Chemistry Reference
In-Depth Information
s salt. In this reaction, addition of 4A
molecular
sieves was found to greatly improve the reactivity of methylation with Meerwein
due to the Lewis acidic nature of Meerwein
s salt and
Proton Sponge (1) in combination and gave desired ether in 95% yield (Scheme 8.4).
Similar effects of 4 A
molecular sieves were reported by Shea et al. and Hansen et al.,as
depicted in Scheme 8.5 [12,13].
For the synthesis of various ethers from alcohols, the combination of alkyl triflate and
Proton Sponge (1) is available. The O-alkylation reaction of symmetrical alcohol proceeds
in high yield with octyl triflate in the presence of Proton Sponge (1) as a base (Scheme 8.6a).
Low yields were obtained with triethylamine or DMAP instead of Proton Sponge (1) [14].
Octadecenyl triflatewas also effectively alkylatedwith the alcohol in the presence of Proton
Sponge (1) in 66% yield [15] (Scheme 8.6b).
Proton Sponge (1) is effective for the ether-type alcohol protection reaction. Reaction
of hindered tertiary and secondary alcohols in a modified triol system with benzyl
chloromethyl ether (BOM-Cl) and Proton Sponge (1) in the presence of sodium iodide
(NaI) gave bis-BOM ether in 84% yield [16] (Scheme 8.7a). Perhydroxylated hexane
derivative was also protected as the bis-BOM ether without generating the acyl
migration product [17] (Scheme 8.7b). Allyl alcohol was protected with (trichlor-
oethoxy)methyl ether using (2,2,2-trichloroethoxy)methyl bromide and Proton Sponge
(1) in 70% yield [18] (Scheme 8.7c). Triisopropylsilyl (TIPS) ether formation of the
secondary alcohol with TIPS triflate and Proton Sponge (1) was observed with an
excellent yield, although other bases, such as 2,6-lutidine and H
unig
s base were not
€
effective [19] (Scheme 8.7d).
8.2.2.1
Thioether Synthesis by Hetero Michael Reaction
Kanemasa et al. reported an asymmetric conjugate addition reaction of thiol to N-crotonyl
oxazolidinone in the presence of Ni(II)-DBFOX/Ph catalyst [20] (Table 8.2). In this
reaction, Proton Sponge (1) is indispensable for high enantioselectivity, and thioether was
obtained in 84-99% yield with 91-94% ee.
8.3 Amide Formation
Sigurdsson et al. developed the synthesis of isocyanates from aliphatic amines [21]. Thus,
the reaction of an aliphatic amine with trichloromethyl chloroformate (diphosgene) in the
presence of Proton Sponge (1) (2 equiv.) at 0
C gave isocyanate in 81% yield. Azide
isocyanate was synthesized from azide amine by means of a similar procedure by Keyes
et al. [22] (Scheme 8.8).
Benzoyl quinidine (BQ) (3) catalysed
-lactam synthesis from acid chloride and tosyl
imine, as reported by Taggi et al. [23]. In this reaction, ketene generated from the acid
chloride with BQ (3) via dehydrohalogenation reacted with imine to give cis-
b
-lactam
selectively with quite high ee (Table 8.3). The BQ (3)-HCl was regenerated to BQ (3) with
Proton Sponge (1).
b
b
-Amino acid synthesis was achieved via BQ (3) catalysed alcoholysis of
-lactam
b
obtained from the N-benzoyl-
-chloroglycine and acid chlorides in the presence of Proton
Sponge (1) with high selectivity [24] (Table 8.4).
a
