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First total synthesis of (6S,1′S,2′S)-hydroxypestalotin
Accepted Manuscript First total synthesis of (6S,1’ S,2’ S)-Hydroxypestalotin Venkata Satyanarayana Mallula, Batthula Srinivas, Palakodety Radha Krishna PII: DOI: Reference:
S0040-4039(15)00990-9 http://dx.doi.org/10.1016/j.tetlet.2015.06.009 TETL 46401
To appear in:
Tetrahedron Letters
Received Date: Revised Date: Accepted Date:
24 April 2015 2 June 2015 3 June 2015
Please cite this article as: Satyanarayana Mallula, V., Srinivas, B., Radha Krishna, P., First total synthesis of (6S, 1’ S,2’ S)-Hydroxypestalotin, Tetrahedron Letters (2015), doi: http://dx.doi.org/10.1016/j.tetlet.2015.06.009
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
2 Graphical Abstract
First total synthesis of (6S,1'S,2'S)-Hydroxypestalotin Leave this area blank for abstract info.
Venkata Satyanarayana Mallula, Batthula Srinivas and Palakodety Radha Krishna*
1
Tetrahedron Letters j o ur na l hom epa ge : w ww . el s ev i er . co m
First total synthesis of (6S,1'S,2'S)-Hydroxypestalotin Venkata Satyanarayana Mallula, Batthula Srinivas and Palakodety Radha Krishna* D-211, Discovery Laboratory, Organic & Biomolecular Chemistry Division CSIR-Indian Institute of Chemical Technology, Hyderabad-500 007, India.
A R TI C L E I N F O
A B S TR AC T
Article history: Received Received in revised form Accepted Available online
The first total synthesis of (6S,1'S,2'S)-Hydroxypestalotin via regioselective opening of epoxides and tandem conjugative addition-lactonization as the key steps is reported. The absolute stereochemistry of all the chiral centers of the target molecule was confirmed by chemical synthesis.
Keywords: Hydroxypestalotin, Regioselective ring-opening of epoxide, Conjugative addition-lactonization. 1. Introduction The higher mangrove plant Sonneratia caseolaris have widely distributed endophytic fungus, Pestalotiopsis microspora. The cultures of this fungal strain is considered an immeasurable reservoir of bioactive natural products1 that offers potential leads towards the treatment of various human diseases and control of certain plant diseases. The ethyl acetate extract contains new α-pyrone derivatives, one of them is a new diastereomer of 2-hydroxypestalotin (1) and its structure was assigned as depicted below (Figure 1).
the crucial intermediate 2 and the other one is on the internal epoxide 4 with titanium(IV) mediated ring-opening reaction with acid en route to epoxide 3. O OH
O OBn OH
O
O
O 2
OBn
OH 1
OBn O
OH O 1′ 6 2′ OH
O
OH
O
4
3
OBn
Scheme 1. Retrosynthesis of (6S,1'S,2'S)-Hydroxypestalotin 1
O
(6S,1'S,2'S )-Hydroxypestalotin 1 Figure 1 Structure of (6S ,1'S,2'S)-Hydroxypestalotin
As a part of our interest on the total synthesis of bioactive α,βunsaturated δ-lactone containing natural products,2 herein we report the first total synthesis of (6S,1'S,2'S)-Hydroxypestalotin 1 by a linear synthetic strategy involving regioselective ring-opening of epoxide and conjugative addition-lactonization as the crucial steps (Scheme 1). The absolute stereochemistry at C6, C1' and C2' of 1 were assigned based on the comparison of the spectral data of all the isolated 2-hydroxypestalotin natural products 3a-c as well as the synthetic diastereoisomers of 2-hydroxypestalotin3d,e and it was confirmed that 1 is 6S,1'S and 2'S, which is an enantiomer of (6R,1'R,2'R)-LL-P880β.3c Hence, it was thought logical to synthesize and unequivocally establish the absolute configuration of all the chiral centres in 1. This became necessary since the stereochemistry was assigned by a comparative data analysis. Accordingly, retrosynthetic analysis revealed that the target compound 1 could be obtained from 2 by a tandem conjugative addition-lactonization using NaOMe in methanol followed by deprotection of the benzyl groups. In order to access 2, it was proposed to perform two regioselective openings; the first one is on terminal epoxide 3 with lithiated anion of ethyl propiolate to access
Thus, to begin the synthesis, the known epoxide 4 was accessed via Sharpless asymmetric epoxidation of commercially available trans-2hexen-1-ol.4 The regioselective ring-opening reaction of epoxy alcohol 4 under titanium(IV) mediated protocol5 using benzoic acid as the nucleophile furnished the corresponding 1,2-diol monoester derivative 5 (Scheme 2) as the sole product in good yield (85%). The regioselectivity of the product was assigned based on our earlier report.5 Additional confirmation of the regioselectivity of the ringopening reaction was carried out by subjecting the product thus obtained to a NaIO4 treatment wherein an aldehyde was found formed, thus ensuring that a C3 opening occurred and the structure of the product was as assigned. This experiment also confirmed the absolute streochemistry of the compound 5 which essentially draws from epoxide 3.
2 Later, the primary hydroxy group of the resulting benzoate 5 was protected as silyl ether (Bu2SnO/imidazole/TBS-Cl/CH2Cl2/ 0 oC to rt) to afford 6 (90%). Next, ester 5 on deprotection (K2CO3/MeOH/0 o C to rt) furnished the alcohol 7 (86%). The hydroxyl groups of the resulting silyl ether 7 were protected as their benzyl ethers (NaH/BnBr/THF/ 0 oC to rt) to afford the all protected intermediate 8 (80%). Compound 8 on silyl group deprotection under conventional TBAF conditions furnished the primary alcohol 9. Next, the primary alcohol 9 was oxidized under Swern conditions (Scheme 3) to afford the corresponding aldehyde which on 1C Wittig olefination (Ph3PCH3+I-/KOtBu/THF) gave the alkene 10 (78% over two steps). Further, compound 10 on exposure to m-CPBA gave a diasteromeric mixture of epoxide 3 (90%), which on regioselective ring-opening reaction with lithiated anion of ethylpropiolate in dry THF under acidic conditions resulted in homopropargylic ester 2 (86%) as an isomeric mixture.6 OBn
OBn OH
OBn
a 10 OBn
O
OBn O
OH
TiCl4, CH 2Cl2 O
O
0 oC-rt, 75%
O
OBn syn- 11
OH 1
Towards establishing the absolute stereochemistry of synthetic 1, firstly a CD spectrum was recorded which showed ∆Ɛ247.7 = -12.4 (Please see Supporting Information for the CD spectrum) indicating that the stereochemistry at C6 stereogenic carbon as S, consistent with the reported values of a similar compound.1,3a,f Since the other two stereogenic carbons, namely C1' and C2' were garnered from a well established chemistry and consequently assigned as 'S,S'. Stitching together the complete information, it could be inferred that the absolute stereochemistry of 1 (at C6,C1',C2') as S,S,S.
O
b
9 OBn
O
3 OBn c
O
O OBn OH
OBn
O
OH 2'
2
3'
Scheme 3. Reagents and conditions: a) i) (COCl)2 , DMSO, Et3N, CH2Cl 2, -78 oC, 1 h; ii) Ph3PCH 3+I −, ΚΟtΒu, THF, -10 oC to 0 oC, 2 h, 78% (over two steps); b) m-CPBA, CHCl3, 0 oC, 4 h, 90%; c) HCCCO 2Et, n-BuLi, BF 3.OE t2, THF, -78 oC, 2 h, 86%.
As per the synthetic plan, the formation of desired pyrone took advantage of an efficient, tandem conjugate addition-lactonization sequence in one pot. Thus, the isomeric ester 2 when treated with NaOMe in MeOH led to a conjugate addition first onto the acetylenic moiety followed by the tandem intramolecular lactonization to furnish the separable lactones syn-11 and anti-11 (85% combined yield). The lactone ring-formation maybe explained presumably through the formation of E/Z geometric intermediates after the conjugate addition onto the acetylenic moiety, and then the alkoxide of E-isomer favouring the concomitant lactone formation, while the Z-isomer equilibrates towards the E-isomer (Figure 2) thus enriching the product formation. Each of separable lactones, syn-11 and anti11, were thoroughly investigated by NMR spectral analysis and found that the data of compound syn-11 was comparable with the earlier reported ent-11.3c O OBn O O OBn syn -11 (45%)
O OBn OH
OBn
O 2
NaOMe
1' OH
5
3 4 O
Figure 3. NOESY Spectrum of 1 (CDCl3) Further confirmation was obtained from the 1H-1H NOESY experiment of 1 (Figure 3) which revealed the absence of NOE between C6H/C2’H suggesting that both protons are in opposite spatial orientation. Extrapolating these observations to its precursor (compound 11 major isomer) it was deduced that C6H and C2’H shares an anti-relationship between them and hence designated as anti-11 while its C6 epimer was christened as syn-11. Additional confirmation of compound 1 was achieved from the other observed NOE correlations between C6H/C5H, C6H/C1’H, C3H/C5H and C2’H/C3’H. In conclusion, we have demonstrated a short and efficient synthetic route for the total synthesis of target compound 1 and unambiguously established its absolute stereochemistry as (6S,1'S,2'S). The key steps involved in this synthesis are regioselective ring-opening reactions of epoxides and conjugative addition-lactonization sequence. The total synthesis of (6S,1'S,2'S)-hydroxypestalotin 1 was achieved in eleven steps in an overall yield of 20.25%. Acknowledgements. Two of the authors (M. V. S and B.S) thank the CSIR and UGC, New Delhi for the financial support in the form of fellowships.
O
MeOH, 0 oC-rt, 85%
2
1 O
OBn O O
References and Notes
OBn anti -11 (30%) O
Nu - O OH
O
R Nu-
HO
H (Z)
R
O O
O
EtO HO
H (E)
R
O
Nu = OMe Figure 2: Geometry of in-situ formed enolic ether intermediates
Finally, global debenzylation of compound syn-11 (TiCl4/CH2Cl2/0 o C-rt) gave the target compound 1 (65%).7 Compound 1 was characterized from the spectral analysis and found consistent with the reported data.1,8
1. Cabrera Rönsberg, DDai, H.; Kurtán, T.; Proksch, P.; Aly, A. H.; Debbab, A.; Mándi, A.; Wray, V.; Tetrahedron Lett. 2013, 54, 3256. 2. (a) Radha Krishna, P.; Srinivas Reddy, P. Tetrahedron 2007, 63, 3995; (b) Radha Krishna, P.; Srinivas, R. Tetrahedron Lett. 2007, 48, 2013; (c) Radha Krishna, P.; Srinivas, R. Tetrahedron: Asymmetry 2007, 18, 2197; (d) Radha Krishna, P.; Srinivas, P. Tetrahedron Lett. 2010, 51, 2295. 3. (a) McGahren, W. J.; Ellestad, G. A.; Morton, G. O.; Kunstmann, M. P.; Mullen, P. J. Org. Chem. 1973, 38, 3542; b) Kirihata, M.; Ohta, K.; Ichimoto, I.; Ueda, H. Agric. Biol. Chem. 1990, 54, 2401; c) Venkatasubbaiah, P.; Van Dyke, C. G.; Phytochemistry 1991, 30, 1471; d) Kirihata, M.; Ohe, M.; Ichimoto, I.; Ueda, H. Biosci. Biotechnol. Biochem. 1992, 56, 1825; e) Kirihata, M.;
3 Kamihisa, Y.; Ichimoto, I.; Ueda, H. Chem. Express 1992, 7, 837; f) Mallula, V. S.; Srinivas, B.; Radha Krishna, P. Tetrahedron Lett. 2015, 56, 1115. 4. (a) Sharpless, K. B.; Behrens, H. C.; Katsuki, T.; Lee, M. W. A.; Martin, S. V.; Takatani, M.; Viti, M. S.; Walker, J. F.; Woodard, S. S. Pure Appl. Chem. 1983, 55, 589; (b) McKee, H. B.; Thomas, K. H.; Sharpless, K. B. J. Org. Chem. 1991, 56, 6966.
5. a) Radha Krishna, P.; Rajesh, N.; Rama Krishna, K. V. S. Tetrahedron Lett. 2014, 55, 3381; b) Radha Krishna, P.; Prabhakar, S.; Rama Krishna, K. V. S. RSC Adv. 2013, 3, 23343. 6. Hanessian, S.; Focken, T.; Oza, R. Org. Lett. 2010, 12, 3172. 7. Udawant, S. P.; Chakraborty, T. K. J. Org. Chem., 2011, 6, 6331. 8. For spectral data and experimental procedures, please see Supporting Information.
S0040-4039(15)00990-9 http://dx.doi.org/10.1016/j.tetlet.2015.06.009 TETL 46401
To appear in:
Tetrahedron Letters
Received Date: Revised Date: Accepted Date:
24 April 2015 2 June 2015 3 June 2015
Please cite this article as: Satyanarayana Mallula, V., Srinivas, B., Radha Krishna, P., First total synthesis of (6S, 1’ S,2’ S)-Hydroxypestalotin, Tetrahedron Letters (2015), doi: http://dx.doi.org/10.1016/j.tetlet.2015.06.009
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
2 Graphical Abstract
First total synthesis of (6S,1'S,2'S)-Hydroxypestalotin Leave this area blank for abstract info.
Venkata Satyanarayana Mallula, Batthula Srinivas and Palakodety Radha Krishna*
1
Tetrahedron Letters j o ur na l hom epa ge : w ww . el s ev i er . co m
First total synthesis of (6S,1'S,2'S)-Hydroxypestalotin Venkata Satyanarayana Mallula, Batthula Srinivas and Palakodety Radha Krishna* D-211, Discovery Laboratory, Organic & Biomolecular Chemistry Division CSIR-Indian Institute of Chemical Technology, Hyderabad-500 007, India.
A R TI C L E I N F O
A B S TR AC T
Article history: Received Received in revised form Accepted Available online
The first total synthesis of (6S,1'S,2'S)-Hydroxypestalotin via regioselective opening of epoxides and tandem conjugative addition-lactonization as the key steps is reported. The absolute stereochemistry of all the chiral centers of the target molecule was confirmed by chemical synthesis.
Keywords: Hydroxypestalotin, Regioselective ring-opening of epoxide, Conjugative addition-lactonization. 1. Introduction The higher mangrove plant Sonneratia caseolaris have widely distributed endophytic fungus, Pestalotiopsis microspora. The cultures of this fungal strain is considered an immeasurable reservoir of bioactive natural products1 that offers potential leads towards the treatment of various human diseases and control of certain plant diseases. The ethyl acetate extract contains new α-pyrone derivatives, one of them is a new diastereomer of 2-hydroxypestalotin (1) and its structure was assigned as depicted below (Figure 1).
the crucial intermediate 2 and the other one is on the internal epoxide 4 with titanium(IV) mediated ring-opening reaction with acid en route to epoxide 3. O OH
O OBn OH
O
O
O 2
OBn
OH 1
OBn O
OH O 1′ 6 2′ OH
O
OH
O
4
3
OBn
Scheme 1. Retrosynthesis of (6S,1'S,2'S)-Hydroxypestalotin 1
O
(6S,1'S,2'S )-Hydroxypestalotin 1 Figure 1 Structure of (6S ,1'S,2'S)-Hydroxypestalotin
As a part of our interest on the total synthesis of bioactive α,βunsaturated δ-lactone containing natural products,2 herein we report the first total synthesis of (6S,1'S,2'S)-Hydroxypestalotin 1 by a linear synthetic strategy involving regioselective ring-opening of epoxide and conjugative addition-lactonization as the crucial steps (Scheme 1). The absolute stereochemistry at C6, C1' and C2' of 1 were assigned based on the comparison of the spectral data of all the isolated 2-hydroxypestalotin natural products 3a-c as well as the synthetic diastereoisomers of 2-hydroxypestalotin3d,e and it was confirmed that 1 is 6S,1'S and 2'S, which is an enantiomer of (6R,1'R,2'R)-LL-P880β.3c Hence, it was thought logical to synthesize and unequivocally establish the absolute configuration of all the chiral centres in 1. This became necessary since the stereochemistry was assigned by a comparative data analysis. Accordingly, retrosynthetic analysis revealed that the target compound 1 could be obtained from 2 by a tandem conjugative addition-lactonization using NaOMe in methanol followed by deprotection of the benzyl groups. In order to access 2, it was proposed to perform two regioselective openings; the first one is on terminal epoxide 3 with lithiated anion of ethyl propiolate to access
Thus, to begin the synthesis, the known epoxide 4 was accessed via Sharpless asymmetric epoxidation of commercially available trans-2hexen-1-ol.4 The regioselective ring-opening reaction of epoxy alcohol 4 under titanium(IV) mediated protocol5 using benzoic acid as the nucleophile furnished the corresponding 1,2-diol monoester derivative 5 (Scheme 2) as the sole product in good yield (85%). The regioselectivity of the product was assigned based on our earlier report.5 Additional confirmation of the regioselectivity of the ringopening reaction was carried out by subjecting the product thus obtained to a NaIO4 treatment wherein an aldehyde was found formed, thus ensuring that a C3 opening occurred and the structure of the product was as assigned. This experiment also confirmed the absolute streochemistry of the compound 5 which essentially draws from epoxide 3.
2 Later, the primary hydroxy group of the resulting benzoate 5 was protected as silyl ether (Bu2SnO/imidazole/TBS-Cl/CH2Cl2/ 0 oC to rt) to afford 6 (90%). Next, ester 5 on deprotection (K2CO3/MeOH/0 o C to rt) furnished the alcohol 7 (86%). The hydroxyl groups of the resulting silyl ether 7 were protected as their benzyl ethers (NaH/BnBr/THF/ 0 oC to rt) to afford the all protected intermediate 8 (80%). Compound 8 on silyl group deprotection under conventional TBAF conditions furnished the primary alcohol 9. Next, the primary alcohol 9 was oxidized under Swern conditions (Scheme 3) to afford the corresponding aldehyde which on 1C Wittig olefination (Ph3PCH3+I-/KOtBu/THF) gave the alkene 10 (78% over two steps). Further, compound 10 on exposure to m-CPBA gave a diasteromeric mixture of epoxide 3 (90%), which on regioselective ring-opening reaction with lithiated anion of ethylpropiolate in dry THF under acidic conditions resulted in homopropargylic ester 2 (86%) as an isomeric mixture.6 OBn
OBn OH
OBn
a 10 OBn
O
OBn O
OH
TiCl4, CH 2Cl2 O
O
0 oC-rt, 75%
O
OBn syn- 11
OH 1
Towards establishing the absolute stereochemistry of synthetic 1, firstly a CD spectrum was recorded which showed ∆Ɛ247.7 = -12.4 (Please see Supporting Information for the CD spectrum) indicating that the stereochemistry at C6 stereogenic carbon as S, consistent with the reported values of a similar compound.1,3a,f Since the other two stereogenic carbons, namely C1' and C2' were garnered from a well established chemistry and consequently assigned as 'S,S'. Stitching together the complete information, it could be inferred that the absolute stereochemistry of 1 (at C6,C1',C2') as S,S,S.
O
b
9 OBn
O
3 OBn c
O
O OBn OH
OBn
O
OH 2'
2
3'
Scheme 3. Reagents and conditions: a) i) (COCl)2 , DMSO, Et3N, CH2Cl 2, -78 oC, 1 h; ii) Ph3PCH 3+I −, ΚΟtΒu, THF, -10 oC to 0 oC, 2 h, 78% (over two steps); b) m-CPBA, CHCl3, 0 oC, 4 h, 90%; c) HCCCO 2Et, n-BuLi, BF 3.OE t2, THF, -78 oC, 2 h, 86%.
As per the synthetic plan, the formation of desired pyrone took advantage of an efficient, tandem conjugate addition-lactonization sequence in one pot. Thus, the isomeric ester 2 when treated with NaOMe in MeOH led to a conjugate addition first onto the acetylenic moiety followed by the tandem intramolecular lactonization to furnish the separable lactones syn-11 and anti-11 (85% combined yield). The lactone ring-formation maybe explained presumably through the formation of E/Z geometric intermediates after the conjugate addition onto the acetylenic moiety, and then the alkoxide of E-isomer favouring the concomitant lactone formation, while the Z-isomer equilibrates towards the E-isomer (Figure 2) thus enriching the product formation. Each of separable lactones, syn-11 and anti11, were thoroughly investigated by NMR spectral analysis and found that the data of compound syn-11 was comparable with the earlier reported ent-11.3c O OBn O O OBn syn -11 (45%)
O OBn OH
OBn
O 2
NaOMe
1' OH
5
3 4 O
Figure 3. NOESY Spectrum of 1 (CDCl3) Further confirmation was obtained from the 1H-1H NOESY experiment of 1 (Figure 3) which revealed the absence of NOE between C6H/C2’H suggesting that both protons are in opposite spatial orientation. Extrapolating these observations to its precursor (compound 11 major isomer) it was deduced that C6H and C2’H shares an anti-relationship between them and hence designated as anti-11 while its C6 epimer was christened as syn-11. Additional confirmation of compound 1 was achieved from the other observed NOE correlations between C6H/C5H, C6H/C1’H, C3H/C5H and C2’H/C3’H. In conclusion, we have demonstrated a short and efficient synthetic route for the total synthesis of target compound 1 and unambiguously established its absolute stereochemistry as (6S,1'S,2'S). The key steps involved in this synthesis are regioselective ring-opening reactions of epoxides and conjugative addition-lactonization sequence. The total synthesis of (6S,1'S,2'S)-hydroxypestalotin 1 was achieved in eleven steps in an overall yield of 20.25%. Acknowledgements. Two of the authors (M. V. S and B.S) thank the CSIR and UGC, New Delhi for the financial support in the form of fellowships.
O
MeOH, 0 oC-rt, 85%
2
1 O
OBn O O
References and Notes
OBn anti -11 (30%) O
Nu - O OH
O
R Nu-
HO
H (Z)
R
O O
O
EtO HO
H (E)
R
O
Nu = OMe Figure 2: Geometry of in-situ formed enolic ether intermediates
Finally, global debenzylation of compound syn-11 (TiCl4/CH2Cl2/0 o C-rt) gave the target compound 1 (65%).7 Compound 1 was characterized from the spectral analysis and found consistent with the reported data.1,8
1. Cabrera Rönsberg, DDai, H.; Kurtán, T.; Proksch, P.; Aly, A. H.; Debbab, A.; Mándi, A.; Wray, V.; Tetrahedron Lett. 2013, 54, 3256. 2. (a) Radha Krishna, P.; Srinivas Reddy, P. Tetrahedron 2007, 63, 3995; (b) Radha Krishna, P.; Srinivas, R. Tetrahedron Lett. 2007, 48, 2013; (c) Radha Krishna, P.; Srinivas, R. Tetrahedron: Asymmetry 2007, 18, 2197; (d) Radha Krishna, P.; Srinivas, P. Tetrahedron Lett. 2010, 51, 2295. 3. (a) McGahren, W. J.; Ellestad, G. A.; Morton, G. O.; Kunstmann, M. P.; Mullen, P. J. Org. Chem. 1973, 38, 3542; b) Kirihata, M.; Ohta, K.; Ichimoto, I.; Ueda, H. Agric. Biol. Chem. 1990, 54, 2401; c) Venkatasubbaiah, P.; Van Dyke, C. G.; Phytochemistry 1991, 30, 1471; d) Kirihata, M.; Ohe, M.; Ichimoto, I.; Ueda, H. Biosci. Biotechnol. Biochem. 1992, 56, 1825; e) Kirihata, M.;
3 Kamihisa, Y.; Ichimoto, I.; Ueda, H. Chem. Express 1992, 7, 837; f) Mallula, V. S.; Srinivas, B.; Radha Krishna, P. Tetrahedron Lett. 2015, 56, 1115. 4. (a) Sharpless, K. B.; Behrens, H. C.; Katsuki, T.; Lee, M. W. A.; Martin, S. V.; Takatani, M.; Viti, M. S.; Walker, J. F.; Woodard, S. S. Pure Appl. Chem. 1983, 55, 589; (b) McKee, H. B.; Thomas, K. H.; Sharpless, K. B. J. Org. Chem. 1991, 56, 6966.
5. a) Radha Krishna, P.; Rajesh, N.; Rama Krishna, K. V. S. Tetrahedron Lett. 2014, 55, 3381; b) Radha Krishna, P.; Prabhakar, S.; Rama Krishna, K. V. S. RSC Adv. 2013, 3, 23343. 6. Hanessian, S.; Focken, T.; Oza, R. Org. Lett. 2010, 12, 3172. 7. Udawant, S. P.; Chakraborty, T. K. J. Org. Chem., 2011, 6, 6331. 8. For spectral data and experimental procedures, please see Supporting Information.