Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

SYNTHESIS OF NITRONATES 439
These reactions produce mixtures of stereoisomeric nitronates, in which the
isomer ratio varies in a wide range depending on the structure of the starting AN,
the reaction conditions, and the nature of the diazo compound used.
3.2.1.1.2. Alkylation with Oxonium Salts Oxonium salts are the most efÞcient
alkylating agents and can react with both activated and nonactivated AN, including
nitromethane and 2-nitropropane (Scheme 3.4).
These reactions also produce mixtures of stereoisomeric nitronates, in which
the isomer ratio varies in a wide range (4, 7–9).
The starting trialkyloxonium salts can be rather simply prepared from the corresponding
dialkyl ethers. However, only trimethyl-and triethyloxonium boroßuorides
have been used in the synthesis of nitronates to prepare O-methyl and
O-ethyl nitronates, respectively.
3.2.1.1.3. Use of other Alkylating Agents Rather recently, a procedure has been
developed for the preparation of alkyl nitronates containing various substituents
adjacent to the oxygen atom under mild conditions with the use of the corresponding
alcohols as the alkylating agents (Mitsunobu reaction (10, 11)). Unfortunately,
for now, the efÞciency of this method in the synthesis of alkyl nitronates has been
demonstrated only for nitroacetate derivatives (Scheme 3.5).
Conventional alkylating agents (alkyl halides, sulfates, etc.) are rather rarely
used in the synthesis of alkyl nitronates. This is associated with relatively low
reactivity of these compounds which, combined with low thermal stability of
nitronates (for more details, see Section 3.3.1.1), does not allow one to isolate
target products in the individual state.
R
H
R′ NO 2
R′′3O + BF4 −
Base
R
R
O
N
OR′′
R = R′ = H, R′′ = Et Ref.6 ; R = H, R′ = (CH 2) 3NO 2, R′′ = Et Ref.7 ;
R = H, R′ = Alkyl, EWG, R′′ = Et Ref.4 ; R = H or Alkyl,
R′′ = Et Ref.8
EtO2CCH2NO2 + ROH
Scheme 3.4
EtO2CN NCO 2Et
PPh3; −20 ÷ 20°C
EtO2C
R = Me (82%) Ref.10 ; C6H5CH2CH2 (27%) Ref.11 ; n-C14H29 (19%) Ref.11
OR
Scheme 3.5
N
O