Acid Halolactonization, selective

Similar asymmetric induction is observed on halolactonization of 7,8-unsaturated thioamides. These substrates, unlike the amides, undergo highly selective protiothiolac-tonization with (f-(-l-)-IO-camphorsulfonic acid,... 

Iodolactonization of yfi-unsaturated amides. Halolactonization (I2 or NBS) of a-substituted "y,8-unsaturated amides in DME/H,0 at room temperature gives predominantly fra/w-2,4-disubstituted y-butyrolactones (equation I). This 1,3-stereoselectivity is in sharp contrast to the moderate 1,3-cw-selectivity observed with a-substituted y,8-unsaturated acids (8,257 9,248). Both diastereomers of a, 3-disubstituted y.S-unsaturated amides are converted into 2.4-rram-2,3,4-trisub tituted lactones (equation II). 

The effect of a 2-nitrogen substituent on the halolactonization reaction of 4-pentenoic acid derivatives has been examined40. A relatively high 1,3-cm selectivity is observed, in contrast to the selectivities observed with a nonpolar substituent (e.g., R = CH3, C6H5). 

High cis selectivity has been observed in the halolactonization of 2-amino-5-substituted (Z)- and ( )-4-pentenoie acids. Treatment of (S)-(Z)-2-/m-butoxycarbonylamino-6-hydroxy-4-hexenoic acid [(S)-(Z)-8] with A -bromosuccinitrude in tetrahydrofuran at 0 °C for 5 minutes gives a 90 10 mixture of the cisjtrans-y-lactone 9 in 95% yield41. The major n.s-isomer can be purified by crystallization from chloroform/hexane. 

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