O-Glucals

Several reactions were carried out with chiral olefines. For example, only one stereoisomer 11 was isolated from the Paterno-BUchi reaction of o-glucal triacetate 10 with acetone (Scheme 3) [9]. 

The observation that L-arabinal adopted the conformation exclusively and that the and conformations of o-glucal were comparably populated led Curran and Suh" to propose a vinylogous anomeric effect , similar to the anomeric effect. Additional stabilisation of an electronegative substituent in an axial orientation was available from a p-type lone pair on oxygen, separated by a double bond, via overlap of the C X a orbital and v /2 of the enol ether (Figure 6.70). No experimental estimates of its magnitude are available, but it is likely to be smaller than the anomeric effect itself, not least because any electrostatic component is much smaller. 

Hydroxylation of the double bond is also an important result of the irradiation of o-glucal in aqueous solution in the presence of barium carbonate. n-Glucose and n-mannose are formed in considerable proportions, in the approximate ratio 1 0.8, when the irradiation is carried out in vacuo or in the presence of oxygen D-arabinose (formed by cleavage of... 

I. Lundt and C. Pedersen, Preparation of tri-O-benzoyl-2-deoxy-a-D-ribo-hexopyranosyl fluoride from derivatives of o-glucal and anhydrous hydrogen fluoride, Acta Chem. Scand., 24 (1970) 240-246. 

G. Bellucci, C. Chiappe, F. D Andrea, and G. Lo Moro, Stereoelectronic control in two-step additions to tri-O-benzyl-o-glucal initiated by electrophilic halogens. Tetrahedron, 53 (1997) 3417-3424. 

The Miller group [71] continued investigation on the cychzation of /3-hydroxy-O-benzyl hydroxamates in the presence of the Mitsimobu reagent leading to N-benzyloxy- -lactams. The Perrier rearrangement of o-glucal 91 followed by formation of hydroxamate and deacetylation provided a substrate 92 suitable for cychzation. hi the Mitsunobu reaction conditions 92 was converted into the -lactam 93, which was oxidized to the lactone 94 (Scheme 24). 

In a radical-based method for preparing C-2 branched-chain sugars by way of addition of dimethyl malonate to tri-O-acetyl-o-glucal (see Chapter 14), 1,4,6-tri-0-acetyl-2,3-dideoxy-a,p-D-erytAro-hex-2-enopyranose was formed as a byproduct in a small amount. 

From Unsaturated Sugars. — Allylic rearrangement of ally cyanates, formed in situ from carbamates such as 24 (derived originally from o-glucal triacetate), yielded isocyanates, e.g. 25, that could then be converted to allylic amine derivatives, e.g. 26 (Scheme 6). Trapping of an allylic... 

Likewise, in the production of 2,3-unsaturated aryl C-glycosides, mixed products with H or OAc at C-3 are obtained on treatment of tri-O-acetyl-o-glucal with aromatic hydrocarbons in acetic anhydride containing palladium acetate [31]. When benzene itself is used the latter type predominates (54%), and the former is formed in small proportions (10%), both compounds having the a configuration. 

Simple thermal rearrangement of tri-O-acetyl-o-glucal, or treatment in benzene containing boron trifluoride [50], or heating in acetic anhydride containing nickel(II) chloride [51], all result at equilibrium not just in the expected 2,3-unsaturated glycosyl acetates 32 (65%) but in some of the starting material (15%) and its C-3 epimer tri-O-acetyl-D-allal 33 (20%) (Scheme 12). 

Methylenecyclohexane is an example of an alkene that adds an electrophile to give a stable cationic intermediate, and with tri-O-acetyl-o-glucal and tin(IV) bromide as catalyst it gives the diene indicated in Scheme 10 stereospecificaUy in 94% yield. On the other hand, reaction of methylenecyclobutane (EtAlCl2 as catalyst) results in the chlorinated adduct 46, since the ionic reaction intermediate favours addition rather than elimination [39]. 

It follows that glycals themselves, being vinyl ethers, could react in inter-molecular fashion in the presence of Lewis acids, and in a control experiment conducted in early work on the reaction of tri-O-acetyl-o-glucal with alcohols it was found that dimerization does occur, as indicated in Scheme 17 [50]. 

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