Unusual reaction of allylic systems: homo- and cross-cyclizations leading to four-carbon rings

Article abstract of DOI:10.1016/j.tet.2009.01.016

The reaction of stabilized allylic-type carbocations with electron rich olefins was investigated. In most cases a facile cyclobutane ring formation was observed by [2+2] cycloaddition reaction promoted by Br?nsted or Lewis acids. Some attempts were made t

Full text of DOI:10.1016/j.tet.2009.01.016

Tetrahedron 65 (2009) 2351–2355
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Unusual reaction of allylic systems: homo- and cross-cyclizations leading
to four-carbon rings
*
Renata Siedlecka
Department of Chemistry, Faculty of Organic Chemistry, Wrocław University of Technology, Wyb. Wyspian´skiego 27, 50-370 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of stabilized allylic-type carbocations with electron rich olefins was investigated. In most cases
a facile cyclobutane ring formation was observed by [2þ2] cycloaddition reaction promoted by Brønsted or
Lewis acids. Some attempts were made to investigate the mechanistic pathway. After de-racemization the
chiral cyclobutane containing diols were obtained as a potential chiral platform for new ligands.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 23 September 2008
Received in revised form 4 December 2008
Accepted 5 January 2009
Available online 14 January 2009
1. Introduction
[a-acetoxy-benzyl]-cyclobutane 5a (60% yield, 5b <5%) with all
groups tethered to the cyclobutane ring in trans positions.
Syntheses of the cyclobutane system have gained remarkable
importance in the synthesis of natural products and other com-
pounds of biological significance.1,2 The principal strategies for
their synthesis are based on [2þ2] photochemical or catalytic cy-
cloaddition, 1,4-cyclization of acyclic precursors and a ring expan-
sion of cyclopropanes.1,3 Cationic [2þ2] cycloadditions employing
simple allylic compounds for the preparation of cyclobutanes are
not often represented, though that pathway has a biosynthetic
origin.4 The literature reported relevant examples,5 and mostly the
reactions of enol ethers6 or silyl enol ethers,7 vinyl acetals8 or
A similar dimerization of 1 in the presence of HBr has already
been observed by Staab and Kurmeier in 1968.11 To the best of our
knowledge, this observation was neglected for the next 40 years
with no published further investigation nor application of this re-
action. Spectral properties of 5a were in agreement with those al-
ready reported. Its all-trans geometry was proved by chemical
correlation with 3
-truxilic acid (derivative of natural alkaloid).11
Additionally, the 1H NMR spectrum of 5a in the presence of Eu(hfc)3
revealed the splitting of OCOCH3 signal (Dd¼0.06 ppm), that
corresponds with equimolar amounts of two enantiomers (R,R) and
(S,S); no meso form has been observed. The reaction can be con-
sidered formally as the ionic [2þ2] cycloaddition with the allylic
system. The use of dichloromethane as a solvent decreased the
selectivity keeping the chemical yield almost unchanged. Thus,
along with the all-trans product 5a (27%), its stereoisomer 5b was
isolated (30%). The geometry of this isomer was established using
1H NMR. The signal from the o-protons of the phenyl ring in the
cis-position to the acetoxy groups was considerably shifted to the
higher field. The significant effect of cyclobutane proton differ-
entiation was also clearly seen. This observed homo-dimerization
prompted us to examine the corresponding cross-cyclization.
Thus, several olefins were reacted with 1 under the same condi-
tions (Fig. 2). The reactions gave products of [2þ2] cycloaddition
(see Table 1) together with the products of homo-cyclization
(5aþ5b).
allylsilanes9,10 with olefins or
a,b-unsaturated carbonyl compounds
were exploited for this purpose.
2. Results and discussion
During our recent work dealing with allylic alcohols, we have
found an unusual reaction leading to the formation of the four-
carbon ring system. The treatment of 1,3-diphenyl-2-propen-1-ol
(1) with acetic anhydride in the presence of catalytic sulfuric acid,
instead of the expected allylic acetate furnished the head-to-tail
dimer (Fig. 1). The product was identified as 1,3-diphenyl-2,4-bis-
OAc
Ph
OAc
Ph
H+, Ac2O
OH
Ph
Ph
Ph
Ph
Ph
Ph
Ph
5b
Ph
5a
OAc
OAc
1
R1
R2
H+, Ac2O
OH
Ph
Figure 1.
R2
Ph
+
R1
Ph
Ph
2
1
OAc
5-8
* Corresponding author. Tel.: þ48 71 320 3224; fax: þ48 71 328 4064.
E-mail address: renata.siedlecka@pwr.wroc.pl
Figure 2.
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.01.016

2352
R. Siedlecka / Tetrahedron 65 (2009) 2351–2355
Table 1
D
OAc
Ph
D
D
Cyclization of 1 with different olefins 2
Ph
H
D
H+
Ac2O
DO
D
2
R1, R2
Producta
Yieldb (%)
Ph
AcO
Ph
Ph
Ph
Ph
Ph
a
b
c
R1¼Ph, R2¼CH(OH)Ph
5
6
7
8
65
40
50
30
11
R1¼R2¼Ph
D
12
R1¼Ph, R2¼H
d
R1¼C(CH3)¼CH2, R2¼H
Figure 5.
a
Products of cross-cyclization were obtained together with 5aþ5b.
b
Yields of isolated compounds.
molecule. The newly formed carbocation 3 can cyclize to the
cyclobutane ring 4 via the second addition stage. The ensuing
acetylation gives the final product (5–8). It seems that the re-
action stereoselectivity would be determined in the second ad-
dition step.
In order to check briefly the scope and limitations the other
acids have been applied for generation of allyl cations in the homo-
cyclization reaction. As summarized in Table 2, the Lewis acids
(except TiCl4) appeared to be useful and gave 5 in even better yield
as compared to sulfuric acid. However, the stereoselectivity was
slightly lower in those cases.
We have also tested the homo-dimerization with 3-phenyl-2-
propen-1-ol (13) as a substrate. However, in this case only the first
stage of the described process (Scheme 1) took place. The benzyl
carbocation corresponding to 3 (see Scheme 1, R1¼Ph, R2¼CH2OH),
obtained after the first addition seems to be quenched with acetic
acid faster then the cyclization occurs. The corresponding product
Ph
OH
H+, Ac2O
OMe
+
OMe
Ph
Ph
Ph
1
9
Figure 3.
The stereochemistry of 6, the main product of the cross-re-
action with trans-stilbene, was defined on the basis of 1H NMR
also as all-trans. The formation of the second isomer was also
observed (less then 10% by NMR), but it could not be isolated as
a pure compound. The geometry of other products (7 and 8) was
not established. Beside 5a all the obtained cyclobutane derivatives
were hitherto unknown. In the case of norbornene the cycload-
dition did not occur and solely the product of esterification of 1
was isolated (63%).
(2-(a-acetoxy-benzyl)-5-phenylpent-4-enyl acetate, 14) was iso-
lated as a mixture of two diastereoisomers (2:1, 30% yield) next to
cinnamyl acetate (Fig. 6).
In experiments designed to gain information about the mech-
anism of this process, we first attempted to detect the reaction
intermediate. The addition of anisole (1 equiv) to the reaction
mixture (Fig. 3) resulted in the formation of the typical product of
electrophilic allylation 912 (65%) along with 5 (20%). This result
suggests the formation of carbocation during the reaction pathway.
Also the reaction of 1 with 3,4-dihydro-2H-pyran, well known as
a ‘trapping agent’ for carbocations, in the same conditions led to
compound 10, being the product of allylation (Fig. 4). This result
supported our conclusion about carbocation formation in this
reaction.
Although compound 5a was obtained as a racemate, it seemed
interesting because of its rigid structure and well defined ge-
ometry (all-trans) around the cyclobutane ring. We reacted the
racemic diol (15), obtained after the hydrolysis of 5a, with (R)-O-
methylatrolactic acid13 used as a resolving agent (Scheme 2). The
respective enantiomeric diols gave monoesters: 16a and 16b,
along with bisester 17a (formed much faster than isomer 17b), all
separable by \column chromatography. After the hydrolysis of
separated mono- and bisesters it was possible to isolate the
enriched (þ)- and (ꢀ)-isomers of (2,4-diphenyl-cyclobutane-1,3-
diyl)bis(phenyl-methanol)-15. In the case of (þ)-15 obtained from
16a the ee was 85% whereas from 17a, the ee was 95% (con-
firmed by HPLC). Thus, this esterification allows for the sub-
stantial enantiomeric enrichment of the isolated products. Diols
(15) offer a promising chiral platform for the construction of new
chiral ligands and further investigations in this direction are
underway.
We have also made an experiment with the deuterated alcohol
11, obtained by LiAlD4 reduction of chalcone (Fig. 5).
In the 1H NMR spectrum of obtained product 12 we observed
decreasing by half an integration of signals from all protons at the
benzylic positions (two from cyclobutane ring (m, 3.00–3.07 ppm)
and two close to the acetyl groups (d, 5.85 ppm)). It suggests that
before the cyclization the delocalized allylic carbocation has been
formed.
It should be noted that there is no evidence to suggest, which
process is faster: carbocation formation or acetylation of allyl
alcohol. To answer this question we used the corresponding allyl
acetate as a substrate for the dimerization. The results were
similar to those obtained with alcohol 1 itself, namely the yield
was slightly better, but the selectivity was worse (51% 5a, 34%
5b). A plausible stepwise mechanism for the [2þ2] cycloaddition
with allylic carbocation is depicted in Scheme 1. In the first step
the formation of allylic cation takes place (fast process), followed
by the addition reaction to the double bond of the second
3. Conclusion
The cationic [2þ2] cycloaddition with allylic system generated
in the presence of Brønsted or Lewis acid was investigated. Several
new cyclobutane derivatives were obtained as the products of
homo- or cross-cyclization. The stepwise mechanism involving the
carbocation formation followed by its addition to the double bond
and the final cyclization was proposed. The sequential esterification
with the resolving acid followed by hydrolysis allowed for the
separation of racemic diol 15.
Ph
Ph
Ph
Ph
H
- H+
+
O
O
Ph
O
Ph
O
Ph
Ph
10
Figure 4.

R. Siedlecka / Tetrahedron 65 (2009) 2351–2355
2353
R1
R2
Ph
R2
R1
R2
Ph
R1
R2
R1
OH
Ph
H+
Ac2O
- AcOH
2
Ph
Ph
Ph
Ph
Ph
Ph
- H2O
Ph
1
OAc
3
4
5
Scheme 1. Postulated mechanism of cyclization.
NaOH and extracted with dichloromethane. The organic solution
was dried (Na2SO4 anhyd) and evaporated in vacuo. The product
was isolated after column chromatography (silica gel; AcOEt/
hexane, 1:5 v/v).
Table 2
Application of different acids as catalysts in homo-dimerization of 1
Substrate
Acid
Yielda (%)
5a
5b
4.2. Analytical data of obtained products
1
1
1
1
1
H2SO4
SnCl4
BF3$Et2O
TiCl4
65
70
82
28
5b
<5b
20
14
4.2.1. all-trans-1,3-Diphenyl-2,4-bis-[a-acetoxy-benzyl]-
cyclobutane (5a)
21
White crystals, mp 167–170 ꢁC (toluene); Rf (AcOEt/hexane, 1:5)
0.42; dH (300 MHz, CDCl3): 1.76 (s, 6H, CH3), 2.91–2.97 (m, 2H, CH),
3.00–3.07 (m, 2H, CH), 5.92 (d, 2H, J 6.8 Hz, CH), 7.01 (dd, 4H, J 6.6,
1.5 Hz, ArH), 7.08–7.13 (m, 16H, ArH); dC (300 MHz, CDCl3): 20.9,
42.7, 51.8, 77.5, 126.2, 127.1, 127.2, 128.0, 128.1, 128.2, 137.6, 141.9,
170.3; nmax (KBr): 3063, 3030, 2932, 2917, 1734, 1496, 1456, 1369,
1248, 1024, 756, 696; HRMS (ESI, MeOH): [MþNa]þ calcd for
C34H32O4Na: 527.21928, found: 527.22183.
p-TsOH
7b
a
Yields of isolated compounds.
b
Yields calculated from the 1H NMR spectrum.
Ph
AcO
Ph
OAc
14
Figure 6.
4.2.2. cis,trans,trans-1,3-Diphenyl-2,4-bis-[a-acetoxy-benzyl]-
cyclobutane (5b)
White crystals, mp 140–142 ꢁC (EtOH); Rf (AcOEt/hexane, 1:5)
0.37; dH (300 MHz, CDCl3): 1.81 (s, 6H, CH3), 2.91–2.93 (m, 3H, CH),
3.13–3.18 (m,1H, CH), 5.89 (d, 2H, J 5.7 Hz), 6.87 (dd, 2H, J 7.5,1.8 Hz,
ArH), 7.04–7.26 (m, 18H, ArH); dC (300 MHz, CDCl3): 20.9, 41.4, 44.0,
51.9, 77.4, 126.1, 126.4, 127.0, 127.1, 127.2, 127.4, 128.0, 128.2, 128.3,
137.7, 141.4, 142.4, 170.2; nmax (KBr): 3086, 3059, 3027, 3005, 2932,
1745, 1723, 1495, 1457, 1249, 1235, 1024, 760, 696; HRMS (ESI,
MeOH): [MþNa]þ calcd for C34H32O4Na: 527.21928, found:
527.22127.
4. Experimental part
Commercially available reagents and solvents were used except
the (R)-O-methylatrolactic acid. Separation of products by chro-
matography was performed on silica gel 60 (230–400 mesh) pur-
chased from Fluka. Melting points are uncorrected. 1H and 13C NMR
spectra of samples in CDCl3 or DMSO were recorded on Bruker
Avance DRX (1H, 300 MHz) spectrometer, using TMS as an internal
standard. IR spectra were recorded on a Perkin Elmer 1600 FTIR
spectrometer. The high-resolution mass spectra (HRMS) were
recorded on a Mariner PE Biosystems unit. Observed rotations at
589 nm were measured using an Optical Activity Ltd Model AA-5
automatic polarimeter.
4.2.3. all-trans-Phenyl(2,3,4-triphenylcyclobutyl)methyl acetate (6)
White solid, mp 108–110 ꢁC (hexane); Rf (AcOEt/hexane, 1:5)
0.44; dH (300 MHz, CDCl3): 1.88 (s, 3H, CH3), 3.13 (ddd, 1H, J 6.9, 9.3,
9.3 Hz, CH), 3.30 (t,1H, J 9.3 Hz, CH), 3.44 (t,1H, J 9.3 Hz, CH), 3.53 (t,
1H, J 9.3 Hz, CH), 6.09 (d, 1H, J 6.9, CH), 7.05–7.32 (m, 20H, ArH); dC
(300 MHz, CDCl3): 21.1, 46.9, 48.1, 51.2, 52.5, 78.1,126.4,126.6,126.6,
126.9, 127.2, 127.2, 127.3, 128.1, 128.3, 128.3, 128.5, 128.5, 138.0,
142.0, 142.5, 142.6, 170.3; nmax (KBr): 3083, 3061, 3027, 2934, 1733,
1494, 1229, 1026, 769, 699; HRMS (ESI, MeOH): [MþNa]þ calcd for
C31H28O2Na: 455.1982, found: 455.1965.
4.1. General procedure of cyclization
To a solution of 2 (3 mmol) in 3 mL of acetic anhydride a few
drops of concd H2SO4 (w2%) was added. Then the allylic alcohol 1
(3 mmol) was added in portions and the mixture was stirred
(temp <20 ꢁC) for 2 h. A strong coloration (mostly red) was ob-
served during the reaction. After that time the reaction mixture
was poured into the ice–water mixture, neutralized with 1 M
4.2.4. (2,3-Diphenylcyclobutyl)(phenyl)methyl acetate (7)
Oil, mixture of two stereoisomers (65%:35%); Rf (AcOEt/hexane,
1:5) 0.45; dH (300 MHz, CDCl3): 2.02 (s, 65% 3H, CH3), 2.04 (s, 35%
OCOR*
Ph
OH
OCOR*
Ph
Ph
Ph
Ph
Ph
i
ii
5a
+
Ph
Ph
Ph
Ph
Ph
Ph
OCOR*
OH
OH
rac-15
16a + 16b
17a + 17b
i
i
(-)-15
(+)-15
Scheme 2. (i) LiOH, MeOH, reflux, 3 h; (ii) RꢂCOOH, DCC, CH2Cl2, rt, 5 days.

2354
R. Siedlecka / Tetrahedron 65 (2009) 2351–2355
3H, CH3), 2.23–2.32 (m, 1H, CHAHB), 2.48–2.58 (m, 1H, CHAHB),
3.42–3.49 (m, 1H, CH), 5.65–5.70 (m, 35% 1H, CH), 5.71–5.76 (m,
65% 1H, CH), 6. 31(d, 1H, J 7.2 Hz, CH–O), 6.34–6.41 (m, 1H. CH),
7.24–7.36 (m, 15H, ArH); dC (300 MHz, CDCl3): 65% 21.3, 35% 21.4,
65% 42.1, 35% 42.2, 45.9, 74.6, 76.3, 126.4, 126.8, 126.9, 127.4, 127.7,
127.8, 128.2, 35% 128.6, 65% 128.7, 65% 128.9, 35% 128.9, 35% 130.0,
65% 130.2, 35% 133.0, 65% 133.2, 65% 137.3, 35% 137.4, 140.6, 65%
143.2, 35% 143.4, 170.2; nmax (CCl4): 3086, 3065, 3031, 2937, 1741,
1495, 1454, 1371, 1237, 1026, 965; HRMS (ESI, MeOH): [MþNa]þ
calcd for C25H24O2Na: 379.1669, found: 379.1665.
2880,1601,1496,1454,1297,1053,1007, 754, 696; HRMS (ESI, MeOH):
[MþNa]þ calcdforC30H28O2Na:443.19815,found:443.19766;(þ)-15:
20
[a]
þ33 (c 0.5, CHCl3, 95% ee); HPLC: tR¼12.2 (Chiracel OD-H, hex-
D
20
ane–i-PrOH, 9:1, v/v); (ꢀ)-15: [
a]
ꢀ17.7 (c 0.23, CHCl3, 48% ee);
D
HPLC: tR¼16.8 (Chiracel OD-H, hexane–i-PrOH, 9:1, v/v).
4.3. Esterification of rac-15
Preparation of the diastereomeric esters with (R)-O-methyla-
trolactic acid was carried out using DCC-DMAP procedure accord-
ing to the literature.14 Diol 15 (0.51 g, 1.2 mmol) was added in one
portion to the solution of (R)-O-methylatrolactic acid (0.4 g,
2.2 mmol) and DCC (0.5 g, 2.4 mmol) in dichloromethane (10 mL),
followed by the addition of DMAP (30 mg, 0.24 mmol). The turbid
solution was stirred at room temperature for 5 days. After that time
the reaction mixture was diluted with diethyl ether (saturated with
water) and stirred for 15 min. The precipitated solid was filtered off
and the solution was evaporated to gave the oily product. Mono-
esters 16a, 16b and bisesters 17a, 17b were isolated by column
chromatography (SiO2, AcOEt/hexane¼1:3, v/v).
4.2.5. Phenyl(2-phenyl-4-(prop-1-en-2-yl)cyclobutyl)methyl
acetate (8)
Oil, equimolar mixture of two stereoisomers; Rf (AcOEt/hexane,
1:5) 0.47; dH (300 MHz, CDCl3): 1.72 (s, 3H, CH3), 1.95 (s, 3H, CH3),
2.08–2.13(m, 1H, CHAHB), 2.22–2.25 (m, 1H, CHAHB), 2.54–2.56 (m,
1H, CH), 3.62–3.70 (m, 2H, 2ꢃCH), 4.49 (br d, 2H, J 6.5 Hz, CH2),
5.74–5.78 (m, 1H, CHOAc), 7.21–7.42 (m, 10H, ArH); dC (300 MHz,
CDCl3): 21.3, 21.3, 21.4, 21.4, 39.8, 40.1, 41.8, 46.8, 46.9, 76.3, 79.7,
110.7, 126.4, 126.6, 128.3, 128.4, 128.7, 130.1, 130.2, 140.1, 140.2, 141.3,
141.6, 147.0, 170.2; nmax (CCl4): 3085, 3064, 3029, 2934, 1741, 1495,
1453, 1367, 1233, 1029, 964; HRMS (ESI, MeOH): [MþK]þ calcd for
C22H24O2K: 359.1408, found: 359.1598.
4.3.1. Monoester 16a
20
Crystallizing oil, Rf (AcOEt/hexane, 1:3) 0.29; [
a]
þ21 (c 1.2,
D
CHCl3, 85% ee); dH (300 MHz, CDCl3): 1.54 (s, 3H, CH3), 1.88 (br s, 1H,
OH), 2.71 (ddd, 1H, J 6.0, 9.0, 9.0 Hz, CH), 2.86 (ddd, 1H, J 6.0, 9.1,
9.1 Hz, CH), 3.00–3.06 (m, 1H, CH), 3.10–3.15 (m, 1H, CH), 3.17 (s, 3H,
CH3), 4.59 (d, 1H, J 6.0 Hz, CH–O), 5.88 (d, 1H, J 5.7 Hz, CH–O), 6.87–
7.34 (m, 25H, ArH); dC (300 MHz, CDCl3): 21.1, 41.8, 51.7, 51.9, 54.3,
76.4, 78.0, 81.4, 126.0, 126.1, 126.2, 126.4, 126.6, 127.3, 127.5, 127.7,
127.8, 127.9, 128.0, 128.1, 128.1, 128.3, 137.5, 140.6, 141.8, 142.4, 142.5,
172.0; nmax (film): 3487, 3086, 3062, 3029, 3004, 2987, 2935, 2856,
1739, 1603, 1496, 1455, 1247, 1138, 1115, 1074, 1050, 911, 759, 697,
548; HRMS (ESI, MeOH): [MþNa]þ calcd for C40H38O4Na: 605.2662,
found: 605.2676.
4.2.6. 1,3-Diphenyl-1-(4-methoxyphenyl)prop-2-ene (9)
Oil, Rf (AcOEt/hexane, 1:5) 0.47; dH (300 MHz, CDCl3): 3.80 (s, 3H,
CH3), 4.86 (d,1H, J 7.5 Hz, CH), 6.34 (d,1H, J 15.9 Hz, CH), 6.67 (dd,1H,
J 15.9, 7.5 Hz, CH), 6.87 (d, 2H, J 8.7 Hz, ArH), 7.16 (d, 2H, J 8.7 Hz, ArH),
7.23–7.42 (m,10H, ArH); dC (300 MHz, CDCl3): 53.5, 55.4,114.0,126.5,
126.6, 127.5, 128.7, 128.7, 128.8, 129.8, 131.4, 133.1, 135.8, 137.5, 144.0,
158.3; In agreement with data reported in the literature.12
4.2.7. (E)-5-(1,3-Diphenylallyl)-3,4-dihydro-2H-pyran (10)
Oil, Rf (AcOEt/hexane, 1:5) 0.67; dH (300 MHz, CDCl3): 1.86–1.92
(m, 4H, 2ꢃCH2), 3.97 (t, 2H, J 5.0 Hz, CH2), 4.05 (d, 1H, J 6.9 Hz, CH),
6.40 (d, 1H, J 15.9 Hz, CH), 6.44 (s, 1H, CH), 6.51 (dd, 1H, J 15.9,
6.9 Hz, CH), 7.24–7.44 (m, 10H, ArH); dC (300 MHz, CDCl3): 22.4,
22.7, 52.3, 65.6, 114.7, 126.3, 126.5, 127.3, 128.4, 128.5, 128.6, 131.0,
131.4, 137.5, 141.5, 142.5; nmax (CCl4): 3083, 3062, 3028, 2971, 2948,
2931, 2895, 1661, 1494, 1150, 967, 700; HRMS (EI): [Mþ] calcd for
C20H20O: 276.15142, found: 276.15210.
4.3.2. Bisester 17a
White solid, mp 137–138.5 ꢁC (toluene/hexane); Rf (AcOEt/
20
hexane, 1:3) 0.41; [
a]
þ10.3 (c 0.76, CHCl3, 95% ee); dH (300 MHz,
D
CDCl3): 1.38 (s, 6H, CH3), 2.80 (br s, 4H, CH), 3.08 (s, 6H,
OCH3),5.69 (d, 2H, J 2.7 Hz, CH–O), 6.78 (d, 4H, J 6.6 Hz, ArH), 6.93–
7.27 (m, 26H, ArH); dC (300 MHz, CDCl3): 20.7, 42.5, 51.7, 52.2, 78.2,
81.1, 126.2, 126.6, 127.4, 127.8, 127.9, 128.1, 128.2, 137.1, 140.3, 141.6,
171.7; nmax (KBr): 3092, 3059, 3041, 3029, 2992, 2955, 2830, 1719,
1601, 1497, 1449, 1227, 1137, 1112, 944, 775, 753, 698, 551; HRMS
(ESI, MeOH): [MþNa]þ calcd for C50H48O6Na: 767.3343, found:
767.3374.
4.2.8. 2-(a-Acetoxy-benzyl)-5-phenylpent-4-enyl acetate (14)
Oil, mixture of two diastereoisomers (2:1), Rf (AcOEt/hexane,
1:5) 0.41; dH (300 MHz, CDCl3): 2.01 (s, 1/3 3H, CH3), 2.03 (s, 2/3 3H,
CH3), 2.08 (s, 2/3 3H, CH3), 2.11 (s, 1/3 3H, CH3), 2.14–2.21 (m, 1H,
CHAHB), 2.32–2.39 (m, 2H, CHAHB, CH), 3.85 (dd, 1/3 1H, J 11.4,
5.4 Hz, CHAHB), 4.05–4.12 (m 1H, CHAHB), 4.30 (dd, 2/3 1H, J 11.4,
5.4 Hz, CHAHB), 5.83 (d, 2/3 1H, J 7.5 Hz, CH), 5.89 (d, 1/3 1H, J 6.3 Hz,
CH), 6.03–6.18 (m, 1H, CH), 6.32 (d, 2/3 1H, J 15.5 Hz, CH), 6.39 (d, 1/
3 1H, J 15.6 Hz, CH), 7.20–7.39 (m, 10H, ArH); dC (300 MHz, CDCl3):
1/3 20.9, 2/3 21.0, 21.2, 1/3 31.0, 2/3 31.5, 2/3 43.3, 1/3 43.4, 2/3 62.8,
1/3 63.5, 2/3 75.4, 1/3 75.5, 126.1, 126.7, 126.8, 127.1, 127.3, 1/3 128.2,
2/3 128.3, 128.6, 1/3 132.3, 2/3 132.6, 2/3 137.3, 1/3 137.3, 2/3 138.7,
1/3 138.8, 2/3 170.0, 1/3 170.1, 1/3 170.9, 2/3 171.0; nmax (film): 3082,
3061, 3029, 2956, 1740, 1496, 1451, 1370, 1236, 1026, 967, 745, 701;
HRMS (ESI, MeOH): [MþNH4]þ calcd for C22H28O4N: 370.2013,
found: 370.2008.
Acknowledgements
I am grateful to Dr. Rafa1 Kowalczyk of this Laboratory for
a sample of (R)-O-methylatrolactic acid.
References and notes
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Whitesolid,mp198–200 ꢁC(toluene);Rf (AcOEt/hexane,1:3)0.15;
dH (300 MHz, CDCl3): 2.00 (br s, 2H, OH), 2.78 (ddd, 2H, J 5.4, 8.7,
8.7 Hz, CH), 3.35 (t, 2H, J 8.7 Hz, CH), 4.84 (d, 2H, J 5.4 Hz, CH), 6.99–
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