Lithiated glycal reactions with

Dotz and coworkers prepared several interesting and novel glycosylidene carbenes (259-261) by reaction of lithiated glycal 258 with the appropriate metal carbonyl derivative. The synthetic utility of the formed carbenes was demonstrated by reaction with 3-hexyne to give a mixture of complexed and uncomplexed adducts 262 and 263, respectively. The anomeric chromium carhene 264 was converted to 265 hy exposure to ethoxy ethyne (Scheme 48) [70]. The same workers have carried out a similar chromium-mediated henzannulation (266 —> 268), this time with the chromium on the aromatic fragment [71]. Other sugar-hased carhenes have also been prepared [72]. 

Expanding upon the lithiation of 2-phenylsulfinyl glycals, Schmidt and Dietrich [33] effected coupling reactions with benzaldehyde (Scheme 7.59). As illustrated, this reaction resulted in an approximate yield of 86%. An important observation is the induction of stereochemistry at the newly formed center with a diastereomeric ratio of approximately 4 1. 

Continuing with the direct metallation of glycals, 2-phenylsulfinyl derivatives have found utility. Their formation and subsequent lithiation, shown in Scheme 3.1.4, is accomplished on reaction of glycals with phenylsulfenyl chloride under basic conditions. Subsequent oxidation with mCPBA yields the sulfinyl compound ready for lithiation on treatment with lithium diisopropylamide. Advantageous to the formation of this species is the stabilization of the anion by chelation of the sulfoxide to the metal. This procedure reported by Schmidt, et al.,5 was utilized in the preparation of C-disaccharides, discussed in Chapter 8. 

Schmidt and coworkers have described the direct lithiation at the C-2 atom of the 1-phenylsulfinyl substituted glycal (46) using LDA followed by reaction with C-electrophiles to give products such as (47) (Scheme 5.15) [51]. 

The reaction of l-(tributylstannyl)-3,4,6-tris-0-(triisopropylsilyl)-D-glucal with n-butyllithium then triflate 29 afforded the protected undecose backbone 30 found in the herbicidins. The preparation of a carbon-bridged analogue of N-acetyllactosamine by reaction of a 1-C-lithiated glycal with a 4-C-formyl protected glucosamine derivative is covered in Chapter 3. 

Under the same conditions, glycals are also formed from l-thioglycoside-2-xanthates. Phenyl thioglycopyranosides easily undergo reductive lithiation with lithium naphtalenide in tetrahy-drofuran at low temperatures and subsequent elimination of C-2 substituents [85,86]. Such reaction conditions are compatible with labile acid- and base-protecting groups and yields... 

As already discussed in Scheme 3.1.3, Hanessian, etal.,8 prepared the Ci stannyl glycal shown. Further work by this group demonstrated the ability to transform this glycal analog to a lithiated species in preparation for coupling with an aldehyde. The reaction, shown in Scheme 3.1.7, produced a 68% yield of the C-glycoside as a mixture of isomers at the newly formed stereogenic center. 

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