Other Reactions of Glycals

A facile aza-Claisen [3,3] sigmatropic rearrangement takes place on treating furanoid glycal 18 with sodium hydride-trichloroacetonitrile at 0 C to give 19 as a useful intermediate for preparing dideoxynucleotides.  

The silyl enol ether 20 has been reported to undergo a facile rearrangement in the presence of Lewis acids to give levoglucosenone as the major product.  

3-Deoxyglycals undergo an iodine(III)-promoted azide transfer reaction when treated with the reagent PhI(N3)2- Thus 4,6-di-0-acetyl-3-deoxy-D-glucal affords 

0-Protected and partially 0-protected glycals (but not 3,4,6-tri-0-/erf-butyldi-methylsilyl-D-glucal) can be readily cyclopropanated with diiodomethane-diethyl-zinc to afford intermediate 1,2-C-methylene derivatives. Treatment of these latter compounds with Lewis acids was expected to produce intermediate ring-expanded oxonium ions 24 which should be readily trapped witii a nucleophile to give oxepane derivatives. This proved to be the case when cyclopropane derivative 25 was treated with trimethylsilyl triflate in the presence of trimethyl-silyl cyanide as internal nucleophile to give oxepane 26. Cyclopropane derivative 27, under similar conditions but with allyltrimethylsilane as nucleophile, reacted intramolecularly to oxepane 

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. 

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