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Abstract

The multicomponent reaction (MCR) of aromatic aldehydes 1 and malononitrile (2) with active methylenes 5a–h in the presence of L-proline produced pyrans and thiopyrans 6a–h stereospecifically and in good yields. Moreover a novel MCR of ethyl propiolate (8) with 1 and 2 in the presence of L-proline to afford (R)-polysubstituted pyran is also reported. X-ray structures, e.e. and optical activity of the synthesized compounds indicated that L-proline as a catalyst is responsible for the observed enantioselectivity in the studied reactions.

2. Results and Discussion

First in an attempt to synthesize the chiral pyranopyrazoles 4, we have reacted benzaldehyde (1), malononitrile (2) and pyrazolon-5-one (3) with 10% mol L-proline as the only catalyst. In contrast to Muramulla et al’s. findings that for the same reaction using L-proline alone as a catalyst under the reported reaction conditions no product was obtained, in our case after the reaction mixture was refluxed in ethanol for 4 h, the pyranopyrazole 4 was isolated in 81% yield (). To initially test if L-proline has induced any enantioselectivity in this reaction, compound 4 was tested for optical activity and found to be optically active with a specific rotation of +247.02 ([α]D, 25 °C, c = 1, DMF).

3.2. General Experimental Procedure for the Synthesis of Pyrans 6a–h and Compound 7

A mixture of benzaldehyde (1, 0.01 mol), malononitrile (2, 0.01 mol) and 10% mol L-proline was stirred at r.t. for 2 min. then active methylenes 5a–h (0.01 mol) were added. The mixture was refluxed in ethanol (10 mL) for 4–6 h followed by TLC. The crude compounds formed were recrystallized from ethanol and further purified using column chromatography using 2:1 petroleum ether/ethyl acetate as an eluent.

2. Results and Discussion

Specific rotation measurements for some selected synthesized compounds revealed that these compounds are optically active, which supports the assumption that L-proline when used as a catalyst brings about enantioselectivity in such reactions. The specific rotation of some selected compounds is listed in .

2. Results and Discussion

In addition a novel synthesis of pyran 13 could be achieved by mixing benzaldehyde (1), and malononitrile (2) with ethyl propiolate (8) in ethanol and 10% L-proline as a catalyst. It is believed that initially L-proline (9) adds to ethyl propiolate (8) affording the enamine ester 10, while benzaldehyde (1) condenses with malononitrile (2) affording 2-benzylidenemalononitrile (11). This was followed by the addition of the electron rich β-carbon in the enamine ester to the electron poor π system in the benzylidine-malononitrile 11, affording an adduct. This adduct 12 is then hydrolyzed by H2O and cyclizes into 13 (). Compound 13 was also tested for optical activity and found to be optically active with a specific rotation of +272.0 ([α]D 25 °C, c = 1, DMF). The structure of 13 has been confirmed with certainty via X-ray crystal structure determination ().