Radikalische Cyclisierung von Dienen, IX. Über die Stereoselektivität der radikalischen 4‐<i>exo</i>‐trig‐Cyclisierung von optisch aktiven (2<i>E</i>)‐6‐Oxohex‐2‐ensäureestern mit Samarium(II)‐iodid

Abstract

Radical‐Type Cyclization of Dienes, IX. ‐ On the Stereoselectivity of the Radical 4‐ exo ‐trig‐Cyclization of Optically Active Ethyl (2 E )‐6‐Oxohex‐2‐enoates with Samarium(II) Iodide Ethyl (5 R , 2 E )‐5‐(benzyloxy)‐4,4‐dimethyl‐6‐oxohex‐2‐enoate ( 9 ) was prepared from ( R )‐(‐)‐pantolactone ( 3 ) by a series of straightforward reactions. The reaction of 9 with samarium(II) iodide produces enantiomerically pure ethyl (1 S ,3 R ,4 R )‐(‐)‐[3‐(benzyloxy)‐4‐hydroxy‐2,2‐dimethylcyclo‐butyl]acetate ( 10 ) in 60% yield. The absolute configuration at C‐1 and C‐3 was confirmed by the conversion of 10 to the crystalline 15a , whose absolute configuration is known from its synthesis from (1 S ,5 S )‐( + )‐α‐pinene ( 12 ). The configuration of the hydroxy group at C‐4 in 10 was determined from NOE studies on the epimiric 17 . The radical 4‐ exo ‐trig‐cyclization of optically active (2 E )‐6‐oxohex‐2‐enoates with samarium(II) iodide opens a new pathway to enantiomerically pure cyclobutanol and cyclobutanone derivatives, which are of interest as building blocks in natural products synthesis.