Ethyl propiolate Diels-Alder reactions

Functionalized nitroalkenes are important chenophiles in the Diels-Alder ri example, fE -methyl fi-nitroactylate is an impottant reagent in organic synthesis The nitre group can be readily eliminated the Diels-Alder reaction of fi-nitroactylate is equivalent to that of ethyl propiolate with an inverse regiochemistry fEq. 8.4. ... 

The iV-aminopyrrole - benzene ring methodology has been applied to a synthesis of the 9,10-dihydrophenanthrene juncusol (218) (81TL1775). Condensation of the tetralone (213) with pyrrolidine and reaction of the enamine with ethyl 3-methoxycarbonylazo-2-butenoate gave pyrrole (214). Diels-Alder reaction of (214) with methyl propiolate produced a 3 1 mixture of (215) and its isomer in 70% yield. Pure (215) was reduced selectively with DIBAL to the alcohol, reoxidized to aldehyde, and then treated with MCPBA to generate formate (216). Saponification to the phenol followed by O-methylation and lithium aluminum hydride reduction of the hindered ester afforded (217), an intermediate which had been converted previously to juncusol (Scheme 46). 

Attempts to isolate 2,3-dimethoxyfuran (156) have, as yet, been fruitless (79JCS(P1)1893), but it may be generated in situ and trapped with the propiolate (155) the initial adducts (157) are unstable under the acidic conditions and yield the biphenyls (158) and (159) (Scheme 67). 2,5-Bis(trimethylsilyloxy)furans, readily available from succinic anhydrides in one step, are also more reactive than furan in Diels-Alder reactions (80TL3423). They readily undergo reaction with both DMAD and ethyl acrylate. Thus at 50 °C in carbon tetrachloride the furan (160) with DMAD followed by detrimethylsilylation gave only the quinone (163). At 80 °C, however, the hydroquinone (164) is the major product. Both the intermediates (161) and (162) may be detected by ]H NMR spectroscopy. The formation... 

The hetero-Diels-Alder reaction is amongst the most efficient processes for the synthesis of six-membered heterocyclic ring systems. Solvent-free conditions have been used to improve reactions of heterodienophiles and heterodynes with low reactivities. Cado et al. (1997) have described the hetero-Diels-Alder reaction of ethyl lH-perimidine-2-acetate as heterocyclic ketene aminal with ethyl propiolate nnder solvent-free conditions with focused microwave irradiation. The new fused perimi-dines (23) were obtained in good yields (67-98%). 

The pyranothiophenes (577), prepared in reasonable yields from commerically available thio-phene-3-acetic acid in a single step, are stable thiophene-2,3-quinodimethane equivalents <88TL5817, 90JCS(PI)68I>. Thus, they undergo Diels-Alder reaction with DMAD to give benzo[6]thiophenes (578) after loss of carhon dioxide, and with ethyl propiolate to give isomeric mixtures of (57 ) and... 

The intramolecular [2+2+1] cycloaddition promoted by pentacarbonyliron can also be extended to silyl protected yne-ynamides (Scheme 4-7) and yne-ynol ethers (Scheme 4—8) ° to give after demetalation cyclopentadienones with fused N- or 0-heterocycles of different ring size. Subsequent Diels-Alder reaction with dimethyl acetylenedicarboxylate (DMAD) or ethyl propiolate transforms the cyclopentadienone moiety into the corresponding benzene derivative. 

Vinylpyrazoles undergo Diels-Alder cycloadditions within 6-30 min to give acceptable yields of easily purified products. This methodology eliminates the most important drawbacks of the classical conditions and allows the reaction to be extended to poorly reactive dienophiles, such as ethyl phenyl propiolate, not accessible by classical heating [Eq. (79)]. 

The 1,2-thiaphospholes (20) react thermally with norbornadiene, norbornene, or diethyl azo-dicarboxylate to produce double Diels Alder cycloadducts (24) and (25). With mixtures of norbornadiene and methyl acrylate or acrylonitrile, crossed double Diels-Alder cycloadducts are obtained. In the presence of AICI3 the 1,2-thiaphospholes (20) react with acrylic esters, acrylonitrile or methyl vinyl ketone to give tandem Diels-Alder/Michael adducts such as (26). The AICI3-promoted reaction of (20) with ethyl propiolate affords with the elimination of a sulfur atom the stable phosphinine (27) (Scheme 4) <94BCJ2785>. 

Reactions of Oxazoles. 2-, 4-, and 5-Nitro-oxazoles can be prepared by the action of dinitrogen tetroxide on the corresponding iodo-compounds. Whereas oxazoles are metallated at C-5, the acid (371) undergoes lithiation at the methyl substituent.The reaction of 2,4-dimethyloxazole with ethyl propiolate yields a mixture of the furans (373) and (374) these are formed by extrusion of acetonitrile from intermediate Diels—Alder adducts such as (372). s3... 

---