or substituted acetic acid unit and can, in principle, be effected with tert-butyl, methyl, and trimethylsilyl ester Supplementary Material Available: Detailed spectroscopic data (IR and NMR) for compounds 10a,d, 11,12a,b, and 14a,b (2 pages). Ordering information is given on any current masthead page.(21) Under these reaction conditions trans olefinic -keto esters 14a and 15a did not undergo ester hydrolysis or isomerization to the cis isomers. Treatment of pure trans-15a with HBF4 for 24 h afforded only recovered ester. The crude products were a mixture of -pyrones 20 and 21, trans -keto esters 14a and 15a, and uncyclized cis -keto acids 14b and 15b, respectively. Exposure of the initial 1,2-addition product to HBF4 for shorter periods of time (2-4 h) afforded larger quantities of -keto acid 14b. Attempted separation of these product mixtures by chromatography on silica gel afforded pure a-pyrones 20 and 21 in yields that were unaffected by the quantity of -keto acids 14b and 15b present in the original mixture. Similarly, -keto acid 11 underwent cyclization to -pyrone 17 when subjected to medium-pressure liquid chromatography (MPLC) on silica gel.(22) When the reaction of 8 with ieri-butyl lithioacetate was quenched with 1.2 equiv of 10% HBF4 14a was isolated as a 5:1 Z/E mixture of stereoisomers. The structures of these stereoisomers were assigned on the basis of proton NMR chemical shifts for the methylene protons (CDC13; E 3.01, Z 3.53). A methyl substituent cis to the carbonyl functionality in /3-methyl-a,/S-unsaturated carbonyl compounds resonates downfield in the NMR spectrum relative to the trans methyl substituent:
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