2-Lithio-l ,3-dioxane

Dialkoxymethyllithium compounds, for example 2-lithio-l,3-dioxan (311), are generated in situ as shown in equation 69, either by reductive lithiation of a phenyl thioether with a lithium arene or by transmetallation of the corresponding trialkylstannyl compound. Subsequent quenching with electrophiles leads to the usual alkylated or functionalized species ... 

The reactivity of anions 433-436 with electrophiles has been studied by Shiner and coworkers. The cyclic derivatives 435 and 436 are relatively stable at —78 and —45 °C, respectively, the transformation of 2-lithio-l,3-dioxane 436 (generated from the corresponding precursor 439) into derivatives 440 occurring in high yields (Scheme 115)649. Michael addition to cyclohex-2-enone took place in the presence of CuI P(Bu-n)3 and BF3 OEl2. 

As already demonstrated in the previous natural product synthesis, the alkylation of 2,2-dimethyl-l,3-dioxan-5-one SAMP/RAMP hydrazones is a reliable tool with which to synthesize chiral 4-substituted 2,2-dimethyl-l,3-dioxan-5-ones in gram quantities and with high enantiomeric excesses [68]. Thus, after metalla-tion of the RAMP hydrazone (R) -96 the corresponding lithio azaenolate was alkyl-... 

The reaction of bis(phenylsulfanyl)alkyllithiums with trialkylboranes provides the adduct 144, after elimination of phenylsulfanyllithium. Final oxidation with hydrogen peroxide-dioxane in aqueous sodium acetate afforded aldehydes or ketones in good yields (Scheme 40)151,152. However, this process cannot be carried out with 2-lithio-l,3-dithiane because the initially formed borate did not undergo the spontaneous alkyl migration reaction152. 

Dialkoxymethyllithiums 43 3649 and 434649 651, 2-lithio-l,3-dioxolane 435649 and 2-lithio-1,3-dioxane 436649 are formyl anion equivalents, which have been prepared either by reductive lithiation of 2-(phenylsulfanyl) substituted precursors at —95 °C649 or by trans-metallation of 2-(tri-n-butylstannyl) substituted compounds at —110649 or —78°C651. The starting acyclic phenylsulfanyl precursors can be prepared from the corresponding orthoformates by reaction with (phenylsulfanyl)trimethylsilane and trimethylsilyl triflate as catalyst (for compounds 433 and 434). The cyclic derivatives (435, 436) were prepared from l,2-bis(l,3-dioxolan-2-yloxy)ethane and propane, in the same way649. 

In contrast, with cuprates derived from 2-lithiotetrahydropyrans and 4-lithio-l,3-dioxanes, bearing at least one further stereogenic center, the stereochemical integrity is maintained. 

Silyl, germanyl and stannyl alk-l-ynyl ketones have been prepared from 2-lithio-2-(trimethylsilylethynyl)-l,3-dioxolane 448. The deprotonation of the dioxane 447 with n-BuLi at — 65 °C afforded the acyl anion 448 which, after reaction with trimethylsilyl, trimethylgermanyl and trimethylstannyl chloride, gave the expected derivatives (Scheme 117)658. Hydrolysis of these products with 0.01 M sulfuric acid at room temperature in aqueous acetone gave the corresponding acyl derivatives 449. On the other hand, the reaction of the intermediate 448 with alkyl halides allows the synthesis of acetylenic ketones659. 

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