Claisen rearrangement

Tetrahedron 70 (2014) 8908e8913

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Synthesis of vinylcyclopropanes by allylation/ring-closing metathesis/Claisen rearrangement Meng-Yang Chang *, Yi-Chia Chen, Chieh-Kai Chan Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 807, Taiwan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 July 2014 Received in revised form 22 September 2014 Accepted 26 September 2014 Available online 2 October 2014

A facile double allylation/ring-closing metathesis/Claisen rearrangement route for preparing vinylcyclopropanes 6 is developed. The efficient synthesis includes O-allylation of a-allyl-a-sulfonylketones 8 with allylic bromides, ring-closing metathesis of diallyl compounds 9 and sequential Claisen rearrangement of the resulting oxepines. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Vinylcyclopropane a-Sulfonyl ketone Claisen rearrangement

1. Introduction Vinylcyclopropane (VCP), a small-ring core in modern synthetic organic chemistry, is important as it can undergo various synthetically useful conversions and act as an active three-membered ring to achieve many cycloadditions.1 Transition metals, Pd(0) or Rh(I), also force VCP with different substituents to undergo different reactions.2,3 Besides transition metal-catalyzed reactions, thermaland radical-mediated reactions of VCPs are also very important in synthesis.4 VCPs in many cases can be important synthetic intermediates in complex natural product synthesis.5 For VCP analogs, 1-amino-2-vinyl-cyclopropanecarboxylic acid (1) with the combination of VCP and a-amino acid is increasingly drawing interest because it is a fragment of BILN 2061 (2) under evaluation as a potent anti-hepatitis C agent.6 Furthermore, many natural products also include the unit of VCP, such as trans-chrysanthemic acid (3)7a and ambruticin S (4),7b as shown in Fig. 1. In recent years, Craig’s efforts have focused on synthesizing skeleton 5 with the combination of VCP with a sulfonyl group via the key transannular, decarboxylative Claisen rearrangement.8a Other synthetic utilities of sulfonyl cyclopropanes have been reported with a CeC bond formation, such as conjugated addition of substituted a-sulfonyl enones.8bee Herein, the article presents a two-step, efficient and stereocontrolled synthesis of chimera 6 bearing the combination of VCP and a-sulfonyl arylketone.

* Corresponding author. Tel.: þ886 7 3121101x2220; e-mail addresses: [email protected], [email protected] (M.-Y. Chang). http://dx.doi.org/10.1016/j.tet.2014.09.085 0040-4020/Ó 2014 Elsevier Ltd. All rights reserved.

Me HO2C

S NH

N

MeO

N

H N Me 2

Me

H N

O

H N

N O

HO O

OH

Me

O

O

Me 3

2

Me

Me HO

HO2C Me

Me CO2H

O

O

Me O

Tol BILN 2061 (1)

S

Me

4

O 5

R1

O

Ar O R

S

O

6

Fig. 1. Representative structures with a VCP fragment.

2. Results and discussion As shown in Scheme 1, the synthesis of skeleton 6 started by identifying a-sulfonyl ketone 7 as a readily available substrate. aSulfonyl ketone (b-ketosulfone) constitutes a diverse structural class associated with important synthetic and medicinal applications.9 By the double allylation (C-/O-) of skeleton 7 or mono-Oallylation of skeleton 8, two allyl substituents of skeleton 9 were formed via the base-mediated regioselective allylation. On the other hand, ring-closing metathesis of diallyl skeleton 9 with Grubbs 2nd catalyst, followed by spontaneous Claisen rearrangement of the corresponding oxepines 10, was another key step. Using the synthetic sequence, skeleton 6 was provided in good yields,

M.-Y. Chang et al. / Tetrahedron 70 (2014) 8908e8913

and its quaternary C1 center and tertiary C2 center were constructed stereoselectively under mild reaction conditions.10,11 O Ar O

S

R

allylation O

LiTMP

K2CO3 allylBr

ring-closing metathesis Claisen rearrangement R1 R1 O O O Grubbs II Ar Ar R1

8 Ar

R1

O

O

Br

Ar

S

R

O

S

R

O O

R

S

O O

9 O

1

2

S

O

R

10

6

7

Scheme 1. Synthetic route toward 6.

a-Benzoyl methanesulfone (7a, Ar¼Ph, R¼Me, prepared by nucleophilic substitution of 2-bromoacetophenone and MeSO2NaeH2O), was selected as the model substrate to achieve optimal allylation. Initially, mono-allylation of 7a with 1.05 equiv of allyl bromide and K2CO3 in acetone at reflux afforded sole 8a with a Callylated group in a 92% yield. Furthermore, treatment of 7a with excess allyl bromide (5 equiv) in the presence of K2CO3 in boiling acetone afforded a mixture of O- and C-diallyl products 9a (28%) and 11a (52%) with E- and Z-isomers and C-/C-diallyl product 12a (14%), as shown in Table 1. 11a was isolated as the major component due to the less repulsion between the phenyl and sulfonyl group via a plausible trans-configured transition state. When this reaction condition was changed to room temperature, no allyl product was isolated (entry 2). Entry 3 shows that a major unknown mixture (w40%) replaced the diallyl products via the NaNH2-mediated double allylation in refluxing THF. They should be amine or imine products from the reaction of ketone and free NH3 in the reaction. From the results, we found that reaction temperature and basicity of the base are key factors affecting the distribution of diallyl products. With this idea in mind, three lithium amides (LDA, LiHMDS and LiTMP) served as the bases for determining for the optimal reaction conditions at lower temperatures (0  C or 78  C). For LDA and LiHMDS, the yield ratio of 11a and 9a was observed in a range of 2/1e3/1 (entries 4e9). For entries 10e12, LiTMP gave major results of 9a (85%, 67% and 73%). Table 1 Optimization of reaction conditionsa O

O base, allylBr Ph

Ph O

S

Me 7a

conditions O

O Me

O Ph

S

O 9a

O Ph O S Me O S O Me O

11a

12a

O Ph O Me

O

8a (92%, K2CO3 1 eq of allylBr)

Entry

Base (eq),b solvent, temp ( C)

Yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12

K2CO3 (5), acetone, 56 K2CO3 (5), acetone, 25 NaNH2 (5), THF, 66d LDA (3), THF, 78 LDA (3), THF, 0 LDA (5), THF, 0 LiHMDS (3), THF, 78 LiHMDS (3), THF, 0 LiHMDS (5), THF, 78 LiTMP (3), THF, 78 LiTMP (3), THF, 0 LiTMP (5), THF, 78

18 62 No reactionc 18 16 26 52 32 45 35 48 22 64 23 60 20 66 85 <5 67 12 73 15

9a

S

11a

12a 12 20 14 14 10 8 10 8 Trace 12 w5

a The one-pot reaction was run on a 1.0 mmol scale with starting material 7a and allyl bromide (1.0 M in THF, 5 equiv, 5 mmol) in THF or acetone (10 mL) for 16 h. b Isolated yield. c >95% of 7a was recovered. d 40% of unknown mixture was isolated.

8909

For a possible reaction mechanism, 8a was first generated from the enolate of 7a via C-allylation, as shown in Scheme 2.12 Using the enolate of 8a (generated in situ from a different base), two intermediates of E-enolate and Z-enolate could be formed. For the Callylation of the enolate of 8a, the stronger steric hindrance of tertiary carbanion should inhibit the formation of 12a. Hence, Oallylation is a major pathway for the resulting enolate of 8a after the C-allylation of the enolate of 7a. From the experimental results (see Table 1), we envisioned that LiTMP forced the formation of the Eenolate via less repulsion between the sulfonyl group and 2,2,6,6tetra-methyl substituents. Therefore, LiTMP-mediated deprotonation provided a possible syn-configuration between sulfonyl and phenyl group via the six-membered transition state A. The major 9a was isolated at 78  C or 0  C via O-allylation of the E-enolate ion of 8a. Conversely, for LDA or LiHMDS-mediated deprotonation of 8a, intermediate B was preferred to generate the Z-enolate via less repulsion between the sulfonyl and phenyl group and between sulfonyl and two trimethylsilyl (TMS) or isopropyl (IPA) substituents. 11a was isolated as a major product at 78  C and 0  C via the O-allylation of the Z-enolate ions of 8a. Adjusting the equivalents of the bases, temperatures and solvents, entry 10 showed that LiTMP should be the optimal base to form the desired 9a with good yields.

LiTMP-mediated O-allylation less repulsion O

Me S O Ph

O Me H N Li Me Me Me

A

LDA or LiHMDS-mediated O-allylation

stronger repulsion Me O

S

O

less repulsion Ph Ph

O S O H Me O Li

Ph O

S O-Li O Me N less TMS TMS repulsion (IPA) (IPA) B E-enolate Z-enolate O-Li

Scheme 2. Possible reaction mechanism.

However, 9a was converted to complex products at rt in the presence of a CDCl3 solution. As shown in Scheme 3, pure E-9a with stronger repulsion was easily transformed into a mixture of Z-11a and 8a at rt within 1 h under the CDCl3 solution via an E-/Zisomerization between intermediates C and D, followed by sequential hydrolysis. The ratio of Z-11a and 8a was determined as 1:2 by the 1H NMR analysis of allylic CH protons. It is also very likely that the trace acid in CDCl3 catalyzed the isomerization of enol ether E-9a. To prevent this unexpected result, 9a was immediately reacted with a Grubbs II catalyst. By screening different reaction conditions, the stereoisomer 6a was isolated in a 91% yield in refluxing (CH2Cl)2 for 1 h via one-pot tandem RCM/Claisen rearrangement. No isolation of dihydro-oxepine 10a was found during the cascade process.13 A reversible Claisen rearrangement process of 6a to 10a was not observed. Zhang reported an atom-economic synthesis of bicyclic VCP by the Rh NHC-catalyzed tandem [5þ2] cycloaddition/Claisen rearrangement of vinylic oxiranes with alkynes.11a However, there have been no reports on the utilization of a one-pot RCM/Claisen rearrangement cascade route as the construction of a VCP core structure. The resulting framework of cyclic intermediate 10a exhibited the characteristics of an electrondonating oxygen atom and an electron-withdrawing sulfonyl group with the electronic ‘push-pull’ nature. Based on these phenomena, one-pot conversion from 7a to 6a was examined next (see Scheme 4). Following the prepared protocol, 6a was isolated in a 55% yield of two-step via LiTMPmediated double allylation in THF followed by treatment of the

8910

M.-Y. Chang et al. / Tetrahedron 70 (2014) 8908e8913

O Ph O

S

Me

bond rotation Ph

O CDCl3

Ph O S

O

O

Me

D

C

O

(CH2Cl)2

O Me

O 9a

O O

S

Me

10a

6a (91%)

Ar O

resulting 9a with Grubbs 2 catalyst in THF. Regarding the formation of 6a with a lower yield, the main reason could be that THF inhibits the RCM efficiency of intermediate 10a.14

LiTMP (3 eq), THF,

O

S

Me

O

Br(2 eq), -78oC to rt, 16h

O Ph O

then Grubbs II (2.3 mol%), THF, reflux, 2h

S

Me

7a

O

6a (55%)

Scheme 4. One-pot synthesis of 6a.

Under optimal reaction conditions, a series of skeleton 6 was prepared via the two-step route (see Table 2). For different Ar and R substituents on the skeleton 7, no influence on the formation of products 6aeo was observed (entries 1e15). The provided yields were distributed in a range of 70e84%. According to the above experimental results, we inferred that the present transformation was tolerant of tethers incorporating electron-donating and electron-withdrawing aromatic functionalities. To extend this twostep protocol, 8b and 8ced were chosen as the starting materials (see Scheme 5). 6p with a methylvinyl (isopropene) group was isolated in a 70% yield via LiTMP-mediated alkylation of 8b with 2-methylallyl bromide, followed by ring-closing metathesis of the resulting diallyl product. Changing the a-substituent of Table 2 Two-step synthesis of 6aec O 1) LiTMP, THF,

Ar O

S R O

Br

2) Grubbs II, (CH2Cl)2

7

O Ar O

S R O 6

Entry

7, Ar¼, R¼,

6, yield¼(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

7a, Ph, Me 7b, 4-PhPh, Me 7c, Ph, Ph 7d, Ph, 4-MePh 7e, Ph, 3-MePh 7f, Ph, 4-FPh 7g, Ph, 4-nBuPh 7h, Ph, 4-tBuPh 7i, 4-FPh, 4-MePh 7j, 2,5-(MeO)2Ph, 4-MePh 7k, 4-NO2Ph, 4-MePh 7l, 3-NO2Ph, 4-MePh 7m, 4-MePh, 4-MePh 7n, 4-PhPh, 4-MePh 7o, 4-MeOPh, 4-MePh

6a, 78 6b, 75 6c, 72 6d, 84 6e, 79 6f, 75 6g, 78 6h, 80 6i, 82 6j, 80 6k, 70 6l, 72 6m, 82 6n, 76 6o, 75

a The reaction was run on the 1.0 mmol scale with 7, LiTMP (3 equiv), allyl bromide (1 equiv and then 2 equiv) at 78  C. b The reaction was run on the resulting product with Grubbs II catalyst (20 mg, 2.3 mmol%) in (CH2Cl)2 (10 mL) at reflux. c Isolated yields.

Me

O

S

2) Grubbs II, (CH2Cl)2

O 6p (70%)

Me O 1) LiTMP, THF,

S

Me

O

O

nd

O

O

Ph

Br

8b

Me

O

Scheme 3. Reaction of 9a.

Ph

S

Ph S

O

Me 1) LiTMP, THF,

O

O

Ph S

2-butadienyl cyclopropane group were found in a range of 72e74% yields, respectively, via the above methodology.

Ph

O

Grubbs II

O Ph O

8a + Z-11a

O S Me O

more repulsion

E-9a

Me

b-ketosulfone from allyl to propargyl group, two 6q and 6r with the

O

Br

Ar O

2) Grubbs II, (CH2Cl)2

8c, Ar = Ph 8d, Ar = 4-PhPh

Me

S

O 6q (72%) 6r (74%)

Scheme 5. Synthesis of 6per.

3. Conclusion In summary, we have described the double allylation/ringclosing metathesis/Claisen rearrangement route for preparing a series of substituted vinylcyclopropanes 6. The facile synthetic route begins with simple starting materials and reagents, and provides a potential methodology for chemical biology research. 4. Experimental section 4.1. General All other reagents and solvents were obtained from commercial sources and used without further purification. Reactions were routinely carried out under an atmosphere of dry air or nitrogen with magnetic stirring. Products in organic solvents were dried with anhydrous MgSO4 before concentration in vacuo. Melting points were determined with a SMP3 melting apparatus. 1H and 13C NMR spectra were recorded on a Varian INOVA-400 spectrometer operating at 400 and at 100 MHz, respectively. Chemical shifts (d) are reported in parts per million (ppm) and the coupling constants (J) are given in Hertz. High resolution mass spectra (HRMS) were measured with a mass spectrometer Finnigan/Thermo Quest MAT 95XL. Elemental analyses were carried out with Heraeus Vario IIINCSH, Heraeus CHN-OS-Rapid Analyzer or Elementar Vario EL III. A representative procedure of skeleton 6 is as follows: n-BuLi (1.6 M in hexane, 3.8 mL, 6.0 mmol) was added to a solution of TMPHBr (666 mg, 3.0 mmol) in THF (6 mL) at 78  C. After 30 min, a solution of skeleton 7 (1.0 mmol) in THF (2 mL) was added to at 78  C. The reaction mixture was stirred at 78  C for 30 min. A solution of allyl bromide (125 mg, 1.0 mmol) in THF (1 mL) was added to the resulting reaction mixture at 78  C. After 1 h, a solution of allyl bromide (250 mg, 2.0 mmol) in THF (1 mL) was added to the resulting reaction mixture at 78  C. The reaction mixture was stirred at 78  C for 16 h, warmed to rt, and the solvent was concentrated. The residue was diluted with water (10 mL) and the mixture was extracted with CH2Cl2 (330 mL). The combined organic layers were washed with brine, dried, filtered and evaporated to afford crude product. Without further purification, Grubbs secondary catalyst (20 mg, 2.3 mmol%) was added to the resulting diallyl compounds in 1,2-dichloroethane (10 mL) at rt. The reaction mixture was stirred at reflux for 2 h, cooled to rt, and the solvent was concentrated. The residue was diluted with water (10 mL) and the mixture was extracted with CH2Cl2 (320 mL). The combined organic layers were washed with brine, dried, filtered and evaporated to afford crude product. Purification on silica gel (hexanes/ EtOAc¼10/1e3/1) afforded skeleton 6.

M.-Y. Chang et al. / Tetrahedron 70 (2014) 8908e8913

4.1.1. Compound (6a). Yield¼78% (195 mg); Colorless gum; HRMS (ESI, Mþþ1) calcd for C13H15O3S 251.0742, found 251.0749; 1H NMR (400 MHz, CDCl3): d 8.03e8.01 (m, 2H), 7.61e7.59 (m, 1H), 7.49e7.46 (m, 2H), 5.36e5.27 (m, 1H), 5.17e5.08 (m, 2H), 3.02 (s, 3H), 2.80e2.74 (m, 1H), 2.16 (dd, J¼6.0, 9.6 Hz, 1H), 1.68 (dd, J¼5.2, 6.4 Hz, 1H); 13C NMR (100 MHz, CDCl3): d 191.71, 135.69, 134.21, 132.79, 130.67 (2), 128.39 (2), 119.15, 52.86, 40.13, 28.68, 17.53. 4.1.2. Compound (6b). Yield¼75% (245 mg); Colorless solid; mp 134e136  C (recrystallized from hexanes and EtOAc); HRMS (ESI, Mþþ1) calcd for C19H19O3S 327.1055, found 327.1053; 1H NMR (400 MHz, CDCl3): d 8.11 (d, J¼8.4 Hz, 2H), 7.69 (d, J¼8.0 Hz, 2H), 7.64e7.62 (m, 2H), 7.49e7.39 (m, 3H), 5.37e5.33 (m, 1H), 5.21e5.10 (m, 2H), 3.03 (s, 3H), 2.85e2.79 (m, 1H), 2.16 (dd, J¼6.0, 9.6 Hz, 1H), 1.71 (t, J¼6.4 Hz, 1H); 13C NMR (100 MHz, CDCl3): d 191.02, 146.92, 139.56, 134.28, 132.89, 131.39 (2), 128.96 (2), 128.46, 127.31 (2), 126.99 (2), 119.16, 52.88, 40.07, 28.58, 17.52; Anal. Calcd for C19H18O3S: C, 69.91; H, 5.56. Found: C, 70.12; H, 5.73. 4.1.3. Compound (6c). Yield¼72% (225 mg); Colorless gum; HRMS (ESI, Mþþ1) calcd for C18H17O3S 313.0899, found 313.0902; 1H NMR (400 MHz, CDCl3): d 7.91e7.89 (m, 2H), 7.63e7.56 (m, 4H), 7.48e7.41 (m, 4H), 5.38e5.34 (m, 1H), 5.16e5.05 (m, 2H), 3.13e3.07 (m, 1H), 2.08 (dd, J¼6.0, 9.6 Hz, 1H), 1.74 (dd, J¼6.0, 7.2 Hz, 1H); 13C NMR (100 MHz, CDCl3): d 190.58, 138.41, 135.90, 133.88 (2), 132.85, 130.80 (2), 129.04 (2), 128.39 (2), 128.08 (2), 119.34, 55.47, 30.26, 18.01. 4.1.4. Compound (6d). Yield¼84% (274 mg); Colorless gum; HRMS (ESI, Mþþ1) calcd for C19H19O3S 327.1055, found 327.1061; 1H NMR (400 MHz, CDCl3): d 7.92 (d, J¼8.0 Hz, 2H), 7.60e7.56 (m, 1H), 7.49e7.46 (m, 2H), 7.44e7.40 (m, 2H), 7.24 (d, J¼8.0 Hz, 2H), 5.37e5.33 (m, 1H), 5.15e5.04 (m, 2H), 3.11e3.05 (m, 1H), 2.41 (s, 3H), 2.06 (dd, J¼6.0, 9.6 Hz, 1H), 1.70 (dd, J¼6.0, 7.2 Hz, 1H); 13C NMR (100 MHz, CDCl3): d 190.71, 144.93, 135.94, 135.48, 133.81, 132.98, 130.83 (2), 129.68 (2), 128.39 (2), 128.02 (2), 119.16, 55.56, 30.16, 21.63, 17.95. 4.1.5. Compound (6e). Yield¼79% (258 mg); Colorless gum; HRMS (ESI, Mþþ1) calcd for C19H19O3S 327.1055, found 327.1059; 1H NMR (400 MHz, CDCl3): d 7.89e7.87 (m, 2H), 7.59e7.55 (m, 1H), 7.44e7.32 (m, 6H), 5.37 (dd, J¼2.0, 15.2 Hz, 1H), 5.17e5.06 (m, 2H), 3.13e3.06 (m, 1H), 2.34 (s, 3H), 2.07 (dd, J¼6.0, 9.6 Hz, 1H), 1.74 (dd, J¼6.0, 7.2 Hz, 1H); 13C NMR (100 MHz, CDCl3): d 190.64, 139.24, 138.23, 136.01, 134.69, 133.77, 132.88, 130.72 (2), 128.88, 128.74, 127.99 (2), 125.48, 119.28, 55.68, 30.25, 21.22, 17.93. 4.1.6. Compound (6f). Yield¼75% (248 mg); Colorless gum; HRMS (ESI, Mþþ1) calcd for C18H16FO3S 331.0804, found 331.0812; 1H NMR (400 MHz, CDCl3): d 7.93e7.90 (m, 2H), 7.64e7.57 (m, 3H), 7.46e7.42 (m, 2H), 7.17e7.12 (m, 2H), 5.38e5.32 (m, 1H), 5.16e5.06 (m, 2H), 3.10e3.03 (m, 1H), 2.08 (dd, J¼5.6, 9.6 Hz, 1H), 1.74 (dd, J¼6.0, 7.2 Hz, 1H); 13C NMR (100 MHz, CDCl3): d 190.52, 165.89 (d, J¼255.5 Hz), 135.79, 134.04, 134.02, 132.73, 131.27 (d, J¼9.9 Hz, 2), 130.79 (2), 128.16 (2), 119.52, 116.41 (d, J¼22.8 Hz, 2), 55.47, 30.28, 18.10. 4.1.7. Compound (6g). Yield¼78% (287 mg); Colorless gum; HRMS (ESI, Mþþ1) calcd for C22H25O3S 369.1524, found 369.1530; 1H NMR (400 MHz, CDCl3): d 7.91e7.88 (m, 2H), 7.59e7.54 (m, 1H), 7.50e7.48 (m, 2H), 7.47e7.39 (m, 2H), 7.24 (d, J¼8.0 Hz, 2H), 5.38e5.33 (m, 1H), 5.16e5.04 (m, 2H), 3.12e3.05 (m, 1H), 2.67e2.64 (m, 2H), 2.06 (dd, J¼6.0, 9.6 Hz, 1H), 1.72 (dd, J¼6.0, 7.2 Hz, 1H), 1.64e1.56 (m, 2H), 1.39e1.30 (m, 2H), 0.93 (t, J¼7.2 Hz, 3H); 13C NMR (100 MHz, CDCl3): d 190.71, 149.74, 135.98, 135.63, 133.75,

8911

132.97, 130.76 (2), 129.00 (2), 128.40 (2), 128.00 (2), 119.17, 55.59, 35.57, 32.99, 30.19, 22.23, 17.93, 13.82. 4.1.8. Compound (6h). Yield¼80% (294 mg); Colorless solid; mp 118e120  C (recrystallized from hexanes and EtOAc); HRMS (ESI, Mþþ1) calcd for C22H25O3S 369.1524, found 369.1528; 1H NMR (400 MHz, CDCl3): d 7.89e7.86 (m, 2H), 7.58e7.50 (m, 3H), 7.46e7.38 (m, 4H), 5.38e5.34 (m, 1H), 5.17e5.05 (m, 2H), 3.13e3.06 (m, 1H), 2.06 (dd, J¼6.0, 9.6 Hz, 1H), 1.73 (dd, J¼6.0, 7.2 Hz, 1H), 1.32 (s, 9H); 13C NMR (100 MHz, CDCl3): d 190.75, 157.72, 136.04, 135.47, 133.72, 132.97, 130.71 (2), 128.24 (2), 128.00 (2), 126.03 (2), 119.20, 55.55, 35.20, 30.99 (3), 30.28, 17.82; Anal. Calcd for C22H24O3S: C, 71.71; H, 6.56. Found: C, 72.01; H, 6.43. 4.1.9. Compound (6i). Yield¼82% (282 mg); Colorless gum; HRMS (ESI, Mþþ1) calcd for C19H18FO3S 345.0961, found 345.0966; 1H NMR (400 MHz, CDCl3): d 7.87 (d, J¼8.8 Hz, 2H), 7.46 (d, J¼8.4 Hz, 2H), 7.39 (d, J¼8.4 Hz, 2H), 7.25 (d, J¼8.8 Hz, 2H), 5.39e5.32 (m, 1H), 5.11e5.01 (m, 2H), 3.13e3.07 (m, 1H), 2.40 (s, 3H), 2.01 (dd, J¼6.0, 9.6 Hz, 1H), 1.67 (dd, J¼5.6, 6.8 Hz, 1H); 13C NMR (100 MHz, CDCl3): d 189.55, 145.08, 140.47, 135.26, 134.16, 132.75, 132.24 (2), 129.73 (2), 128.40 (2), 128.27 (2), 119.46, 55.50, 30.15, 21.60, 17.85. 4.1.10. Compound (6j). Yield¼80% (309 mg); Colorless gum; HRMS (ESI, Mþþ1) calcd for C21H23O5S 387.1266, found 387.1271; 1H NMR (400 MHz, CDCl3): d 7.43 (d, J¼8.4 Hz, 2H), 7.19 (d, J¼8.0 Hz, 2H), 6.97 (dd, J¼3.2, 8.8 Hz, 1H), 6.88 (d, J¼3.2 Hz, 1H), 6.74 (d, J¼9.2 Hz, 1H), 5.35e5.30 (m, 2H), 5.14e5.10 (m, 1H), 3.71 (s, 3H), 3.67 (s, 3H), 3.07e3.01 (m, 1H), 2.39 (s, 3H), 2.08 (dd, J¼6.0, 9.6 Hz, 1H), 2.00 (dd, J¼6.0, 7.2 Hz, 1H); 13C NMR (100 MHz, CDCl3): d 191.28, 152.65, 152.58, 144.27, 136.66, 132.60, 129.33 (2), 128.27 (2), 127.34, 119.94, 118.56, 114.94, 112.45, 57.93, 55.62, 55.57, 33.60, 21.51, 18.78. 4.1.11. Compound (6k). Yield¼70% (260 mg); Pale yellowish solid; mp 80e82  C (recrystallized from hexanes and EtOAc); HRMS (ESI, Mþþ1) calcd for C19H18NO5S 372.0906, found 372.0912; 1H NMR (400 MHz, CDCl3): d 8.24 (d, J¼8.8 Hz, 2H), 7.88 (d, J¼9.2 Hz, 2H), 7.62 (d, J¼8.4 Hz, 2H), 7.28 (d, J¼8.0 Hz, 2H), 5.96e5.88 (m,1H), 5.34e5.25 (m, 2H), 5.23e5.16 (m, 1H), 3.27 (dd, J¼10.4, 14.8 Hz, 1H), 2.91 (dd, J¼8.4, 14.8 Hz, 1H), 2.41 (s, 3H); 13C NMR (100 MHz, CDCl3): d 159.98, 148.83, 144.31, 137.99, 135.12, 134.47, 130.69 (2), 129.80 (2), 126.98 (2), 122.82 (2), 118.35, 112.98, 82.68, 37.69, 21.52; Anal. Calcd for C19H17NO5S: C, 61.44; H, 4.61. Found: C, 61.68; H, 4.87. 4.1.12. Compound (6l). Yield¼72% (267 mg); Colorless gum; HRMS (ESI, Mþþ1) calcd for C19H18NO5S 372.0906, found 372.0905; 1H NMR (400 MHz, CDCl3): d 8.61 (t, J¼2.0 Hz, 1H), 8.41 (ddd, J¼0.8, 2.0, 8.0 Hz, 1H), 8.27 (dt, J¼2.0, 8.0 Hz, 1H), 7.66 (t, J¼8.0 Hz, 1H), 7.44 (d, J¼8.4 Hz, 2H), 7.26 (d, J¼8.0 Hz, 2H), 5.48e5.42 (m, 1H), 5.14e5.04 (m, 2H), 3.22e3.16 (m, 1H), 3.41 (s, 3H), 2.05 (dd, J¼5.6, 9.2 Hz, 1H), 1.76 (dd, J¼6.0, 7.2 Hz, 1H); 13C NMR (100 MHz, CDCl3): d 189.17, 147.87, 145.44, 137.07, 136.48, 135.09, 132.26, 129.92 (2), 129.30, 128.17 (2), 127.81, 125.33, 120.35, 55.71, 30.68, 21.61, 17.76. 4.1.13. Compound (6m). Yield¼82% (279 mg); Colorless gum; HRMS (ESI, Mþþ1) calcd for C20H21O3S 341.1212, found 341.1215; 1H NMR (400 MHz, CDCl3): d 7.85 (d, J¼8.4 Hz, 2H), 7.49 (d, J¼8.4 Hz, 2H), 7.26e7.22 (m, 4H), 5.37e5.33 (m, 1H), 5.14e5.03 (m, 2H), 3.11e3.05 (m, 1H), 2.42 (s, 3H), 2.41 (s, 3H), 2.02 (dd, J¼6.0, 9.6 Hz, 1H), 1.67 (dd, J¼6.0, 7.2 Hz, 1H); 13C NMR (100 MHz, CDCl3): d 190.02, 144.99, 144.84, 135.45, 133.41, 133.15, 131.08 (2), 129.63 (2), 128.76 (2), 128.35 (2), 118.97, 55.42, 29.95, 21.77, 21.60, 17.89. 4.1.14. Compound (6n). Yield¼76% (306 mg); Colorless gum; HRMS (ESI, Mþþ1) calcd for C25H23O3S 403.1368, found 403.1371; 1H NMR

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M.-Y. Chang et al. / Tetrahedron 70 (2014) 8908e8913

(400 MHz, CDCl3): d 8.02 (d, J¼8.0 Hz, 2H), 7.68e7.63 (m, 4H), 7.54e7.46 (m, 4H), 7.43e7.39 (m, 1H), 7.27 (d, J¼8.0 Hz, 2H), 5.40e5.36 (m, 1H), 5.18e5.06 (m, 2H), 3.15e3.09 (m, 1H), 2.42 (s, 3H), 2.07 (dd, J¼5.6, 9.6 Hz, 1H), 1.72 (dd, J¼6.0, 7.2 Hz, 1H); 13C NMR (100 MHz, CDCl3): d 190.10, 146.46, 144.95, 139.66, 135.40, 134.58, 133.08, 131.52 (2), 129.70 (2), 128.92 (2), 128.39 (2), 128.36, 127.28 (2), 126.66 (2), 119.19, 55.50, 30.12, 21.64, 17.93. 4.1.15. Compound (6o). Yield¼75% (267 mg); Colorless gum; HRMS (ESI, Mþþ1) calcd for C20H21O4S 357.1161, found 357.1163; 1H NMR (400 MHz, CDCl3): d 7.95 (d, J¼8.8 Hz, 2H), 7.49 (d, J¼8.4 Hz, 2H), 7.25 (d, J¼8.4 Hz, 2H), 6.91 (d, J¼8.8 Hz, 2H), 5.37e5.31 (m, 1H), 5.12e5.02 (m, 2H), 3.88 (s, 3H), 3.11e3.04 (m, 1H), 2.41 (s, 3H), 2.00 (dd, J¼6.0, 9.6 Hz, 1H), 1.63 (dd, J¼6.0, 7.2 Hz, 1H); 13C NMR (100 MHz, CDCl3): d 188.56, 164.25, 144.85, 135.41, 133.57, 133.38 (2), 129.65 (2), 128.87, 128.38 (2), 118.87, 113.31 (2), 55.50, 55.28, 29.78, 21.64, 17.98. 4.1.16. Compound (6p). Yield¼70% (238 mg); Colorless gum; HRMS (ESI, Mþþ1) calcd for C20H21O3S 341.1212, found 341.1213; 1H NMR (400 MHz, CDCl3): d 7.95e7.92 (m, 2H), 7.59 (d, J¼8.4 Hz, 2H), 7.58e7.54 (m, 1H), 7.45e7.41 (m, 2H), 7.31e7.28 (m, 2H), 4.74 (t, J¼1.2 Hz, 1H), 4.60 (d, J¼1.2 Hz, 1H), 2.71 (t, J¼8.8 Hz, 1H), 2.44 (s, 3H), 2.21 (dd, J¼6.4, 10.0 Hz, 1H), 1.86 (dd, J¼6.4, 8.8 Hz, 1H), 1.51 (s, 3H); 13C NMR (100 MHz, CDCl3): d 190.88, 144.93, 138.82, 136.51, 135.36, 133.59, 130.11 (2), 129.55 (2), 128.83 (2), 128.16 (2), 113.86, 54.89, 32.37, 21.82, 21.68, 16.41. 4.1.17. Compound (6q). Yield¼72% (253 mg); Colorless gum; HRMS (ESI, Mþþ1) calcd for C21H21O3S 353.1212, found 353.1212; 1H NMR (400 MHz, CDCl3): d 7.82e7.80 (m, 2H), 7.56e7.50 (m, 3H), 7.38e7.33 (m, 2H), 7.27e7.25 (m, 2H), 6.29 (dd, J¼11.2, 17.6 Hz, 1H), 5.45 (d, J¼17.6 Hz, 1H), 5.18 (d, J¼11.2 Hz, 1H), 4.91 (s, 1H), 4.61 (s, 1H), 3.11 (t, J¼8.8 Hz, 1H), 2.42 (s, 3H), 2.19 (dd, J¼6.4, 10.0 Hz, 1H), 2.03 (dd, J¼6.4, 8.8 Hz, 1H); 13C NMR (100 MHz, CDCl3): d 190.55, 145.03, 138.95, 137.80, 136.53, 135.48, 133.41, 130.15 (2), 129.64 (2), 128.82 (2), 127.91 (2), 116.60, 115.53, 56.66, 26.62, 21.66, 16.06. 4.1.18. Compound (6r). Yield¼74% (317 mg); Colorless gum; HRMS (ESI, Mþþ1) calcd for C27H25O3S 429.1524, found 429.1533; 1H NMR (400 MHz, CDCl3): d 7.93e7.90 (m, 2H), 7.66e7.26 (m, 11H), 6.32 (dd, J¼11.2, 17.6 Hz, 1H), 5.50 (d, J¼17.6 Hz, 1H), 5.21 (d, J¼11.2 Hz, 1H), 4.93 (s, 1H), 4.64 (s, 1H), 3.16 (t, J¼8.8 Hz, 1H), 2.42 (s, 3H), 2.18 (dd, J¼6.4, 10.0 Hz, 1H), 2.05 (dd, J¼6.4, 8.8 Hz, 1H); 13C NMR (100 MHz, CDCl3): d 189.88, 146.07, 145.05, 139.74, 138.98, 137.92, 135.47, 135.18, 130.84 (2), 129.66 (2), 128.93 (2), 128.81 (2), 128.31, 127.26 (2), 126.53 (2), 116.57, 115.49, 56.75, 26.50, 21.66, 15.98. A representative procedure of skeleton 8 is as follows: K2CO3 (400 mg, 2.9 mmol) was added to a solution of skeleton 7 (1.0 mmol) in acetone (10 mL) at rt for 10 min. Allyl bromide or propargyl bromide (1.05 mmol) was added to the reaction mixture at rt. The reaction mixture was stirred at reflux for 8 h. The reaction mixture was cooled to rt and the solvent was concentrated. The residue was diluted with water (10 mL) and the mixture was extracted with CH2Cl2 (320 mL). The combined organic layers were washed with brine, dried, filtered and evaporated to afford crude product. Purification on silica gel (hexanes/EtOAc¼8/1e4/1) afforded skeleton 8. 4.1.19. Compound (8b). Yield¼93% (292 mg); Colorless solid; mp 109e111  C (recrystallized from hexanes and EtOAc); HRMS (ESI, Mþþ1) calcd for C18H19O3S 315.1055, found 315.1061; 1H NMR (400 MHz, CDCl3): d 7.94e7.91 (m, 2H), 7.62 (d, J¼8.4 Hz, 2H), 7.60e7.56 (m, 1H), 7.47e7.43 (m, 2H), 7.29 (d, J¼8.0 Hz, 2H),

5.61e5.50 (m, 1H), 5.10 (dd, J¼4.0, 11.2 Hz, 1H), 5.01 (dq, J¼1.2, 17.2 Hz, 1H), 4.94 (dq, J¼1.2, 17.2 Hz, 1H), 2.86e2.71 (m, 2H), 2.41 (s, 3H); 13C NMR (100 MHz, CDCl3): d 191.98, 145.47, 137.14, 133.93, 133.19, 131.91, 129.81 (2), 129.54 (2), 129.01 (2), 128.71 (2), 118.96, 69.24, 32.41, 21.67. 4.1.20. Compound (8c). Yield¼90% (281 mg); Colorless gum; HRMS (ESI, Mþþ1) calcd for C18H17O3S 313.0899, found 313.0902; 1H NMR (400 MHz, CDCl3): d 7.98e7.96 (m, 2H), 7.64e7.59 (m, 3H), 7.50e7.46 (m, 2H), 7.30 (d, J¼8.0 Hz, 2H), 5.26 (dd, J¼6.4, 8.0 Hz, 1H), 2.97 (dd, J¼1.6, 2.8 Hz, 1H), 2.95 (d, J¼2.8 Hz, 1H), 2.43 (s, 3H), 1.89 (t, J¼2.8 Hz, 1H); 13C NMR (100 MHz, CDCl3): d 190.89, 145.85, 136.81, 134.10, 132.74, 129.73 (2), 129.68 (2), 129.14 (2), 128.71 (2), 78.18, 71.20, 68.14, 21.68, 18.22. 4.1.21. Compound (8d). Yield¼90% (349 mg); Colorless gum; HRMS (ESI, Mþþ1) calcd for C24H21O3S 389.1212, found 389.1214; 1H NMR (400 MHz, CDCl3): d 8.08e8.05 (m, 2H), 7.72e7.69 (m, 2H), 7.66e7.62 (m, 4H), 7.51e7.47 (m, 2H), 7.45e7.41 (m, 1H), 7.32 (d, J¼8.0 Hz, 2H), 5.30 (dd, J¼6.4, 8.0 Hz, 1H), 2.99 (dd, J¼1.6, 2.8 Hz, 1H), 2.97 (d, J¼2.8 Hz, 1H), 2.43 (s, 3H), 1.91 (t, J¼2.8 Hz, 1H); 13C NMR (100 MHz, CDCl3): d 190.27, 146.84, 145.85, 139.46, 135.44, 132.71, 129.80 (2), 129.74 (2), 129.71 (2), 128.99 (2), 128.51, 127.33 (2), 127.28 (2), 78.24, 71.19, 68.19, 21.68, 18.21. Acknowledgements The authors would like to thank the Ministry of Science and Technology of the Republic of China for its financial support (NSC 102-2113-M-037-005-MY2). Supplementary data Scanned photocopies of 1H and 13C NMR spectral data were supported. Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.tet.2014.09.085. References and notes 1. For the reviews of cyclopropanes, see: (a) Lebel, H.; Marcoux, J.-F.; Molinaro, C.; Charette, A. B. Chem. Rev. 2003, 103, 977; (b) Sydnes, L. K. Chem. Rev. 2003, 103, 1133; (c) Rubin, M.; Rubina, M.; Gevorgyan, V. Chem. Rev. 2007, 107, 3117; (d) Carson, C. A.; Kerr, M. A. Chem. Soc. Rev. 2009, 38, 3051; (e) Zhang, D.; Song, H.; Qin, Y. Acc. Chem. Res. 2011, 44, 447; (f) Tang, P.; Qin, Y. Synthesis 2012, 44, 2969; (g) Reissig, H.-U.; Zimmer, R. Chem. Rev. 2003, 103, 1151; (h) Brandi, A.; Cicchi, S.; Cordero, F. M.; Goti, A. Chem. Rev. 2003, 103, 1213; (i) Yu, M.; Pagenkopf, B. L. Tetrahedron 2005, 61, 321; (j) De Simone, F.; Waser, J. Synthesis 2009, 3353; (k) Shi, M.; Lu, J.-M.; Wei, Y.; Shao, L.-X. Acc. Chem. Res. 2012, 45, 641; (l) Baldwin, J. E. Chem. Rev. 2003, 103, 1197. 2. For the Pd-catalyzed coupling reactions, see: (a) Shimizu, I.; Ohashi, Y.; Tsuji, J. Tetrahedron Lett. 1985, 26, 3825; (b) Trost, B. M.; Morris, P. J. Angew. Chem., Int. Ed. 2011, 50, 6167; (c) Mei, L.-y.; Wei, Y.; Xu, Q.; Shi, M. Organometallics 2012, 31, 7591. 3. For the Rh-catalyzed coupling reactions, see: (a) Wender, P. A.; Haustedt, L. O.; Lim, J.; Love, J. A.; Williams, T. J.; Yoon, J.-Y. J. Am. Chem. Soc. 2006, 128, 6302; (b) Shintani, R.; Nakatsu, H.; Takatsu, K.; Hayashi, T. Chem.dEur. J. 2009, 15, 8692; (c) Yu, Z.-X.; Wender, P. A.; Houk, K. N. J. Am. Chem. Soc. 2004, 126, 9154; (d) Jiao, L.; Ye, S.; Yu, Z.-X. J. Am. Chem. Soc. 2008, 130, 7178; (e) Li, Q.; Jiang, G.-J.; Jiao, L.; Yu, Z.-X. Org. Lett. 2010, 12, 1332; (f) Jiao, L.; Lin, M.; Yu, Z.-X. Chem. Commun. 2010, 1059; (g) Jiao, L.; Lin, M.; Yu, Z.-X. J. Am. Chem. Soc. 2011, 133, 447; (h) Lin, M.; Kang, G.-Y.; Guo, Y.-A.; Yu, Z.-X. J. Am. Chem. Soc. 2012, 134, 398; (i) Jiao, L.; Lin, M.; Zhuo, L.-G.; Yu, Z.-X. Org. Lett. 2010, 12, 2528. 4. For thermal and radical-mediated reactions of VCP, see: (a) Hudlicky, T.; Reed, J. W. Angew. Chem., Int. Ed. 2010, 49, 4864; (b) Feldman, K. S.; Berven, M. H.; Weinreb, P. H. J. Am. Chem. Soc. 1993, 115, 11364; (c) Zhang, H.; Jeon, K. O.; Hay, E. B.; Geib, S. J.; Curran, D. P.; LaPorte, M. G. Org. Lett. 2014, 16, 94. 5. For the VCP intermediates in the synthesis of natural products, see: (a) Shimokawa, J.; Harada, T.; Yokoshima, S.; Fukuyama, T. J. Am. Chem. Soc. 2011, 133, 17634; (b) Zhang, M.; Curran, D. P. J. Am. Chem. Soc. 2011, 133, 10376; (c) Lian, Y.; Miller, L. C.; Born, S.; Sarpong, R.; Davies, H. M. L. J. Am. Chem. Soc. 2010, 132, 12422. 6. (a) Nicola, T.; Brenner, M.; Donsbach, K.; Kreye, P. Org. Process Res. Dev. 2005, 9, 513; (b) Poirer, M.; Aubry, N.; Boucher, C.; Ferland, J.-M.; LaPlante, S.; Tsantrizos, Y. S. J. Org. Chem. 2005, 70, 10765; (c) Yee, N. K.; Farina, V.; Houpis, I. N.; Haddad,

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