Scope and Facial Selectivity of the Prins-Pinacol Synthesis of Attached Rings
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1 Scope and Facial Selectivity of the Prins-Pinacol Synthesis of Attached Rings By Larry E. verman and Emile J. Velthuisen Leading Reference J. rg. Chem. 2006, 71, Presented by Zhenyu Zhong Journal Club Presentation April 3 rd 2006
2 Presentation utline verview of the Prins-Pinacol Reaction Early developments of the acid-promoted Prins-pinacol rearrangement and its potential to be the key strategic element in the total synthesis of heterocyclic and carbocyclic natural products Exploration of the scope of the Prins-pinacol reaction for assembling attached rings with various sizes and ring substitutions Rationalization of facial stereoselection in the Prins cyclization Summary Future Work Zhenyu Zhong Journal Club 2
3 Prins Reaction: The acid-catalyzed condensation of alkenes with aldehyde Nuc H H H H R R R R H + Nuc 1)H 2 2)H + H H H H H H R R carbocation Condensation of formaldehyde with some unsaturated compounds H. J. Prins, Chemisch Weekblad, 16, 64, 1072, Pinacol Rearrangement H R 1 R 2 H R 3 R 4 Lewis Acid H R 1 R 2 R 3 R 4 1,2-shift R 1 R 4 R 2 R 3 Migratory aptitude: aryl > tbu>> cyclopropyl > 2 alkyl > 1 alkyl R. Fittig. Justus Liebigs Ann. Chem. 110, (1859). Zhenyu Zhong Journal Club 3
4 Serendipitous discovery of the Prins-Pinacol Reaction H K-10 Clay HC H Acetone Reflux H Pinnacol Rearrangment H Prins cyclization H xocarbenium ion β-hydroxy carbenium ion Results two C-C bonds, one C- bond and two new stereocenters Completely Stereoselective Martinet, P.; Mousset, G.; Michel, M. C. R. Acad. Sci. Paris, Ser. C 1969, 268, Zhenyu Zhong Journal Club 4
5 ther Examples of Prins-Pinacol Reaction R 3 R 1 SnCl 4 R R 4 R 2 R 1 1) Prins Cylization 2) Pinacol Rearrangement R 3 R 1 R 4 R 3 R and R 2 = CH 3 or R 4 verman et al. J. Am. Chem. Soc. 1987, 109, Me Me TIPS SnCl 4 MeN 2 0 C 75% Me H CH verman et al Can. J. Chem. 2000, 78, 732 Zhenyu Zhong Journal Club 5
6 Natural Products Containing Attached Rings Rings joined by a C-C σ bond are prevalent in many oxacyclic and carbocyclic natural products. Prins-Pinacol reaction, like many sequential, tandem, cascade, or domino reactions, allows the construction of such rings with high stereoselective from relatively simple starting materials. Zhenyu Zhong Journal Club 6
7 Early Study on Prins-Pinacol Reaction Due to A 1,3 minimized chair conformer of the Z-alkene, a greater regio-selectivity can be achieved to form the desired Prins-Pinacol products as a mixture of methoxy epimers. Prins-Pinacol reaction on E-alkene shows less regio-selectivity. Zhenyu Zhong Journal Club 7
8 Preparation of the Prins-Pinacol Precursors Me Me H n t-buli Hexanes Tf2 Tf n 3 PdCl 2 (dppf) NaH/THF Me n n=1, 65% 2steps n=2, 58% 2steps n=3, 79% 2steps 1) LAH 2)TIPSTf Me TIPS n 20: n=1, 43% 2steps 21: n=2, 42% 2steps 22: n=3, 27% 2steps 1a: n=1 1b: n=2 1c: n=3 1d: n=0 4= 3= (Me) 2 HC (Me) 2 HC B B 4 PdCl 2 (dppf) NaH/THF Me Me n n=0, 39% 2steps n=2, 66% 2steps n=3, 46% 2steps 1) LAH 2)TIPSTf Me Me TIPS n 31: n=0, 40% 2steps 32: n=2, 64% 2steps 33: n=3, 50% 2steps Relatively short sequence to prepare the (Z)alkylidenecycloalkylnols Low overall yield Zhenyu Zhong Journal Club 8
9 Results for Various Attached Rings systems Scope and limitations: When the starting ring is Five membered, the cation produced upon Prins cyclization would not undergo Pinacol ring contraction, due to high ring constrain associated with forming of cyclobutane ring. Zhenyu Zhong Journal Club 9
10 Effect of Ring substitution upon Prins-Pinacol reaction Preparation of the Precursors H 1) t-buli Hexanes, then Tf 2 2) 4 PdCl 2 (dppf) NaH/THF Me Me 1) LAH 2)TIPSTf Me Me TIPS 32% over 2 steps 9 73% over 2steps H 1) t-buli Hexanes, then Tf 2 2) 4 PdCl 2 (dppf) NaH/THF Me Me 1) LAH 2)TIPSTf Me Me TIPS 55% over 2 steps 10 87% over 2 steps H 1) t-buli Hexanes, then Tf 2 2) 4 PdCl 2 (dppf) NaH/THF Me Me 1) LAH 2)TIPSTf Me Me TIPS 58% over 2 steps 11 69% over 2 steps Zhenyu Zhong Journal Club 10
11 Effect of Ring substitution upon Prins-Pinacol reaction cont d Geminally disubstituted precursor (48) give desire Prins- Pinacol products in moderate yield. Whereas the precursor (49) affords the Prins-Pinacol in a higher yield. Zhenyu Zhong Journal Club 11
12 Effect of Ring Substitution upon Prins-Pinacol reaction cont d Disfavoring ring contraction of carbocation was due to the developing steric interactions between the axial methyl and pseudoaxial cyclohexyl subsitituent, thus, resulting lower yield for the desired Prins-Pinacol product. Zhenyu Zhong Journal Club 12
13 Prins-Pinacol Reaction on Enatiomerically Riched Precursor Zhenyu Zhong Journal Club 13
14 Analysis of stereochemical outcome Zhenyu Zhong Journal Club 14
15 Summary A variety of attached ring systems were prepared using Prins-pinacol reaction on simple precursors, cyclohexyl- and cyclopentyl triisopropylsiloxy ethers bearing (2Z)-(6,6-dimethoxyhexylidene) or (2Z)-(6,6-dimethoxyhexylidene) side chains. The facial selectivity of Prins reaction was rationalized by the through-space electrostatic stabilizing interactions between the α-alkyloxycarbenium ion and an axially positioned oxygen subsituent. Future Work Improve the efficiency for preparation of precurors of Prins-pinacol reaction. Zhenyu Zhong Journal Club 15
16 Acknowledgements Dr. Wipf All members of Chem 2320 Thank you! Zhenyu Zhong Journal Club 16
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