Glycal Derivatives
3.6
727
⊡ Scheme 51
lacking in the case of previously described methods. In particular, this method is suitable for
the synthesis of C-linked disaccharides and their analogs via reaction of 1-thio-sugars with
iodo-sugars as shown in > Scheme 51.
The application of the Ramberg–Bäcklund reaction for the synthesis of C-glycosides and related compounds has been recently discussed in detail [247].
10 Miscellaneous Methods
Specifically substituted glycals, which are often used as chiral synthons or key substrates, can
be prepared in many different ways. Double bond formation frequently precedes the assembly
of the complete carbon framework designed for a cyclic enol ether intermediate. Of particular
interest here are glycals substituted at C-1 and C-2, which are obtained by functionalization of
the existing double bond. For example, C-2-acylated glycals offer a variety of ways to generate
synthetically useful transformations. C-2 formyl glycals were first obtained by syntheses based
on dithiane [248] and enol ether [249]. Exhaustive hydrogenation of the enone, which leads
to C-2-methyl pyrane, has been applied as the final step in the total synthesis of the antibiotic
restricitin [250]. In an alternative approach, direct formylation of methyl 157 or benzyl ethers
of glycals with Vilsmeier–Haack reagent has been achieved [251], and it has been shown that
these products 158 can undergo BF 3 /Et 2 O catalyzed C-3 arylation to 159 ( > Scheme 52) [252].
The C-2 formyl glycals have an obvious connection, through Wittig chemistry, with C-2 vinyl
glycals. The C-2 vinyl glycals, in their turn, have been studied as the diene components of
Diels–Alder cycloaddition reactions [253]. Interestingly, C-2 formylation can also be achieved
by free radical-promoted cyclization-fragmentation of suitably arranged pyranose derivatives.
For example, in one study, 2-deoxy-2-iodo– mannoside 160 was converted into formylated
glycal 161 [254] in 65% yield ( > Scheme 53), while the corresponding benzylated glycoside
afforded saturated C-2 branched products.
3.6
727
⊡ Scheme 51
lacking in the case of previously described methods. In particular, this method is suitable for
the synthesis of C-linked disaccharides and their analogs via reaction of 1-thio-sugars with
iodo-sugars as shown in > Scheme 51.
The application of the Ramberg–Bäcklund reaction for the synthesis of C-glycosides and related compounds has been recently discussed in detail [247].
10 Miscellaneous Methods
Specifically substituted glycals, which are often used as chiral synthons or key substrates, can
be prepared in many different ways. Double bond formation frequently precedes the assembly
of the complete carbon framework designed for a cyclic enol ether intermediate. Of particular
interest here are glycals substituted at C-1 and C-2, which are obtained by functionalization of
the existing double bond. For example, C-2-acylated glycals offer a variety of ways to generate
synthetically useful transformations. C-2 formyl glycals were first obtained by syntheses based
on dithiane [248] and enol ether [249]. Exhaustive hydrogenation of the enone, which leads
to C-2-methyl pyrane, has been applied as the final step in the total synthesis of the antibiotic
restricitin [250]. In an alternative approach, direct formylation of methyl 157 or benzyl ethers
of glycals with Vilsmeier–Haack reagent has been achieved [251], and it has been shown that
these products 158 can undergo BF 3 /Et 2 O catalyzed C-3 arylation to 159 ( > Scheme 52) [252].
The C-2 formyl glycals have an obvious connection, through Wittig chemistry, with C-2 vinyl
glycals. The C-2 vinyl glycals, in their turn, have been studied as the diene components of
Diels–Alder cycloaddition reactions [253]. Interestingly, C-2 formylation can also be achieved
by free radical-promoted cyclization-fragmentation of suitably arranged pyranose derivatives.
For example, in one study, 2-deoxy-2-iodo– mannoside 160 was converted into formylated
glycal 161 [254] in 65% yield ( > Scheme 53), while the corresponding benzylated glycoside
afforded saturated C-2 branched products.