摘要:
The reactions of 2,3-dimethyl-1,3-butadiene with (E)-2-cyanocinnamates of (S)-ethyl lactate (1a) and (R)-pantolactone (1b), catalysed by TiCl4, afford enantiomerically pure cycloadducts which are easily converted into the enantiomers (1R, 6S) and (1S, 6R) of the methyl 1-cyano-3,4-dimethyl-6-phenyl-3-cyclohexen-1-carboxylate. The reactions with TiCl4 can be explained by the dienophile-TiCl4 chelate model proposed by Helmchen. The non-catalysed reaction of (1b) with the same diene takes place at 100-degrees-C, with a moderately high d.e. and reversal of selectivity.