Schlosser s base

Appropriate substituents at C (2) can facilitate direct C (1 )-lithiation of glycals. Schmidt and coworkers [68] demonstrated that Schlosser s base achieves de-... 

To determine the diastereoselectivity of the above bora-ene reaction, boronate 193 derived from a-pinene was synthesized. Reaction of a-pinene 192 with Schlosser s base (BunLi + KOBu ) furnishes the allyl carbanion, which upon treatment with triisopropyl borate and subsequent transesterification with pinacol yields a-pinanyl pinacol boronate 193. Bora-ene reaction with this allyl boronate and S02 at — 78 °C in CH2CI2 yields the mixed anhydride 194 as a 2.3 1 mixture of diastereomers upon removal of excess S02. Treatment of this mixture of anhydrides with aryl Grignard led to the formation of two diastereomers of aryl sulfoxides 195 in 3.2 1 ratio (Scheme 33) <2006TL2783>. 

In the wake of the successful use of Schlosser s base, a variety of superbasic reagents have been studied, showing sometimes a modulated behavior. It must be pointed out, however, that the simple substitution of one component with a similar reagent (e.g., potassium tert-butoxide with another potassium alcoholate) affects the overall behavior of the base only to a small extent. In contrast, the use of different organolithium components (butyl-, sec-butyl, tert-butyl, and methyllithium) is often responsible for marked differences in the reactivity profiles.40... 

The presence of a heterosubstituent facilitates deprotonation. 4-Methyl-4H-pyran and 1,4-dimethyldihydropyr-idine are regioselectively metallated at the position next to the heteroatom by LIC-KOR or trimethylsilylmethylpotassium.47 The metallation of cyclic vinyl ethers with Schlosser s base has been successfully used in the synthesis of C-glycosides48 50 via metallation of the glucal followed by a reaction with tributyltin chloride to afford the corresponding tin derivative that could be submitted to coupling reactions with various electrophiles.48... 

The selective and efficient metallation of alkenes in the allylic position by the Schlosser s base is certainly one of the major achievements in the field of superbase chemistry. A complete list of successful alkene metallations by superbases would be too long for the purpose of this chapter but some representative examples are shown in Tables 2 and 3 as a general trend for the unsubstituted and heterosubstituted alkenes, respectively. 

Schlosser s base also metallates 1,1,3,3-tetramethylindane and 1,1,2,2,3,3-hexamethylindane, the r/ -analogs of the dirr-butyI bcnzcncs, in hexane at 25°C.148 4,4 -Di-/fV/-butylbiphenyl is efficiently doubly metallated by the... 

Thus, as shown in Scheme 11.63, compound 215 was treated with propanedithiol in the presence of boron trifluoride etherate at 0°C to give dithiane 294 in 80% yield. Treatment of the latter with Schlosser s base (butyllithium/tBuOK) in pentane at — 78°C followed by addition of oxalate 295 gave a 60% yield of the coupling product 296. Subsequent removal of the dithiane ring afforded the hemiacetal 297 in good yield. This method is the umpolung version of the method used by Ireland to solve the same synthetic problem (see Section 11.4.2.2) [155]. Another example of... 

The mixture t-BuLi/t-BuOK is known as Schlosser s base, and is one of the most powerfui bases known, itwiii abstract protons from aiiyiic or benzyiic positions, and wiii even deprotonate benzene. 

Deprotonation of an allylic proton in both isomers of bicyclic ether 173 a and 173b using Schlosser s base led to dienol 174, Eq. 112 [166]. 

While allyl and glycidyl ethers are converted into a mixture of oxetane and oxepine products with xec-butyllithium, Mordini and co-workers reported that allyl, benzyl, and propargyl epoxy ethers can be regioselectively converted into 2-vinyl, 2-phenyl, or 2-aIkynyl-3-(hydroxyalkyl) oxetanes upon treatment with either Schlosser s base or other mixed metal bases. Some of the best results were obtained with the LDA/potassium ferf-butoxide mixture (LIDAKOR, ref 194). While rearrangement of propargylic or benzylic epoxide ethers formed exclusively the four-membered oxetanes, rearrangements of allyl oxiranyl ethers show a selectivity for cyclization to the seven-membered ring. Trialkylsilyl-substituted epoxide allyl ethers also show a preference for the oxepine, and mixtures are obtained as the size of the silyl substituents is increased (Scheme 17). 

Alkenylboronic acids can be synthesized from reactive alkenylmetal species in a way similar to that described above for arylboronic acids (Section 1.3.1.1.1) [224]. Typically, alkenyl bromides or iodides are treated sequentially with n-Buli and a trialkylbo-rate (entry 1, Table 1.4). A nonpolar trienylboronic acid was synthesized using this approach [226]. As described in Section 1.2.2.2, small boronic adds tend to be highly soluble in water and may be difficult to isolate when made using the traditional approach involving an aqueous workup. In these cases, exemplified with the polymerization-prone ethyleneboronic acid synthesized from vinyknagnesium bromide, it has proved more convenient to isolate the product as a dibutyl ester by extraction of the addic aqueous phase with butanol [227]. Recently, aUcoxy-functionaHzed butadi-enyl- and styrenyl boronic esters were synthesized from a,P-unsaturated acetals by treatment with Schlosser s base and subsequent trapping with triisopropylborate (entry 2) [228]. 

Let s begin with the preparation of a 113, the precursor of 107. Known aldehyde 109 was treated with fm -crotyltributylstannane in the presence of a Lewis acid to provide 110. The relative stereochemistry between Cis and Ci9 was chelation controlled. The relative stereochemistry between C17 and C18 resulted from presumed addition of the stannane to the aldehyde via a chair-like transition state with Sn coordinated to the carbonyl oxygen. An interesting procedure was used to convert 110 to the homologous aldehyde, which cyclized to a mixture of anomeric hemiacetals 111. Dehydration of the mixture provided 112. Metallation of 112 using Schlosser s base, followed by stannylation of the resulting vinyl anion, provided 113. ... 

Ley s TPAP oxidation of triol 80 gave pentacycle 81 in 90% yield (Scheme 12.16). Schlosser s base, prepared by deprotonating diisopropylamine with n-BuLi in the presence of KO-t-Bu, was effective in the selenation of... 

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