found 288.1922.
(2S,3S,E)-5-Cyclohexyl-3-(dimethyl(phenyl)silyl)-2-methyl-
pent-4-enal (2e). The title compound was prepared from 1e (129
mg, 0.412 mmol). Isomerization was complete in 1 h, and heating for
35 h achieved the Claisen rearrangement. Purification was performed
using chromatography (1:1 CH2Cl2−hexane) to afford the title
compound (106 mg, 82%, 96:4 dr) as a colorless oil: Rf (1:1
DCM−hexane) = 0.53; IR νmax (thin film)/cm−1 2936, 1700, 1115,
1
838, 721, 465; H NMR (400 MHz, CDCl3) δ 9.48 (1H, d, J = 2.5
Hz), 7.54−7.46 (2H, m), 7.40−7.31 (3H, m), 5.29−5.16 (2H, m),
2.46 (1H, qd, J = 7.0, 2.5 Hz), 1.93 (1H, dd, J = 7.2, 2.0 Hz), 1.75−
1.59 (5H, m), 1.31−1.00 (6H, m), 0.95 (3H, d, J = 7.0 Hz), 0.34 (3H,
s), 0.32 (3H, s); 13C NMR (100 MHz, CDCl3) δ 205.5, 138.0, 137.8,
134.0, 129.1, 127.8, 124.6, 47.0, 41.0, 35.5, 33.3, 26.1, 26.0, 14.4, −2.9,
−3.7; HRMS (EI+) calcd for C19H27OSi [M − CH3]+ 299.1831, found
299.1841.
(2S,3S)-3-(Dimethyl(phenyl)silyl)-2-methyl-5,5-diphenyl-
pent-4-enal (2f). The title compound was prepared from 1f (93 mg,
0.242 mmol). Isomerization was complete in 2.5 h and heating for 2 h
achieved the Claisen rearrangement. Purification was performed using
chromatography (1:1 CH2Cl2−hexane) to afford the title compound
(80 mg, 86%, 94:6 dr) as a colorless oil which solidified upon standing
in the freezer for 3 days: Rf (1:1 DCM−hexane) = 0.39; IR νmax (thin
film)/cm−1 2957, 1722. 1660, 1597, 1446, 1250, 1112, 764 ; 1H NMR
(400 MHz, CDCl3) δ 9.59 (1H, d, J = 2.5 Hz), 7.44−7.39 (2H, m),
7.36−7.26 (6H, m), 7.25−7.16 (3H, m), 7.15−7.10 (2H, m), 6.98−
6.94 (2H, m), 6.00 (1H, d, J = 12.4 Hz), 2.55 (1H, qdd, J = 6.8, 6.8,
2.5 Hz), 2.32 (1H, dd, J = 12.4, 6.8 Hz), 1.03 (3H, d, J = 6.8 Hz), 0.39
(3H, s), 0.33 (3H, s); 13C NMR (100 MHz, CDCl3) δ 204.8, 142.6,
142.0, 139.8, 137.4, 134.0, 129.9, 129.3, 128.3, 128.1, 127.9, 127.1,
127.0, 126.9, 126.9, 47.9, 33.0, 14.4, −2.5, −3.3; HRMS (EI+) calcd
for C26H27OSi [M − H]+ 383.1831, found 383.1829.
(2S,3S)-3-(Dimethyl(phenyl)silyl)-2,5-dimethylhex-4-enal
(2a). The title compound was prepared from 1a (155 mg, 0.60 mmol).
Isomerization was complete in 6 h, and heating for 8 h achieved the
Claisen rearrangement. Purification was performed using chromatog-
raphy (1:1 CH2Cl2−hexane) to afford the title compound (141 mg,
86%, 96:4 dr) as a colorless oil: Rf (CH2Cl2) = 0.45; IR: νmax (thin
1
film)/cm−1 2925, 1628, 1427, 1251, 1115, 828, 700 ; H NMR (400
MHz, CDCl3) δ 9.46 (1H, d, J = 2.8 Hz), 7.52−7.46 (2H, m), 7.36−
7.31 (3H, m), 4.97 (1H, ddq, J = 11.8, 1.2, 1.2 Hz), 2.42 (1H, qdd, J =
7.0, 7.0, 2.8 Hz), 2.18 (1H, dd, J = 11.8, 7.0 Hz), 1.69 (3H, d, J = 1.2
Hz), 1.46 (3H, d, J = 1.2 Hz), 0.97 (3H, d, J = 7.0 Hz), 0.34 (3H, s),
0.30 (3H, s); 13C NMR (100 MHz, CDCl3) δ 205.4, 138.0, 133.8,
132.1, 129.1, 127.7, 121.7, 47.6, 31.4, 25.8, 18.0, 14.3, −2.9, −3.9;
HRMS (EI+) calcd for C16H24OSi [M]+ 260.1596, found 260.1598.
(2S,3S,E)-3-(Dimethyl(phenyl)silyl)-2-methyl-5-phenylpent-
4-enal (2b). The title compound was prepared from 1b (mg, 0.166
mmol). Isomerization was complete in 45 min, and heating for 1.5 h
achieved the Claisen rearrangement. Purification was performed using
chromatography (1:1 CH2Cl2−hexane) to afford the title compound
(46 mg, 90%, 93:7 dr) as a pale yellow oil: Rf (1:1 DCM−hexane) =
1
0.45; IR νmax (thin film)/cm−1 2898, 1710,1422, 753, 710, 454; H
(2S,3S)-4-Cyclopentylidene-3-(dimethyl(phenyl)silyl)-2-
methylbutanal (2g). The title compound was prepared from 1g (64
mg, 0.224 mmol). Isomerization was complete in 2.5 h, and heating for
11 h achieved the Claisen rearrangement. Purification was performed
using chromatography (1:1 CH2Cl2−hexane) to afford the title
compound (58 mg, 91%, 96:4 dr) as a colorless oil: Rf (1:1 DCM−
hexane) = 0.55; IR νmax (thin film)/cm−1 2957, 1716, 1427, 1115, 832,
NMR (400 MHz, CDCl3) δ 9.58 (1H, d, J = 2.3 Hz), 7.67−7.49 (2H,
m), 7.42−7.35 (3H, m), 7.32−7.25 (4H, m), 7.23−7.18 (1H, m), 6.27
(1H, d, J = 15.8 Hz), 6.10 (1H, dd, J = 15.8, 10.3 Hz), 2.63 (1H, qd, J
= 7.0 2.3 Hz), 2.27 (1H, ddd, J = 10.3, 6.5, 0.5 Hz), 1.09 (3H, d, J =
7.0 Hz), 0.43 (3H, s), 0.40 (3H, s); 13C NMR (100 MHz, CDCl3) δ
204.9, 137.5, 137.3, 134.0, 130.4, 129.4, 128.5, 128.4, 127.9, 126.9,
125.9, 47.1, 36.7, 14.6, −2.8, −3.6; HRMS (EI+) calcd for C20H23OSi
[M − H]+ 307.1518, found 307.1511.
(2S,3S,E)-3-(Dimethyl(phenyl)silyl)-2-methylnon-4-enal (2c).
The title compound was prepared from 1c (100 mg, 0.347 mmol).
Isomerization was complete in 3 h, and heating for 27 h achieved the
Claisen rearrangement. Purification was performed using chromatog-
1
700; H NMR (400 MHz, CDCl3) δ 9.46 (1H, d, J = 2.4 Hz), 7.52−
7.48 (2H, m), 7.36−7.31 (3H, m), 5.10 (1H, dt, J = 11.5, 2.3 Hz), 2.42
(1H, qdd, J = 6.8, 6.8, 2.4 Hz), 2.25−2.15 (2H, m), 2.10−2.00 (1H,
m), 2.03 (1H, dd, J = 11.5, 6.8 Hz), 1.90−1.78 (1H, m), 1.62−1.47
(4H, m), 0.98 (3H, d, J = 6.8 Hz), 0.36 (3H, s), 0.31 (3H, s);13C NMR
(100 MHz, CDCl3) δ 205.6, 114.3, 138.1, 133.9, 127.7, 129.0. 127.7,
2061
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The Journal of Organic Chemistry
Note
117.2, 47.7, 33.7, 33.2, 29.2, 26.3, 26.2, 14.4, −3.0, −3.8; HRMS (EI+)
calcd for C18H26OSi [M]+ 286.1753, found 286.1730.
126.1 (major), 125.8 (minor), 125.7 (major), 124.3 (minor), 47.8
(major), 47.8 (minor), 32.8 (major), 31.5 (minor), 16.3 (major), 14.5
(major), 14.3 (minor), 11.7 (minor),-2.6 (minor), −2.8 (major), −3.5
(minor),-3.6 (major); HRMS (EI+) calcd for C21H30NOSi 340.2097,
found 340.2093.
(2S,3S)-4-Cyclohexylidene-3-(dimethyl(phenyl)silyl)-2-meth-
ylbutanal (2h). The title compound was prepared from 1h (164 mg,
0.58 mmol). Isomerization was complete in 4 h, and heating for 5 h
achieved the Claisen rearrangement. Purification was performed using
chromatography (1:1 CH2Cl2−hexane) to afford the title compound
(141 mg, 86%, 97:3 dr) as a colorless oil: Rf (1:1 DCM−hexane) =
(2S,3S,E)-3-(Dimethyl(phenyl)silyl)-2,5,7-trimethyloct-4-enal
(2l). The title compound was prepared from 1l (157 mg, 0.520 mmol).
Isomerization was complete in 45 min, and heating at 40 °C for 5 h
achieved the Claisen rearrangement. Purification was performed using
chromatography (1:1 CH2Cl2−hexane) to afford the title compound
(137 mg, 87%, 98: 2 dr, 56:44 E/Z isomers) as a pale yellow oil: Rf
(1:1 DCM−hexane) = 0.43; IR νmax (thin film)/cm−1 2977, 1700.
1
0.43; IR νmax (thin film)/cm−1 2930, 1642, 1380, 1259, 1051, 749; H
NMR (400 MHz, CDCl3) δ 9.47 (1H, d, J = 2.5 Hz), 7.53−7.46 (2H,
m), 7.36−7.31 (3H, m), 4.91 (1H, d, J = 11.8 Hz), 2.43 (1H, qdd, J =
7.0, 7.0, 2.5 Hz), 2.23 (1H, dd, J = 11.8, 7.0 Hz), 2.11−2.03 (3H, m),
1.96−1.88 (1H, m), 1.56−1.25 (6H, m), 0.97 (3H, d, J = 7.0 Hz), 0.35
(3H, s), 0.31 (3H, s); 13C NMR (100 MHz, CDCl3) δ 205.6, 140.4,
138.0, 133.9, 129.0, 127.7, 118.3, 47.6, 37.4, 30.2, 28.9, 28.6, 27.4, 26.8,
14.3, −2.9, −3.8; HRMS (EI+) calcd for C18H25OSi [M − CH3]+
285.1675, found 285.1685.
1
1662, 1177, 764, 477; H NMR (400 MHz, CDCl3) δ 9.51 (major,
0.56H, d, J = 2.6 Hz), 9.50 (minor, 0.44H, d, J = 2.6 Hz), 7.55−7.50
(2H, m), 7.41−7.33 (3H, m), 5.08 (minor, 0.44H, dq, J = 11.8, 1.0
Hz), 4.99 (major, 0.56H, dq, J = 11.8, 1.2 Hz), 2.50−2.40 (1H, m),
2.29 (minor, 0.44, dd, J = 11.8, 7.2 Hz), 2.24 (major, 0.56H, dd, J =
11.8, 7.2 Hz), 1.92−1.70 (3H, m), 0.99 (3H, d, J = 6.8 Hz), 0.91−0.80
(6H, m), 0.37 (major, 1.68H, s), 0.36 (minor, 1.32H, s), 0.34 (major,
1.68H, s), 0.33 (minor, 1.32H, s); 13C NMR (100 MHz, CDCl3) δ
205.5 (minor), 205.4 (major), 138.0 (minor), 135.4 (major), 135.3
(minor), 133.9 (minor), 133.9 (major), 129.1 (minor), 129.1 (major),
127.9 (major), 127.8 (minor), 123.3 (minor), 122.9 (major), 49.8,
47.9 (minor), 47.7 (major), 41.2, 31.6 (minor), 31.4 (major), 30.8,
26.3 (minor), 26.2 (major), 23.7 (major), 23.3 (minor), 22.6 (minor),
22.5 (major), 22.4 (major), 22.0 (minor), 16.3, 14.4 (major), 14.4
(minor), 14.1, −2.7 (minor), −2.8 (major), −3.8 (minor), −3.8
(major); HRMS (EI+) calcd for C19H30OSiNa [M + Na]+ 325.1964,
found 325.1966.
Ethyl 4-((2S,3S)-2-(Dimethyl(phenyl)silyl)-3-methyl-4-
oxobutylidene)piperidine-1-carboxylate (2i). The title com-
pound was prepared from 1i (104 mg, 0.279 mmol). Isomerization
was complete in 24 h, and heating for 15 h achieved the Claisen
rearrangement. Purification was performed using chromatography (1:1
CH2Cl2−hexane) to afford the title compound (82 mg, 79%, 93.5:6.5
dr) as a colorless oil: Rf (1:1 EtOAc−hexane = 0.8; IR νmax (thin film)/
1
cm−1 1700, 1614, 1428. 1236, 1063, 898, 826; H NMR (400 MHz,
CDCl3) δ 9.50 (1H, J = 2.5 Hz), 7.50−7.45 (2H, m), 7.40−7.35 (3H,
m), 5.07 (1H, d, J = 11.9 Hz), 4.12 (2H, q, J = 7.0 Hz), 3.39−3.21
(3H, m), 3.14−3.01 (1H, m), 2.49 (1H, td, J = 7.0, 2.5 Hz), 2.22 (1H,
dd, J = 11.9, 7.0 Hz), 2.15−2.04 (3H, m), 1.97−1.88 (1H, m), 1.25
(3H, t, J = 7.0 Hz), 1.02 (3H, d, J = 7.0 H) 0.36 (3H, s), 0.31 (3H, s);
13C NMR (100 MHz, CDCl3) δ 204.9, 155.4, 137.6, 135.4, 133.8,
129.3, 127.8, 121.4, 61.2, 47.5, 45.6, 44.2, 30.5, 14.7, 14.3, −3.3, −3.4;
HRMS (EI+) calcd for C20H28O3NSi [M − CH3]+ 358.1838, found
358.1841.
(2S,3S,E)-3-(Dimethyl(phenyl)silyl)-2-ethyl-5-phenylpent-4-
enal (4a). The title compound was prepared from a 75:25 (E/Z)
mixture of 3a (60 mg, 0.184 mmol). Isomerization was complete in 36
h, and heating for 6 h achieved the Claisen rearrangement. Purification
was performed using chromatography (1:1 CH2Cl2−hexane) to afford
the title compound (31 mg, 51%, 97:3 dr) as a yellow oil: Rf (1:1
DCM−hexane) = 0.53 ; IR νmax (thin film)/cm−1 2825, 1721, 1427,
(2S,3S)-4-(Dihydro-2H-pyran-4(3H)-ylidene)-3-(dimethyl-
(phenyl)silyl)-2-methylbutanal (2j). The title compound was
prepared from 1j (38 mg, 0.126 mmol). Isomerization was complete
in 2.5 h, and heating for 5 h achieved the Claisen rearrangement.
Purification was performed using chromatography (1:1 CH2Cl2−
hexane) to afford the title compound (32 mg, 84%, 91:9 dr) as a
yellow oil: Rf = 0.15; IR νmax (thin film)/cm−1 3100, 2955, 1600, 1522,
1
1112, 831, 735; H NMR (400 MHz, CDCl3) δ 9.54 (1H, d, J = 3.3
Hz), 7.52−7.49 (2H, m), 7.39−7.33 (3H, m), 7.31−7.25 (4H, m),
7.22−7.17 (1H, m), 6.23 (1H, d, J = 15.8 Hz), 6.08 (1H, dd, J = 15.8,
10.5 Hz), 2.41−2.33 (1H, m), 2.25 (1 H, dd, J = 10.5, 6.3 Hz), 1.73−
1.59 (1H, m), 1.50−1.39 (1H, m), 0.83 (3H, t, J = 7.4 Hz), 0.40 (3H,
s), 0.38 (3H, s); 13C NMR (100 MHz, CDCl3) δ 205.0, 137.6, 137.3,
134.0, 130.5, 129.3, 128.5, 128.0, 127.9, 126.9, 125.9, 54.1, 34.9, 30.0,
22.3, 11.7, −3.0, −3.7; HRMS (ES+) calcd for C21H26OSiNa [M +
Na]+ 345.1651, found 345.1652.
1
1000, 827, 411; H NMR (400 MHz, CDCl3) δ 9.52 (1H, d, J = 2.5
Hz), 7.52−7.47 (2H, m), 7.39−7.33 (3H, m), 5.04 (1H, d, J = 11.8
Hz), 3.65−3.47 (3H, m), 3.34 (1H, ddd, J = 10.9, 6.8, 4.1 Hz), 2.5
(1H, td, J = 6.8, 2.5 Hz), 2.22 (1H, dd, J = 11.8, 6.8 Hz), 2.20−2.10
(3H, m), 2.00 (1H, qdd, J = 7.0, 4.1, 1.0 Hz), 1.03 (3H, d, J = 7.0 Hz),
0.38 (3H, s), 0.35 (3H, s); 13C NMR (100 MHz, CDCl3) δ 205.0,
137.7, 135.0, 133.9, 129.3, 127.8, 120.4, 69.6, 68.3, 47.5, 37.2, 30.3,
29.9, 14.4, −3.2, −3.4 ; HRMS (EI+) calcd for C17H24O2Si [M −
CH3]+ 288.1546, found 288.1542.
(2S,3S,E)-3-(Dimethyl(phenyl)silyl)-2-methyl-5-phenylhex-4-
enal (2k). The title compound was prepared from 1k (115 mg, 0.356
mmol mmol). Isomerization was complete in 1.5 h and heating at 40
°C for 2 h achieved the Claisen rearrangement. Purification was
performed using chromatography (1:1 CH2Cl2−hexane) to afford the
title compound (98 mg, 85%, 94:6 dr, 80:20 E/Z isomers) as a
colorless oil: Rf (1:1 DCM−hexane) = 0.45; IR νmax (thin film)/cm−
2961, 1722, 1380, 1025, 823, 699, 471 ; 1H NMR (400 MHz, CDCl3)
δ 9.55 (major, 0.8H, d, J = 2.5 Hz), 9.48 (minor, 0.2H, d, J = 2.5 Hz)
7.55−7.50 (major, 1.6H, m), 7.41−7.28 (6H, m), 7.25−7.17 (2H, m),
6.94−6.90 (minor, 0.4H, m), 5.62 (minor, 0.8H, qd, J = 12.0, 1.4
Hz),5.34 (0.2H, qd, J = 12.0, 1.4 Hz), 2.57 (1H, qd, J = 7.0, 2.5 Hz),
2.42 (1H, dd, J = 12.0, 7.0 Hz), 2.00 (minor, 0.6H, d, J = 1.4 Hz), 1.85
(major, 2.4H, d, J = 1.4 Hz), 1.05 (major, 2.6H, d, J = 7.0 Hz), 0.94
(minor, 0.6H, d, J = 7.0 Hz), 0.42 (major, 2.4H, s), 0.37 (major, 2.4H,
s), 0.33 (minor, 0.6H, s), 0.28 (minor, 0.6H, s); 13C NMR (100 MHz,
CDCl3) δ 205.3 (minor), 205.0 (major), 143.9 (major), 142.0
(minor), 137.7 (minor), 135.2 (major), 134.0 (minor), 133.9 (major),
129.3 (major), 129.2 (minor), 128.2 (major), 128.2 (minor),127.9
(major), 127.8 (major), 127.8 (minor),126.6 (major),126.4 (minor),
(2S,3S)-4-Cyclohexylidene-3-(dimethyl(phenyl)silyl)-2-ethyl-
butanal (4b). The title compound was prepared from a 85:15 (E/Z)
mixture of 3b (128 mg, 0.431 mmol). Isomerization was complete in
1.5 h, and heating for 1 h achieved the Claisen rearrangement.
Purification was performed using chromatography (1:1 CH2Cl2−
hexane) to afford the title compound (119 mg, 93%, 97:3 dr) as a
colorless oil: Rf (1:1 DCM−hexane) = 0.56 ; IR νmax (thin film)/
1
cm−12929, 1709, 1427, 1115, 830, 700, 471; H NMR (400 MHz,
CDCl3) δ 9.46 (1H, d, J = 3.5 Hz), 7.53−7.47 (2H, m), 7.38−7.32
(3H, m), 4.90 (1H, d, J = 11.8 Hz), 2.31 (1H, dd, J = 11.8, 6.3 Hz),
2.26−2.20 (1H, m), 1.92 (2H, qd, J = 7.2, 5.0 Hz), 1.64−1.26 (10H,
m), 0.79 (3H, t, J = 7.2 Hz), 0.36 (3H, s), 0.32 (3H, s); 13C NMR
(100 MHz, CDCl3) δ 205.6, 140.3, 138.0, 133.9, 129.1, 127.7, 117.9,
54.7, 37.5, 28.3, 27.5, 26.9, 22.0, 11.7, −3.1, −3.8; HRMS (ES+) calcd
for C20H34NOSi [M + NH4]+ 332.2410, found 332.2420.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra of all reported compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
2062
dx.doi.org/10.1021/jo202560g | J. Org. Chem. 2012, 77, 2058−2063

The Journal of Organic Chemistry
Note
Org. Lett. 2007, 9, 2325. (e) Kerrigan, N. J.; Bungard, C. J.; Nelson, S.
G. Tetrahedron 2008, 64, 6863.
(21) A slow Claisen rearrangement occurs at room temperature;
however, because of the prolonged reaction times (3 days), the
aldehyde diastereoselectivity is very poor.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: m.cook@qub.ac.uk.
(22) (a) Hirsch, J. A. Top. Stereochem. 1967, 3, 199. (b) Jensen, F. R.;
Bushweller, C. H. Adv. Alicycl. Chem. 1971, 3, 139. (c) Eliel, E. L.;
Wilen, S. H. Stereochemistry of Organic Compounds; Wiley: New York,
1993; p 696. (d) Eliel, E. L.; Wilen, S. H.; Doyle, M. P. Basic Organic
Stereochemistry; Wiley: New York, 2001; p 443.
ACKNOWLEDGMENTS
■
This work was generously supported by Queen’s University
Belfast, The Department of Employment and Learning,
Northern Ireland (a studentship to M.G.M.), and the
Engineering and Physical Research Council (EPSRC) (Grant
No. EP/I006605/1). We also gratefully acknowledge Glax-
oSmithKline and AstraZeneca for the generous donation of
equipment.
REFERENCES
■
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