oxidation pathway

Tetrahedron Letters xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet

Transformation of ethers into aldehydes or ketones: a catalytic aerobic deprotection/oxidation pathway Zhenlu Shen a,⇑, Meng Chen a, Tiantian Fang b, Meichao Li a, Weimin Mo a, Baoxiang Hu a, Nan Sun a, Xinquan Hu a,c,⇑ a b c

College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China College of Pharmaceutical Sciences, Zhejiang University of Technology, Hangzhou 310014, China State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China

a r t i c l e

i n f o

Article history: Received 20 February 2015 Revised 6 April 2015 Accepted 8 April 2015 Available online xxxx Keywords: Functional group interconversion PMB ethers Carbonyl compounds Oxidation Oxygen

a b s t r a c t A facile and efficient protocol for direct transformation of p-methoxybenzyl (PMB) ethers to aldehydes or ketones via a catalytic aerobic oxidation process has been developed. The reaction was performed with the combination of catalytic amounts of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), 2,2,6,6tetramethylpiperidine N-oxy (TEMPO), and tert-butyl nitrite (TBN), with molecular oxygen as terminal oxidant. A variety of PMB ether substrates derived from benzylic alcohols, heteroaromatic alcohols, and aliphatic alcohols, were converted to their corresponding carbonyl compounds in good conversions and selectivities. Ó 2015 Elsevier Ltd. All rights reserved.

Functional group interconversion is the process of the transformation of one functional group to another and is part of the basic toolkit of organic chemistry.1 Aldehydes and ketones, two kinds of carbonyl compounds, are valuable intermediates for further elaboration into complex organic molecules. Normally, they are obtained from alcohols via conversion of hydroxyl groups to carbonyl groups.2 As we know, hydroxyl groups are usually protected in advance during the multi-step syntheses.3 There are many types of protecting groups for the hydroxyl, while ether groups are the most common ones owing to the high chemical stability of ethers under a wide variety of synthetic procedures and reaction conditions.4,5 In many cases, the hydroxyl groups need to be further oxidized into carbonyl groups after removal of protecting groups, especially in the syntheses of those complicated natural products. Typically, a two-step process, sequential deprotection and selective oxidation of alcohols, is required to realize such a chemical transformation.6 On these considerations, the direct transformation of ether groups to carbonyl groups is a potentially important and useful transformation in organic synthesis. Because of the stability of ethers, it is not easy to directly convert ethers to carbonyl compounds. Up to now, only a few reports showed the direct

⇑ Corresponding authors. Tel.: +86 571 88320416; fax: +86 571 88320103. E-mail addresses: [email protected] (Z. Shen), [email protected] (X. Hu).

transformation of ethers to carbonyl compounds.7 However, these reported methods have some drawbacks: (1) the oxidants were somewhat dangerous and expensive; (2) limited substrates could be used. Recently, a catalytic oxidation of ethers to carbonyl compounds using Mn complex as catalyst and mCPBA as oxidant has been reported.8 The reagent system mCPBA/CCl3CN/MeCN was also developed to the direct oxidation of ethers.9 Very recently, Oxone/ KBr system was successfully applied to oxidation of benzyl ethers.10 It should be noted that oxidants mCPBA and Oxone were used in stoichiometric amounts, and ethers derived from primary alcohols were oxidized to carboxylic acid rather than aldehydes (Scheme 1). The direct transformation of ethers to corresponding aldehydes or ketones using molecular oxygen as the environmentally benign terminal oxidant has not been reported.pMethoxybenzyl (PMB) ethers are one of the most widely used hydroxyl protecting groups due to their deprotection conditions being orthogonal to other protecting and functional groups, and due to their stability under acidic and/or basic reaction condition, feasibility of introduction.5,11 We recently reported 2,3-dichloro5,6-dicyano-1,4-benzoquinone (DDQ)/tert-butyl nitrite (TBN)/O2 catalytic oxidation system to replace the stoichiometric amount of DDQ for the oxidative deprotection of benzyl-type ethers (including PMB, p-phenylbenzyl, and benzyl ethers), in which DDQ was employed as a catalyst, TBN as a co-catalyst, and molecular oxygen as the terminal oxidant.12 In this transition metal-free

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Z. Shen et al. / Tetrahedron Letters xxx (2015) xxx–xxx

ref 9 O

R1

O mCPBA (2 equiv) CCl3CN/MeCN (1/1, 0.2 M)

ref 10

Oxone R (1.5 equiv) KBr (1.0 equiv)

R2 1

R

O

Ph

this work H (or R2) PMB R1 O

MeCN

O R

DDQ/TEMPO/TBN (cat.) O2 (0.3 MPa) CH2ClCH2Cl

R2

1

O 1

R

H(or R2)

OH 1

R

H(or R2)

Scheme 1. Transformation of ethers into carbonyl compounds.

system, the key point is that TBN was served as the NO equivalent to activate molecular oxygen. Very recently, a similar oxidation system, DDQ/NaNO2/O2, was also applied to PMB ether deprotection.13 In fact, the DDQ/TBN/O2 catalytic oxidation system was expanded from our previous catalytic oxidation systems, such as 2,2,6,6-tetramethylpiperidin-N-oxyl (TEMPO)/TBN/O2,14 TEMPO/ HBr/TBN/O2,15 TEMPO/1,3-dibromo-5,5-dimethylhydantoin/NaNO2/ O2,16 and TEMPO/Br2/NaNO2/O2.17 These oxidation systems have been successfully applied to selective oxidation of alcohols into corresponding carbonyl compounds with their unique advantages. Based on our understandings of both oxidations, we envisioned that a three-component catalyst system, DDQ/TEMPO/TBN, can fulfill the direct transformation of PMB ethers into carbonyl compounds via aerobic deprotection/oxidation (Scheme 1). Herein, we reported our research results of this potential tandem reaction, that is, PMB ethers were deprotected into alcohols, which were sequentially oxidized into carbonyl compounds. Although deprotection of PMB ethers and aerobic oxidation of alcohols have been extensively studied,11,18 to the best of our knowledge, this work is the first example of direct transformation of PMB ethers to aldehydes or ketones via deprotection/oxidation reaction catalyzed by DDQ/TEMPO/TBN with molecular oxygen as the oxidant. To find a reliable and robust condition for the deprotection/oxidation of PMB ethers into carbonyl compounds, PMB ether of benzyl alcohol (1a) was selected as the model substrate. Our initial experiment of transformation 1a to benzaldehyde was carried out with 10 mol % of DDQ, 5 mol % of TEMPO, and 10 mol % of TBN in 1,2-dichloroethane at 100 °C under 0.3 MPa of oxygen. It was found that 1a was completely converted into benzaldehyde with a certain amount of benzoic acid due to over-oxidation. This preliminary result showed that DDQ/TEMPO/TBN/O2 system for deprotection/oxidation of PMB ethers into carbonyl compounds designed in the Scheme 1 was feasible, and inspired us to further search the suitable reaction conditions. After optimization of oxygen pressure, reaction temperature, and the amounts of DDQ, TEMPO, and TBN, we finally concluded that 3 mol % of DDQ, 3 mol % of TEMPO, and 3 mol % of TBN at 100 °C under 0.3 MPa of oxygen in 2.5 h were suitable for the ideal transformation of 1a to benzaldehyde.18 Under such a condition, 1a underwent a complete deprotection/oxidation to furnish benzaldehyde in excellent selectivity (high than 99%). Several solvents, such as ethylene glycol diethyl ether (EGDE), PhCl, toluene, PhCF3, t-BuOH, 1,4-dioxane, and DMF were also examined, the conversions of 1a were 100%, 45%, 29%, 18%, 27%, 6%, and 4%, respectively.19 Although 1a could

be fully converted in EGDE, which was a good solvent for the DDQ/TBN-catalyzed aerobic oxidative deprotection of PMB ethers,12a the selectivity to benzaldehyde was only 76%, the intermediate benzyl alcohol could not be oxidized completely into benzaldehyde. On the basis of these results, we then focused our studies on the application DDQ/TEMPO/TBN/O2 system to a variety of PMB ethers. The results of PMB ethers derived from benzylic alcohols and their heteroaromatic analogs are summarized in Table 1. PMB ethers of methylbenzyl alcohol (1b–1d), 4-methoxybenzyl alcohol (1e), and 4-chlorobenzyl alcohol (1f) readily reacted, quantitatively afforded their corresponding benzaldehydes in 2.5–4 h (entries 1–6), and the isolated yields of 2b–2f were all higher than 90%. The reaction of PMB ether of 3-chlorobenzyl alcohol (1g) required a longer reaction time, and 3-chlorobenzaldehyde (2g) could be obtained in 96% selectivity along with 4% of 3-chlorobenzyl alcohol in 6 h (entry 7). However, PMB ether 1h, containing a 2-Cl on phenyl ring, needed an increased catalysts loading and extended reaction time. A full conversion of 1h with 96% selectivity and 89% isolated yield of 2chlorobenzaldehyde (2h) was achieved in 24 h by increasing the dosage of DDQ, TEMPO, and TBN to 12 mol % (entry 8). The slow rate of oxidation could be ascribed to the steric hinderance and electron-withdrawing properties. For PMB ether of 4-methylthiobenzyl alcohol (1i), this catalytic oxidation system could provide clean 4-methylthiobenzaldehyde (2i) without any observable oxidation of the methylthio group, and the isolated yield of 4(methylthio)benzaldehyde (2i) was 91% (entry 9). When PMB ethers of heteroaromatic analogs (1j and 1k) were submitted to the deprotection/oxidation reactions, the selectivities to furan-2carbaldehyde (2j) and thiophene-2-carbaldehyde (2k) were also higher than 99% (entries 10 and 11). Then PMB ethers of secondary benzylic alcohols (1l–1o) were subject to the deprotection/oxidation, a longer reaction time or increased catalyst loading was needed, comparing with PMB ethers of primary benzylic alcohols (entries 12–15). When PMB ether of diphenylmethanol (1p) was used as the substrate, it could be converted to benzophenone (2p) in 93% isolated yield (entry 16).20 A 90% selectivity of 2chloro-5,6-dihydrocyclopenta[b]pyridin-7-one (2q), along with 10% of 2-chloro-6,7-dihydro-5H-cyclopenta[b]pyridin-7-ol, could be obtained in the 10:20:20 molar ratio of DDQ/TEMPO/TBN (entry 17). 2-Chloro-6,7-dihydroquinolin-8(5H)-one (2r), a useful intermediate for organic synthesis, was achieved from PMB ether of 2-chloro-5,6,7,8-tetrahydroquinolin-8-ol (1r) in 90% isolated yield (entry 18). With these results in hand, we tried the newly established DDQ/ TEMPO/TBN/O2 system for transformation of PMB ethers derived from aliphatic alcohols into their corresponding aldehydes or ketones. The representative results summarized in Table 2 demonstrate the effectiveness of this catalytic oxidation system for direct transformation of PMB ethers of aliphatic alcohols to aldehydes or ketones via deprotection/oxidation reaction. PMB ethers of primary aliphatic alcohols 3a and 3b could be smoothly converted into their corresponding aldehydes 4a and 4b in excellent selectivities (entries 1 and 2). PMB octan-2-yl ether (3c), a PMB ether derived from secondary aliphatic alcohol, was also successfully converted to the desired octan-2-one (4c) in 93% isolated yield (entry 3). When PMB ether of cyclohexanol (3d) was subjected to the deprotection/oxidation reaction, cyclohexanone (4d) was obtained in 95% selectivity, accompanying 5% of cyclohexanol (entry 4). The successful conversion of PMB ether of N-Boc-piperidin-4-yl-methanol (3f) to N-Boc-piperidine-4-carbaldehyde (4f) showed that this reaction could tolerate the Boc group (entry 6), and the isolated yield of 4f was 87%. The tolerance of the DDQ/ TEMPO/TBN/O2 system toward other protecting groups was also explored. A series of hexane-1,6-diol derivatives were prepared and submitted to the deprotection/oxidation reactions (entries 7–

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Z. Shen et al. / Tetrahedron Letters xxx (2015) xxx–xxx Table 1 Transformation of PMB ethers derived from benzyl alcohols into carbonyl compoundsa Entry

Substrate

Product

OPMB

1

OPMB

2

OPMB 3

OPMB

4

OPMB 5

Yieldb (%)

2.5

100

>99

97c

3:3:3

4

100

>99

92

2c

3:3:3

3

100

>99

91

2d

3:3:3

4

100

>99

90

2e

3:3:3

4

100

>99

94

2f

3:3:3

3

100

>99

93

2g

3:3:3

6

100

96

91

2h

12:12:12

24

100

95

89

3:3:3

9

100

>99

91

8:8:8

3

100

>99

87

2k

5:5:5

14

100

>99

89

2l

3:3:3

12

100

>99

90

2m

8:8:8

7

100

99

92

2n

5:5:5

3

100

>99

93

2o

5:5:5

3

100

>99

95

8:8:8

24

100

>99

93

2q

10:20: 20

24

100

90

86

2r

3:30:30

24

100

94

90

2b CHO

1c

CHO

1d

CHO

1e O

CHO

1f Cl

CHO

1g

7

Cl

Cl

OPMB 8

Cl

CHO

1h Cl

OPMB

11

Select. (%)

CHO

OPMB

10

Conv. (%)

3:3:3

2a

1b

Cl

9

Time (h)

CHO

1a

O

OPMB 6

Catalyst (mol %)

S

CHO

1i

2i S

OPMB O OPMB S

1j 1k

O

CHO

S

CHO

2j

O

OPMB

12

1l O

OPMB

13

1m O

OPMB

14

1n

Cl

Cl

O OPMB 15

1o

Cl

Cl O

OPMB 16

17

1p

Cl

1q

N

2p

Cl

N O

OPMB 18

Cl

1r

N OPMB

Cl

N O

a All reactions were carried out with 2 mmol of substrate in 10 mL of ClCH2CH2Cl. Catalyst = DDQ/TEMPO/TBN in the specified molar ratio, O2 (0.3 MPa), 100 °C (oil bath), the conversion and selectivity were determined by GC with area normalization. b Isolated yield. c GC yield with the external standard method.

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Z. Shen et al. / Tetrahedron Letters xxx (2015) xxx–xxx

Table 2 Transformation of PMB ethers and benzyl ethers derived from aliphatic alcohols into carbonyl compoundsa Entry

Substrate

Product

OPMB 1d 2 3

OPMB

4

7 8 9 10 11e

CH3(CH2)6CHO

3c

O O

3d

OPMB

5 6

3b

OPMB

3e

Boc N OPMB MeO(CH2)6OPMB MOMO(CH2)6OPMB AcO(CH2)6OPMB BzO(CH2)6OPMB

OBn

Time (h)

Conv. (%)

Select. (%)

Yieldb (%)

O 3a

CH3(CH2)7OPMB

Catalyst (mol %)

3f

O Boc N

4a

3:8:8

5

100

>99

97c

4b

3:10:10

9

100

>99

98c

4c

3:10:10

7

100

>99

93

4d

3:10:10

15

100

95

94c

4e

3:20:20

16

96

99

97c

4f

10:20:20

8

100

93

87

4g 4h 4i 4j

3:20:10 3:10:10 3:10:10 3:10:10

24 12 15 18

100 100 100 100

98 98 97 96

91 93 94 90

4b

15:15:15

24

100

97

92

O 3g 3h 3i 3j 5a

MeO(CH2)5CHO MOMO(CH2)5CHO AcO(CH2)5CHO BzO(CH2)5CHO

O

a All reactions were carried out with 2 mmol of substrate in 10 mL of ClCH2CH2Cl. Catalyst = DDQ/TEMPO/TBN in the specified molar ratio, O2 (0.3 MPa), 100 °C (oil bath), the conversion and selectivity were determined by GC with area normalization. b Isolated yield. c GC yield with the external standard method. d O2 (0.2 MPa), 80 °C (oil bath). e PhCl as the solvent, 120 °C (oil bath).

10). As can be seen, not only the stable methyl ether (3g), but also the acid-sensitive MOM group (3h), proceeded through the tandem PMB deprotection and selective alcohol oxidation to afford 4g and 4h in excellent selectivities (entries 7 and 8). Ester groups, including acetate and benzoate, were also fully stable under the deprotection/oxidation conditions, and the isolated yields of 4i and 4j were 94% and 90%, respectively (entries 9 and 10). As a comparison, benzyl ether derived from octan-2-ol (5a) was employed as a representative benzyl ether substrate in the DDQ/ TEMPO/TBN/O2 system, to test the catalytic capability of deprotection/oxidation. The experiment result showed that using more catalysts, longer reaction time and higher temperature could complete the deprotection/oxidation. Up to 93% selectivity of octan-2-one, along with 7% selectivity of octan-2-ol, was observed (entry 11 vs entry 3). Benzyl ether of octan-1-ol (5b) was also tested, but the results were not satisfying. A 65% conversion of 5b in 33% selectivity to octanal could be achieved in 24 h with 10 mol % of DDQ, 10 mol % of TEMPO, and 10 mol % of TBN at 100 °C under 0.3 MPa of oxygen. The poor conversion might be due to the difficulty of deprotection of the benzyl group at 100 °C. While the temperature was increased to 120 °C with chlorobenzene as the reaction solvent, a great deal of octanoic acid was obtained and the selectivity to octanal was fairly low.21 Compound 7 is a key intermediate for the synthesis of indeno[1,2-c]isoquinolines, which possesses high biological activities. In general, 7 was synthesized from PMB ether 6 in two steps, deprotection of the PMB group with stoichiometric DDQ followed by pyridinium dichromate (PDC) oxidation. The overall yield of this two-step process was 56%.6b With our newly developed deprotection/oxidation process, compound 6 was subjected to the DDQ/ TEMPO/TBN/O2 system, the reaction could be smoothly performed and 78% isolated yield of 7 was obtained (Scheme 2). A plausible overall reaction mechanism for the transformation of PMB ethers into carbonyl compounds via deprotection/oxidation with DDQ/TEMPO/TBN catalytic system in the presence of molecular oxygen is shown in Scheme 3. Under a certain temperature TBN can release NO, which will be easily oxidized into NO2 by O2. A

DDQ (10 mol%) TEMPO (10 mol%) TBN (10 mol%)

PMBO N 6

O

O

O2 (0.3 MPa) CH2ClCH2Cl 100 oC (oil bath), 16 h

N O 7 (yield 78%)

ref 6b HO 6

DDQ, H2O CH2Cl2 (yield 73%)

PDC N O

7

CH2Cl2 (yield 77%)

8

total yield: 56%

Scheme 2. Application of DDQ/TEMPO/TBN/O2 catalytic oxidation system.

PMB ether substrate is oxidative deprotected to its corresponding alcohol by DDQ, and DDQ is converted to DDHQ. The generated alcohol is selectively oxidized to its corresponding aldehyde or ketone by TEMPO+, which is reduced to TEMPOH at the same time. Then DDQ oxidizes TEMPOH to TEMPO+ and generates DDHQ. DDHQ is subsequently oxidized to DDQ by NO2, which turns into NO immediately. In our previous research, the role of each component of DDQ, TEMPO, and TBN in the aerobic alcohol oxidation in the presence of acetic acid or TFA at ambient temperature has been revealed by detailed kinetic studies.22 In conclusion, we have successfully applied the DDQ/TEMPO/ TBN/O2 system, a metal-free catalytic oxidation system, for direct transformation of PMB ethers into their corresponding aldehydes or ketones via a new tandem deprotection/oxidation reaction. Under the optimal reaction conditions, a variety of PMB ether substrates derived from benzylic alcohols, heteroaromatic alcohols, and aliphatic alcohols, can be converted to their corresponding aldehydes and ketones in good conversions and selectivities, with excellent functional group tolerance. This method is environmentally benign and possesses the potential application in multistep synthesis.

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Z. Shen et al. / Tetrahedron Letters xxx (2015) xxx–xxx

O N OH TEMPOH

O O2

NC

NO

Cl

H2O

H(or R2)

N + O TEMPO

O DDQ

H (or R2)

OH NO2

R

Cl

NC TBN

1

NC

Cl

NC

Cl

R1

O

PMB OH 1

R

OH DDHQ

H(or R2)

Scheme 3. A proposed overall mechanism of the DDQ/TEMPO/TBN/O2 system for the transformation of PMB ethers into carbonyl compounds.

Acknowledgement This work was financially supported by the National Natural Science Foundation of China (21376224, 21206147) and Opening Foundation of Zhejiang Key Course of Chemical Engineering and Technology, Zhejiang University of Technology (20130211).

8. 9. 10. 11.

Supplementary data Supplementary data (experimental details and copies of 1H NMR, 13C NMR, HRMS for the compounds 1a–1r, 2b–2r, 3f, 4c, 4f–4j, 5a, 5b, 6 and 7) associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.04. 033. References and notes 12. 1. (a) Larock, R. C. Comprehensive Organic Transformation, 2nd ed.; Wiley-VCH: New York, 1999; (b) Katritzky, A. R.; Taylor, R. J. K. Comprehensive Organic Functional Group Transformations II, 2nd ed.; Elsevier: Oxford, 2004. 2. (a) Hudlicky, M. Oxidations in Organic Chemistry; American Chemical Society: Washington D.C., 1990; (b) Arends, I. W. C. E.; Sheldon, R. A. In Modern Oxidation Methods; Backvall, J.-E., Ed.; Wiley-VCH: Weinheim, 2004; (c) Fernandez, M.; Tojo, G. In Oxidation of Alcohols to Aldehydes and Ketones: A Guide to Current Common Practice; Tojo, G., Ed.; Springer: New York, 2006. 3. (a) Wuts, P. G. M.; Greene, T. W. Protective Groups in Organic Synthesis, 4th ed.; Wiley and Sons: New York, 2006; (b) Kocienski, P. J. Protecting Groups, 3rd ed.; Thieme: Stuttgart, 2005. 4. Some examples of protection of hydroxyl groups, see: (a) Laserna, V.; Fiorani, G.; Whiteoak, C. J.; Martin, E.; Escudero-Adan, E.; Kleij, A. W. Angew. Chem., Int. Ed. 2014, 53, 10416; (b) Shimura, S.; Ishima, M.; Nakajima, S.; Fujii, T.; Himeno, N.; Ikeda, K.; Izaguirre-Carbonell, J.; Murata, H.; Takeuchi, T.; Kamisuki, S.; Suzuki, T.; Kuramochi, K.; Watashi, K.; Kobayashi, S.; Sugawara, F. J. Am. Chem. Soc. 2013, 135, 18949; (c) Erhard, T.; Ehrlich, G.; Metz, P. Angew. Chem., Int. Ed. 2011, 50, 3892; (d) Wu, K.-L.; Mercado, E. V.; Pettus, T. R. R. J. Am. Chem. Soc. 2011, 133, 6114. 5. For some reviews, see: (a) Steven, A. W.; Daniel, Z. Tetrahedron 2005, 61, 7833; (b) Jarowicki, K.; Kocienski, P. J. Chem. Soc., Perkin Trans. 1 2001, 2109; (c) Sartori, G.; Ballini, R.; Bigi, F.; Bosica, G.; Maggi, R.; Righi, P. Chem. Rev. 2004, 104, 199. 6. (a) Van, H. T. M.; Yang, S. H.; Khadka, D. B.; Kim, Y.-C.; Cho, W.-J. Tetrahedron 2009, 65, 10142; (b) Van, H. T. M.; Le, Q. M.; Lee, K. Y.; Lee, E.-S.; Kwon, Y.; Kim, T. S.; Le, T. N.; Lee, S.-H.; Cho, W.-J. Bioorg. Med. Chem. Lett. 2007, 17, 5763; (c) Burke, E. P. G. J. Am. Chem. Soc. 2008, 130, 14084; (d) Sugimura, H.; Sato, S.; Tokudome, K.; Yamada, T. Org. Lett. 2014, 16, 3384; (e) Boonsompat, J.; Padwa, A. J. Org. Chem. 2011, 76, 2753; (f) An, C.; Jurica, J. A.; Walsh, S. P.; Hoye, A. T.; Smith, A. B. J. Org. Chem. 2013, 78, 4278; (g) Kadota, I.; Takamura, H.; Nishii, H.; Yamamoto, Y. J. Am. Chem. Soc. 2005, 127, 9246. 7. (a) Amone, A.; Bernardi, R.; Cavicchioli, M.; Resnati, G. J. Org. Chem. 1996, 60, 2314; (b) van Heerden, F. R.; Dixon, J. T.; Holzapfel, C. W. Tetrahedron Lett. 1992, 33, 7399; (c) Nishiguchi, T.; Bougauchi, M. J. Org. Chem. 1990, 55, 5606; (d)

13. 14. 15. 16. 17. 18.

19. 20.

21. 22.

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