Alcohols anchimeric assistance

The dehydration of 1,4-cyclohexane diols emphasises the critical importance of stereochemical arrangement of the hydroxy functions in a substrate . The m-diol eliminates to give the cyclohexenol as the major product but the trans isomer yields mainly 1,4-epoxycyclohexane. As elimination from trans-1,4-cyclohexanediol occurs more rapidly than from t-butyl alcohol, anchimeric assistance was suggested, the intramolecular concerted ring closure occurring through a boat conformation, viz. 

At present, this rule fails only when functional neighboring substituents, capable of anchimeric assistance and in a convenient position with respect to the developing positive charge, can compete with bromine in the charge stabilization of the cationic intermediate (ref. 15). For example, the reaction of some unsaturated alcohols (ref. 16) goes through five- or six-membered cyclic oxonium ions, rather than through bromonium ions. 

An interesting alcoholysis of epoxides has been reported by Masaki and coworkers <96BCSJ195>, who examined the behavior of epoxides in the presence of a catalytic amount of the Tt-acid tetracyanoethylene (TCNE, 85) in alcoholic media. Ring-opening is very facile under these conditions, typically proceeding via normal C-2 attack, as exemplified by styrene oxide (86). Certain epoxy ethers (e.g., 89) undergo C-1 attack due to anchimeric assistance. Analysis of the reaction mixtures revealed the presence of captodative ethylenes (e.g., 85) formed in situ, whieh were shown to be aetive in eatalyzing the reaction. The proposed mode of catalysis is represented by the intermediate 87. The affinity of these captodative olefins for... 

Dimethylaminoethane-2-ol (20) is a compound that, by virtue of its nucleophilic center (Me2NH+C2H40), is employed to convert protected segments bound to supports as benzyl esters into acids by transesterification into dimethylaminoethyl esters [C(=0)0C2H4NMe2] that are hydrolyzable by a dimethylformamide-water (1 1) mixture. Compound 20 readily forms esters from acid chlorides. The hydrolysis and esterification are facilitated by anchimeric assistance by the adjacent nitrogen atom (see Section 2.10). The amino alcohol also reacts with dichloromethane. 

Acid-induced wagner-meerwein rearrangements in chiral alcohols. In view of the considerable interest on ion-molecule complexes involved in gas-phase analogues of solvolytic reactions," ° " a sustained research effort has been directed to the study of Wagner-Meerwein rearrangements in cationized )8-arylaIkyl systems, under conditions excluding nucleophilic assistance by the solvent which in these systems normally interferes with anchimeric assistance of groups adjacent to the... 

There is a strong parallel between elimination reactions in solution and the dehydration of alcohols over alumina. The trans elimination reactions and the anchimeric assistance of alcohols over aluminas suggest that the dehydration must occur within either the submicroscopical pores, or crevices, or channels of the aluminas. The aluminas therefore must surround the alcohol molecules providing acid sites to act as proton donors or electron acceptors and basic sites to act as proton acceptors or electron donors. For that reason the aluminas seem to act as solvating agents and therefore may be considered as a pseudosolvents for dehydration reactions. 

Toshimitsu, A. Hirosawa, C. Tamao, K. Retention of configuration in the Ritter-type substitution reaction of chiral /j-arylthio alcohols through the anchimeric assistance of the arylthio group. Tetrahedron 1994, 50, 8997— 9008. 

The type 2 reaction is in fact the same reaction that is favoured by the cyclization possibility if there is a heteroatom in a convenient position. The latter probably also provides anchimeric assistance to the expulsion of a neutral fragment. The example of cyclohexanediols provides a good illustration. As is shown by the following data, the comparison of ty-amino alcohols H2N(CH2) OH with various linear chain lengths also... 

The azide procedure for peptide synthesis and particularly for fragment condensations is considered to be a mainly racemization free method. This low racemization tendency of azides was explained by several theories, which have been reviewed.t l The most plausible cause of racemization is the formation of oxazoles (Scheme 3) and the related enolization. In presence of bases the a-carbon proton is readily abstracted to form an anionic oxazol-5(4//)-one resonance system.For the formation of the oxazol-5(4//)-one the influence of the substituent Y on the a-carbonyl is essential. Since the a-carbonyl group of amino acid azides are less activated and thus relatively insensitive to oxygen containing nucleophiles such as water and alcohols, oxazol-5(4//)-one formation is largely prevented. It was proposed that the soft electron shell of the azide shields the a-carbonyl atom, so that only strong nucleophiles can attack it.t 1 The reactivity towards amines can be explained in a manner analogous to the aminolysis of anchimerically assisted active esters.h 1... 

There is much additional evidence to support the postulate that the effect of neighboring sulfur is due to anchimeric assistance. Cyclohexyl chloride undergoes solvolysis in ethanol-water to yield a mixture of alcohol and ether. As usual for secondary alkyl substrates, reaction is SnI with nucleophilic assistance from the solvent (see Sec. 14.17). A C5H5S— group on the adjacent carbon can speed... 

The addition of a magnesium hydride to propargylic alcohols proceeds with a Cp2TiCl2 catalyst, in which the oxomagnesium species may provide anchimeric assistance for the addition [26] (see Section 15.3( 10)). 

Brown has shown that on introduction of a double bond to isomeric p-nitro-benzoates 52 and 53 the rate of solvolysis in either case decreases equally (about fivefold) which is due to the inductive double-bond effect and clearly points to the lacking anchimeric assistance in the solvolysis of206 this is also evidenced by the absence of tricyclic compounds among the reaction prcKiucts. Tlte isomers 206 and 207 are characterized by a high exotendo rate ratio and pr iominant formation of exo alcohol. 

Further, the olefinic alcohols (11a and 11b) derived from an (l )-glyceraldehyde derivative, upon PdCl2- and Pd(OAc)2-mediated cyclization, gave hitherto important 2-deoxy C-alkyl (12) and C-vinyl glycosides (13), respectively, albeit as 1 1 mixtures, due to the absense of anchimeric assistance in stereodefining (Scheme 22.4). 

Thionyl chloride Is highly electrophilic, and converts the alcohol to the corresponding alkyl chloride via an addition-elimination process (with neighbouring group or anchimeric assistance of the SMe group). 

A rather careful study of the competition between 0 -5, 0 -6, and 0 -3 participation in the reactions of a number of O-toluene-p-sulfonyl-alditols and -deoxyalditols has appeared. While the order of anchimeric assistance in the alkaline hydrolysis of cu-chloro alcohols is O -3 > 0 -5 > 0"-6, Hartman and Barker (second reference in Ref. 108) found the order primary 0 -5 > secondary 0 -5 = secondary 0 -3 > primary 0 -6 from their studies of primary O-toluene-p-sulfonyldeoxyalditol displacements. Thus, alkaline treatment of (206) yields (207) directly and not via the epoxide (208), since epoxide (209) yielded only glycol and none of the tetra-... 

---