Glycals with substituents

Glycals with C-2 substituents that have unsaturated, electron-withdrawing character undergo nucleophilic addition at C-l and are therefore potential glycosylating agents. The use of tri-O-benzyl-2-nitro-D-galactal in the synthesis of 2-amino-2-deoxy-a- and /1-galactosides by this approach has already been mentioned (Section II.2.a.i). 

They observed mainly a-glycoside products in their examples using glycals with all equatorial substituents. Danishefsky [84] has obtained, in allal series, a P-glycoside with the BMLF protocol. Earlier physical organical studies had shown that the hydration of enol ethers occurs via an AdE2 (addition-electrophilic-bimo-lecular) mechanism, i.e., a first step of protonation to form an oxocarbenium ion followed by a second step of nucleophilic addition [85, 86] (Fig. 5.45). 

Using this approach, different isomers of di-substituted exo-glycals can be prepared as stereochemically pure isomers by simply choosing the order of introduction of the aromatic or heteroaromatic substituents (compare 9d and 9e in Scheme 4). Interesting exo-glycals with non-aromatic substituents can be obtained by Stille type coupling provided that this substituent is not introduced first as it was sensitive to Suzuki conditions. 

Refluxing acetylated glycals with phenols in chlorobenzene without any acid catalyst gives the usual 0-linked rearranged compounds as o/p-mixtures. Use of phenols with NO2 and Bu substituents allows for easy crystallization of pure a-anomers and consequently multigram quantities of these products can be made. The products are readily deacetylated and converted to 0-benzyl ether derivatives. ... 

The synthesis of tributylstannylglycals by treating glycals with /-butyllithium then tributyltin chloride is successful when the 0-3 is unsubstituted otherwise furans are formed. However, Scheme 1 shows how glycal phenyl sulfones with 0-3 substituents can be converted to the C-stannyl analogues, useful for preparing 1-C-arylglycais. ... 

Reactions of some furanoid glycals having free 3-hydroxyl (518 and 522) and 3-0-benzyl groups (520 and 524) with AcOF gave different products in the former and the latter groups of compounds, respectively, addition reaction occurred mainly from the same (to give 519 and 523, respectively) and from the opposite sides [to give 521 and 525 (with 526), respectively] of the substituents at C-3 (see Table V). 

Reaction with glycals.1 (E)-l reacts with glycals in the presence of BF, eth-erate to give pyrans substituted at C2 with a 3-methylpropenyl group that is trans to a substituent at C6. 

The term glycal is used to define sugar derivatives having a double bond between C-1 and C-2. Accordingly, C-1 glycals are A1,2 unsaturated carbohydrate derivatives with a carbon substituent at the anomeric position. These compounds are versatile synthetic intermediates, owing to the variety of transformations associated with their enol ether functionality, and have found ample use in the preparation of C-glycosyl compounds,1 carbohydrate mimics,2 and natural products.3... 

This caveat could be circumvented with the use of triisopropylsilyl (TIPS) protecting groups (Scheme 2a,b),12 16 or when 6-deoxy glycals (devoid of the C-6 oxygen substituent) are used as substrates (Scheme 2c,d,e).13,17,18... 

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