2,3-Allenoates, enantioselective synthesis

SCHEME 14.39. Enantioselective synthesis of y-thioethers from 2,3-allenoates. 

In 2012, the Sun group developed an efficient NHC-catalyzed internal redox reaction of alkynals bearing a y-leaving group. This process provides a new route to a range of allenoates in good yields. Preliminary results demonstrate that the enantioselective variant can also be achieved (one example with 89% yield and 30% ee). Albeit with low enantioselectivity, this first successful exploitation of an allenolate intermediate for allenoate formation provides a versatile platform for further development of new NHC-catalyzed reactions of alkynals. Without the use of transition metals and the production of phosphine oxide waste, this process serves as a green alternative for the synthesis of allenoates (Scheme 7.92). 

The enantioselective Lewis-acid-promoted 2 -t- 2-cycloaddition of trifluoropyruvate with various alkynes produced stable oxetene derivatives for use in the synthesis of pharmaceuticals and agrochemicals. l,4-diazabicyclo[2.2.2]octane (DABCO)-catalysed 2- -2-cycloaddition of allenoates and trifluoromethylketones produced 2-alkyleneoxetanes in good yields and with good diastereo-selectivities dr > 20 1). A cinchona alkaloid (20) catalysed the 2- -2-cycloaddition between tV-sulfonylimines and alkyl 2,3-butadienoates to form / -configured 2,4-disubstituted azetidines in high yields and with high enantioselectivities. ... 

Phosphine-Catalyzed Reactions Enantioselective [3+2] cycloaddition of allenes with enones leads to the synthesis of cyclopentenes, which can be further transformed to cyclopentanes (Scheme 6.4). The phosphine addition to allenoate generates an allylic carbanion, which undergoes addition to the enone in the a and the p positions followed by the phosphine elimination to afford 3 and 4, respeetively [6]. 

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