C-Aryl glucals

Furthermore, coupling to form C-aryl glucals, as precursors to C-aryl glycosides, was carried out both from the C-metallated glucal154 and onto the C-iodo glucal (equations 90 and 91)155. A synthesis of vineomycinone B2 methyl ester has resulted from this chemistry. A vinyl-vinyl coupling reaction has also formed the cornerstone of aryl steroid synthesis (equation 92)156. 

Friesen, R W, Loo, R W, Preparation of C-aryl glucals via the palladium catalyzed coupling of metalated aromatics with l-iodo-3,4,6-tri-0-(triisopropylsilyl)-D-glucal, J. Org. Chem., 56, 4821-4823, 1991. 

C-Aryl glucals. l-lodo-3,4,5-tri-o(triisopropylsilyl)-l>glucal I can be prepared in... 

In the presence of a Pd catalyst, CIPd[P(Cf,Hvinyl iodide couples with mctalatcd aromatics, particularly arylzinc chloride or arylboronic acids. Aryl Grignard reagents, however, couple in very low yield. The resulting C-aryl glucals can be converted to C-aryl glycosides by functionalization of the enol ether double bond.2... 

The use of an organoindium glucal in palladium catalyzed [Pd(PPh3)2] cross-coupling reactions with aryl iodides gave moderate yields (40-60%) of C-l aryl glucals.45... 

In one additional example of the formation of C-arylglycosides from glycals, Ichikawa, et al.,65 illustrated the applicability of this chemistry to heterocyclic aryl species. Specifically, as shown in Scheme 2.4.15, tri-O-acetoxy glucal was treated with furan and borontrifluoride etherate. The result was a 54% yield of the desired C-furanoglycoside as a 1 1 anomeric mixture. 

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. 

Likewise, activated aromatic compounds give mainly -linked aryl C-glyco-sides from tri-O-acetyl-D-glucal (1) in the presence of Lewis acids, presumably because the bulky aryl groups can attain the favoured -equatorial orientation by isomerization via acyclic C-1 carbenium ions which are stabilized by both the allylic and aryl rr-systems [44]. 

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