- Chiral Synthesis via Organoboranes. 35. Simple Procedures for the Efficient Recycling of the Terpenyl Chiral Auxiliaries and Convenient Isolation of the Homoallylic Alcohols in Asymmetric Allyl- and Crotylboration of Aldehydes
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Asymmetric allyl- and crotylboration of aldehydes, RCHO, with terpenyl-based allyl- and crotylborane reagents Ter2*BAll (1), Ter2*BCrtZ (2), and Ter2*BCrtE (3, Ter* = Ipc, 4-Icr and 2-Icr; All = allyl and Crt = crotyl), afford Ter2*BOCH*(R)C*(1R)(2R)CH=CH2 intermediates 4.In these reactions, the isolation of homoallylic alcohols, HOCH*(R)C*(1R)(2R)CH=CH2 (5), can be accomplished via oxidation of 4 with alkaline hydrogen peroxide.Unfortunately, oxidative workup destroys the chiral auxiliary and produces a large amount of nonrecyclable byproduct, terpenol (Ter*OH).Further, isolation of the pure homoallylic alcohol by distillation can be difficult if it boils in the range of the abundant byproduct.Therefore, in order to recycle the chiral auxiliaries and isolate the product homoallylic alcohols in an efficient manner, we have developed the following procedures: (1) elimination workup, in which enantiomerically pure α-pinene and Δ2- and Δ3-carenes are liberated from terpenylborinates 4 by treatment with isobutyraldehyde and 1 mol percent BF3*OEt2; (2) ethanolamine workup involving treatment of 4 with ethanolamine (EA) to achieve the precipitation of the ethanolamine adducts (EA-BTer2*, Ter* = Ipc and 2-Icr, 11 and 12) from which the Ter2*BOMe can be easily regenerated; and (3) 8-hydroxyquinoline workup, involving treatment of 4 with 8-hydroxyquinoline (8-HQ) to precipitate the 8-HQ adducts (8-HQ-BTer2*, Ter* = Ipc, 4-Icr and 2-Icr, 13-15), from which the various Ter2*BOMe intermediates can be conveniently liberated.It is hoped that these procedures will significantly enhance the scope of asymmetric allyl-/crotylboration of aldehydes with Ter2*BAll (1), Ter2*BCrtZ (2), and Ter2*BCrtE (3) and serve as excellent alternatives for any catalytic versions yet to be discovered.
- Brown, Herbert C.,Racherla, Uday S.,Liao, Y.,Khanna, Vijay V.
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- Chiral Synthesis via Organoboranes. Part 32. Synthesis of B-(Cycloalk-2-enyl)-diisopinocampheylboranes of High Enantiomeric Purity via the Asymmetric Hydroboration of Cycloalka-1,3-dienes. Successful Asymmetric Allylborations of Aldehydes with B-(Cycloalk-2-enyl)diisopinocampheylboranes
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The hydroboration of cycloalka-1,3-dienes (CnH2n-4, n = 6, 7 and 8) with diisopinocampheylborane (dIpc2BH or lIpc2BH) at -25 deg C provides highly enantiomerically pure B-(cycloalk-2-enyl)diisopinocampheylboranes, l or dIpc2BCnH2n-3 (>/= 93percent ee).Surprisingly, these allylic borane derivatives retain their stereochemical integrity at -25 deg C although such compounds are capable of undergoing racemization through rapid allylic rearrangements.Furthermore, the B-(cycloalk-2-enyl)diisopinocampheylboranes achieve allylborations of aldehydes at -78 deg C and afford 1-(cycloalk-2-enyl)alkan-1-ols in 90-95percent ee and 100percent syn-diastereoselectivity.
- Brown, Herbert C.,Bhat, Krishna S.,Jadhav, Prabhakar K.
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p. 2633 - 2638
(2007/10/02)
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- Hydrated ?-bonded organometallic cations in organic synthesis. I. Allyl-, crotyl-, 1-methylallyl-, cyclohex-2-enyl-, and cinnamyl-stannation of carbonyl compounds in water
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Homoallylic alcohols can be prepared in water by allyl-, crotyl-, 1-methylallyl-, cyclohex-2-enyl-, or cinnamyl-stannation of such carbonyl compounds as aldehydes, dialdehydes, and ketones, as well as acetals.The procedure is based on: Bu2RSnCl + R'COR" + (1/2)H2O -> R(HO)CR'R" + (1/2)(Bu2SnCl)2O where R = allyl, crotyl, 1-methylallyl, cyclohex-2-enyl, or cinnamyl group, R' = H or alkyl group, R" R'= alkyl group.In most cases, the reaction products are obtained rapidly in high yields (80-100percent).Hydrated organometallic cations Bu2RSn+(aq) are partly involved.These results, together with those already published on 2-propynyl-and allenyl-stannation, indicate the value of this procedure.
- Furlani, Donatella,Marton, Daniele,Tagliavini, Giuseppe,Zordan, Michele
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p. 345 - 356
(2007/10/02)
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- Synthesis with Organoboranes; 4. Transformation of Olefins into Homoallylic Alcohols, β,γ- and α,β-Unsaturated Ketones via Allylic Diethylboranes
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Allylic diethylboranes are obtained from 1-methylcycloalkenes and 2-alkenes, respectively, via metalation with trimethylsilylmethylpotassium (generated in situ from bis(trimethylsilylmethyl)mercury and potassium sand) and subsequent reaction with chlorodiethylborane.The allylic boranes react diastereoselectively with aldehydes to give homoallylic alcohols.The alcohols are oxidized with pyridinium dichromate to β,γ-unsaturated ketones, which in turn are isomerized with aluminum oxide to conjugated ketones.The transformation provides access to 2-methylene-1-acylcycloalkanes, 3-acyl-1-alkenes and 3-acyl-2-alkenes not readily available by Friedel-Crafts acylation of olefins.
- Zaidlewicz, Marek
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p. 701 - 703
(2007/10/02)
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- Stereochemical Aspects of Aldehyde Additions to Cyclohex-2-enylstannanes
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Cyclohex-2-enylation of aldehydes, mediated with boron trifluoride etherate, involves γ-equatorial approach of the presumed aldehyde-Lewis acid adduct (RCHO-BF3), irrespective of starting stannane stereochemistry.Marginal threo selectivity characterizes homoallyl alcohol formation from benzaldehyde, whereas alkanals react with high erythro selectivity, so that 1-(1'-hydroxyalkyl)cyclohex-2-enes in this diastereomeric form are available based on allyltin chemistry.
- Young, David,Kitching, William
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p. 1767 - 1777
(2007/10/02)
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