Allyls conformations

NMR spectroscopy has also proven useful in studying rates of C-C bond rotation in allyl and pentadienyl species. The allyl systems are simpler to analyze since only one allyl conformation is possible (ignoring substituent orientations), whereas the pentadienyl... 

The Kishi synthesis ot monensin teatures allylic conformational analysis to predict stereochemistry of hydroboratlon-oxidations in acyclic systems. The Still synthesis features acyclic diastereoselection in carbonyl addition reactions (chelation control and Felkin-Ahn control). 

Kobylecki RJ, Lane AC, Smith CFC, Wakelin LPG, Cruse WBT, Egert E, Kennard O (1982) N-methylnalorphine Definition of N-allyl conformation for antagonist at the opiate receptor. J Mec Chem 25 1278-1280... 

Pyran-2-one, 5,6-dibromo-5,6-dihydro-reactions, 3, 735 Pyran-2-one, 5,6-dihydro-allylic bromination, 3, 799 dehydrogenation, 3, 724, 799 H NMR,3, 581 synthesis, 3, 841, 843 Pyran-2-one, 4,6-dimethyl-irradiation, 3, 677 photochemistry, 3, 678 Pyran-2-one, 5,6-dimethyl-chloromethylation, 3, 680 conformation, 3, 631 Pyran-2-one, 5-f ormyl-IR Spectra, 3, 595 Pyran-2-one, 6-formyl-IR spectra, 3, 595 Pyran-2-one, 5-halo-synthesis, 3, 799 Pyran-2-one, 3-hydroxy-IR spectra, 3, 595 Pyran-2-one, 4-hydroxy-methylation, 2, 57 3, 676 pyran-4-one synthesis from, 3, 816 reactions with phosphorus oxychloride, 2, 57 synthesis, 3, 792, 794, 795, 798 tautomerism, 2, 56 3, 642 Pyran-2-one, 4-hydroxy-6-methyl-methylation, 3, 692 reactions... 

Allyl strain influences the conformation of Z-alkenes. A 4-substituted 2-alkene will prefer conformation C over D or E to minimize the steric interaction with the C—1 methyl group. ... 

Fonnation of allylic products is characteristic of solvolytic reactions of other cyclopropyl halides and sulfonates. Similarly, diazotization of cyclopropylamine in aqueous solution gives allyl alcohol. The ring opening of a cyclopropyl cation is an electrocyclic process of the 4 + 2 type, where n equals zero. It should therefore be a disrotatory process. There is another facet to the stereochemistry in substituted cyclopropyl systems. Note that for a cri-2,3-dimethylcyclopropyl cation, for example, two different disrotatory modes are possible, leading to conformationally distinct allyl cations ... 

The transition state for such processes is represented as two interacting allyl fragments. When the process is suprafacial in both groups, an aromatic transition state results, and the process is thermally allowed. Usually, a chairlike transition state is involved, but a boatlike conformation is also possible. 

Alkyl derivatives of 1,3-butadiene usually undergo photosensitized Z-E isomerism when photosensitizers that can supply at least 60 kcal/mol are used. Two conformers of the diene, the s-Z and s-E, exist in equilibrium, so there are two nonidentical ground states from which excitation can occur. Two triplet excited states that do not readily interconvert are derived from the s-E and s-Z conformers. Theoretical calculations suggest that at their energy minimum the excited states of conjugated dienes can be described as an alkyl radical and an orthogonal allyl system called an allylmethylene diradical ... 

Wlien tlie diiral molybdenum -K-allyl-substituted enone 147 was treated witli litliium dimetliylciiptate, formation of adduct 148 witli fait selectivity was observed tSdieme 6.29) [69], Interestingly, bigber selectivities were obtained in tlie presetice of boron ttlbuotlde etlierate. It is assumed tliat Lewis acid coordination induces tlie s-trans reactive conformation 149 [64], Consequently, nudeopb de attack anti to tlie molybdetiLim ftagmetit sbould afford tlie major diastereomer 148. 

For acyclic allylic substrates die situation is mote complex, since a larger number of reactive conformations, and betice corcesponding transition states, compete. Hius, mediyl ciimamyl derivatives 163 tX= O.Acj, upon treatment witli litliiiim dimetliylcuprate, mainly gave tlie S 2 substitution product 166 fentry 1, Tab. 6.6 and Sdieme 6.34) [80]. Hie preference for die S 2 product is expected, since de-conjugation of die alkene system is electronically imfavorable. 

The synthesis of key intermediate 6 begins with the asymmetric synthesis of the lactol subunit, intermediate 8 (see Scheme 3). Alkylation of the sodium enolate derived from carboximide 21 with allyl iodide furnishes intermediate 26 as a crystalline solid in 82 % yield and in >99 % diastereomeric purity after recrystallization. Guided by transition state allylic strain conformational control elements5d (see Scheme 4), the action of sodium bis(trimethylsilyl)amide on 21 affords chelated (Z)-enolate 25. Chelation of the type illustrated in 25 prevents rotation about the nitrogen-carbon bond and renders... 

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