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431.
Jinbo Zhao Yu Liu Qiwen He Dr. Yuxue Li Prof. Dr. Shengming Ma Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(42):11361-11372
The highly selective tuning of the isomerization from 1‐arylalka‐1,2‐dien‐1‐yllithium to 1‐arylalka‐1,2‐dien‐3‐yllithium has been realized in the deprotonation of 1‐arylalk‐1‐yne (conditions A and B) and carbolithiation of 1‐arylbut‐3‐en‐1‐yne with alkyllithium (conditions C and D). Subsequent transmetallation and Pd‐catalyzed Negishi coupling reactions afforded 1,1‐diaryl or 1,3‐diaryl allenes with high selectivity. Deuterium‐labeling cross experiments indicated that an intermolecular lithiation process occurred in both 1,3‐lithium shift conditions (conditions B and D). 1‐Arylalka‐1,2‐diene was confirmed experimentally to be the intermediate. A computational study at the B3LYP level for the isomerization indicated that the acidity of H at the 3‐position is higher than that of the H at the 1‐position of 1‐phenyl‐1,2‐butadiene. Under conditions B, iPr2NH acts as a proton carrier to finish the 1,3‐lithium shift. The overall activation barrier for the rate‐determining step in the solvated models is ≈21.0 kcal mol?1, indicating that the isomerization is reasonable at room temperature. For the isomerization under conditions D, DFT calculations indicated that the addition of TMEDA (tetramethylethylenediamine) and HMPA (hexamethylphosphoramide) changes the global minimum of the system; among the possible mechanisms (P1–P5) considered, the mechanism catalyzed by dilithiated species (P5) is the most probable one. The overall activation barriers for isomerization in THF and TMEDA solvated models are 22.6 and 19.7 kcal mol?1, respectively, proving that the isomerization may proceed at RT in THF or at ?78 °C with TMEDA, due to the fact that the solvation of the additives may increase the concentration of 1‐phenyl‐1,2‐butadienyllithium monomer by a deaggregation effect. 相似文献
432.
Claire Kammerer Guillaume Prestat Dr. David Madec Dr. Giovanni Poli Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(17):4224-4227
Free to decide : Various 4‐(α‐styryl) γ‐lactams are synthesized in 61–88 % yield by a phosphine‐free palladium‐catalyzed carbopalladation/allylic alkylation domino sequence (see scheme). The cyclization is totally regio‐ and diastereoselective in favor of the 3,4‐trans‐disubstituted γ‐lactam. The process is successfully applied to the synthesis of a new aza analogue of the naturally occurring lignan (+)‐oxo‐parabenzlactone.
433.
434.
José Luis García Ruano Prof. Dr. Vanesa Marcos José Alemán Dr. 《Angewandte Chemie (International ed. in English)》2009,48(17):3155-3157
No hurdle is too high : The regioselectivity and stereoselectivity of the halohydroxylation of non‐activated allenes are controlled by a remote sulfinyl group through anchimeric assistance (see scheme). The resulting halohydrines are excellent chiral targets for the preparation of optically pure propargylic alcohols and Baylis–Hillman‐type products.
435.
Xiaobing Zhang Prof. Dr. Chunling Fu Prof. Yihua Yu Prof. Dr. Shengming Ma 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(42):13501-13509
A highly stereoselective iodolactonization of 4‐allenoic acids with a new sterically demanding electrophilic iodination reagent to afford optically active γ‐butyrolactones has been developed. The reaction shows high efficiency of axial chirality transfer and excellent Z/E selectivity and has been applied to the synthesis of chiral cis‐β,γ‐disubstituted γ‐butyrolactones to give very high diastereomeric and enantiomeric excess values. The reaction has been successfully utilized in the synthesis of naturally occurring compounds (+)‐cis‐whisky lactone and (+)‐cis‐3‐methyl‐4‐decanolide. 相似文献
436.
Treatment of the osmabenzene [Os{CHC(PPh3)CHC(PPh3)CH} Cl2(PPh3)2]Cl ( 1 ) with excess 8‐hydroxyquinoline produces monosubstituted osmabenzene [Os{CH C(PPh3) CHC(PPh3)CH}(C9H6NO)Cl(PPh3)]Cl ( 2 ) or disubstituted osmabenzene [Os{CHC(PPh3)CHC(PPh3)CH} (C9H6NO)2]Cl ( 3 ) under different reaction conditions. Osmabenzene 2 evolves into cyclic η2‐allene‐coordinated complex [Os{CH?C(PPh3)CH=(η2‐C?CH2)}(C9H6NO)(PPh3)2]Cl ( 4 ) in the presence of excess PPh3 and NaOH, presumably involving a P? C bond cleavage of the metallacycle. Reaction of 4 with excess 8‐hydroxyquinoline under air affords the SNAr product [(C9H6NO)Os{CHC(PPh3)CHCHC} (C9H6NO)(PPh3)]Cl ( 5 ). Complex 4 is fairly reactive to a nucleophile in the presence of acid, which could react with water to give carbonyl complex [Os{CH?C(PPh3)CH?CH2}(C9H6NO) (CO)(PPh3)2]Cl ( 6 ). Complex 4 also reacts with PPh3 in the presence of acid and results in a transformation to [Os {CHC(PPh3)CHCHC}(C9H6NO)Cl (PPh3)2]Cl ( 7 ) and [Os{CH?C(PPh3) CH=(η2‐C?CH(PPh3))}(C9H6NO) Cl(PPh3)]Cl ( 8 ). Further investigation shows that the ratio of 7 and 8 is highly dependent on the amount of the acid in the reaction. 相似文献
437.
All(enes) great! A novel Rh(III)-catalyzed oxidative coupling with allenes under mild conditions provides heterocycles with exocyclic double bonds. This reaction features low catalyst loadings, high regio- and stereoselectivity, and excellent substrate scope. The products were derivatized and preliminary mechanistic studies were conducted. 相似文献
438.
Dr. Fabian Pfrengle Prof. Dr. Hans‐Ulrich Reissig 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(39):11915-11925
A stereodivergent synthesis of differently configured C2‐branched 4‐amino sugar derivatives was accomplished. The Lewis acid mediated rearrangement of phenylthio‐substituted 1,2‐oxazines delivered glycosyl donor equivalents that can directly be employed in glycosidation reactions. Treatment with methanol provided internally protected amino sugar equivalents that have been transformed into the stereoisomeric methyl glycosides 28 , ent‐ 28 , 29 , ent‐ 29 and 34 in two simple reductive steps. Reaction with natural carbohydrates or bicyclic amino sugar precursors allowed the synthesis of homo‐oligomeric di‐ and trisaccharides 44 , 46 and 47 or a hybrid trisaccharide 51 with natural carbohydrates. Access to a bivalent amino sugar derivative 54 was accomplished by reaction of rearrangement product 10 with 1,5‐pentanediol. Alternatively, when a protected L ‐serine derivative was employed as glycosyl acceptor, the glycosylated amino acid 60 was efficiently prepared in few steps. In this report we describe the synthesis of unusual amino sugar building blocks from enantiopure 1,2‐oxazines that can be attached to natural carbohydrates or natural product aglycons to produce new natural product analogues with potential applications in medicinal chemistry. 相似文献
439.
440.