C–C Bond Formation
2.5
333
⊡ Scheme 51
⊡ Scheme 52
triethyl orthoacetate in the presence of propanoic acid and hydroquinone, obtaining ethyl
2-{(2S,3S,6S)-2-allyloxy-6-[(tert-butyldiphenylsilanyloxy)-methyl]-3,6-dihydro-2H-pyran-3yl} acetate 190 as the only product via Claisen–Johnson rearrangement [71] ( > Scheme 52).
Krohn reported a base-catalyzed vinyl tosylate-cuprate cross coupling to form regio- and stereodiverse C-branched 1,6-anhydrosugar building blocks [72]. As shown in > Scheme 53, treatment of epoxide 1,6:3,4-dianhydro-2-O-tolylsulfonyl-β-D-galactopyranose 191 in THF with
Gilman methyl cuprate afforded 1,6-anhydro-2,3-dideoxy-2-methyl-β-D-threo-hex-2-enopyranose 192 as the sole product. The reaction was initiated from the rearrangement of epoxide
to allyl alcohol under basic conditions, and followed by a subsequent cross-coupling between
vinyl tosylate and the cuprate. Under the same reaction conditions, 1,6:2,3-dianhydro4-O-p-tolylsulfonyl-β-D-mannopyranose 193 and 1,6:3,4-dianhydro-2-O-p-toluolsulfonylβ-D-altropyranose 195 was converted into 1,6-anhydro-3,4-dideoxy-4-methyl-β-D-threohex3-enopyranose 194, and 1,6-anhydro-2,3-dideoxy-2-methyl-β-D-erythrohex-2-enopyranose
196b, respectively. Compound 195 was also reacted with the corresponding n-butyl Gilman
cuprate to afford 1,6-anhydro-2-butyl-2,3-dideoxy-β-D-erythrohex-2-enopyranose 196c in
68% yield, in addition to 22% of 1,6-anhydro-2,3-dideoxy-β-D-erythrohex-2-enopyranose
196a. The product 196a may be formed by hydrolysis of intermediates, such as metalated
vinyl species.
An efficient route towards the syntheses of chiral 2-C-methylene-O-glycosides and chiral
pyrano[2,3-b][1] benzopyrans using InCl 3 as the catalyst was reported [73]. Accordingly, reaction of 2-C-acetoxymethyl glycal derivatives and aliphatic (or aromatic) hydroxyl compounds
in the presence of InCl 3 via Ferrier rearrangement furnished the corresponding 2-C-methylene
glycosides ( > Scheme 54). InCl 3 , In(OTf) 3 , and Yb(OTf) 3 were tested to catalyze the Ferrier rearrangement of 2-C-acetoxymethyl-3,4,6-tri-O-benzyl-D-glucal 197a and the best result
was obtained with 30 mol% InCl 3 . The substrates 197a and 197b, on treatment with benzyl
2.5
333
⊡ Scheme 51
⊡ Scheme 52
triethyl orthoacetate in the presence of propanoic acid and hydroquinone, obtaining ethyl
2-{(2S,3S,6S)-2-allyloxy-6-[(tert-butyldiphenylsilanyloxy)-methyl]-3,6-dihydro-2H-pyran-3yl} acetate 190 as the only product via Claisen–Johnson rearrangement [71] ( > Scheme 52).
Krohn reported a base-catalyzed vinyl tosylate-cuprate cross coupling to form regio- and stereodiverse C-branched 1,6-anhydrosugar building blocks [72]. As shown in > Scheme 53, treatment of epoxide 1,6:3,4-dianhydro-2-O-tolylsulfonyl-β-D-galactopyranose 191 in THF with
Gilman methyl cuprate afforded 1,6-anhydro-2,3-dideoxy-2-methyl-β-D-threo-hex-2-enopyranose 192 as the sole product. The reaction was initiated from the rearrangement of epoxide
to allyl alcohol under basic conditions, and followed by a subsequent cross-coupling between
vinyl tosylate and the cuprate. Under the same reaction conditions, 1,6:2,3-dianhydro4-O-p-tolylsulfonyl-β-D-mannopyranose 193 and 1,6:3,4-dianhydro-2-O-p-toluolsulfonylβ-D-altropyranose 195 was converted into 1,6-anhydro-3,4-dideoxy-4-methyl-β-D-threohex3-enopyranose 194, and 1,6-anhydro-2,3-dideoxy-2-methyl-β-D-erythrohex-2-enopyranose
196b, respectively. Compound 195 was also reacted with the corresponding n-butyl Gilman
cuprate to afford 1,6-anhydro-2-butyl-2,3-dideoxy-β-D-erythrohex-2-enopyranose 196c in
68% yield, in addition to 22% of 1,6-anhydro-2,3-dideoxy-β-D-erythrohex-2-enopyranose
196a. The product 196a may be formed by hydrolysis of intermediates, such as metalated
vinyl species.
An efficient route towards the syntheses of chiral 2-C-methylene-O-glycosides and chiral
pyrano[2,3-b][1] benzopyrans using InCl 3 as the catalyst was reported [73]. Accordingly, reaction of 2-C-acetoxymethyl glycal derivatives and aliphatic (or aromatic) hydroxyl compounds
in the presence of InCl 3 via Ferrier rearrangement furnished the corresponding 2-C-methylene
glycosides ( > Scheme 54). InCl 3 , In(OTf) 3 , and Yb(OTf) 3 were tested to catalyze the Ferrier rearrangement of 2-C-acetoxymethyl-3,4,6-tri-O-benzyl-D-glucal 197a and the best result
was obtained with 30 mol% InCl 3 . The substrates 197a and 197b, on treatment with benzyl