Synthesis of pseudopterane analogues by intramolecular SE2′ cyclization

Intramolecular Barbier-type additions of allylmetal intermediates derived from the allylic halides 13 and 14 and the allylic acetate 18 to aldehydes were examined as possible routes to pseudopterolide natural products. The halides cyclized upon treatment with SnCl2 or CrCl2 to afford a mixture of diastereomeric products 16a-d. In contrast, the acetate 18 afforded largely the 1,7-syn products 16a and 16b upon treatment with 5 mol % Pd(PPh3)(4) and Et2Zn. When InI was employed in place of the Et2Zn, the syn,trans product 16a was formed exclusively. This intermediate was converted to the syn,cis- and syn,trans-butenolides 24 and 26. In the latter case, thermolysis of the tert-butyl ester at 210 degrees C followed by esterification with TMSCHN2 gave the methyl ester 35, an analogue of pseudopterolide.