STEREOSELECTIVE SYNTHESIS OF 2-OXAZOLINE-4-CARBOXYLATES THROUGH LEWISACID-CATALYZED FORMAL [3-ALKOXYOXAZOLES WITHALDEHYD ES - CATALYTIC EFFECT OF METHYLALUMINUM BETA-BINAPHTHOXIDE ON CIS-SELECTI VITY(2] CYCLOADDITIONS OF 5)

The formal [3 + 2] cycloadditions of 5-methoxy-2-(p-methoxyphenyl)oxaz ole with benzaldehyde, para- and meta-substituted benzaldehydes, propa nal, cinnamaldehyde, and heterocyclic carboxaldehydes in the presence of methylaluminum beta-binaphthoxide gave the corresponding methyl kyl -2-(p-methoxyphenyl)-2-oxazoline-4-carboxylates with high cis-selectiv ity (up to 98%). The use of ortho-substituted benzaldehydes resulted i n a decrease in the cis-selectivity. The reaction of 5-methoxy-2-(p-me thoxyphenyl)oxazole with benzaldehyde in the presence of titanium(IV) chloride or tin(IV) chloride gave the corresponding trans-2-oxazoline- 4-carboxylate with 85-86% trans-selectivity. The reaction of ethyl gly oxylate with 5-ethoxy-2-phenyloxazole catalyzed by a 1:1 mixture of ti tanium(IV) chloride and titanium tetraisopropoxide gave diethyl 2-phen yl-2-oxazoline-4,5-dicarboxylate with a preference for cis-selectivity (cis/trans = 84:6). The cis-selectivity of the reaction in the presen ce of methylaluminum beta-binaphthoxide can be explained by an antiper iplanar approach of the C4-C5 double bond of the oxazole to the aldehy de coordinated to the catalyst, followed by ring opening of the oxazol e through a stepwise pathway involving zwitterionic intermediates.