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Studies directed toward the syntheses of amphidinolides: formal total synthesis of (−)-amphidinolide P
TETRAHEDRON LETTERS Tetrahedron Letters 42 (2001) 3387–3390
Pergamon
Studies directed toward the syntheses of amphidinolides: formal total synthesis of (−)-amphidinolide P Tushar K. Chakraborty* and Sanjib Das Indian Institute of Chemical Technology, Hyderabad 500 007, India Received 21 December 2000; revised 20 February 2001; accepted 15 March 2001
Abstract—A formal total synthesis of (−)-amphidinolide P (1) has been achieved via an efficient convergent strategy for the stereoselective construction of the two advanced intermediates 2 and 3, used recently by Williams et al. in their synthesis of the same molecule. © 2001 Elsevier Science Ltd. All rights reserved.
Amphidinolides represent a family of cytotoxic marine natural products with diverse structural features and pronounced biological activities that include very potent anticancer activities displayed by many of these molecules against various tumour cell lines and an increase in rabbit skeletal muscle actomyosin ATPase activity by one of them.1–3 Recently, Williams et al. have achieved the total synthesis of (−)-amphidinolide P (1),4 which turned out to be the enantiomer of the natural product isolated by Kobayashi.5 In continuation of our work on the synthesis of amphidinolides O and P,6 we now report a formal total synthesis of (−)-amphidinolide P (1) via the stereoselective construction of the two advanced intermediates 2 and 3 following a concise strategy. A Stille coupling of 2 and 3 and the conversion of the resulting acyclic precursor to the final macrolide 1 in two steps have already been reported.4
gyl alcohol 6 was alkylated with 2,3-dibromopropene using ethylmagnesium bromide and a catalytic amount of cuprous bromide,7 which was followed by acid-catalyzed THP-deprotection to give the enyne 7 in 75% yield. Hydride reduction of the acetylenic moiety of 7 using Red-Al gave a ‘1,4-diene’ intermediate with an allylic alcohol moiety that was subjected to a Sharpless asymmetric epoxidation reaction using natural diisopropyl L-(+)-tartrate to furnish the chiral epoxy alcohol 8 (>98% ee)8 in 60% yield from 7. Oxidation of 8 using SO3–py complex gave the C7–C11 aldehyde 4 in 70% yield. The two chiral centres in the other component 5 were generated by an Evans aldol reaction between the chiral oxazolidinone 9 and the aldehyde 10,9 prepared from monoPMB-protected propane-1,3-diol by oxidation. The syn aldol adduct 11 was transformed into the methyl ester intermediate 12 in three steps in 70% overall yield: treatment with DDQ under anhydrous conditions to convert the PMB-ether to the PMP-acetal, oxidative removal of the chiral auxiliary and conversion of the acid to an ester using CH2N2. Next, the ester 12 was reacted with trimethylsi-
We planned the construction of the vinyl bromide 2 from two subunits, the C7–C11 aldehyde 4 and the C1–C6 allylsilane component 5. The actual synthesis of 2 is outlined in Scheme 1. The THP-protected propar-
Bu3Sn H
Me
O
H
H H
O
OH Me
H
O
Me
O
1: (-)-amphidinolide P
12
Br
O
H OTBS O 1 H CO2Me
7
Me
OH
17
Me
11
3 H
Br
O
2
* Corresponding author. 0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 0 - 4 0 3 9 ( 0 1 ) 0 0 4 5 0 - 6
6
O
4
O 1
H
7 CHO
Me
PMP Me3Si
Me 5
T. K. Chakraborty, S. Das / Tetrahedron Letters 42 (2001) 3387–3390
3388
1. EtMgBr, THF, then CuBr (cat.), followed by Br
1. Red-Al, Et2O 2. (+)-DIPT, Ti(OiPr)4, TBHP, 4Ao MS, CH2Cl2
Br
Br
H
OTHP 2. TsOH (cat.), MeOH 6
OH
7
75%
60%
Br
O
SO3-py, DMSO, Et3N, CH2Cl2 4
H
HO
70%
8
1. DDQ, 4Ao MS, CH2Cl2 O
Bu2BOTf, Et3N, CH2Cl2 then PMBO CHO 10
O N
O
OH O N
PMBO
9
H
4, BF3.Et2O, CH2Cl2
1. Me3SiCH2MgCl, CeCl3, THF 2. Silica gel, CH2Cl2
70%
Me 12
Br
O H
1. TBSOTf, 2,6-lutidine, CH2Cl2 2. NaCNBH3, TMSCl, CH3CN
PMP
OH O
O 85%
40%
60%
O
MeO2C
7
5
O
3. CH2N2, Et2O O
Me Bn 11
92%
Bn
PMP
2. LiOH, H2O2, THF-H2O
O
Me 13 H
Br
O
H
H OTBS OH
Br
O
H OTBS OPMB
+
OH
OPMB Me 14
(3:2)
Me 15
1. TESOTf, 2,6-lutidine, O CH2Cl2 H 2. DDQ, CHCl3-H2O 14 85%
Br H OTBS OTES
1. SO3-py, DMSO, Et3N, CH2Cl2 2. NaClO2, 2-methyl-2-butene, O H NaH2PO4, t-BuOH 3. CH2N2, Et2O 90% OH
15 92%
1. CSA (cat.), CH2Cl2-MeOH 2. (COCl)2, DMSO, Et3N, CH2Cl2
H OTBS OTES CO2Me
Me
Me 16 1. SO3-py, DMSO, Et3N, CH2Cl2 2. NaClO2, 2-methyl-2-butene, O NaH2PO4, t-BuOH H 3. CH2N2, Et2O
Br
2 60%
17
Br H OTBS OPMB CO2Me
Me
1. DDQ, CHCl3-H2O 2. (COCl)2, DMSO, Et3N, CH2Cl2 60%
2
18
Scheme 1. Stereoselective synthesis of 2.
lylmethylmagnesium chloride in the presence of anhydrous cerium(III) chloride.10,11 The resulting b-hydroxysilane underwent Peterson-type elimination in the presence of silica gel to furnish the allylsilane 5 in 60% yield. The Sakurai reaction12,13 between 4 and 5 provided the required adduct 13 in 40% yield, the major product being an acid-catalyzed rearranged product from 5. The only isomer that could be detected was the expected product 13. The stereochemistry of the newly generated
C7 centre was assigned based on the known diastereofacial selectivity found in the nucleophilic additions to chiral epoxy aldehydes.14 Silylation of 13 was followed by acetal deprotection. Most of the acid-catalyzed deprotection procedures resulted in the decomposition of the starting material. Finally, a silyl-promoted acetal ring opening was successfully carried out using sodium cyanoborohydride in the presence of TMSCl.15 Two PMB-ethers 14 and 15 were formed in 3:2 ratio in 85% total yield. Although the acetal opening failed to give any selectivity, both 14 and 15 were useful and
T. K. Chakraborty, S. Das / Tetrahedron Letters 42 (2001) 3387–3390
3389
1. (+)-DET, Ti(OiPr)4, 1. NaH, BnBr, TBAI (cat.), THF 2. Red-Al, Et2O HO
TBHP, 4Ao MS, CH2Cl2
BnO
OH
1. TBDPSCl, Et3N, DMAP (cat.), CH2Cl2 2. PMBOC(=NH)CCl3, TfOH (cat.), CH2Cl2-cyclohexane 3. MeLi, Et2O
Me OPiv
HO OH
23
TBDPSO PMBO 24 Bu3Sn
1. CHI3, CrCl3-LAH, THF 2. n-BuLi, n-Bu3SnCl, Et2O
1. TBAF, THF 2. (COCl)2, DMSO, Et3N, CH2Cl2
Me
OH
85%
OPMB
Me OHC PMBO
Me 25
Me DDQ, CHCl3-H2O
PMBO 72%
Me
22
Me
21
TBDPSO
50%
1. (COCl)2, DMSO, Et3N, CH2Cl2 2. MeMgI, Et2O 3. Step 1 4. Ph3P=CH2, Et2O 65%
OH
72%
20
92%
OH
BnO
60%
19
1. Piv-Cl, CH2Cl2-pyridine 2. Pd-C, H2, MeOH
Me
2. Me2CuLi, Et2O OH
Me
3
26
Scheme 2. Stereoselective synthesis of 3.
could be converted easily to the desired b-ketoester 2 by routine functional group manipulations. In the case of 14, TES-protection of the secondary hydroxyl group was followed by deprotection of the C1-hydroxyl group. A two-step oxidation protocol was followed to oxidize the primary hydroxyl group of 16 to a carboxylic acid that was esterified with CH2N2 to give 17. Selective removal of the TES-group was followed by Swern oxidation of the resulting C3-hydroxyl group to furnish 2. On the other hand, the primary hydroxyl group of 15 was first transformed into the ester 18 and was subsequently subjected to PMB-deprotection and Swern oxidation to reach the target 2. The starting material for the synthesis of 3 was 2-butyne1,4-diol (19, Scheme 2). A slight modification of a reported procedure16 was followed to prepare the allylic alcohol 20 from 19, in 60% yield, by monobenzylation and Red-Al reduction. Sharpless asymmetric epoxidation of 20 using natural diethyl L-(+)-tartrate and Ti(OiPr)4 in stoichiometric amounts (>98% ee)17 was followed by epoxide ring opening using dimethylcopper lithium to give the 1,3-diol 21 in 72% overall yield in two steps. The minor 1,2-diol was removed from the crude product by oxidative cleavage using NaIO4. Selective protection of the primary hydroxyl group of 21 as a pivalate was followed by the Bn-deprotection to give the 1,2-diol 22 in 92% yield. Routine functional group manipulations transformed 22 into 23 (50% yield in three steps), which was subsequently converted to the olefin 24 in four steps of which the first three were to prepare a methylketo intermediate that was subjected to a Wittig olefination reaction in the fourth step giving an overall
yield of 65% from 23. Desilylation of 24 was followed by Swern oxidation to furnish the aldehyde 25 in 85% yield. The aldehyde 25 was reacted with CHI3 in the presence of anhydrous CrCl2, generated in situ from CrCl3 and LAH, following the Takai protocol18 to introduce an E-vinyl iodide moiety that was next subjected to an iodine–tin exchange to give the E-vinyl stannane 26 in 72% yield. Finally, deprotection of the PMB-ether furnished the desired intermediate 3. The spectroscopic data and specific rotations of 2 and 3 prepared by us were identical with the reported values.4 In conclusion, the convergent synthetic protocol reported herein for the stereoselective construction of the two advanced intermediates 2 and 3 that were used by Williams et al. in their synthesis of (−)-amphidinolide P (1) demonstrates a formal total synthesis of the molecule. Acknowledgements The authors wish to thank Drs. A. C. Kunwar and M. Vairamani for NMR and mass spectroscopic assistance, respectively; UGC, New Delhi for a research fellowship (S.D.) and CSIR, New Delhi for a Young Scientist Award Research Grant (T.K.C.). References 1. Kobayashi, J.; Ishibashi, M. Chem. Rev. 1993, 93, 1753– 1769.
3390
T. K. Chakraborty, S. Das / Tetrahedron Letters 42 (2001) 3387–3390
2. Ishibashi, M.; Kobayashi, J. Heterocycles 1997, 44, 543– 572. 3. Matsunaga, K.; Nakatani, K.; Ishibashi, M.; Kobayashi, J.; Ohizumi, Y. Biochim. Biophys. Acta 1999, 1427, 24–32. 4. Williams, D. R.; Myers, B. J.; Mi, L. Org. Lett. 2000, 2, 945–948. 5. Ishibashi, M.; Takahashi, M.; Kobayashi, J. J. Org. Chem. 1995, 60, 6062–6066. 6. Chakraborty, T. K.; Das, S. Chem. Lett. 2000, 80–81. 7. Terrell, L. R.; Ward, III, J. S.; Maleczka, Jr., R. E. Tetrahedron Lett. 1999, 40, 3097–3100. 8. The other enantiomer could not be detected by 1H NMR analysis of the Mosher ester of 8. 9. Williams, D. R.; Clark, M. P.; Berliner, M. A. Tetrahedron Lett. 1999, 40, 2287–2290. 10. Narayanan, B. A.; Bunnelle, W. H. Tetrahedron Lett. 1987, 28, 6261–6264.
.
11. Lee, T. V.; Channon, J. A.; Cregg, C.; Porter, J. R.; Roden, F. S.; Yeoh, H. T.-L. Tetrahedron 1989, 45, 5877–5886. 12. Hosomi, A.; Endo, M.; Sakurai, H. Chem. Lett. 1976, 941–942. 13. Sakurai, H.; Hosomi, A. Tetrahedron Lett. 1976, 16, 1295–1298. 14. Nacro, K.; Baltas, M.; Escudier, J.-M.; Gorrichon, L. Tetrahedron 1996, 52, 9047–9056. 15. Johansson, R.; Samuelsson, B. J. Chem. Soc., Chem. Commun. 1984, 201–202. 16. Kiyota, H.; Abe, M.; Ono, Y.; Oritani, T. Synlett 1997, 1093–1095. 17. Katsuki, T.; Lee, A. W. M.; Ma, P.; Martin, V. S.; Masamune, S.; Sharpless, K. B.; Tuddenham, D.; Walker, F. J. J. Org. Chem. 1982, 47, 1373–1377. 18. Takai, K.; Nitta, K.; Utimoto, K. J. Am. Chem. Soc. 1986, 108, 7408–7410.
.
Pergamon
Studies directed toward the syntheses of amphidinolides: formal total synthesis of (−)-amphidinolide P Tushar K. Chakraborty* and Sanjib Das Indian Institute of Chemical Technology, Hyderabad 500 007, India Received 21 December 2000; revised 20 February 2001; accepted 15 March 2001
Abstract—A formal total synthesis of (−)-amphidinolide P (1) has been achieved via an efficient convergent strategy for the stereoselective construction of the two advanced intermediates 2 and 3, used recently by Williams et al. in their synthesis of the same molecule. © 2001 Elsevier Science Ltd. All rights reserved.
Amphidinolides represent a family of cytotoxic marine natural products with diverse structural features and pronounced biological activities that include very potent anticancer activities displayed by many of these molecules against various tumour cell lines and an increase in rabbit skeletal muscle actomyosin ATPase activity by one of them.1–3 Recently, Williams et al. have achieved the total synthesis of (−)-amphidinolide P (1),4 which turned out to be the enantiomer of the natural product isolated by Kobayashi.5 In continuation of our work on the synthesis of amphidinolides O and P,6 we now report a formal total synthesis of (−)-amphidinolide P (1) via the stereoselective construction of the two advanced intermediates 2 and 3 following a concise strategy. A Stille coupling of 2 and 3 and the conversion of the resulting acyclic precursor to the final macrolide 1 in two steps have already been reported.4
gyl alcohol 6 was alkylated with 2,3-dibromopropene using ethylmagnesium bromide and a catalytic amount of cuprous bromide,7 which was followed by acid-catalyzed THP-deprotection to give the enyne 7 in 75% yield. Hydride reduction of the acetylenic moiety of 7 using Red-Al gave a ‘1,4-diene’ intermediate with an allylic alcohol moiety that was subjected to a Sharpless asymmetric epoxidation reaction using natural diisopropyl L-(+)-tartrate to furnish the chiral epoxy alcohol 8 (>98% ee)8 in 60% yield from 7. Oxidation of 8 using SO3–py complex gave the C7–C11 aldehyde 4 in 70% yield. The two chiral centres in the other component 5 were generated by an Evans aldol reaction between the chiral oxazolidinone 9 and the aldehyde 10,9 prepared from monoPMB-protected propane-1,3-diol by oxidation. The syn aldol adduct 11 was transformed into the methyl ester intermediate 12 in three steps in 70% overall yield: treatment with DDQ under anhydrous conditions to convert the PMB-ether to the PMP-acetal, oxidative removal of the chiral auxiliary and conversion of the acid to an ester using CH2N2. Next, the ester 12 was reacted with trimethylsi-
We planned the construction of the vinyl bromide 2 from two subunits, the C7–C11 aldehyde 4 and the C1–C6 allylsilane component 5. The actual synthesis of 2 is outlined in Scheme 1. The THP-protected propar-
Bu3Sn H
Me
O
H
H H
O
OH Me
H
O
Me
O
1: (-)-amphidinolide P
12
Br
O
H OTBS O 1 H CO2Me
7
Me
OH
17
Me
11
3 H
Br
O
2
* Corresponding author. 0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 0 - 4 0 3 9 ( 0 1 ) 0 0 4 5 0 - 6
6
O
4
O 1
H
7 CHO
Me
PMP Me3Si
Me 5
T. K. Chakraborty, S. Das / Tetrahedron Letters 42 (2001) 3387–3390
3388
1. EtMgBr, THF, then CuBr (cat.), followed by Br
1. Red-Al, Et2O 2. (+)-DIPT, Ti(OiPr)4, TBHP, 4Ao MS, CH2Cl2
Br
Br
H
OTHP 2. TsOH (cat.), MeOH 6
OH
7
75%
60%
Br
O
SO3-py, DMSO, Et3N, CH2Cl2 4
H
HO
70%
8
1. DDQ, 4Ao MS, CH2Cl2 O
Bu2BOTf, Et3N, CH2Cl2 then PMBO CHO 10
O N
O
OH O N
PMBO
9
H
4, BF3.Et2O, CH2Cl2
1. Me3SiCH2MgCl, CeCl3, THF 2. Silica gel, CH2Cl2
70%
Me 12
Br
O H
1. TBSOTf, 2,6-lutidine, CH2Cl2 2. NaCNBH3, TMSCl, CH3CN
PMP
OH O
O 85%
40%
60%
O
MeO2C
7
5
O
3. CH2N2, Et2O O
Me Bn 11
92%
Bn
PMP
2. LiOH, H2O2, THF-H2O
O
Me 13 H
Br
O
H
H OTBS OH
Br
O
H OTBS OPMB
+
OH
OPMB Me 14
(3:2)
Me 15
1. TESOTf, 2,6-lutidine, O CH2Cl2 H 2. DDQ, CHCl3-H2O 14 85%
Br H OTBS OTES
1. SO3-py, DMSO, Et3N, CH2Cl2 2. NaClO2, 2-methyl-2-butene, O H NaH2PO4, t-BuOH 3. CH2N2, Et2O 90% OH
15 92%
1. CSA (cat.), CH2Cl2-MeOH 2. (COCl)2, DMSO, Et3N, CH2Cl2
H OTBS OTES CO2Me
Me
Me 16 1. SO3-py, DMSO, Et3N, CH2Cl2 2. NaClO2, 2-methyl-2-butene, O NaH2PO4, t-BuOH H 3. CH2N2, Et2O
Br
2 60%
17
Br H OTBS OPMB CO2Me
Me
1. DDQ, CHCl3-H2O 2. (COCl)2, DMSO, Et3N, CH2Cl2 60%
2
18
Scheme 1. Stereoselective synthesis of 2.
lylmethylmagnesium chloride in the presence of anhydrous cerium(III) chloride.10,11 The resulting b-hydroxysilane underwent Peterson-type elimination in the presence of silica gel to furnish the allylsilane 5 in 60% yield. The Sakurai reaction12,13 between 4 and 5 provided the required adduct 13 in 40% yield, the major product being an acid-catalyzed rearranged product from 5. The only isomer that could be detected was the expected product 13. The stereochemistry of the newly generated
C7 centre was assigned based on the known diastereofacial selectivity found in the nucleophilic additions to chiral epoxy aldehydes.14 Silylation of 13 was followed by acetal deprotection. Most of the acid-catalyzed deprotection procedures resulted in the decomposition of the starting material. Finally, a silyl-promoted acetal ring opening was successfully carried out using sodium cyanoborohydride in the presence of TMSCl.15 Two PMB-ethers 14 and 15 were formed in 3:2 ratio in 85% total yield. Although the acetal opening failed to give any selectivity, both 14 and 15 were useful and
T. K. Chakraborty, S. Das / Tetrahedron Letters 42 (2001) 3387–3390
3389
1. (+)-DET, Ti(OiPr)4, 1. NaH, BnBr, TBAI (cat.), THF 2. Red-Al, Et2O HO
TBHP, 4Ao MS, CH2Cl2
BnO
OH
1. TBDPSCl, Et3N, DMAP (cat.), CH2Cl2 2. PMBOC(=NH)CCl3, TfOH (cat.), CH2Cl2-cyclohexane 3. MeLi, Et2O
Me OPiv
HO OH
23
TBDPSO PMBO 24 Bu3Sn
1. CHI3, CrCl3-LAH, THF 2. n-BuLi, n-Bu3SnCl, Et2O
1. TBAF, THF 2. (COCl)2, DMSO, Et3N, CH2Cl2
Me
OH
85%
OPMB
Me OHC PMBO
Me 25
Me DDQ, CHCl3-H2O
PMBO 72%
Me
22
Me
21
TBDPSO
50%
1. (COCl)2, DMSO, Et3N, CH2Cl2 2. MeMgI, Et2O 3. Step 1 4. Ph3P=CH2, Et2O 65%
OH
72%
20
92%
OH
BnO
60%
19
1. Piv-Cl, CH2Cl2-pyridine 2. Pd-C, H2, MeOH
Me
2. Me2CuLi, Et2O OH
Me
3
26
Scheme 2. Stereoselective synthesis of 3.
could be converted easily to the desired b-ketoester 2 by routine functional group manipulations. In the case of 14, TES-protection of the secondary hydroxyl group was followed by deprotection of the C1-hydroxyl group. A two-step oxidation protocol was followed to oxidize the primary hydroxyl group of 16 to a carboxylic acid that was esterified with CH2N2 to give 17. Selective removal of the TES-group was followed by Swern oxidation of the resulting C3-hydroxyl group to furnish 2. On the other hand, the primary hydroxyl group of 15 was first transformed into the ester 18 and was subsequently subjected to PMB-deprotection and Swern oxidation to reach the target 2. The starting material for the synthesis of 3 was 2-butyne1,4-diol (19, Scheme 2). A slight modification of a reported procedure16 was followed to prepare the allylic alcohol 20 from 19, in 60% yield, by monobenzylation and Red-Al reduction. Sharpless asymmetric epoxidation of 20 using natural diethyl L-(+)-tartrate and Ti(OiPr)4 in stoichiometric amounts (>98% ee)17 was followed by epoxide ring opening using dimethylcopper lithium to give the 1,3-diol 21 in 72% overall yield in two steps. The minor 1,2-diol was removed from the crude product by oxidative cleavage using NaIO4. Selective protection of the primary hydroxyl group of 21 as a pivalate was followed by the Bn-deprotection to give the 1,2-diol 22 in 92% yield. Routine functional group manipulations transformed 22 into 23 (50% yield in three steps), which was subsequently converted to the olefin 24 in four steps of which the first three were to prepare a methylketo intermediate that was subjected to a Wittig olefination reaction in the fourth step giving an overall
yield of 65% from 23. Desilylation of 24 was followed by Swern oxidation to furnish the aldehyde 25 in 85% yield. The aldehyde 25 was reacted with CHI3 in the presence of anhydrous CrCl2, generated in situ from CrCl3 and LAH, following the Takai protocol18 to introduce an E-vinyl iodide moiety that was next subjected to an iodine–tin exchange to give the E-vinyl stannane 26 in 72% yield. Finally, deprotection of the PMB-ether furnished the desired intermediate 3. The spectroscopic data and specific rotations of 2 and 3 prepared by us were identical with the reported values.4 In conclusion, the convergent synthetic protocol reported herein for the stereoselective construction of the two advanced intermediates 2 and 3 that were used by Williams et al. in their synthesis of (−)-amphidinolide P (1) demonstrates a formal total synthesis of the molecule. Acknowledgements The authors wish to thank Drs. A. C. Kunwar and M. Vairamani for NMR and mass spectroscopic assistance, respectively; UGC, New Delhi for a research fellowship (S.D.) and CSIR, New Delhi for a Young Scientist Award Research Grant (T.K.C.). References 1. Kobayashi, J.; Ishibashi, M. Chem. Rev. 1993, 93, 1753– 1769.
3390
T. K. Chakraborty, S. Das / Tetrahedron Letters 42 (2001) 3387–3390
2. Ishibashi, M.; Kobayashi, J. Heterocycles 1997, 44, 543– 572. 3. Matsunaga, K.; Nakatani, K.; Ishibashi, M.; Kobayashi, J.; Ohizumi, Y. Biochim. Biophys. Acta 1999, 1427, 24–32. 4. Williams, D. R.; Myers, B. J.; Mi, L. Org. Lett. 2000, 2, 945–948. 5. Ishibashi, M.; Takahashi, M.; Kobayashi, J. J. Org. Chem. 1995, 60, 6062–6066. 6. Chakraborty, T. K.; Das, S. Chem. Lett. 2000, 80–81. 7. Terrell, L. R.; Ward, III, J. S.; Maleczka, Jr., R. E. Tetrahedron Lett. 1999, 40, 3097–3100. 8. The other enantiomer could not be detected by 1H NMR analysis of the Mosher ester of 8. 9. Williams, D. R.; Clark, M. P.; Berliner, M. A. Tetrahedron Lett. 1999, 40, 2287–2290. 10. Narayanan, B. A.; Bunnelle, W. H. Tetrahedron Lett. 1987, 28, 6261–6264.
.
11. Lee, T. V.; Channon, J. A.; Cregg, C.; Porter, J. R.; Roden, F. S.; Yeoh, H. T.-L. Tetrahedron 1989, 45, 5877–5886. 12. Hosomi, A.; Endo, M.; Sakurai, H. Chem. Lett. 1976, 941–942. 13. Sakurai, H.; Hosomi, A. Tetrahedron Lett. 1976, 16, 1295–1298. 14. Nacro, K.; Baltas, M.; Escudier, J.-M.; Gorrichon, L. Tetrahedron 1996, 52, 9047–9056. 15. Johansson, R.; Samuelsson, B. J. Chem. Soc., Chem. Commun. 1984, 201–202. 16. Kiyota, H.; Abe, M.; Ono, Y.; Oritani, T. Synlett 1997, 1093–1095. 17. Katsuki, T.; Lee, A. W. M.; Ma, P.; Martin, V. S.; Masamune, S.; Sharpless, K. B.; Tuddenham, D.; Walker, F. J. J. Org. Chem. 1982, 47, 1373–1377. 18. Takai, K.; Nitta, K.; Utimoto, K. J. Am. Chem. Soc. 1986, 108, 7408–7410.
.