Alkenyllithium reagents

As mentioned above, 1-alkenyl aryl sulphoxides can effectively be a-lithiated by treatment with a slight excess of LDA in THF at — 78°. The 1-(arylsulphinyl)alkenyllithium reagents 331 so generated react cleanly and rapidly with a variety of electrophiles to give 1-substituted 1-alkenyl sulphoxides 332 in high yields (equation 191). 

Arylsulphenyl halides, reactions of 282 Arylsulphenylpiperidines 623 Arylsulphinic acids, reactions of 283 Arylsulphinylalkenes 625 (Arylsulphinyl)alkenyllithium reagents 312 2-Arylsulphinyl-a-arylalkanols, synthesis of 252... 

As with organomagnesium reagents, there is usually loss of stereochemical integrity at the site of reaction during the preparation of alkyllithium compounds.25 Alkenyllithium reagents can usually be prepared with retention of configuration of the double bond.26,27... 

Alkenyllithium reagents can be alkylated in good yields by alkyl iodides and bromides.79... 

Similarly, alkenyllithium reagents add to dimethyl boronate to give adducts that decompose to Z-alkenes on treatment with iodine.27... 

Thus, the alkenyllithium reagent derived from the (X)-y-iodo allylic ether 205 underwent addition of crotylzinc bromide in ether at — 50 °C and, after hydrolysis of the resulting dimetallic reagent, compound 206 was obtained with high diastereoselectivity (anti/syn = 93/7) (equation 101)146. 

Substrate-induced diastereoselection has also been achieved in the case of alkenyllithium reagents derived from (Z)-5-iodo homoallylic ethers152,153. Thus, the allylzincation of the alkenyllithium derived from 230 proceeded efficiently in ether at — 20 °C and led after hydrolysis to 231 as a single diastereomer (equation 112). 

Methanol-0-4 methyl nitrite, and dimethyl disulfide have been examined as potential chemical probes for distinguishing between alkoxides and enolates in the gas phase.171 Methanol-0-d proved to be unsuitable and methyl nitrite reacts too slowly in contrast, the reactive ambident behaviour of dimethyl disulfide results in elimination across the C—S bond on reaction with alkoxides ( hard bases ) and attack at sulfur by enolates ( soft bases ). This probe has been applied to investigation of the anionic oxy-Cope rearrangement. The dianionic oxy-Cope rearrangement is a key step in a squarate ester cascade involving stereoinduced introduction of two alkenyllithium reagents cis to each other.172... 

M. Braun, a-Heteroatom Substituted 1-Alkenyllithium Reagents Carbanions and Carbenoids for C-C Bond Formation , Angew. Chem. Int. Ed. Engl. 1998, 37, 430 451. 

The alkenyllithium reagents are used for stereospecific syntheses of alkenes and functionally substituted alkenes. ... 

The title reaction represents the decomposition of arenesulfonylhydrazones with alkyllithium reagents to furnish unrearranged, less-substituted alkenes almost exclusively. The in situ generated alkenyllithium reagents can be further elaborated by reaction with an electrophile. Since the electrophile can be easily varied, the Shapiro reaction p>ermits the convenient transformation of a ketone into a single substituted alkene in a synthetically useful way. This section will highlight the versatility of this reaction in selective organic synthesis. 

The original Shapiro reaction involves the preparation of unfiinctionalized alkenes from ketone tosyl-hydrazones by quenching of the in situ generated alkenyllithium reagents with water. Recent applications of this reaction in natural product synthesis include the synthesis of 9(0)-methanoprostacyclin (Scheme 12), the in vitro conversion of humulene to A ( -capnellene (Scheme 13), the synthesis of die basic skeleton of isoadsirene (equation S2) ° and the total synthesis of the eudesmanolides rothin A and rothin B (equation 53). ... 

The ability of the trisylhydrazone dianion to undergo alkylation further enhances the synthetic potential of the Shapiro reaction. The initial carbonyl alkylation of acetone trisylhydrazone dianion, followed by fragmentation to the functionalized alkenyllithium reagent and subsequent trapping with carbon diox-... 

Addition of a number of alkyUithium and alkenyllithium reagents and phenyl-lithium to 558 was successful. However, efforts to introduce functionality at C(4) in 561 via organolithiums 562-566 was unsuccessful, despite considerable experimentation. The authors reported that FVP of 560 usually afforded better yields of 561 and cleaner material. Cyclization of 560 with TMSOTf was favored for higher molecular weight substrates. A limitation of the methodology concerned preparation of 558, which was not available by alkylation using LDA. 

Another major contributor to this research area has been the ButenschSn laboratories, whose focus has been on the double addition of alkenyllithium reagents to benzocyclobutenedione chromium complex jg4 220,221 j ej-eas the addition of alkenyllithium reagents to the squarate esters used in the Paquette studies favor the tra 5-addition pathway, addition to 184 gives exclusively sy -addition products, enabling them to undergo facile dianionic oxy-Cope rearrangements. In a similar fashion, the diketones produced are well situated to undergo intramolecular aldol additions. 

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