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

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Abstract
Radical‐Type Cyclization of Dienes, IX. ‐ On the Stereoselectivity of the Radical 4‐exo‐trig‐Cyclization of Optically Active Ethyl (2E)‐6‐Oxohex‐2‐enoates with Samarium(II) IodideEthyl (5R, 2E)‐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 (1S,3R,4R)‐(‐)‐[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 (1S,5S)‐( + )‐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 (2E)‐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.