Chemistry Reference
In-Depth Information
The a- and b-glycosyl isocyanates 1645 can be obtained by oxidation of glycosyl
isocyanide 1644, which, in turn, is produced by stereospecific dehydration with
triphenylphosphane/tetrabromomethane of the corresponding glycosyl formamide
1643 [1225]. The reaction of a- and b-glycosyl isocyanates with amines proceeds
smoothly to provide a- and b-glycosyl ureas with retention of the stereochemistry at
the anomeric position. This method allows the synthesis of a building block that
can be used for the construction of glycopeptide mimics with urea-glycosyl link-
ages 1647.
PPh
3
CBr
4
Et
3
N
CH
2
Cl
2
OAc
OAc
OAc
pyridine
N-oxide
AcO
AcO
AcO
+
H
2
N
O
O
O
1646
-20°C, 3 h
AcO
NC
AcO
N=C=O
AcO
NHCHO
OAc
OAc
1645
OAc
79 %
1644
1643
OAc
AcO
O
O
95 %
AcO
H
H
OAc
1647
Typical procedure. 2,3,4,6-Tetra-O-acetyl-a-
d
-glucopyranosyl isocyanide 1644 [1225]:
To
a solution of 2,3,4,6-tetra-O-acetyl-a-d-glucopyranosyl formamide 1643 (2.80 g, 7.47
mmol), triethylamine (4.48 mL, 32.2 mmol), and tetrabromomethane (8.00 g, 24.1
mmol) in dichloromethane (85 mL) cooled to
20
C under nitrogen atmosphere
was added a solution of triphenylphosphine (6.30 g, 24.1 mmol) in dichloro-
methane (5 mL). After stirring at
20
C for 3 h, the solution was diluted with di-
ethyl ether, washed with aqueous ammonium chloride solution, water, and brine,
and dried over anhydrous sodium sulfate. Concentration and purification by chro-
matography on silica gel (diethyl ether/hexane, 2:1) gave 2,3,4,6-tetra-O-acetyl-a-d-
glucopyranosyl isocyanide 1644 (0.86 g, 79%).
A synthesis of the marine diterpene geranyllinaloisocyanide 1652 has been suc-
cessfully accomplished by a novel regioselective allylamine construction, which is
achieved by an allyl cyanate-to-isocyanate rearrangement 1649
1650. The result-
ing formamide 1651 is dehydrated with triphenylphosphane/tetrabromomethane
to give the geranyllinaloisocyanide 1652 in 82% yield [1226].
!
Typical procedure. Geranyllinaloisocyanide 1652 [1226]:
A solution of the formamide
1651 (204 mg, 0.64 mmol), tetrabromomethane (594 mg, 1.79 mmol), and diiso-
propylethylamine (0.60 mL, 3.45 mmol) in dichloromethane (7 mL) was cooled to
20
C. To this solution was added dropwise a solution of triphenylphosphine
