Highly Stereoselective Iodolactonization of 4,5-Allenoic Acids-An Efficient Synthesis of 5-(1 '-Iodo-1 '(Z)-alkenyl)-4,5-dihydro-2(3H)-furanones

In this paper, it is reported that the efficient iodolactonization of 4,5-allenoic acid with I-2, in cyclohexane in the presence or absence of K2CO3 afforded 5-(I '-iodo-I '(Z)-alkenyl)-4,5-dihydro-2(3H)-furanones highly stereo-selectively. However, the reaction of axially optically active 4,5-allenoic acids (R)-(-)-5a and (R)-(-)-5b with I-2 afforded the corresponding products with a serious loss of chirality. This problem was solved by conducting the iodolactonization with N-iodosuccinimide in CH,Cl, in the presence of Cs2CO3; however, the ZIE selectivity is somewhat lower. The pure optically active Z products were prepared by Subsequent kinetic resolution with Sonogashira coupling. The reaction of the substrates with a substituent at the 3-position of the starting 4,5-allenoic acids afforded the trans-4,5-disubstituted gamma-butyrlactones as the only products. The reaction of the 4.5-allenoic acids (S)-(+)-11, (R)-(-)-11, and (S)(+)-1 m In with a center chirality at the 3-position afforded the trans products with very high enatiopurity and up to 98:2 Z/E selectivity regardless Of the axial chirality of the allene moiety.