Tert- Alkyl Alkylating agent

In cases where Noyori s reagent (see p. 102f.) and other enantioselective reducing agents are not successful, (+)- or (—)-chlorodiisopinocampheylborane (Ipc BCl) may help. This reagent reduces prochiral aryl and tert-alkyl ketones with exceptionally high enantiomeric excesses (J. Chandrasekharan, 1985 H.C. Brown, 1986). The initially formed boron moiety is usually removed hy precipitation with diethanolamine. Ipc2BCl has, for example, been applied to synthesize polymer-supported chiral epoxides with 90% e.e. from Merrifield resins (T. Antonsson, 1989). 

On the other hand, under superacidic conditions, alkanes are readily alkylated via front-side CJ-iasertion by carbocationic alkylating agents. The direct alkylation of the tertiary C—H CJ-bond of isobutylene with isobutane has been demonstrated (71). The stericaHy unfavorable reaction of tert-huty fluoroantimonate with isobutane gave a Cg fraction, 2% of which was 2,2,3,3-tetramethylbutane ... 

Trifluoroacetic acid removes tert-butyl-based protectors by the S vl mechanism, with the cation being trapped by the trifluoroacetate anion however, the tert-butyl trifluoroacetate produced is an alkylating agent, and the acid is not strong enough to protonate the side chains of methionine, tryptophan, and cysteine, so these are acceptors of tert-butyl. A scavenger is required to prevent their alkylation. Anisole... 

Benzoxazinones 141 and 143 have been reacted in a reductive radical alkylation using triethylborane as the alkyl radical source <2004SL2597>. Triethylborane could also be used in catalytic amount with isopropyl, tert-h xVj, or cyclohexyl iodide as the alkylating agent. Zinc with copper iodide could also he used as initiator (Scheme 8). 

Preparative use could be made also from the perturbed redox catalysis (Eqs. (84)—(86), (93)-(95). For example, alkylated aromatic hydrocarbons that are difficult to obtain by Friedel-Crafts alkylation could be synthesized in this way. The most suitable alkylating agent is tert-butyl chloride or bromide -ass) alkyla-... 

It was felt that a similar alkylation reaction would allow the facile introduction of the kainoid C-3 side chain if a bromoacetate ester was used in place of allyl bromide as the alkylating agent. The aim was to attempt to increase stereoselectivity by using the bulky ester tert-butyl bromoacetate. Appropriately protected 4-keto-L-proline 39 derivatives... 

At this point the conversion of /i3-(chloromethylidyne)-tris-(tricarbonylcobalt) is complete, and its reactions with a wide variety of nucleophiles, such as alcohols, thiols, amines, alkylating agents, and the more nucleophilic aromatic compounds, may be carried out. The reaction with tert-butanol is described here. 

The copper(II) carbene complexes were active in the conjugate addition to cyclo-hexenone using ZnEt as alkylation agent. The most efficient catalyst proved to be the monocarbene copper(II) complex with a tert-bxxtyl wingtip group. 

Carboxylic acids can be converted to tert-butyl esters by treatment with tert-butyl 2,2,2-trichloroacetimidate (see 10-10) and BF3<)Et2." Carboxylic esters can be formed from the carboxylate anion and a suitable alkylating agent (10-26). 

The pentacyclic compound 45 a can be smoothly quaternized at the phosphorus atom bearing three C-substituents by treatment with alkylating agents. Oxidizing agents such as tert-butyl hydroperoxide, elemental sulfur, or elemental selenium, on the other hand, preferentially attack at the 0-substituted phosphorus atom. When an excess of the oxidizing agent is employed, both phosphorus atoms are oxidized [45]. 

Electrophilic substitution of aromatic nuclei in tyrosine and tryptophan side chains has frequently been reported in connection with acidolytic removal of blocking groups. C-Benzylation and tert.butylation of the tyrosine side chain and N-alkylation of the indole nucleus in tryptophan are often attributed to the alkyl cations generated in the reaction. This common side reaction is caused, however, mainly by the alkylating agents formed in the process, such as benzyl bromide or tert.butyl trifluoroacetate. The same is true for the S-alkylation of the methionine side chain. Conversion of the thioether to a sulfonium salt can... 

---