Organolithium reagents, addition

The addition of mild nucleophiles occurs preferentially in a (as described below) but with hydride, organomagnesium, or organolithium reagents addition in y can also be observed, leading to isolable y-pyrans. 

The asymmetric addition of organolithium reagents to arylox azolines has been used to construct highly complex polycyclic terpene structures found in natural products. For example, the asymmetric addition of vinyllithium to chiral naphthyloxazoline 3 followed by treatment of the resulting anionic intermediate with iodoethyl dioxolane 61... 

The equatorial selectivity observed with organolithium reagents is enhanced in diethyl ether as the reaction solvent by the addition of lithium perchlorate (Table l)12. I3C-NMR studies47 indicate that the formation of a complex between lithium perchlorate and the carbonyl group, which also leads to a dramatic enhancement of the rate of the addition reaction, accounts for the increased diastereoselectivity. 

Addition of Grignard and organolithium reagents to imines 2. derived from enantiomerically pure (S)-valinol (1), provides a-substituted phenethylamines 3 in moderate to good yield and excellent diastereoselectivity (in each case only one diastereomer can be detected by NMR)15. By appropriate selection of imine and organometallic reagent both diastereomeric amines are accessible (see also refs 16 and 17). 

Table 1. Enantioselectivc Addition of Organolithium Reagents to Imines 1 Mediated by the Enantiomerically Pure Amino Ether 241...

The enantiomerically pure vinylic organolithium reagent 8 is prepared from (A )-ethyl lactate in four steps. Subsequent addition of A-mesitylsulfonylimines 7 (they provide better diastereo-selectivilies than the corresponding jV-Losylimines) at — 110°C in tetrahydrofuran affords the adducts 9a and 9b in moderate yields (20-63%) but high diaslereoselectivities (d.r. 96 4 — 98 2)l0. 

Table 2. Stereoselective Addition of Organolithium Reagents to Racemic Hydrazones l2 OR1...

In order to prove the utility of this method and to ascertain the absolute configuration of the products, (S)-alanine has been enantioselectively prepared. The key step is the addition of methyllithium to the AjA -dimethyl hydrazone acetal 4c, derived from diol 3c. In accordance with 13C-NMR investigations it can therefore be assumed that all major diastereomers resulting from the addition of organolithium reagents to hydrazone acetals 4a-c derived from diols 3a, 3b or 3c (Table 3, entries 1 -6) have an S configuration at the newly formed stereogenic center. 

Excellent results are achieved in the diastereoselectivc addition of Grignard and organolithium reagents to hydrazones 17 or 19 derived from either r,-ephedrine17 19 or L-valinol20. 

Table 5. Addition of Organolithium Reagents R2Li to Oxime Ethers 4 and Reduction of the Resulting Alkoxyamines 5 to Amines 6"...

The reaction of butyllithium with 1-naphthaldehyde cyclohexylimine in the presence of (/C )-l,2-diphenylethane-1,2-diol dimethyl ether in toluene at —78 °C, followed by treatment with acetate buffer, gave 2-butyl-1,2-dihydronaphthalene-l-carbaldehyde, which was then reduced with sodium borohydride in methanol to afford (1 R,2.S)-2-butyl-1 -hydroxymcthyl-1,2-dihydronaphthalene in 80% overall yield with 91 % ee83. Similarly, the enantioselective conjugate addition of organolithium reagents to several a,/J-unsaturated aldimines took place in the presence of C2-symmetric chiral diethers, such as (/, / )-1,2-butanediol dimethyl ether and (/, / )- ,2-diphenylethane-1,2-diol dimethyl ether. 

The addition reactions of alkyllithium-lithium bromide complexes to a-trimethylsilyl vinyl sulfones that have as a chiral auxiliary a y-mono-thioacetal moiety derived from ( + )-camphor are highly diastereoselective. A transition state that involves chelation of the organolithium reagent to the oxygen of the thioacetal moiety has been invoked. The adducts are readily converted via hydrolysis, to chiral a-substituted aldehydes22. 

ADDITION OF ORGANOLITHIUM REAGENTS TO ALLYL ALCOHOL 2-METHYL-l-HEXANOL... 

The Addition of Grignard Reagents and Organolithium Reagents to Aldehydes and Ketones... 

Mechanisms for the addition of organolithium reagents have been investigated... 

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