Glycals stannylated

Scheme 4 Tin-lithium exchange, on 1-stannyl glycals, followed by reaction with carbon electrophiles.

The advantage of this protocol over the direct lithiation method is that the lithiated glycal was produced under less demanding conditions. However, the first lithiation step is not avoided and thence the acidity related restrictions to protecting groups will still apply. In this context, Beau devised a route to 1-stannyl glycals based on a radical-mediated... 

Recent contributions from Vogel s group have shown that, under CO atmosphere and in the presence of Pd2(dba)3 and Ph3As, 1-stannyl glycals can be carbonylated and coupled to organic halides (Scheme 6a),38 or vinyl triflates (Scheme 6b),39 in carbonylative Stifle cross-coupling processes.40 Also of interest is the palladium catalyzed cross-coupling reaction of... 

Scheme 5 Palladium-catalyzed coupling reactions of C-l stannylated glycals with organic halides.

The added advantage of the C (1 )-stannylated glycals is their abUity to participate in palladium-catalyzed coupling reactions with organic halides, a process independently reported by Beau [75] and Friesen [81]. Vinyl stannane 237 can be benzylated, allylated or acylated provided that appropriate catalysts are used [75,77] and representative examples are given in Scheme 59. The C-arylation of... 

Direct C-l deprotonation-lithiation occurs on treatment of the O-benzyl-ated or -silylated glycals with strong bases such as tert-butyllithium at low temperatures, and the vinyllithiums (such as 120) can subsequently be stannylated with tributylstannyl chloride.135 Otherwise, compound 121 can be produced from S-phenyl tetra-C-benzyl-l-thio-/f-D-glucopyranoside via sulfone 122 by treatment with tributylstannane and a radical initiator.132... 

Sulfonylated glycals are also useful precursors of 1-unsaturated C-glycosyl compounds. For example, treatment of 52 with an excess of tri-n-butyltin hydride in the presence of a free radical initiator (AIBN) in refluxing toluene leads to exchange of the anomeric sulfonyl group resulting in 1-stannyl glycal 53, which in turn can be coupled with bromoaryl substrates in a palladium-catalyzed reaction to 54 (O Scheme 17) [153]. 

Aminosugar-derived glycal 141 was obtained by this method from an appropriate chiral precursor 139 [237], Unexpectedly, however, reaction of tungsten intermediate 140 with tri- -butyltin triflate and triethylamine in ether gave the glycal 141 (in 85% yield) instead of the 1-stannyl derivative (O Scheme 47). The molybdenum carbonyl-triethylamine complex afforded the same product, although in a much lower yield. 

Lithio glycal is prepared by direct abstraction of a vinylic proton with butyl lithium or Schlos-ser s base (w-BuLi and t-BuOK). Alternatively, transmetalation of 1-stannyl-glycal with butyl... 

Later work elaborating on the chemistry of glycals demonstrated the ease of formation of 1-stannyl glycals. These compounds, introduced in Scheme 3.1.1, are useful substrates for the direct formation of C-glycosides as well as for metal-metal exchanges with lithium to be discussed later in this chapter. As shown in Scheme 3.1.3, Hanessian, eta/.,4 utilized potassium tert-butoxide and butyllithium to effect the formation of 1-stannyl glycals. 

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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